Revealing the ultrafast outflow in IRAS 13224-3809 through spectral variability

NASA Astrophysics Data System (ADS)

Parker, M. L.; Alston, W. N.; Buisson, D. J. K.; Fabian, A. C.; Jiang, J.; Kara, E.; Lohfink, A.; Pinto, C.; Reynolds, C. S.

2017-08-01

We present an analysis of the long-term X-ray variability of the extreme narrow-line Seyfert 1 galaxy IRAS 13224-3809 using principal component analysis (PCA) and fractional excess variability (Fvar) spectra to identify model-independent spectral components. We identify a series of variability peaks in both the first PCA component and Fvar spectrum which correspond to the strongest predicted absorption lines from the ultrafast outflow (UFO) discovered by Parker et al. (2017). We also find higher order PCA components, which correspond to variability of the soft excess and reflection features. The subtle differences between RMS and PCA results argue that the observed flux-dependence of the absorption is due to increased ionization of the gas, rather than changes in column density or covering fraction. This result demonstrates that we can detect outflows from variability alone and that variability studies of UFOs are an extremely promising avenue for future research.

Multi-phase functionalization of titanium for enhanced photon absorption in the vis-NIR region.

PubMed

Thakur, Pooja; Tan, Bo; Venkatakrishnan, Krishnan

2015-10-19

Inadequate absorption of Near Infrared (NIR) photons by conventional silicon solar cells has been a major stumbling block towards the attainment of a high efficiency "full spectrum" solar cell. An effective enhancement in the absorption of such photons is desired as they account for a considerable portion of the tappable solar energy. In this work, we report a remarkable gain observed in the absorption of photons in the near infrared and visible region (400 nm-1000 nm) by a novel multi-phased oxide of titanium. Synthesised via a single step ultra-fast laser pulse interaction with pure titanium, characterisation studies have identified this oxide of titanium to be multi-phased and composed of Ti3O, (TiO.716)3.76 and TiO2 (rutile). Computed to have an average band gap value of 2.39 eV, this ultrafast laser induced multi-phased titanium oxide has especially exhibited steady absorption capability in the NIR range of 750-1000 nm, which to the best of our knowledge, was never reported before. The unique NIR absorption properties of the laser functionalised titanium coupled with the simplicity and versatility of the ultrafast laser interaction process involved thereby provides tremendous potential towards the photon sensitization of titanium and thereafter for the inception of a "full spectrum" solar device.

Broadband ultrafast nonlinear absorption and nonlinear refraction of layered molybdenum dichalcogenide semiconductors.

PubMed

Wang, Kangpeng; Feng, Yanyan; Chang, Chunxia; Zhan, Jingxin; Wang, Chengwei; Zhao, Quanzhong; Coleman, Jonathan N; Zhang, Long; Blau, Werner J; Wang, Jun

2014-09-21

A series of layered molybdenum dichalcogenides, i.e., MoX₂ (X = S, Se and Te), were prepared in cyclohexyl pyrrolidinone by a liquid-phase exfoliation technique. The high quality of the two-dimensional nanostructures was verified by transmission electron microscopy and absorption spectroscopy. Open- and closed-aperture Z-scans were employed to study the nonlinear absorption and nonlinear refraction of the MoX₂ dispersions, respectively. All the three-layered nanostructures exhibit prominent ultrafast saturable absorption (SA) for both femtosecond (fs) and picosecond (ps) laser pulses over a broad wavelength range from the visible to the near infrared. While the dispersions treated with low-speed centrifugation (1500 rpm) have an SA response, and the MoS₂ and MoSe₂ dispersions after higher speed centrifugation (10,000 rpm) possess two-photon absorption for fs pulses at 1030 nm, which is due to the significant reduction of the average thickness of the nanosheets; hence, the broadening of band gap. In addition, all dispersions show obvious nonlinear self-defocusing for ps pulses at both 1064 nm and 532 nm, resulting from the thermally-induced nonlinear refractive index. The versatile ultrafast nonlinear properties imply a huge potential of the layered MoX2 semiconductors in the development of nanophotonic devices, such as mode-lockers, optical limiters, optical switches, etc.

Optical properties of transiently-excited semiconductor hyperbolic metamaterials

DOE PAGES

Campione, Salvatore; Luk, Ting S.; Liu, Sheng; ...

2015-10-02

Ultrafast optical excitation of photocarriers has the potential to transform undoped semiconductor superlattices into semiconductor hyperbolic metamaterials (SHMs). In this paper, we investigate the optical properties associated with such ultrafast topological transitions. We first show reflectance, transmittance, and absorption under TE and TM plane wave incidence. In the unpumped state, the superlattice exhibits a frequency region with high reflectance (>80%) and a region with low reflectance (<1%) for both TE and TM polarizations over a wide range of incidence angles. In contrast, in the photopumped state, the reflectance for both frequencies and polarizations is very low (<1%) for a similarmore » range of angles. Interestingly, this system can function as an all-optical reflection switch on ultrafast timescales. Furthermore, for TM incidence and close to the epsilon-near-zero point of the longitudinal permittivity, directional perfect absorption on ultrafast timescales may also be achieved. Lastly, we discuss the onset of negative refraction in the photopumped state.« less

Spectroscopic Studies of Carotenoid-to-Bacteriochlorophyll Energy Transfer in LHRC Photosynthetic Complex from Roseiflexus castenholzii

DOE Office of Scientific and Technical Information (OSTI.GOV)

Niedzwiedzki, Dariusz; Collins, Aaron M.; LaFountain, Amy M.

Carotenoids present in the photosynthetic light-harvesting reaction center (LHRC) complex from chlorosome lacking filamentous anoxygenic phototroph, Roseiflexus castenholzii were purified and characterized for their photochemical properties. The LHRC from anaerobically grown cells contains five different carotenoids, methoxy-keto-myxocoxanthin, γ-carotene, and its three derivatives, whereas the LHRC from aerobically grown cells contains only three carotenoid pigments with methoxy-keto-myxocoxanthin being the dominant one. The spectroscopic properties and dynamics of excited singlet states of the carotenoids were studied by steady-state absorption, fluorescence and ultrafast time-resolved optical spectroscopy in organic solvent and in the intact LHRC complex. Time-resolved transient absorption spectroscopy performed in the near-infraredmore » (NIR) on purified carotenoids combined with steady-state absorption spectroscopy led to the precise determination of values of the energies of the S 1(2 1A g -) excited state. Global and single wavelength fitting of the ultrafast spectral and temporal data sets of the carotenoids in solvents and in the LHRC revealed the pathways of de-excitation of the carotenoid excited states.« less

Ultrafast excited-state deactivation of 9-methylhypoxanthine in aqueous solution: A QM/MM MD study.

PubMed

Guo, Xugeng; Yuan, Huijuan; An, Beibei; Zhu, Qiuling; Zhang, Jinglai

2016-04-21

Photoinduced ultrafast non-adiabatic decay of 9-methylhypoxanthine (9MHPX) in aqueous solution was investigated by ab initio surface-hopping dynamics calculations using a combined quantum mechanical/molecular mechanical approach. The absorption spectra of 9MHPX in aqueous solution were also explored by the hybrid cluster-continuum model at the level of time-dependent density functional theory along with the polarizable continuum model (PCM). The static electronic-structure calculations indicate that the absorption spectra of 9MHPX simulated by TD-B3LYP/PCM and TD-X3LYP/PCM can reproduce very well the experimental findings, with the accuracy of about 0.20 eV. According to dynamics simulations, irradiation of 9MHPX populates the bright excited singlet S1 state, which may undergo an ultrafast non-radiative deactivation to the S0 state. The lifetime of the S1 state of 9MHPX in aqueous solution is predicted to be 115.6 fs, slightly longer than that in the gas phase (88.8 fs), suggesting that the solventwater has no significant influence on the excited-state lifetime of 9MHPX. Such a behavior in 9MHPX is distinctly different from its parent hypoxanthine keto-N9H tautomer in which the excited-state lifetime of the latter in watersolution was remarkably enhanced as compared to the gas phase. The significant difference of the photodynamical behaviors between 9MHPX and keto-N9H can be ascribed to their different hydrogen bond environment in aqueous solution.

Ultrafast electron and energy transfer in dye-sensitized iron oxide and oxyhydroxide nanoparticles.

PubMed

Gilbert, Benjamin; Katz, Jordan E; Huse, Nils; Zhang, Xiaoyi; Frandsen, Cathrine; Falcone, Roger W; Waychunas, Glenn A

2013-10-28

An emerging area in chemical science is the study of solid-phase redox reactions using ultrafast time-resolved spectroscopy. We have used molecules of the photoactive dye 2',7'-dichlorofluorescein (DCF) anchored to the surface of iron(III) oxide nanoparticles to create iron(II) surface atoms via photo-initiated interfacial electron transfer. This approach enables time-resolved study of the fate and mobility of electrons within the solid phase. However, complete analysis of the ultrafast processes following dye photoexcitation of the sensitized iron(III) oxide nanoparticles has not been reported. We addressed this topic by performing femtosecond transient absorption (TA) measurements of aqueous suspensions of uncoated and DCF-sensitized iron oxide and oxyhydroxide nanoparticles, and an aqueous iron(III)-dye complex. Following light absorption, excited state relaxation times of the dye of 115-310 fs were found for all samples. Comparison between TA dynamics on uncoated and dye-sensitized hematite nanoparticles revealed the dye de-excitation pathway to consist of a competition between electron and energy transfer to the nanoparticles. We analyzed the TA data for hematite nanoparticles using a four-state model of the dye-sensitized system, finding electron and energy transfer to occur on the same ultrafast timescale. The interfacial electron transfer rates for iron oxides are very close to those previously reported for DCF-sensitized titanium dioxide (for which dye-oxide energy transfer is energetically forbidden) even though the acceptor states are different. Comparison of the alignment of the excited states of the dye and the unoccupied states of these oxides showed that the dye injects into acceptor states of different symmetry (Ti t2gvs. Fe eg).

A Search for H I Lyα Counterparts to Ultrafast X-Ray Outflows

NASA Astrophysics Data System (ADS)

Kriss, Gerard A.; Lee, Julia C.; Danehkar, Ashkbiz

2018-06-01

Prompted by the H I Lyα absorption associated with the X-ray ultrafast outflow at ‑17,300 km s‑1 in the quasar PG 1211+143, we have searched archival UV spectra at the expected locations of H I Lyα absorption for a large sample of ultrafast outflows identified in XMM-Newton and Suzaku observations. Sixteen of the X-ray outflows have predicted H I Lyα wavelengths falling within the bandpass of spectra from either the Far Ultraviolet Spectroscopic Explorer or the Hubble Space Telescope, although none of the archival observations were simultaneous with the X-ray observations in which ultrafast X-ray outflows (UFOs) were detected. In our spectra broad features with FWHM of 1000 km s‑1 have 2σ upper limits on the H I column density of generally ≲2 × 1013 cm‑2. Using grids of photoionization models covering a broad range of spectral energy distributions (SEDs), we find that producing Fe XXVI Lyα X-ray absorption with equivalent widths >30 eV and associated H I Lyα absorption with {N}{{H}{{I}}}< 2× {10}13 {cm}}-2 requires total absorbing column densities {N}{{H}}> 5× {10}22 {cm}}-2 and ionization parameters log ξ ≳ 3.7. Nevertheless, a wide range of SEDs would predict observable H I Lyα absorption if ionization parameters are only slightly below peak ionization fractions for Fe XXV and Fe XXVI. The lack of Lyα features in the archival UV spectra indicates that the UFOs have very high ionization parameters, that they have very hard UV-ionizing spectra, or that they were not present at the time of the UV spectral observations owing to variability.

Carrier-Specific Femtosecond XUV Transient Absorption of PbI 2 Reveals Ultrafast Nonradiative Recombination

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lin, Ming-Fu; Verkamp, Max A.; Leveillee, Joshua

Femtosecond carrier recombination in PbI 2 is measured using tabletop high-harmonic extreme ultraviolet (XUV) transient absorption spectroscopy and ultrafast electron diffraction. XUV absorption from 45 eV to 62 eV measures transitions from the iodine 4d core level to the conduction band density of states. Photoexcitation at 400 nm creates separate and distinct transient absorption signals for holes and electrons, separated in energy by the 2.4 eV band gap of the semiconductor. The shape of the conduction band and therefore the XUV absorption spectrum is temperature dependent, and nonradiative recombination converts the initial electronic excitation to thermal excitation within picoseconds. Ultrafastmore » electron diffraction (UED) is used to measure the lattice temperature and confirm the recombination mechanism. Lastly, the XUV and UED results support a 2nd-order recombination model with a rate constant of 2.5x10 -9 cm 3/s.« less

Carrier-Specific Femtosecond XUV Transient Absorption of PbI 2 Reveals Ultrafast Nonradiative Recombination

DOE PAGES

Lin, Ming-Fu; Verkamp, Max A.; Leveillee, Joshua; ...

2017-11-30

Femtosecond carrier recombination in PbI 2 is measured using tabletop high-harmonic extreme ultraviolet (XUV) transient absorption spectroscopy and ultrafast electron diffraction. XUV absorption from 45 eV to 62 eV measures transitions from the iodine 4d core level to the conduction band density of states. Photoexcitation at 400 nm creates separate and distinct transient absorption signals for holes and electrons, separated in energy by the 2.4 eV band gap of the semiconductor. The shape of the conduction band and therefore the XUV absorption spectrum is temperature dependent, and nonradiative recombination converts the initial electronic excitation to thermal excitation within picoseconds. Ultrafastmore » electron diffraction (UED) is used to measure the lattice temperature and confirm the recombination mechanism. Lastly, the XUV and UED results support a 2nd-order recombination model with a rate constant of 2.5x10 -9 cm 3/s.« less

THz-circuits driven by photo-thermoelectric, gate-tunable graphene-junctions

NASA Astrophysics Data System (ADS)

Brenneis, Andreas; Schade, Felix; Drieschner, Simon; Heimbach, Florian; Karl, Helmut; Garrido, Jose A.; Holleitner, Alexander W.

2016-10-01

For future on-chip communication schemes, it is essential to integrate nanoscale materials with an ultrafast optoelectronic functionality into high-frequency circuits. The atomically thin graphene has been widely demonstrated to be suitable for photovoltaic and optoelectronic devices because of its broadband optical absorption and its high electron mobility. Moreover, the ultrafast relaxation of photogenerated charge carriers has been verified in graphene. Here, we show that dual-gated graphene junctions can be functional parts of THz-circuits. As the underlying optoelectronic process, we exploit ultrafast photo-thermoelectric currents. We describe an immediate photo-thermoelectric current of the unbiased device following a femtosecond laser excitation. For a picosecond time-scale after the optical excitation, an additional photo-thermoelectric contribution shows up, which exhibits the fingerprint of a spatially inverted temperature profile. The latter can be understood by the different time-constants and thermal coupling mechanisms of the electron and phonon baths within graphene to the substrate and the metal contacts. The interplay of the processes gives rise to ultrafast electromagnetic transients in high-frequency circuits, and it is equally important for a fundamental understanding of graphene-based ultrafast photodetectors and switches.

THz-circuits driven by photo-thermoelectric, gate-tunable graphene-junctions

PubMed Central

Brenneis, Andreas; Schade, Felix; Drieschner, Simon; Heimbach, Florian; Karl, Helmut; Garrido, Jose A.; Holleitner, Alexander W.

2016-01-01

For future on-chip communication schemes, it is essential to integrate nanoscale materials with an ultrafast optoelectronic functionality into high-frequency circuits. The atomically thin graphene has been widely demonstrated to be suitable for photovoltaic and optoelectronic devices because of its broadband optical absorption and its high electron mobility. Moreover, the ultrafast relaxation of photogenerated charge carriers has been verified in graphene. Here, we show that dual-gated graphene junctions can be functional parts of THz-circuits. As the underlying optoelectronic process, we exploit ultrafast photo-thermoelectric currents. We describe an immediate photo-thermoelectric current of the unbiased device following a femtosecond laser excitation. For a picosecond time-scale after the optical excitation, an additional photo-thermoelectric contribution shows up, which exhibits the fingerprint of a spatially inverted temperature profile. The latter can be understood by the different time-constants and thermal coupling mechanisms of the electron and phonon baths within graphene to the substrate and the metal contacts. The interplay of the processes gives rise to ultrafast electromagnetic transients in high-frequency circuits, and it is equally important for a fundamental understanding of graphene-based ultrafast photodetectors and switches. PMID:27762291

THz-circuits driven by photo-thermoelectric, gate-tunable graphene-junctions.

PubMed

Brenneis, Andreas; Schade, Felix; Drieschner, Simon; Heimbach, Florian; Karl, Helmut; Garrido, Jose A; Holleitner, Alexander W

2016-10-20

For future on-chip communication schemes, it is essential to integrate nanoscale materials with an ultrafast optoelectronic functionality into high-frequency circuits. The atomically thin graphene has been widely demonstrated to be suitable for photovoltaic and optoelectronic devices because of its broadband optical absorption and its high electron mobility. Moreover, the ultrafast relaxation of photogenerated charge carriers has been verified in graphene. Here, we show that dual-gated graphene junctions can be functional parts of THz-circuits. As the underlying optoelectronic process, we exploit ultrafast photo-thermoelectric currents. We describe an immediate photo-thermoelectric current of the unbiased device following a femtosecond laser excitation. For a picosecond time-scale after the optical excitation, an additional photo-thermoelectric contribution shows up, which exhibits the fingerprint of a spatially inverted temperature profile. The latter can be understood by the different time-constants and thermal coupling mechanisms of the electron and phonon baths within graphene to the substrate and the metal contacts. The interplay of the processes gives rise to ultrafast electromagnetic transients in high-frequency circuits, and it is equally important for a fundamental understanding of graphene-based ultrafast photodetectors and switches.

Dynamic Optoelectronic Properties in Perovskite Oxide Thin Films Measured with Ultrafast Transient Absorption & Reflectance Spectroscopy

NASA Astrophysics Data System (ADS)

Smolin, Sergey Y.

Ultrafast transient absorption and reflectance spectroscopy are foundational techniques for studying photoexcited carrier recombination mechanisms, lifetimes, and charge transfer rates. Because quantifying photoexcited carrier dynamics is central to the intelligent design and improvement of many solid state devices, these transient optical techniques have been applied to a wide range of semiconductors. However, despite their promise, interpretation of transient absorption and reflectance data is not always straightforward and often relies on assumptions of physical processes, especially with respect to the influence of heating. Studying the material space of perovskite oxides, the careful collection, interpretation, and analysis of ultrafast data is presented here as a guide for future research into novel semiconductors. Perovskite oxides are a class of transition metal oxides with the chemical structure ABO3. Although traditionally studied for their diverse physical, electronic, and magnetic properties, perovskite oxides have gained recent research attention as novel candidates for light harvesting applications. Indeed, strong tunable absorption, unique interfacial properties, and vast chemical flexibility make perovskite oxides a promising photoactive material system. However, there is limited research characterizing dynamic optoelectronic properties, such as recombination lifetimes, which are critical to know in the design of any light-harvesting device. In this thesis, ultrafast transient absorption and reflectance spectroscopy was used to understand these dynamic optoelectronic properties in highquality, thin (<50 nm) perovskite oxide films grown by molecular beam epitaxy. Starting with epitaxial LaFeO3 (LFO) grown on (LaAlO 3)0.3(Sr2AlTaO6)0.7 (LSAT), transient absorption spectroscopy reveals two photoinduced absorption features at the band gap of LFO at 2.4 eV and at the higher energy absorption edge at 3.5 eV. Using a combination of temperature-dependent, variable-angle spectroscopic ellipsometry and time-resolved ultrafast optical spectroscopy on a type I heterostructure, we clarify thermal and electronic contributions to spectral transients in LaFeO3. Upon comparison to thermally-derived static spectra of LaFeO3, we find that thermal contributions dominate the transient absorption and reflectance spectra above the band gap. A transient photoinduced absorption feature below the band gap at 1.9 eV is not reproduced in the thermally derived spectra and has significantly longer decay kinetics from the thermallyinduced features; therefore, this long lived photoinduced absorption is likely derived, at least partially, from photoexcited carriers with lifetimes much longer than 3 nanoseconds. LaFeO3 has a wide band gap of 2.4 eV but its absorption can be decreased with chemical substitution of Sr for Fe to make it more suitable for various applications. This type of A-site substitution is a common route to change static optical absorption in perovskite oxides, but there are no systematic studies looking at how A-site substitution changes dynamic optoelectronic properties. To understand the relationship between composition and static and dynamic optical properties we worked with the model system of La1-xSrxFeO 3-delta epitaxial films grown on LSAT, uncovering the effects of A-site cation substitution and oxygen stoichiometry. Variable-angle spectroscopic ellipsometry was used to measure static optical properties, revealing a linear increase in absorption coefficient at 1.25 eV and a red-shifting of the optical absorption edge with increasing Sr fraction. The absorption spectra can be similarly tuned through the introduction of oxygen vacancies, indicating the critical role that nominal Fe valence plays in optical absorption. Dynamic optoelectronic properties were studied with ultrafast transient reflectance spectroscopy with broadband visible (1.6 eV to 4 eV) and near-infrared (0.9 eV to 1.5 eV) probes. The sign of the reflectance change in the near-infrared region in LSFO is indicative of carrier bandfilling of newly created electronic states by photoexcited carriers. Moreover, we find that similar transient spectral trends can be induced with A-site substitution or through oxygen vacancies, which is a surprising result. Probing the near-infrared region reveals similar nanosecond (1-3 ns) photoexcited carrier lifetimes for oxygen deficient and stoichiometric films. These results demonstrate that while the static optical absorption is strongly dependent on nominal Fe valence tuned through cation or anion stoichiometry, oxygen vacancies do not appear to play a significantly detrimental role in long lived recombination kinetics. Although this thesis represents one of the first comprehensive studies using broad band transient absorption and reflectance spectroscopy to study dynamic optoelectronic phenomena in perovskite oxides, it can also serve as a guide for the implementation and interpretation of ultrafast spectroscopy in other material systems. Moreover, the ultrafast work on perovskite oxides indicates that these materials have long nanosecond lifetimes required for light harvesting devices and should be investigated further.

Ultrafast dynamics of hard tissue ablation using fs-lasers.

PubMed

Domke, Matthias; Wick, Sebastian; Laible, Maike; Rapp, Stephan; Huber, Heinz P; Sroka, Ronald

2018-05-29

Several studies on hard tissue laser ablation demonstrated that ultrafast lasers enable precise material removal without thermal side effects. Although the principle ablation mechanisms have been thoroughly investigated, there are still open questions regarding the influence of material properties on transient dynamics. In this investigation, we applied pump-probe microscopy to record ablation dynamics of biomaterials with different tensile strengths (dentin, chicken bone, gallstone, kidney stones) at delay times between 1 ps and 10 μs. Transient reflectivity changes, pressure and shock wave velocities, and elastic constants were determined. The result revealed that absorption and excitation show the typical well-known transient behaviour of dielectric materials. We observed for all samples a photomechanical laser ablation process, where ultrafast expansion of the excited volume generates pressure waves leading to fragmentation around the excited region. Additionally, we identified tensile-strength-related differences in the size of ablated craters and ejected particles. The elastic constants derived were in agreement with literature values. In conclusion, pressure-wave-assisted material removal seems to be a general mechanism for hard tissue ablation with ultrafast lasers. This photomechanical process increases ablation efficiency and removes heated material, thus ultrafast laser ablation is of interest for clinical application where heating of the tissue must be avoided. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.

Photoionization and electron radical recombination dynamics in photoactive yellow protein investigated by ultrafast spectroscopy in the visible and near-infrared spectral region.

PubMed

Zhu, Jingyi; Paparelli, Laura; Hospes, Marijke; Arents, Jos; Kennis, John T M; van Stokkum, Ivo H M; Hellingwerf, Klaas J; Groot, Marie Louise

2013-09-26

Photoinduced ionization of the chromophore inside photoactive yellow protein (PYP) was investigated by ultrafast spectroscopy in the visible and near-infrared spectral regions. An absorption band that extended from around 550 to 850 nm was observed and ascribed to solvated electrons, ejected from the p-hydroxycinnamic acid anion chromophore upon the absorption of two 400 nm photons. Global kinetic analysis showed that the solvated electron absorption decayed in two stages: a shorter phase of around 10 ps and a longer phase of more than 3 ns. From a simulation based on a diffusion model we conclude that the diffusion rate of the electron is about 0.8 Å(2)/ps in wild type PYP, and that the electron is ejected to a short distance of only several angstroms away from the chromophore. The chromophore-protein pocket appears to provide a water-similar local environment for the electron. Because mutations at different places around the chromophore have different effect on the electron recombination dynamics, we suggest that solvated electrons could provide a new method to investigate the local dielectric environment inside PYP and thus help to understand the role of the protein in the photoisomerization process.

Broadband ultrafast nonlinear absorption and ultra-long exciton relaxation time of black phosphorus quantum dots.

PubMed

Chen, Runze; Zheng, Xin; Jiang, Tian

2017-04-03

Black phosphorus (BP) has recently attracted significant attention for its brilliant physical and chemical features. The remarkable strong light-matter interaction and tunable direct wide range band-gap make it an ideal candidate in various application regions, especially saturable absorbers. In this paper, ultrasmall black phosphorus quantum dots (BPQDs), a unique form of phosphorus nanostructures, with average size of 5.7 ± 0.8 nm are synthesized. Compared with BP nanosheets (BPNs) with similar thickness, the ultrafast nonlinear optical (NLO) absorption properties and excited carrier dynamics are investigated in wide spectra. Beyond the saturation absorption (SA), giant two photon absorption (TPA) is observed in BPQDs. BPQDs exhibit quite different excitation intensity and wavelength dependent nonlinear optical (NLO) response from BPNs, which is attributed to the quantum confinement and edge effects. The BPQDs show broadband photon-induced absorption (PIA) under the probe wavelength from 470 nm to 850 nm and a fast and a slow decay time are obtained as long as 92 ± 10 ps and 1100 ± 100 ps, respectively. The substantial independence for ultra-long time scales of pump intensity and temperature reveals that the carrier recombination mechanism may be attributed to a defect-assisted Auger capture process. These findings will help to develop optoelectronic and photonic devices operating in the infrared and visible wavelength region.

Absorption bleaching of squarylium dye J aggregates via a two-photon excitation process

NASA Astrophysics Data System (ADS)

Furuki, Makoto; Tian, Minquan; Sato, Yasuhiro; Pu, Lyong Sun; Tatsuura, Satoshi; Abe, Shuji

2001-08-01

Squarylium dye J aggregates exhibit ultrafast nonlinear optical response of absorption saturation at the resonant wavelength of 770 nm. We studied the two-photon excitation process of J aggregates. By fluorescence measurement, we found the two-photon absorption band at 1.3 μm, which was different from that of the dye solution at 1.2 μm. Absorption saturation at 770 nm via a two-photon excitation process was observed by two-photon resonant excitation at 1.3 μm and also by off-resonant excitation at 1.55 μm, suggesting the possibility of J aggregates for optical switching materials working at the wavelength used in optical communications.

Spectral heterogeneity and carotenoid-to-bacteriochlorophyll energy transfer in LH2 light-harvesting complexes from Allochromatium vinosum.

PubMed

Magdaong, Nikki M; LaFountain, Amy M; Hacking, Kirsty; Niedzwiedzki, Dariusz M; Gibson, George N; Cogdell, Richard J; Frank, Harry A

2016-02-01

Photosynthetic organisms produce a vast array of spectral forms of antenna pigment-protein complexes to harvest solar energy and also to adapt to growth under the variable environmental conditions of light intensity, temperature, and nutrient availability. This behavior is exemplified by Allochromatium (Alc.) vinosum, a photosynthetic purple sulfur bacterium that produces different types of LH2 light-harvesting complexes in response to variations in growth conditions. In the present work, three different spectral forms of LH2 from Alc. vinosum, B800-820, B800-840, and B800-850, were isolated, purified, and examined using steady-state absorption and fluorescence spectroscopy, and ultrafast time-resolved absorption spectroscopy. The pigment composition of the LH2 complexes was analyzed by high-performance liquid chromatography, and all were found to contain five carotenoids: lycopene, anhydrorhodovibrin, spirilloxanthin, rhodopin, and rhodovibrin. Spectral reconstructions of the absorption and fluorescence excitation spectra based on the pigment composition revealed significantly more spectral heterogeneity in these systems compared to LH2 complexes isolated from other species of purple bacteria. The data also revealed the individual carotenoid-to-bacteriochlorophyll energy transfer efficiencies which were correlated with the kinetic data from the ultrafast transient absorption spectroscopic experiments. This series of LH2 complexes allows a systematic exploration of the factors that determine the spectral properties of the bound pigments and control the rate and efficiency of carotenoid-to-bacteriochlorophyll energy transfer.

Unraveling shock-induced chemistry using ultrafast lasers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Moore, David Steven

The exquisite time synchronicity between shock and diagnostics needed to unravel chemical events occurring in picoseconds has been achieved using a shaped ultrafast laser pulse to both drive the shocks and interrogate the sample via a multiplicity of optical diagnostics. The shaped laser drive pulse can produce well-controlled shock states of sub-ns duration with sub-10 ps risetimes, sufficient for investigation offast reactions or phase transformations in a thin layer with picosecond time resolution. The shock state is characterized using ultrafast dynamic ellipsometry (UDE) in either planar or Gaussian spatial geometries, the latter allowing measurements of the equation of state ofmore » materials at a range of stresses in a single laser pulse. Time-resolved processes in materials are being interrogated using UDE, ultrafast infrared absorption, ultrafast UV/visible absorption, and femtosecond stimulated Raman spectroscopy. Using these tools we showed that chemistry in an energetic thin film starts only after an induction time of a few tens of ps, an observation that allows differentiation between proposed shock-induced reaction mechanisms. These tools are presently being applied to a variety of energetic and reactive sample systems, from nitromethane and carbon disulfide, to microengineered interfaces in tunable energetic mixtures. Recent results will be presented, and future trends outlined.« less

Ultrafast excited-state deactivation of 9-methylhypoxanthine in aqueous solution: A QM/MM MD study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Guo, Xugeng, E-mail: xgguo@henu.edu.cn, E-mail: zhangjinglai@henu.edu.cn; Yuan, Huijuan; An, Beibei

Photoinduced ultrafast non-adiabatic decay of 9-methylhypoxanthine (9MHPX) in aqueous solution was investigated by ab initio surface-hopping dynamics calculations using a combined quantum mechanical/molecular mechanical approach. The absorption spectra of 9MHPX in aqueous solution were also explored by the hybrid cluster-continuum model at the level of time-dependent density functional theory along with the polarizable continuum model (PCM). The static electronic-structure calculations indicate that the absorption spectra of 9MHPX simulated by TD-B3LYP/PCM and TD-X3LYP/PCM can reproduce very well the experimental findings, with the accuracy of about 0.20 eV. According to dynamics simulations, irradiation of 9MHPX populates the bright excited singlet S{sub 1}more » state, which may undergo an ultrafast non-radiative deactivation to the S{sub 0} state. The lifetime of the S{sub 1} state of 9MHPX in aqueous solution is predicted to be 115.6 fs, slightly longer than that in the gas phase (88.8 fs), suggesting that the solvent water has no significant influence on the excited-state lifetime of 9MHPX. Such a behavior in 9MHPX is distinctly different from its parent hypoxanthine keto-N9H tautomer in which the excited-state lifetime of the latter in water solution was remarkably enhanced as compared to the gas phase. The significant difference of the photodynamical behaviors between 9MHPX and keto-N9H can be ascribed to their different hydrogen bond environment in aqueous solution.« less

Dynamic surface acoustic response to a thermal expansion source on an anisotropic half space.

PubMed

Zhao, Peng; Zhao, Ji-Cheng; Weaver, Richard

2013-05-01

The surface displacement response to a distributed thermal expansion source is solved using the reciprocity principle. By convolving the strain Green's function with the thermal stress field created by an ultrafast laser illumination, the complete surface displacement on an anisotropic half space induced by laser absorption is calculated in the time domain. This solution applies to the near field surface displacement due to pulse laser absorption. The solution is validated by performing ultrafast laser pump-probe measurements and showing very good agreement between the measured time-dependent probe beam deflection and the computed surface displacement.

Characterization of photo-induced valence tautomerism in a cobalt-dioxolene complex by ultrafast spectroscopy

NASA Astrophysics Data System (ADS)

Beni, A.; Bogani, L.; Bussotti, L.; Dei, A.; Gentili, P. L.; Righini, R.

2005-01-01

The valence tautomerism of low-spin CoIII(Cat-N-BQ)(Cat-N-SQ) was investigated by means of UV-vis pump-probe transient absorption spectroscopy in chloroform. By exciting the CT transition of the complex at 480 nm, an intramolecular electron transfer process is selectively triggered. The photo-induced charge transfer is pursued by a cascade of two main molecular events characterized by the ultrafast transient absorption spectroscopy: the first gives rise to the metastable high-spin CoII(Cat-N-BQ)2 that, secondly, reaches the chemical equilibrium with the reactant species.

Ultrafast relaxation dynamics of nitric oxide synthase studied by visible broadband transient absorption spectroscopy

NASA Astrophysics Data System (ADS)

Hung, Chih-Chang; Yabushita, Atsushi; Kobayashi, Takayoshi; Chen, Pei-Feng; Liang, Keng S.

2017-09-01

Ultrafast dynamics of endothelial nitric oxide synthase (eNOS) oxygenase domain was studied by transient absorption spectroscopy pumping at Soret band. The broadband visible probe spectrum has visualized the relaxation dynamics from the Soret band to Q-band and charge transfer (CT) band. Supported by two-dimensional correlation spectroscopy, global fitting analysis has successfully concluded the relaxation dynamics from the Soret band to be (1) electronic transition to Q-band (0.16 ps), (2) ligand dissociation and CT (0.94 ps), (3) relaxation of the CT state (4.0 ps), and (4) ligand rebinding (59 ps).

Use of interfacial layers to prolong hole lifetimes in hematite probed by ultrafast transient absorption spectroscopy

NASA Astrophysics Data System (ADS)

Paradzah, Alexander T.; Diale, Mmantsae; Maabong, Kelebogile; Krüger, Tjaart P. J.

2018-04-01

Hematite is a widely investigated material for applications in solar water oxidation due primarily to its small bandgap. However, full realization of the material continues to be hampered by fast electron-hole recombination rates among other weaknesses such as low hole mobility, short hole diffusion length and low conductivity. To address the problem of fast electron-hole recombination, researchers have resorted to growth of nano-structured hematite, doping and use of under-layers. Under-layer materials enhance the photo-current by minimising electron-hole recombination through suppressing of back electron flow from the substrate, such as fluorine-doped tin oxide (FTO), to hematite. We have carried out ultrafast transient absorption spectroscopy on hematite in which Nb2O5 and SnO2 materials were used as interfacial layers to enhance hole lifetimes. The transient absorption data was fit with four different lifetimes ranging from a few hundred femtoseconds to a few nanoseconds. We show that the electron-hole recombination is slower in samples where interfacial layers are used than in pristine hematite. We also develop a model through target analysis to illustrate the effect of under-layers on electron-hole recombination rates in hematite thin films.

Ultrafast Decay of the Solvated Electron in a Neat Polar Solvent: The Unusual Case of Propylene Carbonate.

PubMed

Le Caër, Sophie; Ortiz, Daniel; Marignier, Jean-Louis; Schmidhammer, Uli; Belloni, Jacqueline; Mostafavi, Mehran

2016-01-07

The behavior of carbonates is critical for a detailed understanding of aging phenomena in Li-ion batteries. Here we study the first reaction stages of propylene carbonate (PC), a cyclical carbonate, by picosecond pulse radiolysis. An absorption band with a maximum around 1360 nm is observed at 20 ps after the electron pulse and is shifted to 1310 nm after 50 ps. This band presents the features of a solvated electron absorption band, the solvation lasting up to 50 ps. Surprisingly, in this polar solvent, the solvated electron follows an ultrafast decay and disappears with a half time of 360 ps. This is attributed to the formation of a radical anion PC(-•). The yield of the solvated electron is low, suggesting that the radical anions are mainly directly produced from presolvated electrons. These results demonstrate that the initial electron transfers mechanisms are strongly different in linear compared with cyclical carbonates.

Understanding the features in the ultrafast transient absorption spectra of CdSe quantum dots

NASA Astrophysics Data System (ADS)

Zhang, Cheng; Do, Thanh Nhut; Ong, Xuanwei; Chan, Yinthai; Tan, Howe-Siang

2016-12-01

We describe a model to explain the features of the ultrafast transient absorption (TA) spectra of CdSe core type quantum dots (QDs). The measured TA spectrum consists of contributions by the ground state bleach (GSB), stimulated emission (SE) and excited state absorption (ESA) processes associated with the three lowest energy transition of the QDs. We model the shapes of the GSB, SE and ESA spectral components after fits to the linear absorption. The spectral positions of the ESA components take into account the biexcitonic binding energy. In order to obtain the correct weightage of the GSB, SE and ESA components to the TA spectrum, we enumerate the set of coherence transfer pathways associated with these processes. From our fits of the experimental TA spectra of 65 Å diameter QDs, biexcitonic binding energies for the three lowest energy transitions are obtained.

Ultra-fast dynamics in the nonlinear optical response of silver nanoprism ordered arrays.

PubMed

Sánchez-Esquivel, Héctor; Raygoza-Sanchez, Karen Y; Rangel-Rojo, Raúl; Kalinic, Boris; Michieli, Niccolò; Cesca, Tiziana; Mattei, Giovanni

2018-03-15

In this work we present the study of the ultra-fast dynamics of the nonlinear optical response of a honeycomb array of silver triangular nanoprisms, performed using a femtosecond pulsed laser tuned with the dipolar surface plasmon resonance of the nanoarray. Nonlinear absorption and refraction, and their time-dependence, were explored using the z-scan and time-resolved excite-probe techniques. Nonlinear absorption is shown to change sign with the input irradiance and the behavior was explained on the basis of a three-level model. The response time was determined to be in the picosecond regime. A technique based on a variable frequency chopper was also used in order to discriminate the thermal and electronic contributions to the nonlinearity, which were found to have opposite signs. All these findings propel the investigated nanoprism arrays as good candidates for applications in advanced ultra-fast nonlinear nanophotonic devices.

Ultrafast saturable absorption in TiS2 induced by non-equilibrium electrons and the generation of a femtosecond mode-locked laser.

PubMed

Tian, Xiangling; Wei, Rongfei; Liu, Meng; Zhu, Chunhui; Luo, Zhichao; Wang, Fengqiu; Qiu, Jianrong

2018-05-24

Non-equilibrium electrons induced by ultrafast laser excitation in a correlated electron material can disturb the Fermi energy as well as optical nonlinearity. Here, non-equilibrium electrons translate a semiconductor TiS2 material into a plasma to generate broad band nonlinear optical saturable absorption with a sub-picosecond recovery time of ∼768 fs (corresponding to modulation frequencies over 1.3 THz) and a modulation response up to ∼145%. Based on this optical nonlinear modulator, a stable femtosecond mode-locked pulse with a pulse duration of ∼402 fs and a pulse train with a period of ∼175.5 ns is observed in the all-optical system. The findings indicate that non-equilibrium electrons can promote a TiS2-based saturable absorber to be an ultrafast switch for a femtosecond pulse output.

Triplet exciton dissociation and electron extraction in graphene-templated pentacene observed with ultrafast spectroscopy.

PubMed

McDonough, Thomas J; Zhang, Lushuai; Roy, Susmit Singha; Kearns, Nicholas M; Arnold, Michael S; Zanni, Martin T; Andrew, Trisha L

2017-02-08

We compare the ultrafast dynamics of singlet fission and charge generation in pentacene films grown on glass and graphene. Pentacene grown on graphene is interesting because it forms large crystals with the long axis of the molecules "lying-down" (parallel to the surface). At low excitation fluence, spectra for pentacene on graphene contain triplet absorptions at 507 and 545 nm and no bleaching at 630 nm, which we show is due to the orientation of the pentacene molecules. We perform the first transient absorption anisotropy measurements on pentacene, observing negative anisotropy of the 507 and 545 nm peaks, consistent with triplet absorption. A broad feature at 853 nm, observed on both glass and graphene, is isotropic, suggesting hole absorption. At high fluence, there are additional features, whose kinetics and anisotropies are not explained by heating, that we assign to charge generation; we propose a polaron pair absorption at 614 nm. The lifetimes are shorter at high fluence for both pentacene on glass and graphene, indicative of triplet-triplet annihilation that likely enhances charge generation. The anisotropy decays more slowly for pentacene on graphene than on glass, in keeping with the smaller domain size observed via atomic force microscopy. Coherent acoustic phonons are observed for pentacene on graphene, which is a consequence of more homogeneous domains. Measuring the ultrafast dynamics of pentacene as a function of molecular orientation, fluence, and polarization provides new insight to previous spectral assignments.

Laser plasma x-ray source for ultrafast time-resolved x-ray absorption spectroscopy

DOE PAGES

Miaja-Avila, L.; O'Neil, G. C.; Uhlig, J.; ...

2015-03-02

We describe a laser-driven x-ray plasma source designed for ultrafast x-ray absorption spectroscopy. The source is comprised of a 1 kHz, 20 W, femtosecond pulsed infrared laser and a water target. We present the x-ray spectra as a function of laser energy and pulse duration. Additionally, we investigate the plasma temperature and photon flux as we vary the laser energy. We obtain a 75 μm FWHM x-ray spot size, containing ~10 6 photons/s, by focusing the produced x-rays with a polycapillary optic. Since the acquisition of x-ray absorption spectra requires the averaging of measurements from >10 7 laser pulses, wemore » also present data on the source stability, including single pulse measurements of the x-ray yield and the x-ray spectral shape. In single pulse measurements, the x-ray flux has a measured standard deviation of 8%, where the laser pointing is the main cause of variability. Further, we show that the variability in x-ray spectral shape from single pulses is low, thus justifying the combining of x-rays obtained from different laser pulses into a single spectrum. Finally, we show a static x-ray absorption spectrum of a ferrioxalate solution as detected by a microcalorimeter array. Altogether, our results demonstrate that this water-jet based plasma source is a suitable candidate for laboratory-based time-resolved x-ray absorption spectroscopy experiments.« less

Effects of surface and interface traps on exciton and multi-exciton dynamics in core/shell quantum dots

NASA Astrophysics Data System (ADS)

Bozio, Renato; Righetto, Marcello; Minotto, Alessandro

2017-08-01

Exciton interactions and dynamics are the most important factors determining the exceptional photophysical properties of semiconductor quantum dots (QDs). In particular, best performances have been obtained for ingeniously engineered core/shell QDs. We have studied two factors entering in the exciton decay dynamics with adverse effects for the luminescence efficiency: exciton trapping at surface and interface traps, and non-radiative Auger recombination in QDs carrying either net charges or multiple excitons. In this work, we present a detailed study into the optical absorption, fluorescence dynamics and quantum yield, as well as ultrafast transient absorption properties of CdSe/CdS, CdSe/Cd0.5Zn0.5S, and CdSe/ZnS QDs as a function of shell thickness. It turns out that de-trapping processes play a pivotal role in determining steady state emission properties. By studying the excitation dependent photoluminescence quantum yields (PLQY) in different CdSe/CdxZn1-xS (x = 0, 0.5, 1) QDs, we demonstrate the different role played by hot and cold carrier trapping rates in determining fluorescence quantum yields. Finally, the use of global analysis allows us untangling the complex ultrafast transient absorption signals. Smoothing of interface potential, together with effective surface passivation, appear to be crucial factors in slowing down both Auger-based and exciton trapping recombination processes.

Ultrafast transient absorption studies of hematite nanoparticles: the effect of particle shape on exciton dynamics.

PubMed

Fitzmorris, Bob C; Patete, Jonathan M; Smith, Jacqueline; Mascorro, Xiomara; Adams, Staci; Wong, Stanislaus S; Zhang, Jin Z

2013-10-01

Much progress has been made in using hematite (α-Fe2 O3 ) as a potentially practical and sustainable material for applications such as solar-energy conversion and photoelectrochemical (PEC) water splitting; however, recent studies have shown that the performance can be limited by a very short charge-carrier diffusion length or exciton lifetime. In this study, we performed ultrafast studies on hematite nanoparticles of different shapes to determine the possible influence of particle shape on the exciton dynamics. Nanorice, multifaceted spheroidal nanoparticles, faceted nanocubes, and faceted nanorhombohedra were synthesized and characterized by using SEM and XRD techniques. Their exciton dynamics were investigated by using femtosecond transient absorption (TA) spectroscopy. Although the TA spectral features differ for the four samples studied, their decay profiles are similar, which can be fitted with time constants of 1-3 ps, approximately 25 ps, and a slow nanosecond component extending beyond the experimental time window that was measured (2 ns). The results indicate that the overall exciton lifetime is weakly dependent on the shape of the hematite nanoparticles, even though the overall optical absorption and scattering are influenced by the particle shape. This study suggests that other strategies need to be developed to increase the exciton lifetime or to lengthen the exciton diffusion length in hematite nanostructures. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Broadband ultrafast nonlinear absorption and nonlinear refraction of layered molybdenum dichalcogenide semiconductors

NASA Astrophysics Data System (ADS)

Wang, Kangpeng; Feng, Yanyan; Chang, Chunxia; Zhan, Jingxin; Wang, Chengwei; Zhao, Quanzhong; Coleman, Jonathan N.; Zhang, Long; Blau, Werner J.; Wang, Jun

2014-08-01

A series of layered molybdenum dichalcogenides, i.e., MoX2 (X = S, Se and Te), were prepared in cyclohexyl pyrrolidinone by a liquid-phase exfoliation technique. The high quality of the two-dimensional nanostructures was verified by transmission electron microscopy and absorption spectroscopy. Open- and closed-aperture Z-scans were employed to study the nonlinear absorption and nonlinear refraction of the MoX2 dispersions, respectively. All the three-layered nanostructures exhibit prominent ultrafast saturable absorption (SA) for both femtosecond (fs) and picosecond (ps) laser pulses over a broad wavelength range from the visible to the near infrared. While the dispersions treated with low-speed centrifugation (1500 rpm) have an SA response, and the MoS2 and MoSe2 dispersions after higher speed centrifugation (10 000 rpm) possess two-photon absorption for fs pulses at 1030 nm, which is due to the significant reduction of the average thickness of the nanosheets; hence, the broadening of band gap. In addition, all dispersions show obvious nonlinear self-defocusing for ps pulses at both 1064 nm and 532 nm, resulting from the thermally-induced nonlinear refractive index. The versatile ultrafast nonlinear properties imply a huge potential of the layered MoX2 semiconductors in the development of nanophotonic devices, such as mode-lockers, optical limiters, optical switches, etc.A series of layered molybdenum dichalcogenides, i.e., MoX2 (X = S, Se and Te), were prepared in cyclohexyl pyrrolidinone by a liquid-phase exfoliation technique. The high quality of the two-dimensional nanostructures was verified by transmission electron microscopy and absorption spectroscopy. Open- and closed-aperture Z-scans were employed to study the nonlinear absorption and nonlinear refraction of the MoX2 dispersions, respectively. All the three-layered nanostructures exhibit prominent ultrafast saturable absorption (SA) for both femtosecond (fs) and picosecond (ps) laser pulses over a broad wavelength range from the visible to the near infrared. While the dispersions treated with low-speed centrifugation (1500 rpm) have an SA response, and the MoS2 and MoSe2 dispersions after higher speed centrifugation (10 000 rpm) possess two-photon absorption for fs pulses at 1030 nm, which is due to the significant reduction of the average thickness of the nanosheets; hence, the broadening of band gap. In addition, all dispersions show obvious nonlinear self-defocusing for ps pulses at both 1064 nm and 532 nm, resulting from the thermally-induced nonlinear refractive index. The versatile ultrafast nonlinear properties imply a huge potential of the layered MoX2 semiconductors in the development of nanophotonic devices, such as mode-lockers, optical limiters, optical switches, etc. Electronic supplementary information (ESI) available: Electron scattering patterns from TEM characterizations of MX2 nanosheets; CA Z-scan results of graphene dispersions in the ps region. See DOI: 10.1039/c4nr02634a

Ultrafast two-photon absorption generated free-carrier modulation in a silicon nanoplasmonic resonator

NASA Astrophysics Data System (ADS)

Nielsen, M. P.; Elezzabi, A. Y.

2014-03-01

Ultrafast all-optical modulation in Ag/HfO2/Si/HfO2/Ag metal-insulator-semiconductor-insulator-metal (MISIM) nanoring resonators through two-photon absorption photogenerated free-carriers is studied using self-consistent 3-D finite difference time domain (FDTD) simulations. The self-consistent FDTD simulations incorporate the two-photon absorption, free carrier absorption, and plasma dispersion effects in silicon. The nanorings are aperture coupled to Ag/HfO2/Si(100nm)/HfO2/Ag MISIM waveguides by 300nm wide and 50nm deep apertures. The effects of pump pulse energy, HfO2 spacer thickness, and device footprint on the modulation characteristics are studied. Nanoring radius is varied between 540nm and 1μm, the HfO2 spacer thickness is varied between 10nm and 20nm, and the pump pulse energy is explored up to 60pJ. Modulation amplitude, switching time, average generated carrier density, and wavelength resonant shift is studied for each of the device configurations. In a compact device footprint of only 1.4μm2, a 13.1dB modulation amplitude was obtained with a switching time of only 2ps using a modest pump pulse energy of 16.0pJ. The larger bandwidth associated with more compact nanorings and thinner spacer layers is shown to result in increased modulation amplitude.

Ultrafast light matter interaction in CdSe/ZnS core-shell quantum dots

NASA Astrophysics Data System (ADS)

Yadav, Rajesh Kumar; Sharma, Rituraj; Mondal, Anirban; Adarsh, K. V.

2018-04-01

Core-shell quantum dot are imperative for carrier (electron and holes) confinement in core/shell, which provides a stage to explore the linear and nonlinear optical phenomena at the nanoscalelimit. Here we present a comprehensive study of ultrafast excitation dynamics and nonlinear optical absorption of CdSe/ZnS core shell quantum dot with the help of ultrafast spectroscopy. Pump-probe and time-resolved measurements revealed the drop of trapping at CdSe surface due to the presence of the ZnS shell, which makes more efficient photoluminescence. We have carried out femtosecond transient absorption studies of the CdSe/ZnS core-shell quantum dot by irradiation with 400 nm laser light, monitoring the transients in the visible region. The optical nonlinearity of the core-shell quantum dot studied by using the Z-scan technique with 120 fs pulses at the wavelengths of 800 nm. The value of two photon absorption coefficients (β) of core-shell QDs extracted as80cm/GW, and it shows excellent benchmark for the optical limiting onset of 2.5GW/cm2 with the low limiting differential transmittance of 0.10, that is an order of magnitude better than graphene based materials.

Absorption spectrum and ultrafast response of monolayer and bilayer transition-metal dichalcogenides

NASA Astrophysics Data System (ADS)

Turkowski, Volodymyr; Ramirez-Torres, Alfredo; Rahman, Talat S.

2015-03-01

We apply a combined time-dependent density functional theory and many-body theory approach to examine the absorption spectrum and nonequilibrium response of monolayer and bilayer MoS2, MoSe2, WS2 and WSe2 systems. In particular, we evaluate the possibility of existence of bound states - excitons and trions in the undoped systems. In a previous work we have already demonstrated that the binding energies of these states in the monolayer systems are large which makes them available for room temperature applications. We analyze the possibility of ultrafast electron-hole separation in bilayer systems through inter-layer hole transfer, and show that such a possibility exists, in agreement with experimental observations. For doped systems we consider the possibility of Mahan excitonic states in monolayers and show that the binding energy for these states is of the order of 10 meV. We perform a detailed analysis of the relaxation of doped monolayers excited by ultrafast laser pulse by taking into account electron-phonon scattering effects, and demonstrate that ultrafast (10-100fs) processes, including luminescence, may be relevant for these materials. Work supported in part by DOE Grant No. DOE-DE-FG02-07ER46354.

Spatially and temporally resolved exciton dynamics and transport in single nanostructures and assemblies

NASA Astrophysics Data System (ADS)

Huang, Libai

2015-03-01

The frontier in solar energy conversion now lies in learning how to integrate functional entities across multiple length scales to create optimal devices. To address this new frontier, I will discuss our recent efforts on elucidating multi-scale energy transfer, migration, and dissipation processes with simultaneous femtosecond temporal resolution and nanometer spatial resolution. We have developed ultrafast microscopy that combines ultrafast spectroscopy with optical microscopy to map exciton dynamics and transport with simultaneous ultrafast time resolution and diffraction-limited spatial resolution. We have employed pump-probe transient absorption microscopy to elucidate morphology and structure dependent exciton dynamics and transport in single nanostructures and molecular assemblies. More specifically, (1) We have applied transient absorption microscopy (TAM) to probe environmental and structure dependent exciton relaxation pathways in sing-walled carbon nanotubes (SWNTs) by mapping dynamics in individual pristine SWNTs with known structures. (2) We have systematically measured and modeled the optical properties of the Frenkel excitons in self-assembled porphyrin tubular aggregates that represent an analog to natural photosynthetic antennae. Using a combination of ultrafast optical microscopy and stochastic exciton modeling, we address exciton transport and relaxation pathways, especially those related to disorder.

Discovery of an Ultraviolet Counterpart to an Ultrafast X-Ray Outflow in the Quasar PG 1211+143

NASA Astrophysics Data System (ADS)

Kriss, Gerard A.; Lee, Julia C.; Danehkar, Ashkbiz; Nowak, Michael A.; Fang, Taotao; Hardcastle, Martin J.; Neilsen, Joseph; Young, Andrew

2018-02-01

We observed the quasar PG 1211+143 using the Cosmic Origins Spectrograph on the Hubble Space Telescope in 2015 April as part of a joint campaign with the Chandra X-ray Observatory and the Jansky Very Large Array. Our ultraviolet spectra cover the wavelength range 912–2100 Å. We find a broad absorption feature (∼ 1080 {km} {{{s}}}-1) at an observed wavelength of 1240 Å. Interpreting this as H I Lyα, in the rest frame of PG 1211+143 (z = 0.0809), this corresponds to an outflow velocity of ‑16,980 {km} {{{s}}}-1 (outflow redshift {z}{out}∼ -0.0551), matching the moderate ionization X-ray absorption system detected in our Chandra observation and reported previously by Pounds et al. With a minimum H I column density of {log} {N}{{H}{{I}}}> 14.5, and no absorption in other UV resonance lines, this Lyα absorber is consistent with arising in the same ultrafast outflow as the X-ray absorbing gas. The Lyα feature is weak or absent in archival ultraviolet spectra of PG 1211+143, strongly suggesting that this absorption is transient, and intrinsic to PG 1211+143. Such a simultaneous detection in two independent wavebands for the first time gives strong confirmation of the reality of an ultrafast outflow in an active galactic nucleus.

Ultrafast time-resolved X-ray absorption spectroscopy of ferrioxalate photolysis with a laser plasma X-ray source and microcalorimeter array

DOE PAGES

O’Neil, Galen C.; Miaja-Avila, Luis; Joe, Young Il; ...

2017-02-17

The detailed pathways of photoactivity on ultrafast time scales are a topic of contemporary interest. Using a tabletop apparatus based on a laser plasma X-ray source and an array of cryogenic microcalorimeter X-ray detectors, we measured a transient X-ray absorption spectrum during the ferrioxalate photoreduction reaction. With these high-efficiency detectors, we observe the Fe K edge move to lower energies and the amplitude of the extended X-ray absorption fine structure reduce, consistent with a photoreduction mechanism in which electron transfer precedes disassociation. We provide quantitative limits on the Fe–O bond length change. Lastly, we review potential improvements to our measurementmore » technique, highlighting the future potential of tabletop X-ray science using microcalorimeter sensors.« less

Ultrafast time-resolved X-ray absorption spectroscopy of ferrioxalate photolysis with a laser plasma X-ray source and microcalorimeter array

DOE Office of Scientific and Technical Information (OSTI.GOV)

O’Neil, Galen C.; Miaja-Avila, Luis; Joe, Young Il

The detailed pathways of photoactivity on ultrafast time scales are a topic of contemporary interest. Using a tabletop apparatus based on a laser plasma X-ray source and an array of cryogenic microcalorimeter X-ray detectors, we measured a transient X-ray absorption spectrum during the ferrioxalate photoreduction reaction. With these high-efficiency detectors, we observe the Fe K edge move to lower energies and the amplitude of the extended X-ray absorption fine structure reduce, consistent with a photoreduction mechanism in which electron transfer precedes disassociation. We provide quantitative limits on the Fe–O bond length change. Lastly, we review potential improvements to our measurementmore » technique, highlighting the future potential of tabletop X-ray science using microcalorimeter sensors.« less

Ultrafast strong-field photoelectron emission from biased metal surfaces: exact solution to time-dependent Schrödinger Equation

PubMed Central

Zhang, Peng; Lau, Y. Y.

2016-01-01

Laser-driven ultrafast electron emission offers the possibility of manipulation and control of coherent electron motion in ultrashort spatiotemporal scales. Here, an analytical solution is constructed for the highly nonlinear electron emission from a dc biased metal surface illuminated by a single frequency laser, by solving the time-dependent Schrödinger equation exactly. The solution is valid for arbitrary combinations of dc electric field, laser electric field, laser frequency, metal work function and Fermi level. Various emission mechanisms, such as multiphoton absorption or emission, optical or dc field emission, are all included in this single formulation. The transition between different emission processes is analyzed in detail. The time-dependent emission current reveals that intense current modulation may be possible even with a low intensity laser, by merely increasing the applied dc bias. The results provide insights into the electron pulse generation and manipulation for many novel applications based on ultrafast laser-induced electron emission. PMID:26818710

Hard-X-Ray-Induced Multistep Ultrafast Dissociation

NASA Astrophysics Data System (ADS)

Travnikova, Oksana; Marchenko, Tatiana; Goldsztejn, Gildas; Jänkälä, Kari; Sisourat, Nicolas; Carniato, Stéphane; Guillemin, Renaud; Journel, Loïc; Céolin, Denis; Püttner, Ralph; Iwayama, Hiroshi; Shigemasa, Eiji; Piancastelli, Maria Novella; Simon, Marc

2016-05-01

Creation of deep core holes with very short (τ ≤1 fs ) lifetimes triggers a chain of relaxation events leading to extensive nuclear dynamics on a few-femtosecond time scale. Here we demonstrate a general multistep ultrafast dissociation on an example of HCl following Cl 1 s →σ* excitation. Intermediate states with one or multiple holes in the shallower core electron shells are generated in the course of the decay cascades. The repulsive character and large gradients of the potential energy surfaces of these intermediates enable ultrafast fragmentation after the absorption of a hard x-ray photon.

Direct and simultaneous observation of ultrafast electron and hole dynamics in germanium

DOE PAGES

Zurch, Michael; Chang, Hung -Tzu; Borja, Lauren J.; ...

2017-06-01

Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices. However, overlapping spectral features in optical pump-probe spectroscopy often render assignments of separate electron and hole carrier dynamics ambiguous. Here, ultrafast electron and hole dynamics in germanium nanocrystalline thin films are directly and simultaneously observed by ultrafast transient absorption spectroscopy in the extreme ultraviolet at the germanium M 4,5 edge. We decompose the spectra into contributions of electronic state blocking and photo-induced band shifts at a carrier density of 8 × 10 20 cm –3. Separate electron and hole relaxation times are observedmore » as a function of hot carrier energies. A first-order electron and hole decay of ~1 ps suggests a Shockley–Read–Hall recombination mechanism. Furthermore, the simultaneous observation of electrons and holes with extreme ultraviolet transient absorption spectroscopy paves the way for investigating few- to sub-femtosecond dynamics of both holes and electrons in complex semiconductor materials and across junctions.« less

Layer-Dependent Ultrafast Carrier and Coherent Phonon Dynamics in Black Phosphorus.

PubMed

Miao, Xianchong; Zhang, Guowei; Wang, Fanjie; Yan, Hugen; Ji, Minbiao

2018-05-09

Black phosphorus is a layered semiconducting material, demonstrating strong layer-dependent optical and electronic properties. Probing the photophysical properties on ultrafast time scales is of central importance in understanding many-body interactions and nonequilibrium quasiparticle dynamics. Here, we applied temporally, spectrally, and spatially resolved pump-probe microscopy to study the transient optical responses of mechanically exfoliated few-layer black phosphorus, with layer numbers ranging from 2 to 9. We have observed layer-dependent resonant transient absorption spectra with both photobleaching and red-shifted photoinduced absorption features, which could be attributed to band gap renormalization of higher subband transitions. Surprisingly, coherent phonon oscillations with unprecedented intensities were observed when the probe photons were in resonance with the optical transitions, which correspond to the low-frequency layer-breathing mode. Our results reveal strong Coulomb interactions and electron-phonon couplings in photoexcited black phosphorus, providing important insights into the ultrafast optical, nanomechanical, and optoelectronic properties of this novel two-dimensional material.

Direct and simultaneous observation of ultrafast electron and hole dynamics in germanium

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zurch, Michael; Chang, Hung -Tzu; Borja, Lauren J.

Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices. However, overlapping spectral features in optical pump-probe spectroscopy often render assignments of separate electron and hole carrier dynamics ambiguous. Here, ultrafast electron and hole dynamics in germanium nanocrystalline thin films are directly and simultaneously observed by ultrafast transient absorption spectroscopy in the extreme ultraviolet at the germanium M 4,5 edge. We decompose the spectra into contributions of electronic state blocking and photo-induced band shifts at a carrier density of 8 × 10 20 cm –3. Separate electron and hole relaxation times are observedmore » as a function of hot carrier energies. A first-order electron and hole decay of ~1 ps suggests a Shockley–Read–Hall recombination mechanism. Furthermore, the simultaneous observation of electrons and holes with extreme ultraviolet transient absorption spectroscopy paves the way for investigating few- to sub-femtosecond dynamics of both holes and electrons in complex semiconductor materials and across junctions.« less

Direct and simultaneous observation of ultrafast electron and hole dynamics in germanium.

PubMed

Zürch, Michael; Chang, Hung-Tzu; Borja, Lauren J; Kraus, Peter M; Cushing, Scott K; Gandman, Andrey; Kaplan, Christopher J; Oh, Myoung Hwan; Prell, James S; Prendergast, David; Pemmaraju, Chaitanya D; Neumark, Daniel M; Leone, Stephen R

2017-06-01

Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices. However, overlapping spectral features in optical pump-probe spectroscopy often render assignments of separate electron and hole carrier dynamics ambiguous. Here, ultrafast electron and hole dynamics in germanium nanocrystalline thin films are directly and simultaneously observed by ultrafast transient absorption spectroscopy in the extreme ultraviolet at the germanium M 4,5 edge. We decompose the spectra into contributions of electronic state blocking and photo-induced band shifts at a carrier density of 8 × 10 20  cm -3 . Separate electron and hole relaxation times are observed as a function of hot carrier energies. A first-order electron and hole decay of ∼1 ps suggests a Shockley-Read-Hall recombination mechanism. The simultaneous observation of electrons and holes with extreme ultraviolet transient absorption spectroscopy paves the way for investigating few- to sub-femtosecond dynamics of both holes and electrons in complex semiconductor materials and across junctions.

Direct and simultaneous observation of ultrafast electron and hole dynamics in germanium

PubMed Central

Zürch, Michael; Chang, Hung-Tzu; Borja, Lauren J.; Kraus, Peter M.; Cushing, Scott K.; Gandman, Andrey; Kaplan, Christopher J.; Oh, Myoung Hwan; Prell, James S.; Prendergast, David; Pemmaraju, Chaitanya D.; Neumark, Daniel M.; Leone, Stephen R.

2017-01-01

Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices. However, overlapping spectral features in optical pump-probe spectroscopy often render assignments of separate electron and hole carrier dynamics ambiguous. Here, ultrafast electron and hole dynamics in germanium nanocrystalline thin films are directly and simultaneously observed by ultrafast transient absorption spectroscopy in the extreme ultraviolet at the germanium M4,5 edge. We decompose the spectra into contributions of electronic state blocking and photo-induced band shifts at a carrier density of 8 × 1020 cm−3. Separate electron and hole relaxation times are observed as a function of hot carrier energies. A first-order electron and hole decay of ∼1 ps suggests a Shockley–Read–Hall recombination mechanism. The simultaneous observation of electrons and holes with extreme ultraviolet transient absorption spectroscopy paves the way for investigating few- to sub-femtosecond dynamics of both holes and electrons in complex semiconductor materials and across junctions. PMID:28569752

Ultrafast dynamics of differently aligned COOH-DTE-BODIPY conjugates linked to the surface of TiO2

NASA Astrophysics Data System (ADS)

Schweighöfer, Felix; Yüce, Imanuel; Dworak, Lars; Guo, Peng; Zastrow, Marc; Mayer, Kerstin; Barta, Christoph; Liebmann, Diana; Ziebart, Nandor; Rück-Braun, Karola; Wachtveitl, Josef

2018-02-01

The photoinduced dynamics of two DTE-BODIPY conjugates A, B with carboxylic acid anchoring groups coupled to the surface of TiO2 were studied by ultrafast transient absorption spectroscopy. For compound A, with an orthogonal orientation of the BODIPY chromophore and the photoswitchable DTE unit, a charge separated state could not be reliably detected. Nevertheless, besides the energy transfer from the BODIPY to the ring-closed DTE-c, indications for an electron transfer reaction were found by analyzing fluorescence quenching on TiO2 in steady state fluorescence measurements. For compound B with a parallel orientation of chromophore and photoswitch, a charge separated state was conclusively identified for the coupled dyad (TiO2) via the observation of a positive absorption signal (at λ pr  >  610 nm) at later delay times. An electron transfer rate of 7  ×  1010 s-1 can be extracted, indicating slower processes in the dyads in comparison to previously published electron transfer reactions of DTE compounds coupled to TiO2.

Taple-top imaging of the non-adiabatically driven isomerization in the acetylene cation

NASA Astrophysics Data System (ADS)

Beaulieu, Samuel; Ibrahim, Heide; Wales, Benji; Schmidt, Bruno E.; Thiré, Nicolas; Bisson, Éric; Hebeisen, Christoph T.; Wanie, Vincent; Giguere, Mathieu; Kieffer, Jean-Claude; Sanderson, Joe; Schuurman, Michael S.; Légaré, François

2014-05-01

One of the primary goals of modern ultrafast science is to follow nuclear and electronic evolution of molecules as they undergo a photo-chemical reaction. Most of the interesting dynamics phenomena in molecules occur when an electronically excited state is populated. When the energy difference between electronic ground and excited states is large, Free Electron Laser (FEL) and HHG-based VUV sources were, up to date, the only light sources able to efficiently initiate those non-adiabatic dynamics. We have developed a simple table-top approach to initiate those rich dynamics via multiphoton absorption. As a proof of principle, we studied the ultrafast isomerization of the acetylene cation. We have chosen this model system for isomerization since the internal conversion mechanism which leads to proton migration is still under debate since decades. Using 266 nm multiphoton absorption as a pump and 800 nm induced Coulomb Explosion as a probe, we have shoot the first high-resolution molecular movie of the non-adiabatically driven proton migration in the acetylene cation. The experimental results are in excellent agreement with high level ab initio trajectory simulations.

Ultrafast dynamics of differently aligned COOH-DTE-BODIPY conjugates linked to the surface of TiO2.

PubMed

Schweighöfer, Felix; Yüce, Imanuel; Dworak, Lars; Guo, Peng; Zastrow, Marc; Mayer, Kerstin; Barta, Christoph; Liebmann, Diana; Ziebart, Nandor; Rück-Braun, Karola; Wachtveitl, Josef

2018-01-05

The photoinduced dynamics of two DTE-BODIPY conjugates A, B with carboxylic acid anchoring groups coupled to the surface of TiO 2 were studied by ultrafast transient absorption spectroscopy. For compound A, with an orthogonal orientation of the BODIPY chromophore and the photoswitchable DTE unit, a charge separated state could not be reliably detected. Nevertheless, besides the energy transfer from the BODIPY to the ring-closed DTE-c, indications for an electron transfer reaction were found by analyzing fluorescence quenching on TiO 2 in steady state fluorescence measurements. For compound B with a parallel orientation of chromophore and photoswitch, a charge separated state was conclusively identified for the coupled dyad (TiO 2 ) via the observation of a positive absorption signal (at λ pr   >  610 nm) at later delay times. An electron transfer rate of 7  ×  10 10 s -1 can be extracted, indicating slower processes in the dyads in comparison to previously published electron transfer reactions of DTE compounds coupled to TiO 2 .

Selective resolution of photocurrent generating pathways in transition metal dichalcogenides by ultrafast microscopy (Conference Presentation)

NASA Astrophysics Data System (ADS)

Graham, Matthew W.

2017-02-01

Presently, there exists no reliable in-situ time-resolved method that selectively isolates both the recombination and escape times relevant to photocurrent generation in the ultrafast regime. Transport based measurements lack the required time resolution, while purely optical measurement give a convoluted weighted-average of all electronic dynamics, offering no selectivity for photocurrent generating pathways. Recently, the ultrafast photocurrent (U-PC) autocorrelation method has successfully measured the rate limiting electronic relaxation processes in materials such as graphene, carbon nanotubes, and transition metal dichalcogenide (TMD) materials. Here, we unambiguously derive and experimentally confirm a generic U-PC response function by simultaneously resolving the transient absorption (TA) and U-PC response for highly-efficient (48% IQE at 0 bias) WSe2 devices and twisted bilayer graphene. Surprisingly, both optical TA and electrical U-PC responses give the same E-field-dependent electronic escape and recombination rates. These rates further accurately quantify a material's intrinsic PC generation efficiency. We demonstrate that the chirality of the incident light impacts the U-PC kinetics, suggesting such measurements directly access the ultrafast dynamics need to complex electronic physics such as the valley-Hall effect. By combining E-field dependent ultrafast photocurrent with transient absorption microscopy, we have selectively imaged the dominant kinetic bottlenecks that inhibit photocurrent production in devices made from stacked few-layer TMD materials. This provides a new methodology to intelligently select materials that intrinsically avoid recombination bottlenecks and maximize photocurrent yield.

Femtosecond coherent nuclear dynamics of excited tetraphenylethylene: Ultrafast transient absorption and ultrafast Raman loss spectroscopic studies

NASA Astrophysics Data System (ADS)

Kayal, Surajit; Roy, Khokan; Umapathy, Siva

2018-01-01

Ultrafast torsional dynamics plays an important role in the photoinduced excited state dynamics. Tetraphenylethylene (TPE), a model system for the molecular motor, executes interesting torsional dynamics upon photoexcitation. The photoreaction of TPE involves ultrafast internal conversion via a nearly planar intermediate state (relaxed state) that further leads to a twisted zwitterionic state. Here, we report the photoinduced structural dynamics of excited TPE during the course of photoisomerization in the condensed phase by ultrafast Raman loss (URLS) and femtosecond transient absorption (TA) spectroscopy. TA measurements on the S1 state reveal step-wise population relaxation from the Franck-Condon (FC) state → relaxed state → twisted state, while the URLS study provides insights on the vibrational dynamics during the course of the reaction. The TA spectral dynamics and vibrational Raman amplitudes within 1 ps reveal vibrational wave packet propagating from the FC state to the relaxed state. Fourier transformation of this oscillation leads to a ˜130 cm-1 low-frequency phenyl torsional mode. Two vibrational marker bands, Cet=Cet stretching (˜1512 cm-1) and Cph=Cph stretching (˜1584 cm-1) modes, appear immediately after photoexcitation in the URLS spectra. The initial red-shift of the Cph=Cph stretching mode with a time constant of ˜400 fs (in butyronitrile) is assigned to the rate of planarization of excited TPE. In addition, the Cet=Cet stretching mode shows initial blue-shift within 1 ps followed by frequency red-shift, suggesting that on the sub-picosecond time scale, structural relaxation is dominated by phenyl torsion rather than the central Cet=Cet twist. Furthermore, the effect of the solvent on the structural dynamics is discussed in the context of ultrafast nuclear dynamics and solute-solvent coupling.

Femtosecond coherent nuclear dynamics of excited tetraphenylethylene: Ultrafast transient absorption and ultrafast Raman loss spectroscopic studies.

PubMed

Kayal, Surajit; Roy, Khokan; Umapathy, Siva

2018-01-14

Ultrafast torsional dynamics plays an important role in the photoinduced excited state dynamics. Tetraphenylethylene (TPE), a model system for the molecular motor, executes interesting torsional dynamics upon photoexcitation. The photoreaction of TPE involves ultrafast internal conversion via a nearly planar intermediate state (relaxed state) that further leads to a twisted zwitterionic state. Here, we report the photoinduced structural dynamics of excited TPE during the course of photoisomerization in the condensed phase by ultrafast Raman loss (URLS) and femtosecond transient absorption (TA) spectroscopy. TA measurements on the S 1 state reveal step-wise population relaxation from the Franck-Condon (FC) state → relaxed state → twisted state, while the URLS study provides insights on the vibrational dynamics during the course of the reaction. The TA spectral dynamics and vibrational Raman amplitudes within 1 ps reveal vibrational wave packet propagating from the FC state to the relaxed state. Fourier transformation of this oscillation leads to a ∼130 cm -1 low-frequency phenyl torsional mode. Two vibrational marker bands, C et =C et stretching (∼1512 cm -1 ) and C ph =C ph stretching (∼1584 cm -1 ) modes, appear immediately after photoexcitation in the URLS spectra. The initial red-shift of the C ph =C ph stretching mode with a time constant of ∼400 fs (in butyronitrile) is assigned to the rate of planarization of excited TPE. In addition, the C et =C et stretching mode shows initial blue-shift within 1 ps followed by frequency red-shift, suggesting that on the sub-picosecond time scale, structural relaxation is dominated by phenyl torsion rather than the central C et =C et twist. Furthermore, the effect of the solvent on the structural dynamics is discussed in the context of ultrafast nuclear dynamics and solute-solvent coupling.

Tuning the nonlinear response of (6,5)-enriched single-wall carbon nanotubes dispersions

NASA Astrophysics Data System (ADS)

Aréstegui, O. S.; Silva, E. C. O.; Baggio, A. L.; Gontijo, R. N.; Hickmann, J. M.; Fantini, C.; Alencar, M. A. R. C.; Fonseca, E. J. S.

2017-04-01

Ultrafast nonlinear optical properties of (6,5)-enriched single-wall carbon nanotubes (SWCNTs) dispersions are investigated using the thermally managed Z-scan technique. As the (6,5) SWCNTs presented a strong resonance in the range of 895-1048 nm, the nonlinear refractive index (n2) and the absorption coefficients (β) measurements were performed tuning the laser exactly around absorption peak of the (6,5) SWCNTs. It is observed that the nonlinear response is very sensitive to the wavelength and the spectral behavior of n2 is strongly correlated to the tubes one-photon absorption band, presenting also a peak when the laser photon energy is near the tube resonance energy. This result suggests that a suitable selection of nanotubes types may provide optimized nonlinear optical responses in distinct regions of the electromagnetic spectrum. Analysis of the figures of merit indicated that this material is promising for ultrafast nonlinear optical applications under near infrared excitation.

High Harmonic Generation XUV Spectroscopy for Studying Ultrafast Photophysics of Coordination Complexes

NASA Astrophysics Data System (ADS)

Ryland, Elizabeth S.; Lin, Ming-Fu; Benke, Kristin; Verkamp, Max A.; Zhang, Kaili; Vura-Weis, Josh

2017-06-01

Extreme ultraviolet (XUV) spectroscopy is an inner shell technique that probes the M_{2,3}-edge excitation of atoms. Absorption of the XUV photon causes a 3p→3d transition, the energy and shape of which is directly related to the element and ligand environment. This technique is thus element-, oxidation state-, spin state-, and ligand field specific. A process called high-harmonic generation (HHG) enables the production of ultrashort (˜20fs) pulses of collimated XUV photons in a tabletop instrument. This allows transient XUV spectroscopy to be conducted as an in-lab experiment, where it was previously only possible at accelerator-based light sources. Additionally, ultrashort pulses provide the capability for unprecedented time resolution (˜50fs IRF). This technique has the capacity to serve a pivotal role in the study of electron and energy transfer processes in materials and chemical biology. I will present the XUV transient absorption instrument we have built, along with ultrafast transient M_{2,3}-edge absorption data of a series of small inorganic molecules in order to demonstrate the high specificity and time resolution of this tabletop technique as well as how our group is applying it to the study of ultrafast electronic dynamics of coordination complexes.

Capturing local structure modulations of photoexcited BiVO4 by ultrafast transient XAFS.

PubMed

Uemura, Yohei; Kido, Daiki; Koide, Akihiro; Wakisaka, Yuki; Niwa, Yasuhiro; Nozawa, Shunsuke; Ichiyanagi, Kohei; Fukaya, Ryo; Adachi, Shin-Ichi; Katayama, Tetsuo; Togashi, Tadashi; Owada, Shigeki; Yabashi, Makina; Hatada, Keisuke; Iwase, Akihide; Kudo, Akihiko; Takakusagi, Satoru; Yokoyama, Toshihiko; Asakura, Kiyotaka

2017-06-29

Ultrafast excitation of photocatalytically active BiVO 4 was characterized by femto- and picosecond transient X-ray absorption fine structure spectroscopy. An initial photoexcited state (≪500 fs) changed to a metastable state accompanied by a structural change with a time constant of ∼14 ps. The structural change might stabilize holes on oxygen atoms since the interaction between Bi and O increases.

Graphene-clad microfibre saturable absorber for ultrafast fibre lasers.

PubMed

Liu, X M; Yang, H R; Cui, Y D; Chen, G W; Yang, Y; Wu, X Q; Yao, X K; Han, D D; Han, X X; Zeng, C; Guo, J; Li, W L; Cheng, G; Tong, L M

2016-05-16

Graphene, whose absorbance is approximately independent of wavelength, allows broadband light-matter interactions with ultrafast responses. The interband optical absorption of graphene can be saturated readily under strong excitation, thereby enabling scientists to exploit the photonic properties of graphene to realize ultrafast lasers. The evanescent field interaction scheme of the propagating light with graphene covered on a D-shaped fibre or microfibre has been employed extensively because of the nonblocking configuration. Obviously, most of the fibre surface is unused in these techniques. Here, we exploit a graphene-clad microfibre (GCM) saturable absorber in a mode-locked fibre laser for the generation of ultrafast pulses. The proposed all-surface technique can guarantee a higher efficiency of light-graphene interactions than the aforementioned techniques. Our GCM-based saturable absorber can generate ultrafast optical pulses within 1.5 μm. This saturable absorber is compatible with current fibre lasers and has many merits such as low saturation intensities, ultrafast recovery times, and wide wavelength ranges. The proposed saturable absorber will pave the way for graphene-based wideband photonics.

Graphene-clad microfibre saturable absorber for ultrafast fibre lasers

PubMed Central

Liu, X. M.; Yang, H. R.; Cui, Y. D.; Chen, G. W.; Yang, Y.; Wu, X. Q.; Yao, X. K.; Han, D. D.; Han, X. X.; Zeng, C.; Guo, J.; Li, W. L.; Cheng, G.; Tong, L. M.

2016-01-01

Graphene, whose absorbance is approximately independent of wavelength, allows broadband light–matter interactions with ultrafast responses. The interband optical absorption of graphene can be saturated readily under strong excitation, thereby enabling scientists to exploit the photonic properties of graphene to realize ultrafast lasers. The evanescent field interaction scheme of the propagating light with graphene covered on a D-shaped fibre or microfibre has been employed extensively because of the nonblocking configuration. Obviously, most of the fibre surface is unused in these techniques. Here, we exploit a graphene-clad microfibre (GCM) saturable absorber in a mode-locked fibre laser for the generation of ultrafast pulses. The proposed all-surface technique can guarantee a higher efficiency of light–graphene interactions than the aforementioned techniques. Our GCM-based saturable absorber can generate ultrafast optical pulses within 1.5 μm. This saturable absorber is compatible with current fibre lasers and has many merits such as low saturation intensities, ultrafast recovery times, and wide wavelength ranges. The proposed saturable absorber will pave the way for graphene-based wideband photonics. PMID:27181419

Polarization and Thickness Dependent Absorption Properties of Black Phosphorus: New Saturable Absorber for Ultrafast Pulse Generation

PubMed Central

Li, Diao; Jussila, Henri; Karvonen, Lasse; Ye, Guojun; Lipsanen, Harri; Chen, Xianhui; Sun, Zhipei

2015-01-01

Black phosphorus (BP) has recently been rediscovered as a new and interesting two-dimensional material due to its unique electronic and optical properties. Here, we study the linear and nonlinear optical properties of BP flakes. We observe that both the linear and nonlinear optical properties are anisotropic and can be tuned by the film thickness in BP, completely different from other typical two-dimensional layered materials (e.g., graphene and the most studied transition metal dichalcogenides). We then use the nonlinear optical properties of BP for ultrafast (pulse duration down to ~786 fs in mode-locking) and large-energy (pulse energy up to >18 nJ in Q-switching) pulse generation in fiber lasers at the near-infrared telecommunication band ~1.5 μm. We observe that the output of our BP based pulsed lasers is linearly polarized (with a degree-of-polarization ~98% in mode-locking, >99% in Q-switching, respectively) due to the anisotropic optical property of BP. Our results underscore the relatively large optical nonlinearity of BP with unique polarization and thickness dependence, and its potential for polarized optical pulse generation, paving the way to BP based nonlinear and ultrafast photonic applications (e.g., ultrafast all-optical polarization switches/modulators, frequency converters etc.). PMID:26514090

Role of surface states and defects in the ultrafast nonlinear optical properties of CuS quantum dots

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mary, K. A. Ann; Unnikrishnan, N. V., E-mail: nvu100@yahoo.com; Philip, Reji

2014-07-01

We report facile preparation of water dispersible CuS quantum dots (2–4 nm) and nanoparticles (5–11 nm) through a nontoxic, green, one-pot synthesis method. Optical and microstructural studies indicate the presence of surface states and defects (dislocations, stacking faults, and twins) in the quantum dots. The smaller crystallite size and quantum dot formation have significant effects on the high energy excitonic and low energy plasmonic absorption bands. Effective two-photon absorption coefficients measured using 100 fs laser pulses employing open-aperture Z-scan in the plasmonic region of 800 nm reveal that CuS quantum dots are better ultrafast optical limiters compared to CuS nanoparticles.

Modified two-photon absorption and dispersion of ultrafast third-order polarization beats via twin noisy driving fields

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang Yanpeng; Department of Electronic Science and Technology, Xi'an Jiaotong University, Xi'an 710049; Gan Chenli

2006-05-15

We investigate the color-locked twin-noisy-field correlation effects in third-order nonlinear absorption and dispersion of ultrafast polarization beats. We demonstrate a phase-sensitive method for studying the two-photon nondegenerate four-wave mixing (NDFWM) due to atomic coherence in a multilevel system. The reference signal is another one-photon degenerate four-wave-mixing signal, which propagates along the same optical path as the NDFWM signal. This method is used for studying the phase dispersion of the third-order susceptibility and for the optical heterodyne detection of the NDFWM signal. The third-order nonlinear response can be controlled and modified through the color-locked correlation of twin noisy fields.

Ultrafast Laser-Based Spectroscopy and Sensing: Applications in LIBS, CARS, and THz Spectroscopy

PubMed Central

Leahy-Hoppa, Megan R.; Miragliotta, Joseph; Osiander, Robert; Burnett, Jennifer; Dikmelik, Yamac; McEnnis, Caroline; Spicer, James B.

2010-01-01

Ultrafast pulsed lasers find application in a range of spectroscopy and sensing techniques including laser induced breakdown spectroscopy (LIBS), coherent Raman spectroscopy, and terahertz (THz) spectroscopy. Whether based on absorption or emission processes, the characteristics of these techniques are heavily influenced by the use of ultrafast pulses in the signal generation process. Depending on the energy of the pulses used, the essential laser interaction process can primarily involve lattice vibrations, molecular rotations, or a combination of excited states produced by laser heating. While some of these techniques are currently confined to sensing at close ranges, others can be implemented for remote spectroscopic sensing owing principally to the laser pulse duration. We present a review of ultrafast laser-based spectroscopy techniques and discuss the use of these techniques to current and potential chemical and environmental sensing applications. PMID:22399883

Femtosecond dynamics of monolayer MoS2-Ag nanoparticles hybrid probed at 532 nm

NASA Astrophysics Data System (ADS)

Xu, Xuefeng; Shi, Ying; Liu, Xiaochun; Sun, Mengtao

2018-01-01

In this communication, plasmon-exciton couplings of monolayer MoS2/Ag nanoparticles (NPs) hybrids with different sizes are investigated, using transient absorption spectra. Ultrafast dynamics of coupling interactions inside these hybrid structures are carefully examined at 532 nm, which can well interpret the apllication of plasmon-exciton coupling for the co-driven chemical reactions excited at 532 nm. Our experimental results can promote the deeper understanding on the physical mechanism of plasmon-excition interaction, and applications in different fields.

Elucidating ultrafast electron dynamics at surfaces using extreme ultraviolet (XUV) reflection-absorption spectroscopy.

PubMed

Biswas, Somnath; Husek, Jakub; Baker, L Robert

2018-04-24

Here we review the recent development of extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy. This method combines the benefits of X-ray absorption spectroscopy, such as element, oxidation, and spin state specificity, with surface sensitivity and ultrafast time resolution, having a probe depth of only a few nm and an instrument response less than 100 fs. Using this technique we investigated the ultrafast electron dynamics at a hematite (α-Fe2O3) surface. Surface electron trapping and small polaron formation both occur in 660 fs following photoexcitation. These kinetics are independent of surface morphology indicating that electron trapping is not mediated by defects. Instead, small polaron formation is proposed as the likely driving force for surface electron trapping. We also show that in Fe2O3, Co3O4, and NiO, band gap excitation promotes electron transfer from O 2p valence band states to metal 3d conduction band states. In addition to detecting the photoexcited electron at the metal M2,3-edge, the valence band hole is directly observed as transient signal at the O L1-edge. The size of the resulting charge transfer exciton is on the order of a single metal-oxygen bond length. Spectral shifts at the O L1-edge correlate with metal-oxygen bond covalency, confirming the relationship between valence band hybridization and the overpotential for water oxidation. These examples demonstrate the unique ability to measure ultrafast electron dynamics with element and chemical state resolution using XUV-RA spectroscopy. Accordingly, this method is poised to play an important role to reveal chemical details of previously unseen surface electron dynamics.

Observation of sub-100-fs optical response from spin-coated films of squarylium dye J aggregates

NASA Astrophysics Data System (ADS)

Furuki, Makoto; Tian, Minquan; Sato, Yasuhiro; Pu, Lyong Sun; Kawashima, Hitoshi; Tatsuura, Satoshi; Wada, Osamu

2001-04-01

For spin-coated films of squarylium dye J aggregates, ultrafast nonlinear optical responses were investigated by pump-probe measurements. By using a broadband mode-locked titanium:sapphire laser, we succeeded in observing the optical response with a time resolution of better than 60 fs. Time-resolved transmission data are shown for different excitation wavelengths, resonant to the excitonic absorption band and off-resonant. Relaxation times of the absorption saturation were evaluated to be 140 fs (fast component) and 950 fs (slow component) in the case of resonant excitation and 98 fs in the case of off-resonant excitation.

Electronic and nuclear contributions to time-resolved optical and X-ray absorption spectra of hematite and insights into photoelectrochemical performance

DOE PAGES

Hayes, Dugan; Hadt, Ryan G.; Emery, Jonathan D.; ...

2016-11-02

Ultrafast time-resolved studies of photocatalytic thin films can provide a wealth of information crucial for understanding and thereby improving the performance of these materials by directly probing electronic structure, reaction intermediates, and charge carrier dynamics. The interpretation of transient spectra, however, can be complicated by thermally induced structural distortions, which appear within the first few picoseconds following excitation due to carrier–phonon scattering. Here we present a comparison of ex situ steady-state thermal difference spectra and transient absorption spectra spanning from NIR to hard X-ray energies of hematite thin films grown by atomic layer deposition. We find that beyond the firstmore » 100 picoseconds, the transient spectra measured for all excitation wavelengths and probe energies are almost entirely due to thermal effects as the lattice expands in response to the ultrafast temperature jump and then cools to room temperature on the microsecond timescale. At earlier times, a broad excited state absorption band that is assigned to free carriers appears at 675 nm, and the lifetime and shape of this feature also appear to be mostly independent of excitation wavelength. The combined spectroscopic data, which are modeled with density functional theory and full multiple scattering calculations, support an assignment of the optical absorption spectrum of hematite that involves two LMCT bands that nearly span the visible spectrum. Lastly, our results also suggest a framework for shifting the ligand-to-metal charge transfer absorption bands of ferric oxide films from the near-UV further into the visible part of the solar spectrum to improve solar conversion efficiency.« less

Excited state electron and energy relays in supramolecular dinuclear complexes revealed by ultrafast optical and X-ray transient absorption spectroscopy.

PubMed

Hayes, Dugan; Kohler, Lars; Hadt, Ryan G; Zhang, Xiaoyi; Liu, Cunming; Mulfort, Karen L; Chen, Lin X

2018-01-28

The kinetics of photoinduced electron and energy transfer in a family of tetrapyridophenazine-bridged heteroleptic homo- and heterodinuclear copper(i) bis(phenanthroline)/ruthenium(ii) polypyridyl complexes were studied using ultrafast optical and multi-edge X-ray transient absorption spectroscopies. This work combines the synthesis of heterodinuclear Cu(i)-Ru(ii) analogs of the homodinuclear Cu(i)-Cu(i) targets with spectroscopic analysis and electronic structure calculations to first disentangle the dynamics at individual metal sites by taking advantage of the element and site specificity of X-ray absorption and theoretical methods. The excited state dynamical models developed for the heterodinuclear complexes are then applied to model the more challenging homodinuclear complexes. These results suggest that both intermetallic charge and energy transfer can be observed in an asymmetric dinuclear copper complex in which the ground state redox potentials of the copper sites are offset by only 310 meV. We also demonstrate the ability of several of these complexes to effectively and unidirectionally shuttle energy between different metal centers, a property that could be of great use in the design of broadly absorbing and multifunctional multimetallic photocatalysts. This work provides an important step toward developing both a fundamental conceptual picture and a practical experimental handle with which synthetic chemists, spectroscopists, and theoreticians may collaborate to engineer cheap and efficient photocatalytic materials capable of performing coulombically demanding chemical transformations.

Ultrafast time-resolved carotenoid to-bacteriochlorophyll energy transfer in LH2 complexes from photosynthetic bacteria.

PubMed

Cong, Hong; Niedzwiedzki, Dariusz M; Gibson, George N; LaFountain, Amy M; Kelsh, Rhiannon M; Gardiner, Alastair T; Cogdell, Richard J; Frank, Harry A

2008-08-28

Steady-state and ultrafast time-resolved optical spectroscopic investigations have been carried out at 293 and 10 K on LH2 pigment-protein complexes isolated from three different strains of photosynthetic bacteria: Rhodobacter (Rb.) sphaeroides G1C, Rb. sphaeroides 2.4.1 (anaerobically and aerobically grown), and Rps. acidophila 10050. The LH2 complexes obtained from these strains contain the carotenoids, neurosporene, spheroidene, spheroidenone, and rhodopin glucoside, respectively. These molecules have a systematically increasing number of pi-electron conjugated carbon-carbon double bonds. Steady-state absorption and fluorescence excitation experiments have revealed that the total efficiency of energy transfer from the carotenoids to bacteriochlorophyll is independent of temperature and nearly constant at approximately 90% for the LH2 complexes containing neurosporene, spheroidene, spheroidenone, but drops to approximately 53% for the complex containing rhodopin glucoside. Ultrafast transient absorption spectra in the near-infrared (NIR) region of the purified carotenoids in solution have revealed the energies of the S1 (2(1)Ag-)-->S2 (1(1)Bu+) excited-state transitions which, when subtracted from the energies of the S0 (1(1)Ag-)-->S2 (1(1)Bu+) transitions determined by steady-state absorption measurements, give precise values for the positions of the S1 (2(1)Ag-) states of the carotenoids. Global fitting of the ultrafast spectral and temporal data sets have revealed the dynamics of the pathways of de-excitation of the carotenoid excited states. The pathways include energy transfer to bacteriochlorophyll, population of the so-called S* state of the carotenoids, and formation of carotenoid radical cations (Car*+). The investigation has found that excitation energy transfer to bacteriochlorophyll is partitioned through the S1 (1(1)Ag-), S2 (1(1)Bu+), and S* states of the different carotenoids to varying degrees. This is understood through a consideration of the energies of the states and the spectral profiles of the molecules. A significant finding is that, due to the low S1 (2(1)Ag-) energy of rhodopin glucoside, energy transfer from this state to the bacteriochlorophylls is significantly less probable compared to the other complexes. This work resolves a long-standing question regarding the cause of the precipitous drop in energy transfer efficiency when the extent of pi-electron conjugation of the carotenoid is extended from ten to eleven conjugated carbon-carbon double bonds in LH2 complexes from purple photosynthetic bacteria.

Light, Molecules, Action: Broadband UV-visible transient absorption studies of excited state dynamics in photoactive molecules

NASA Astrophysics Data System (ADS)

Sension, Roseanne

2015-03-01

Broadband UV-visible transient absorption spectroscopy provides a powerful tool for the investigation of the dynamics of electronically excited molecules in the condensed phase. It is now possible to obtain transient spectra on a routine basis spanning the range from <300 nm to >800 nm with femtosecond time resolution. We have used this method to study the excited state dynamics and internal conversion of a range of molecular systems with potential application as optically powered molecular devices. The cyclohexadiene ring-opening reaction is the basis of a class of important optical switches and of the biological synthesis of previtamin D3. The ring-opening reaction is ultrafast, occurring on a picosecond to subpicosecond times scale depending on the substituents around the ring. These have a significant influence on the dynamics and electronic structure of the electronically excited molecule. The results of a series of transient absorption studies as a function of chromophore substitution and environment will be presented. The cis-trans isomerization of polyene molecules, especially substituted stilbenes, provides another important class of functional molecular transformations. Again the excited state dynamics can be ultrafast with photochemistry controlled by details of the curve crossings and conical intersections. Finally the photochemistry of the even more complex set of cobalamin chromophores with a photoalabile C-Co bond has been proposed as a tool for spatio-temporal control of molecule delivery including drug delivery. Broadband transient absorption spectroscopy has been used to investigate the ultrafast electronic dynamics of a range of cobalamin compounds with comparison to detailed theoretical calculations. The results of these studies will be presented.

Optical nonlinearities and ultrafast all-optical switching of m-plane GaN in the near-infrared

DOE Office of Scientific and Technical Information (OSTI.GOV)

Fang, Yu; Zhou, Feng; Yang, Junyi

2015-06-22

We reported a systematic investigation on the three-photon absorption (3PA) spectra and wavelength dispersion of Kerr refraction of bulk m-plane GaN crystal with both polarization E⊥c and E//c by femtosecond Z-scan technique in the near-infrared region from 760 to 1030 nm. Both 3PA spectra and Kerr refraction dispersion were in good agreement with two-band models. The calculated nonlinear figure of merit and measured ultrafast nonlinear refraction dynamics via femtosecond pump-probe with phase object method revealed that m-plane GaN would be a promising candidate for ultrafast all-optical switching and autocorrelation applications at telecommunication wavelengths.

Real-Time Time-Frequency Two-Dimensional Imaging of Ultrafast Transient Signals in Solid-State Organic Materials

PubMed Central

Takeda, Jun; Ishida, Akihiro; Makishima, Yoshinori; Katayama, Ikufumi

2010-01-01

In this review, we demonstrate a real-time time-frequency two-dimensional (2D) pump-probe imaging spectroscopy implemented on a single shot basis applicable to excited-state dynamics in solid-state organic and biological materials. Using this technique, we could successfully map ultrafast time-frequency 2D transient absorption signals of β-carotene in solid films with wide temporal and spectral ranges having very short accumulation time of 20 ms per unit frame. The results obtained indicate the high potential of this technique as a powerful and unique spectroscopic tool to observe ultrafast excited-state dynamics of organic and biological materials in solid-state, which undergo rapid photodegradation. PMID:22399879

Preferential Charge Generation at Aggregate Sites in Narrow Band Gap Infrared Photoresponsive Polymer Semiconductors

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sulas, Dana B.; London, Alexander E.; Huang, Lifeng

Infrared organic photodetector materials are investigated using transient absorption spectroscopy, demonstrating that ultrafast charge generation assisted by polymer aggregation is essential to compensate for the energy gap law, which dictates that excited state lifetimes decrease as the band gap narrows. Short sub–picosecond singlet exciton lifetimes are measured in a structurally related series of infrared–absorbing copolymers that consist of alternating cyclopentadithiophene electron–rich “push” units and strong electron–deficient “pull” units, including benzothiadiazole, benzoselenadiazole, pyridalselenadiazole, or thiadiazoloquinoxaline. While the ultrafast lifetimes of excitons localized on individual polymer chains suggest that charge carrier generation will be inefficient, high detectivity for polymer:PC 71BM infrared photodetectorsmore » is measured in the 0.6 < λ < 1.5 µm range. The photophysical processes leading to charge generation are investigated by performing a global analysis on transient absorption data of blended polymer:PC 71BM films. In these blends, charge carriers form primarily at polymer aggregate sites on the ultrafast time scale (within our instrument response), leaving quickly decaying single–chain excitons unquenched. Lastly, the results have important implications for the further development of organic infrared optoelectronic devices, where targeting processes such as excited state delocalization over aggregates may be necessary to mitigate losses to ultrafast exciton decay as materials with even lower band gaps are developed.« less

Preferential Charge Generation at Aggregate Sites in Narrow Band Gap Infrared Photoresponsive Polymer Semiconductors

DOE PAGES

Sulas, Dana B.; London, Alexander E.; Huang, Lifeng; ...

2018-02-13

Infrared organic photodetector materials are investigated using transient absorption spectroscopy, demonstrating that ultrafast charge generation assisted by polymer aggregation is essential to compensate for the energy gap law, which dictates that excited state lifetimes decrease as the band gap narrows. Short sub–picosecond singlet exciton lifetimes are measured in a structurally related series of infrared–absorbing copolymers that consist of alternating cyclopentadithiophene electron–rich “push” units and strong electron–deficient “pull” units, including benzothiadiazole, benzoselenadiazole, pyridalselenadiazole, or thiadiazoloquinoxaline. While the ultrafast lifetimes of excitons localized on individual polymer chains suggest that charge carrier generation will be inefficient, high detectivity for polymer:PC 71BM infrared photodetectorsmore » is measured in the 0.6 < λ < 1.5 µm range. The photophysical processes leading to charge generation are investigated by performing a global analysis on transient absorption data of blended polymer:PC 71BM films. In these blends, charge carriers form primarily at polymer aggregate sites on the ultrafast time scale (within our instrument response), leaving quickly decaying single–chain excitons unquenched. Lastly, the results have important implications for the further development of organic infrared optoelectronic devices, where targeting processes such as excited state delocalization over aggregates may be necessary to mitigate losses to ultrafast exciton decay as materials with even lower band gaps are developed.« less

Temperature dependence of the hydrated electron's excited-state relaxation. II. Elucidating the relaxation mechanism through ultrafast transient absorption and stimulated emission spectroscopy

NASA Astrophysics Data System (ADS)

Farr, Erik P.; Zho, Chen-Chen; Challa, Jagannadha R.; Schwartz, Benjamin J.

2017-08-01

The structure of the hydrated electron, particularly whether it exists primarily within a cavity or encompasses interior water molecules, has been the subject of much recent debate. In Paper I [C.-C. Zho et al., J. Chem. Phys. 147, 074503 (2017)], we found that mixed quantum/classical simulations with cavity and non-cavity pseudopotentials gave different predictions for the temperature dependence of the rate of the photoexcited hydrated electron's relaxation back to the ground state. In this paper, we measure the ultrafast transient absorption spectroscopy of the photoexcited hydrated electron as a function of temperature to confront the predictions of our simulations. The ultrafast spectroscopy clearly shows faster relaxation dynamics at higher temperatures. In particular, the transient absorption data show a clear excess bleach beyond that of the equilibrium hydrated electron's ground-state absorption that can only be explained by stimulated emission. This stimulated emission component, which is consistent with the experimentally known fluorescence spectrum of the hydrated electron, decreases in both amplitude and lifetime as the temperature is increased. We use a kinetic model to globally fit the temperature-dependent transient absorption data at multiple temperatures ranging from 0 to 45 °C. We find the room-temperature lifetime of the excited-state hydrated electron to be 137 ±40 fs, in close agreement with recent time-resolved photoelectron spectroscopy (TRPES) experiments and in strong support of the "non-adiabatic" picture of the hydrated electron's excited-state relaxation. Moreover, we find that the excited-state lifetime is strongly temperature dependent, changing by slightly more than a factor of two over the 45 °C temperature range explored. This temperature dependence of the lifetime, along with a faster rate of ground-state cooling with increasing bulk temperature, should be directly observable by future TRPES experiments. Our data also suggest that the red side of the hydrated electron's fluorescence spectrum should significantly decrease with increasing temperature. Overall, our results are not consistent with the nearly complete lack of temperature dependence predicted by traditional cavity models of the hydrated electron but instead agree qualitatively and nearly quantitatively with the temperature-dependent structural changes predicted by the non-cavity hydrated electron model.

Multiple exciton dissociation in CdSe quantum dots by ultrafast electron transfer to adsorbed methylene blue.

PubMed

Huang, Jier; Huang, Zhuangqun; Yang, Ye; Zhu, Haiming; Lian, Tianquan

2010-04-07

Multiexciton generation in quantum dots (QDs) may provide a new approach for improving the solar-to-electric power conversion efficiency in QD-based solar cells. However, it remains unclear how to extract these excitons before the ultrafast exciton-exciton annihilation process. In this study we investigate multiexciton dissociation dynamics in CdSe QDs adsorbed with methylene blue (MB(+)) molecules by transient absorption spectroscopy. We show that excitons in QDs dissociate by ultrafast electron transfer to MB(+) with an average time constant of approximately 2 ps. The charge separated state is long-lived (>1 ns), and the charge recombination rate increases with the number of dissociated excitons. Up to three MB(+) molecules per QD can be reduced by exciton dissociation. Our result demonstrates that ultrafast interfacial charge separation can effectively compete with exciton-exciton annihilation, providing a viable approach for utilizing short-lived multiple excitons in QDs.

Hot-hole extraction from quantum dot to molecular adsorbate.

PubMed

Singhal, Pallavi; Ghosh, Hirendra N

2015-03-09

Ultrafast thermalized and hot-hole-transfer processes have been investigated in CdSe quantum dot (QD)/catechol composite systems in which hole transfer from photoexcited QDs to the catechols is thermodynamically favorable. A series of catechol derivatives were selected with different electron-donating and -withdrawing groups, and the effect of these groups on hole transfer and charge recombination (CR) dynamics has been investigated. The hole-transfer time was determined using the fluorescence upconversion technique and found to be 2-10 ps depending on the molecular structure of the catechol derivatives. The hot-hole-transfer process was followed after monitoring 2S luminescence of CdSe QDs. Interestingly, hot-hole extraction was observed only in the CdSe/3-methoxycatechol (3-OCH3) composite system owing to the higher electron-donating property of the 3-methoxy group. To confirm the extraction of the hot hole and to monitor the CR reaction in CdSe QD/catechol composite systems, ultrafast transient absorption studies have been carried out. Ultrafast transient-absorption studies show that the bleach recovery kinetics of CdSe QD at the 2S excitonic position is much faster in the presence of 3-OCH3. This faster bleach recovery at the 2S position in CdSe/3-OCH3 suggests hot-hole transfer from CdSe QD to 3-OCH3. CR dynamics in CdSe QD/catechol composite systems was followed by monitoring the excitonic bleach at the 1S position and was found to decrease with free energy of the CR reaction. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A study of the proteorhodopsin primary photoreaction by low-temperature FTIR difference and ultrafast transient infrared spectroscopy

NASA Astrophysics Data System (ADS)

Amsden, Jason J.

Proteorhodopsin (PR), a newly discovered microbial rhodopsin found in marine proteobacteria, functions as a light-driven proton pump similar to bacteriorhodopsin (BR). PR-containing bacteria account for ˜13% of the microorganisms in the oceans' photic zone and are responsible for a significant fraction of the biosphere's solar energy conversion. We study the initial response of proteorhodopsin to photon absorption using a combination of low-temperature (80 K) Fourier transform infrared (FTIR) difference spectroscopy and ultrafast transient infrared (TIR) spectroscopy. Low-temperature FTIR difference spectroscopy combined with site-directed mutagenesis and isotope labeling is used to detect and characterize changes occurring in the conformation of the retinal chromophore, protein, and internal water molecules of green-absorbing PR (GPR) and blue-absorbing PR (BPR) during the initial phototransition. Measurements on cryogenically trapped intermediates do not accurately reflect all native structural changes occurring in PR and other microbial rhodopsins on ultrafast time scales at room temperature. Recent studies demonstrate that photoactive proteins such as photoactive yellow protein, myoglobin, and green-fluorescent protein, can react within several picoseconds to photon absorption by their chromophores. Faster subpicosecond protein responses have been suggested to occur in rhodopsin-like proteins where retinal chromophore photoisomerization may impulsively drive structural changes in nearby protein groups. Here, I test this possibility by investigating the earliest protein and chromophore structural changes occurring in GPR using ultrafast TIR spectroscopy with ˜200 fs time resolution combined with non-perturbing isotope labeling. On the basis of total-15N and retinal C15D (retinal with a deuterium on carbon 15) isotope labeling, the all-trans to 13-cis retinal chromophore isomerization occurs with a 500-700 fs time constant and the amide II mode of one or more peptide bonds in the protein backbone downshifts in frequency on the same 500--700 fs time scale. This downshift represents the weakening of the hydrogen bonding in one or more peptide bonds in the peptide backbone. Preliminary data on BR indicates that this amide II downshift is a universal feature in microbial rhodopsins. In addition, the protein changes we observe on a subpicosecond time-scale may be involved in storage and transfer of the absorbed photon energy subsequently utilized for proton transport.

Investigation of the optical properties of novel organic macromolecules for solar cell applications

NASA Astrophysics Data System (ADS)

Adegoke, Oluwasegun Oluwasina

The search for renewable energy sources to replace fossil fuel has been a major research focus in the energy sector. The sun, with its vast amount of energy, remains the most abundant and ubiquitous energy source that far exceeds the world energy demand. The ability to effectively capture and convert energy from the sun in the form of photons will be the key to its effective utilization. Organic macromolecules have tremendous potentials to replace and out-perform existing materials, due to their low-cost, ease of tunability, high absorption coefficient and "green" nature. In this dissertation, spectroscopic techniques of steady state absorption and time-resolved fluorescence spectroscopy were used to show the improved absorption of the oligothiophene-functionalized ZnPc through ultrafast energy transfer. ZnPc is known for its chemical and thermal stability. The power conversion efficiency (PCE) in ZnPc-based solar devices is however, very low because of the poor absorption of ZnPc in the 300 - 550 nm region of the solar spectrum. Oligothiophenes have good absorption in the spectral region where the absorption of ZnPc is poor. Other groups of organic compounds that have gained prominence in the study for the design of efficient active materials for photovoltaic cells are the polymers. In the dissertation, different factors which can affect the performance of organic polymers in photovoltaics systems were investigated and analyzed. The effects of the alteration of conjugation, donor-acceptor groups, heteroatoms and alkyl side chains on the photophysical properties and ultimately the performance of organic polymers in organic photovoltaics were investigated. The different effects were investigated using ultrafast spectroscopic techniques which are capable of providing insight of fluorescence decay dynamics at very short times in a time scale of femtosecond. The electronic structure calculations of the polymers were carried out to provide further evidence to the experimental findings. PTB7, which has one of the best power conversion efficiency in organic photovoltaics, was one of the investigated polymers. Other novel organic polymers based on thiophene and furan framework were also considered.

Ultrafast Photoresponsive Starburst and Dendritic Fullerenyl Nanostructures for Broadband Nonlinear Photonic Material Applications

DTIC Science & Technology

2014-08-20

antenna located at the opposite cage pole area) by helical st-PMMA led to observation of broadband characteristics at 780 and 980 nm with 2PA cross...absorption (2PA) based photodynamic therapy4‒8 (2γ-PDT) against pathogens and cancer to minimize the damage to surrounding normal tissue. Photoresponsive...periconjugation with a physical separation distance of only ɛ.5 Ǻ between donor and acceptor moieties. This led to the realization of ultrafast intramolecular

Femtosecond transient absorption, Raman, and electrochemistry studies of tetrasulfonated copper phthalocyanine in water solutions.

PubMed

Abramczyk, H; Brozek-Płuska, B; Kurczewski, K; Kurczewska, M; Szymczyk, I; Krzyczmonik, P; Błaszczyk, T; Scholl, H; Czajkowski, W

2006-07-20

Ultrafast time-resolved electronic spectra of the primary events induced in the copper tetrasulfonated phthalocyanine Cu(tsPc)4-) in aqueous solution has been measured by femtosecond pump-probe transient absorption spectroscopy. The primary events initiated by the absorption of a photon occurring within the femtosecond time scale are discussed on the basis of the electron transfer mechanism between the adjacent phthalocyanine rings proposed recently in our laboratory. The femtosecond transient absorption results are compared with the low temperature emission spectra obtained with Raman spectroscopy and the voltammetric curves.

White light Z-scan measurements of ultrafast optical nonlinearity in reduced graphene oxide nanosheets in the 400–700 nm region

DOE Office of Scientific and Technical Information (OSTI.GOV)

Perumbilavil, Sreekanth; Sankar, Pranitha; Priya Rose, T.

Wavelength dispersion of optical power limiting is an important factor to be considered while designing potential optical limiters for laser safety applications. We report the observation of broadband, ultrafast optical limiting in reduced graphene oxide (rGO), measured by a single open aperture Z-scan using a white light continuum (WLC) source. WLC Z-scan is fast when the nonlinearity is to be measured over broad wavelength ranges, and it obviates the need for an ultrafast tunable laser making it cost-economic compared to conventional Z-scan. The nonlinearity arises from nondegenerate two-photon absorption, owing mostly to the crystallinity and extended π conjugation of rGO.

Serial Femtosecond Crystallography and Ultrafast Absorption Spectroscopy of the Photoswitchable Fluorescent Protein IrisFP.

PubMed

Colletier, Jacques-Philippe; Sliwa, Michel; Gallat, François-Xavier; Sugahara, Michihiro; Guillon, Virginia; Schirò, Giorgio; Coquelle, Nicolas; Woodhouse, Joyce; Roux, Laure; Gotthard, Guillaume; Royant, Antoine; Uriarte, Lucas Martinez; Ruckebusch, Cyril; Joti, Yasumasa; Byrdin, Martin; Mizohata, Eiichi; Nango, Eriko; Tanaka, Tomoyuki; Tono, Kensuke; Yabashi, Makina; Adam, Virgile; Cammarata, Marco; Schlichting, Ilme; Bourgeois, Dominique; Weik, Martin

2016-03-03

Reversibly photoswitchable fluorescent proteins find growing applications in cell biology, yet mechanistic details, in particular on the ultrafast photochemical time scale, remain unknown. We employed time-resolved pump-probe absorption spectroscopy on the reversibly photoswitchable fluorescent protein IrisFP in solution to study photoswitching from the nonfluorescent (off) to the fluorescent (on) state. Evidence is provided for the existence of several intermediate states on the pico- and microsecond time scales that are attributed to chromophore isomerization and proton transfer, respectively. Kinetic modeling favors a sequential mechanism with the existence of two excited state intermediates with lifetimes of 2 and 15 ps, the second of which controls the photoswitching quantum yield. In order to support that IrisFP is suited for time-resolved experiments aiming at a structural characterization of these ps intermediates, we used serial femtosecond crystallography at an X-ray free electron laser and solved the structure of IrisFP in its on state. Sample consumption was minimized by embedding crystals in mineral grease, in which they remain photoswitchable. Our spectroscopic and structural results pave the way for time-resolved serial femtosecond crystallography aiming at characterizing the structure of ultrafast intermediates in reversibly photoswitchable fluorescent proteins.

Charge carrier trapping and acoustic phonon modes in single CdTe nanowires.

PubMed

Lo, Shun Shang; Major, Todd A; Petchsang, Nattasamon; Huang, Libai; Kuno, Masaru K; Hartland, Gregory V

2012-06-26

Semiconductor nanostructures produced by wet chemical synthesis are extremely heterogeneous, which makes single particle techniques a useful way to interrogate their properties. In this paper the ultrafast dynamics of single CdTe nanowires are studied by transient absorption microscopy. The wires have lengths of several micrometers and lateral dimensions on the order of 30 nm. The transient absorption traces show very fast decays, which are assigned to charge carrier trapping into surface defects. The time constants vary for different wires due to differences in the energetics and/or density of surface trap sites. Measurements performed at the band edge compared to the near-IR give slightly different time constants, implying that the dynamics for electron and hole trapping are different. The rate of charge carrier trapping was observed to slow down at high carrier densities, which was attributed to trap-state filling. Modulations due to the fundamental and first overtone of the acoustic breathing mode were also observed in the transient absorption traces. The quality factors for these modes were similar to those measured for metal nanostructures, and indicate a complex interaction with the environment.

Excited state electron and energy relays in supramolecular dinuclear complexes revealed by ultrafast optical and X-ray transient absorption spectroscopy

DOE PAGES

Hayes, Dugan; Kohler, Lars; Hadt, Ryan G.; ...

2017-11-28

Here, the kinetics of photoinduced electron and energy transfer in a family of tetrapyridophenazine-bridged heteroleptic homo- and heterodinuclear copper(I) bis(phenanthroline)/ruthenium(II) polypyridyl complexes were studied using ultrafast optical and multi-edge X-ray transient absorption spectroscopies. This work combines the synthesis of heterodinuclear Cu(I)–Ru(II) analogs of the homodinuclear Cu(I)–Cu(I) targets with spectroscopic analysis and electronic structure calculations to first disentangle the dynamics at individual metal sites by taking advantage of the element and site specificity of X-ray absorption and theoretical methods. The excited state dynamical models developed for the heterodinuclear complexes are then applied to model the more challenging homodinuclear complexes. These resultsmore » suggest that both intermetallic charge and energy transfer can be observed in an asymmetric dinuclear copper complex in which the ground state redox potentials of the copper sites are offset by only 310 meV. We also demonstrate the ability of several of these complexes to effectively and unidirectionally shuttle energy between different metal centers, a property that could be of great use in the design of broadly absorbing and multifunctional multimetallic photocatalysts. This work provides an important step toward developing both a fundamental conceptual picture and a practical experimental handle with which synthetic chemists, spectroscopists, and theoreticians may collaborate to engineer cheap and efficient photocatalytic materials capable of performing coulombically demanding chemical transformations.« less

Excited state electron and energy relays in supramolecular dinuclear complexes revealed by ultrafast optical and X-ray transient absorption spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hayes, Dugan; Kohler, Lars; Hadt, Ryan G.

Here, the kinetics of photoinduced electron and energy transfer in a family of tetrapyridophenazine-bridged heteroleptic homo- and heterodinuclear copper(I) bis(phenanthroline)/ruthenium(II) polypyridyl complexes were studied using ultrafast optical and multi-edge X-ray transient absorption spectroscopies. This work combines the synthesis of heterodinuclear Cu(I)–Ru(II) analogs of the homodinuclear Cu(I)–Cu(I) targets with spectroscopic analysis and electronic structure calculations to first disentangle the dynamics at individual metal sites by taking advantage of the element and site specificity of X-ray absorption and theoretical methods. The excited state dynamical models developed for the heterodinuclear complexes are then applied to model the more challenging homodinuclear complexes. These resultsmore » suggest that both intermetallic charge and energy transfer can be observed in an asymmetric dinuclear copper complex in which the ground state redox potentials of the copper sites are offset by only 310 meV. We also demonstrate the ability of several of these complexes to effectively and unidirectionally shuttle energy between different metal centers, a property that could be of great use in the design of broadly absorbing and multifunctional multimetallic photocatalysts. This work provides an important step toward developing both a fundamental conceptual picture and a practical experimental handle with which synthetic chemists, spectroscopists, and theoreticians may collaborate to engineer cheap and efficient photocatalytic materials capable of performing coulombically demanding chemical transformations.« less

Transition-metal dichalcogenides heterostructure saturable absorbers for ultrafast photonics.

PubMed

Chen, Hao; Yin, Jinde; Yang, Jingwei; Zhang, Xuejun; Liu, Mengli; Jiang, Zike; Wang, Jinzhang; Sun, Zhipei; Guo, Tuan; Liu, Wenjun; Yan, Peiguang

2017-11-01

In this Letter, high-quality WS 2 film and MoS 2 film were vertically stacked on the tip of a single-mode fiber in turns to form heterostructure (WS 2 -MoS 2 -WS 2 )-based saturable absorbers with all-fiber integrated features. Their nonlinear saturable absorption properties were remarkable, such as a large modulation depth (∼16.99%) and a small saturable intensity (6.23  MW·cm -2 ). Stable pulses at 1.55 μm with duration as short as 296 fs and average power as high as 25 mW were obtained in an erbium-doped fiber laser system. The results demonstrate that the proposed heterostructures own remarkable nonlinear optical properties and offer a platform for adjusting nonlinear optical properties by stacking different transition-metal dichalcogenides or modifying the thickness of each layer, paving the way for engineering functional ultrafast photonics devices with desirable properties.

Transient GaAs plasmonic metasurfaces at terahertz frequencies

DOE PAGES

Yang, Yuanmu; Kamaraju, N.; Campione, Salvatore; ...

2016-12-09

Here we demonstrate the ultrafast formation of terahertz (THz) metasurfaces through all-optical creation of spatially modulated carrier density profiles in a deep-subwavelength GaAs film. The switch-on of the transient plasmon mode, governed by the GaAs effective electron mass and electron–phonon interactions, is revealed by structured-optical pump THz probe spectroscopy, on a time scale of 500 fs. By modulating the carrier density using different pump fluences, we observe a wide tuning of the electric dipole resonance of the transient GaAs metasurface from 0.5 THz to 1.7 THz. Furthermore, we numerically demonstrate that the metasurface presented here can be generalized to moremore » complex architectures for realizing functionalities such as perfect absorption, leading to a 30 dB modulation depth. In conclusion, the platform also provides a pathway to achieve ultrafast manipulation of infrared beams in the linear and, potentially, nonlinear regime.« less

Ultrafast time-resolved spectroscopy of lead halide perovskite films

NASA Astrophysics Data System (ADS)

Idowu, Mopelola A.; Yau, Sung H.; Varnavski, Oleg; Goodson, Theodore

2015-09-01

Recently, lead halide perovskites which are organic-inorganic hybrid structures, have been discovered to be highly efficient as light absorbers. Herein, we show the investigation of the excited state dynamics and emission properties of non-stoichiometric precursor formed lead halide perovskites grown by interdiffusion method using steady-state and time-resolved spectroscopic measurements. The influence of the different ratios of the non-stoichiometric precursor solution was examined. The observed photoluminescence properties were correlated with the femtosecond transient absorption measurements.

Probing the Ultrafast Energy Dissipation Mechanism of the Sunscreen Oxybenzone after UVA Irradiation.

PubMed

Baker, Lewis A; Horbury, Michael D; Greenough, Simon E; Coulter, Philip M; Karsili, Tolga N V; Roberts, Gareth M; Orr-Ewing, Andrew J; Ashfold, Michael N R; Stavros, Vasilios G

2015-04-16

Oxybenzone is a common constituent of many commercially available sunscreens providing photoprotection from ultraviolet light incident on the skin. Femtosecond transient electronic and vibrational absorption spectroscopies have been used to investigate the nonradiative relaxation pathways of oxybenzone in cyclohexane and methanol after excitation in the UVA region. The present data suggest that the photoprotective properties of oxybenzone can be understood in terms of an initial ultrafast excited state enol → keto tautomerization, followed by efficient internal conversion and subsequent vibrational relaxation to the ground state (enol) tautomer.

Ultrafast chemical reactions in shocked nitromethane probed with dynamic ellipsometry and transient absorption spectroscopy.

PubMed

Brown, Kathryn E; McGrane, Shawn D; Bolme, Cynthia A; Moore, David S

2014-04-10

Initiation of the shock driven chemical reactions and detonation of nitromethane (NM) can be sensitized by the addition of a weak base; however, the chemical mechanism by which sensitization occurs remains unclear. We investigated the shock driven chemical reaction in NM and in NM sensitized with diethylenetriamine (DETA), using a sustained 300 ps shock driven by a chirped Ti:sapphire laser. We measured the solutions' visible transient absorption spectra and measured interface particle and shock velocities of the nitromethane solutions using ultrafast dynamic ellipsometry. We found there to be a volume-increasing reaction that takes place around interface particle velocity up = 2.4 km/s and up = 2.2 km/s for neat NM and NM with 5% DETA, respectively. The rate at which transient absorption increases is similar in all mixtures, but with decreasing induction times for solutions with increasing DETA concentrations. This result supports the hypothesis that the chemical reaction mechanisms for shocked NM and NM with DETA are the same. Data from shocked NM are compared to literature experimental and theoretical data.

Nonlinear absorption properties of silicene nanosheets.

PubMed

Zhang, Fang; Wang, Mengxia; Wang, Zhengping; Han, Kezhen; Liu, Xiaojuan; Xu, Xinguang

2018-06-01

As the cousins of graphene, i.e. same group IVA element, the nonlinear absorption (NLA) properties of silicene nanosheets were rarely studied. In this paper, we successfully exfoliated the two-dimensional silicene nanosheets from bulk silicon crystal using liquid phase exfoliation method. The NLA properties of silicene nanosheets were systemically investigated for the first time, as we have known. Silicene performed exciting saturable absorption and two photon absorption (2PA) behavior. The lower saturable intensity and larger 2PA coefficient at 532 nm excitation indicates that silicene has potential application in ultrafast lasers and optical limiting devices, especially in visible waveband.

Nonlinear absorption properties of silicene nanosheets

NASA Astrophysics Data System (ADS)

Zhang, Fang; Wang, Mengxia; Wang, Zhengping; Han, Kezhen; Liu, Xiaojuan; Xu, Xinguang

2018-06-01

As the cousins of graphene, i.e. same group IVA element, the nonlinear absorption (NLA) properties of silicene nanosheets were rarely studied. In this paper, we successfully exfoliated the two-dimensional silicene nanosheets from bulk silicon crystal using liquid phase exfoliation method. The NLA properties of silicene nanosheets were systemically investigated for the first time, as we have known. Silicene performed exciting saturable absorption and two photon absorption (2PA) behavior. The lower saturable intensity and larger 2PA coefficient at 532 nm excitation indicates that silicene has potential application in ultrafast lasers and optical limiting devices, especially in visible waveband.

Spin relaxation dynamics of holes in intrinsic GaAs quantum wells studied by transient circular dichromatic absorption spectroscopy at room temperature.

PubMed

Fang, Shaoyin; Zhu, Ruidan; Lai, Tianshu

2017-03-21

Spin relaxation dynamics of holes in intrinsic GaAs quantum wells is studied using time-resolved circular dichromatic absorption spectroscopy at room temperature. It is found that ultrafast dynamics is dominated by the cooperative contributions of band filling and many-body effects. The relative contribution of the two effects is opposite in strength for electrons and holes. As a result, transient circular dichromatic differential transmission (TCD-DT) with co- and cross-circularly polarized pump and probe presents different strength at several picosecond delay time. Ultrafast spin relaxation dynamics of excited holes is sensitively reflected in TCD-DT with cross-circularly polarized pump and probe. A model, including coherent artifact, thermalization of nonthermal carriers and the cooperative contribution of band filling and many-body effects, is developed, and used to fit TCD-DT with cross-circularly polarized pump and probe. Spin relaxation time of holes is achieved as a function of excited hole density for the first time at room temperature, and increases with hole density, which disagrees with a theoretical prediction based on EY spin relaxation mechanism, implying that EY mechanism may be not dominant hole spin relaxation mechanism at room temperature, but DP mechanism is dominant possibly.

Ultrafast dynamics induced by the interaction of molecules with electromagnetic fields: Several quantum, semiclassical, and classical approaches.

PubMed

Antipov, Sergey V; Bhattacharyya, Swarnendu; El Hage, Krystel; Xu, Zhen-Hao; Meuwly, Markus; Rothlisberger, Ursula; Vaníček, Jiří

2017-11-01

Several strategies for simulating the ultrafast dynamics of molecules induced by interactions with electromagnetic fields are presented. After a brief overview of the theory of molecule-field interaction, we present several representative examples of quantum, semiclassical, and classical approaches to describe the ultrafast molecular dynamics, including the multiconfiguration time-dependent Hartree method, Bohmian dynamics, local control theory, semiclassical thawed Gaussian approximation, phase averaging, dephasing representation, molecular mechanics with proton transfer, and multipolar force fields. In addition to the general overview, some focus is given to the description of nuclear quantum effects and to the direct dynamics, in which the ab initio energies and forces acting on the nuclei are evaluated on the fly. Several practical applications, performed within the framework of the Swiss National Center of Competence in Research "Molecular Ultrafast Science and Technology," are presented: These include Bohmian dynamics description of the collision of H with H 2 , local control theory applied to the photoinduced ultrafast intramolecular proton transfer, semiclassical evaluation of vibrationally resolved electronic absorption, emission, photoelectron, and time-resolved stimulated emission spectra, infrared spectroscopy of H-bonding systems, and multipolar force fields applications in the condensed phase.

Ultrafast Photoinduced Electron Transfer in a π-Conjugated Oligomer/Porphyrin Complex.

PubMed

Aly, Shawkat M; Goswami, Subhadip; Alsulami, Qana A; Schanze, Kirk S; Mohammed, Omar F

2014-10-02

Controlling charge transfer (CT), charge separation (CS), and charge recombination (CR) at the donor-acceptor interface is extremely important to optimize the conversion efficiency in solar cell devices. In general, ultrafast CT and slow CR are desirable for optimal device performance. In this Letter, the ultrafast excited-state CT between platinum oligomer (DPP-Pt(acac)) as a new electron donor and porphyrin as an electron acceptor is monitored for the first time using femtosecond (fs) transient absorption (TA) spectroscopy with broad-band capability and 120 fs temporal resolution. Turning the CT on/off has been shown to be possible either by switching from an organometallic oligomer to a metal-free oligomer or by controlling the charge density on the nitrogen atom of the porphyrin meso unit. Our time-resolved data show that the CT and CS between DPP-Pt(acac) and cationic porphyrin are ultrafast (approximately 1.5 ps), and the CR is slow (ns time scale), as inferred from the formation and the decay of the cationic and anionic species. We also found that the metallic center in the DPP-Pt(acac) oligomer and the positive charge on the porphyrin are the keys to switching on/off the ultrafast CT process.

Broadband atomic-layer MoS₂ optical modulators for ultrafast pulse generations in the visible range.

PubMed

Zhang, Yuxia; Yu, Haohai; Zhang, Rui; Zhao, Gang; Zhang, Huaijin; Chen, Yanxue; Mei, Liangmo; Tonelli, Mauro; Wang, Jiyang

2017-02-01

Visible lasers are a fascinating regime, and their significance is illustrated by the 2014 Noble prizes in physics and chemistry. With the development of blue laser diodes (LDs), the LD-pumped solid-state visible lasers become a burgeoning direction today. Constrained by the scarce visible optical modulators, the solid-state ultrafast visible lasers are rarely realized. Based on the bandgap structure and optoelectronic properties of atomic-layer MoS₂, it can be proposed that MoS₂ has the potential as a visible optical modulator. Here, by originally revealing layer-dependent nonlinear absorption of the atomic-layer MoS₂ in the visible range, broadband atomic-layer MoS₂ optical modulators for the visible ultrafast pulse generation are developed and selected based on the proposed design criteria for novel two-dimensional (2D) optical modulators. By applying the selected MoS₂ optical modulators in the solid-state praseodymium lasers, broadband mode-locked ultrafast lasers from 522 to 639 nm are originally realized. We believe that this Letter should promote the development of visible ultrafast photonics and further applications of 2D optoelectronic materials.

Ultrafast dynamics induced by the interaction of molecules with electromagnetic fields: Several quantum, semiclassical, and classical approaches

PubMed Central

Antipov, Sergey V.; Bhattacharyya, Swarnendu; El Hage, Krystel; Xu, Zhen-Hao; Meuwly, Markus; Rothlisberger, Ursula; Vaníček, Jiří

2018-01-01

Several strategies for simulating the ultrafast dynamics of molecules induced by interactions with electromagnetic fields are presented. After a brief overview of the theory of molecule-field interaction, we present several representative examples of quantum, semiclassical, and classical approaches to describe the ultrafast molecular dynamics, including the multiconfiguration time-dependent Hartree method, Bohmian dynamics, local control theory, semiclassical thawed Gaussian approximation, phase averaging, dephasing representation, molecular mechanics with proton transfer, and multipolar force fields. In addition to the general overview, some focus is given to the description of nuclear quantum effects and to the direct dynamics, in which the ab initio energies and forces acting on the nuclei are evaluated on the fly. Several practical applications, performed within the framework of the Swiss National Center of Competence in Research “Molecular Ultrafast Science and Technology,” are presented: These include Bohmian dynamics description of the collision of H with H2, local control theory applied to the photoinduced ultrafast intramolecular proton transfer, semiclassical evaluation of vibrationally resolved electronic absorption, emission, photoelectron, and time-resolved stimulated emission spectra, infrared spectroscopy of H-bonding systems, and multipolar force fields applications in the condensed phase. PMID:29376107

Time-dependent nonequilibrium soft x-ray response during a spin crossover

NASA Astrophysics Data System (ADS)

van Veenendaal, Michel

2018-03-01

A theoretical framework is developed for better understanding the time-dependent soft-x-ray response of dissipative quantum many-body systems. It is shown how x-ray absorption and resonant inelastic x-ray scattering (RIXS) at transition-metal L edges can provide insight into ultrafast intersystem crossings of importance for energy conversion, ultrafast magnetism, and catalysis. The photoinduced doublet-to-quartet spin crossover on cobalt in Fe-Co Prussian blue analogs is used as a model system to demonstrate how the x-ray response is affected by the nonequilibrium dynamics on a femtosecond time scale. Changes in local spin and symmetry and the underlying mechanism are reflected in strong broadenings, a collapse of clear selection rules during the intersystem crossing, fluctuations in the isotropic branching ratio in x-ray absorption, crystal-field collapse and/or oscillations, and time-dependent anti-Stokes processes in RIXS.

Ultrafast Dynamics in DNA and RNA Derivatives Monitored by Broadband Transient Absorption Spectrscopy

NASA Astrophysics Data System (ADS)

Brister, Matthew M.; Crespo-Hernández, Carlos E.

2015-06-01

The ultrafast dynamics of nucleic acids have been under scrutiny for the past couple of decades because of the role that the high-energy electronic states play in mutagenesis and carcinogenesis. Kinetic models have been proposed, based on both experimental and theoretical discoveries. Direct experimental evidence of the intersystem crossing rate and population of the triplet state for most nucleic acid bases has yet to be reported, even though the triplet state is thought to be the most reactive species. Utilizing broadband femtosecond transient absorption spectroscopy, we reveal the time scale at which singlet-to-triplet population transfer occurs in several nucleic acid derivatives in the condensed phase. The implication of these results to the current understanding of the DNA and RNA photochemistry will be discussed. The authors acknowledge the CAREER program of the National Science Foundation (Grant No. CHE-1255084) for financial support.

Ultra-fast switching of light by absorption saturation in vacuum ultra-violet region.

PubMed

Yoneda, Hitoki; Inubushi, Yuichi; Tanaka, Toshihiro; Yamaguchi, Yuta; Sato, Fumiya; Morimoto, Shunsuke; Kumagai, Taisuke; Nagasono, Mitsuru; Higashiya, Atsushi; Yabashi, Makina; Ishikawa, Tetsuya; Ohashi, Haruhiko; Kimura, Hiroaki; Kitamura, Hikaru; Kodama, Ryosuke

2009-12-21

Advances in free electron lasers producing high energy photons [Nat. Photonics 2(9), 555-559 (2008)] are expected to open up a new science of nonlinear optics of high energy photons. Specifically, lasers of photon energy higher than the plasma frequency of a metal can show new interaction features because they can penetrate deeply into metals without strong reflection. Here we show the observation of ultra-fast switching of vacuum ultra-violet (VUV) light caused by saturable absorption of a solid metal target. A strong gating is observed at energy fluences above 6J/cm2 at wavelength of 51 nm with tin metal thin layers. The ratio of the transmission at high intensity to low intensity is typically greater than 100:1. This means we can design new nonlinear photonic devices such as auto-correlator and pulse slicer for the VUV region.

An instrumentation project for measuring weak and broadband ultrafast laser signals

NASA Astrophysics Data System (ADS)

Ellis, Armin T.

From our everyday experiences, we know that as light travels through a medium it attenuates due to absorption and scattering. Absorption is the cause of color in tea or grape juice, and it is described by Beer's law. Scattering is the reason why scuba divers have a limited range of vision and why mountain peaks become harder to see the further away they are. Precursors, although not fully understood, are transient light transmission effects and have been shown to exhibit lower attenuation through media than that predicted by Beer's law for steady-state light. In this thesis we present an instrumentation based approach for studying precursors by measuring spectral evolution and pure attenuation over distance. We will also introduce a new instrument concept, RotaryFROG, capable of simultaneous measurement of intensity, phase, and polarization versus frequency of low-intensity broadband pulses for use with ultrafast lasers.

Subpiosecond Third Order Nonlinear Response in Polythiophene and Thiopene Based Thin Films

NASA Technical Reports Server (NTRS)

Harris, D.; Royer, E.; Dorsinville, R.

1995-01-01

Ultrafast relaxation kinetics of the third order nonlinear susceptibility of polythiophene and polycondensed thiophene-based polymer was determined by the forward degenerate four-wave mixing technique. Deep into the absorption band the nonlinear response shows only a fast component (less than 900 fs at 587 nm) while at the edge of the absorption band at 642 nm a much slower and complex decay was measured.

Dynamics of Carotenoids Probed by Femtosecond Absorption, Fluorescence, and Raman Spectroscopy

NASA Astrophysics Data System (ADS)

Yoshizawa, M.; Kosumi, D.; Komukai, M.; Yanagi, K.; Hashimoto, H.

Ultrafast optical responses in β-carotene and lycopene depend on the pump wavelength. Excess vibrational energy induced by the photoexcitation remains longer than several picoseconds in the excited states and slows down the relaxation kinetics.

Bimodal Exciplex Formation in Bimolecular Photoinduced Electron Transfer Revealed by Ultrafast Time-Resolved Infrared Absorption.

PubMed

Koch, Marius; Licari, Giuseppe; Vauthey, Eric

2015-09-03

The dynamics of a moderately exergonic photoinduced charge separation has been investigated by ultrafast time-resolved infrared absorption with the dimethylanthracene/phthalonitrile donor/acceptor pair in solvents covering a broad range of polarity. A distinct spectral signature of an exciplex could be identified in the -C≡N stretching region. On the basis of quantum chemistry calculations, the 4-5 times larger width of this band compared to those of the ions and of the locally excited donor bands is explained by a dynamic distribution of exciplex geometry with different mutual orientations and distances of the constituents and, thus, with varying charge-transfer character. Although spectrally similar, two types of exciplexes could be distinguished by their dynamics: short-lived, "tight", exciplexes generated upon static quenching and longer-lived, "loose", exciplexes formed upon dynamic quenching in parallel with ion pairs. Tight exciplexes were observed in all solvents, except in the least polar diethyl ether where quenching is slower than diffusion. The product distribution of the dynamic quenching depends strongly on the solvent polarity: whereas no significant loose exciplex population could be detected in acetonitrile, both exciplex and ion pair are generated in less polar solvents, with the relative population of exciplex increasing with decreasing solvent polarity. These results are compared with those reported previously with donor/acceptor pairs in different driving force regimes to obtain a comprehensive picture of the role of the exciplexes in bimolecular photoinduced charge separation.

Two-dimensional infrared spectroscopy of intermolecular hydrogen bonds in the condensed phase.

PubMed

Elsaesser, Thomas

2009-09-15

Hydrogen bonding plays a key role in the structural, physical, and chemical properties of liquids such as water and in macromolecular structures such as proteins. Vibrational spectroscopy is an important tool for understanding hydrogen bonding because it provides a way to observe local molecular geometries and their interaction with the environment. Linear vibrational spectroscopy has mapped characteristic changes of vibrational spectra and the occurrence of new bands that form upon hydrogen bonding. However, linear vibrational spectroscopy gives very limited insight into ultrafast dynamics of the underlying molecular interactions, such as the motions of hydrogen-bonded groups, energy dissipation and delocalization, and the fluctuations within hydrogen-bonded structures that occur in the ultrafast time domain. Nonlinear vibrational spectroscopy with its femtosecond time resolution can discern these dynamic processes in real time and has emerged as an important tool for unraveling molecular dynamics and for quantifying interactions that govern the vibrational and structural dynamics of hydrogen bonds. This Account reviews recent progress originating from third-order nonlinear methods of coherent multidimensional vibrational spectroscopy. Ultrafast dynamics of intermolecular hydrogen bonds are addressed for a number of prototype systems: hydrogen-bonded carboxylic acid dimers in an aprotic liquid environment, the disordered fluctuating hydrogen-bond network of liquid water, and DNA oligomers interacting with water. Cyclic carboxylic acid dimers display a rich scheme of vibrational couplings, resulting in OH stretching absorption bands with highly complex spectral envelopes. Two-dimensional spectroscopy of acetic acid dimers in a nonpolar liquid environment demonstrates that multiple Fermi resonances of the OH stretching mode with overtones and combination tones of fingerprint vibrations dominate both the 2D and linear absorption spectra. The coupling of the OH stretching mode with low-frequency hydrogen-bonding modes leads to additional progressions and coherent low-frequency hydrogen-bond motions in the subpicosecond time domain. In water, the 2D spectra reveal ultrafast spectral diffusion on a sub-100 fs time scale caused by the ultrafast structural fluctuations of the strongly coupled hydrogen-bond network. Librational motions play a key role for the ultrafast loss of structural memory. Spectral diffusion rates are enhanced by resonant transfer of OH stretching quanta between water molecules, typically occurring on a 100 fs time scale. In DNA oligomers, femtosecond nonlinear vibrational spectroscopy resolves NH and OH stretching bands in the highly congested infrared spectra of these molecules, which contain alternating adenine-thymine pairs. Studies at different levels of hydration reveal the spectral signatures of water molecules directly interacting with the phosphate groups of DNA and of a second water species forming a fluctuating environment around the DNA oligomers. We expect that the application of 2D infrared spectroscopy in an extended spectral range will reveal the intrinsic coupling between water and specific functional units of DNA.

Strong-field two-photon transition by phase shaping

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Sangkyung; Lim, Jongseok; Ahn, Jaewook

2010-08-15

We demonstrate the ultrafast coherent control of a nonlinear two-photon absorption in a dynamically shifted energy level structure. We use a spectrotemporal laser-pulse shaping that is programed to preserve the resonant absorption condition during the intense laser-field interaction. Experiments carried out in the strong-field regime of two-photon absorption in the ground state of atomic cesium reveal that the analytically obtained offset and curvature of a laser spectrum compensate the effect of both static and dynamic energy shifts of the given light-atom interaction.

Probing Exciton and Charge Dynamics in Organic Thin Films and Photovoltaics with Nonlinear Spectroscopy

NASA Astrophysics Data System (ADS)

McDonough, Thomas J.

Emerging organic solar cell technologies offer unique advantages over silicon solar cells, such as solution processability and the use of flexible substrates, but the efficiencies of these devices do not yet match the efficiency of silicon. Ultrafast nonlinear spectroscopies can probe the fates of photoexcited species on timescales in which these species are lost to channels that do not result in electric current. In the first study, I compare the ultrafast dynamics of singlet fission and charge generation in pentacene films grown on glass and graphene. The molecular orientation is different on the two substrates: the long axis of the pentacene molecules are "standing-up" (normal to the surface) on glass and "lying-down" (parallel to the surface) on graphene. By studying the fluence and polarization dependence of the transient absorption of pentacene on these two substrates, I am able to clarify previous spectral assignments. I identify a broad, isotropic absorption at 853 nm as due in significant part to hole absorption, in contrast to this feature's typical assignment to T1-T2 absorption. At high fluence, additional peaks at 614 and 688 (on glass) nm appear, whose kinetics and anisotropies are not explained by heating, which I assign to charge generation. In the second study, I utilize two-dimensional white-light spectroscopy to study the morphology dependence of exciton diffusion in semiconducting carbon nanotubes. I analyze the spectral diffusion of the S 1-S1 2D-WL lineshape via the center line slope method to separate the homogeneous and inhomogeneous contributions to the lineshape in each sample. I determine a morphology independent homogeneous linewidth of 10 meV, but I find that the inhomogeneous linewidth is sensitive to the particular sample environment. I model our experimental spectra with kinetic Monte Carlo simulations of exciton diffusion in a 1D potential. I also present preliminary bias-dependent transient absorption and 2D-WL measurements of carbon nanotube solar cell devices. I observe increasing positive trion absorption with increasing forward bias. The kinetics in the device are much different than the kinetics in the thin film, and there is an interesting change in kinetics with bias voltage that requires further investigation.

Excited state dynamics & optical control of molecular motors

NASA Astrophysics Data System (ADS)

Wiley, Ted; Sension, Roseanne

2014-03-01

Chiral overcrowded alkenes are likely candidates for light driven rotary molecular motors. At their core, these molecular motors are based on the chromophore stilbene, undergoing ultrafast cis/trans photoisomerization about their central double bond. Unlike stilbene, the photochemistry of molecular motors proceeds in one direction only. This unidirectional rotation is a result of helicity in the molecule induced by steric hindrance. However, the steric hindrance which ensures unidirectional excited state rotation, has the unfortunate consequence of producing large ground state barriers which dramatically decrease the overall rate of rotation. These molecular scale ultrafast motors have only recently been studied by ultrafast spectroscopy. Our lab has studied the photochemistry and photophysics of a ``first generation'' molecular motor with UV-visible transient absorption spectroscopy. We hope to use optical pulse shaping to enhance the efficiency and turnover rate of these molecular motors.

Protonation-induced ultrafast torsional dynamics in 9-anthrylbenzimidazole: a pH activated molecular rotor.

PubMed

Nandi, Amitabha; Kushwaha, Archana; Das, Dipanwita; Ghosh, Rajib

2018-03-07

We report the photophysical properties and excited state dynamics of 9-anthrylbenzimidazole (ANBI) which exhibits protonation-induced molecular rotor properties. In contrast to the highly emissive behavior of neutral ANBI, protonation of the benzimidazole group of ANBI induces efficient nonradiative deactivation by ultrafast torsional motion around the bond connecting the anthracene and benzimidazole units, as revealed by ultrafast transient absorption and fluorescence spectroscopy. Contrary to viscosity-independent fluorescence of neutral dyes, protonated ANBI is shown to display linear variation of emission yield and lifetime with solvent viscosity. The protonation-induced molecular rotor properties in the studied system are shown to be driven by enhanced charge transfer and are corroborated by quantum chemical calculations. Potential application as a microviscosity sensor of acidic regions in a heterogeneous environment by these proton-activated molecular rotor properties of ANBI is discussed.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kay, Jeffrey J.; Park, Samuel; Kohl, Ian Thomas

In this work, shock-induced reactions in high explosives and their chemical mechanisms were investigated using state-of-the-art experimental and theoretical techniques. Experimentally, ultrafast shock interrogation (USI, an ultrafast interferometry technique) and ultrafast absorption spectroscopy were used to interrogate shock compression and initiation of reaction on the picosecond timescale. The experiments yielded important new data that appear to indicate reaction of high explosives on the timescale of tens of picoseconds in response to shock compression, potentially setting new upper limits on the timescale of reaction. Theoretically, chemical mechanisms of shock-induced reactions were investigated using density functional theory. The calculations generated important insightsmore » regarding the ability of several hypothesized mechanisms to account for shock-induced reactions in explosive materials. The results of this work constitute significant advances in our understanding of the fundamental chemical reaction mechanisms that control explosive sensitivity and initiation of detonation.« less

Analyte-induced spectral filtering in femtosecond transient absorption spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Abraham, Baxter; Nieto-Pescador, Jesus; Gundlach, Lars

Here, we discuss the influence of spectral filtering by samples in femtosecond transient absorption measurements. Commercial instruments for transient absorption spectroscopy (TA) have become increasingly available to scientists in recent years and TA is becoming an established technique to measure the dynamics of photoexcited systems. Furthermore, we show that absorption of the excitation pulse by the sample can severely alter the spectrum and consequently the temporal pulse shape. This “spectral self-filtering” effect can lead to systematic errors and misinterpretation of data, most notably in concentration dependent measurements. Finally, the combination of narrow absorption peaks in the sample with ultrafast broadbandmore » excitation pulses is especially prone to this effect.« less

Analyte-induced spectral filtering in femtosecond transient absorption spectroscopy

DOE PAGES

Abraham, Baxter; Nieto-Pescador, Jesus; Gundlach, Lars

2017-03-06

Here, we discuss the influence of spectral filtering by samples in femtosecond transient absorption measurements. Commercial instruments for transient absorption spectroscopy (TA) have become increasingly available to scientists in recent years and TA is becoming an established technique to measure the dynamics of photoexcited systems. Furthermore, we show that absorption of the excitation pulse by the sample can severely alter the spectrum and consequently the temporal pulse shape. This “spectral self-filtering” effect can lead to systematic errors and misinterpretation of data, most notably in concentration dependent measurements. Finally, the combination of narrow absorption peaks in the sample with ultrafast broadbandmore » excitation pulses is especially prone to this effect.« less

Bright Two-Photon Emission and Ultra-Fast Relaxation Dynamics in a DNA-Templated Nanocluster Investigated by Ultra-Fast Spectroscopy

DTIC Science & Technology

2012-01-01

Michigan 3003 S. State St Ann Arbor, MI 48109 -1274 REPORT DOCUMENTATION PAGE b . ABSTRACT UU c. THIS PAGE UU 2. REPORT TYPE New Reprint 17. LIMITATION OF...Figure 1: Steady state absorption for Au25 Au55, Au140, Au2406 and Mie theory calculation using parameter similar to Au25.7 B . Emission Mechanism of...short-lived (hundreds of fs), and it is most likely to be associated with the metal core (State B ).7,17 The near-infrared emission is related to the

Free electron laser-driven ultrafast rearrangement of the electronic structure in Ti

PubMed Central

Principi, E.; Giangrisostomi, E.; Cucini, R.; Bencivenga, F.; Battistoni, A.; Gessini, A.; Mincigrucci, R.; Saito, M.; Di Fonzo, S.; D'Amico, F.; Di Cicco, A.; Gunnella, R.; Filipponi, A.; Giglia, A.; Nannarone, S.; Masciovecchio, C.

2015-01-01

High-energy density extreme ultraviolet radiation delivered by the FERMI seeded free-electron laser has been used to create an exotic nonequilibrium state of matter in a titanium sample characterized by a highly excited electron subsystem at temperatures in excess of 10 eV and a cold solid-density ion lattice. The obtained transient state has been investigated through ultrafast absorption spectroscopy across the Ti M2,3-edge revealing a drastic rearrangement of the sample electronic structure around the Fermi level occurring on a time scale of about 100 fs. PMID:26798835

Ultrafast lattice dynamics of single crystal and polycrystalline gold nanofilms☆

NASA Astrophysics Data System (ADS)

Hu, Jianbo; Karam, Tony E.; Blake, Geoffrey A.; Zewail, Ahmed H.

2017-09-01

Ultrafast electron diffraction is employed to spatiotemporally visualize the lattice dynamics of 11 nm-thick single-crystal and 2 nm-thick polycrystalline gold nanofilms. Surprisingly, the electron-phonon coupling rates derived from two temperature simulations of the data reveal a faster interaction between electrons and the lattice in the case of the single-crystal sample. We interpret this unexpected behavior as arising from quantum confinement of the electrons in the 2 nm-thick gold nanofilm, as supported by absorption spectra, an effect that counteracts the expected increase in the electron scattering off surfaces and grain boundaries in the polycrystalline materials.

Pump-probe micro-spectroscopy by means of an ultra-fast acousto-optics delay line.

PubMed

Audier, Xavier; Balla, Naveen; Rigneault, Hervé

2017-01-15

We demonstrate femtosecond pump-probe transient absorption spectroscopy using a programmable dispersive filter as an ultra-fast delay line. Combined with fast synchronous detection, this delay line allows for recording of 6 ps decay traces at 34 kHz. With such acquisition speed, we perform single point pump-probe spectroscopy on bulk samples in 80 μs and hyperspectral pump-probe imaging over a field of view of 100 μm in less than a second. The usability of the method is illustrated in a showcase experiment to image and discriminate between two pigments in a mixture.

Ultrafast optical measurements of surface waves on a patterned layered nanostructure

NASA Astrophysics Data System (ADS)

Daly, Brian; Bjornsson, Matteo; Connolly, Aine; Mahat, Sushant; Rachmilowitz, Bryan; Antonelli, George; Myers, Alan; Yoo, Hui-Jae; Singh, Kanwal; King, Sean

2015-03-01

We report ultrafast optical pump-probe measurements of 12 - 54 GHz surface acoustic waves (SAWs) on patterned layered nanostructures. These very high frequency SAWs were generated and detected on the following patterned film stack: 25 nm physically vapor deposited TiN / 180 nm porous PECVD-grown a-SiOC:H dielectric / 12 nm non-porous PECVD-grown a-SiOC:H etch-stop / 100 nm CVD-grown a-SiO2 / Si (100) substrate. The TiN layer was dry plasma etched to form lines of rectangular cross section with pitches of 420 nm, 250 nm, 180 nm, and 168 nm and the lines were oriented parallel to the [110] direction on the wafer surface. The absorption of ultrafast pulses from a Ti:sapphire oscillator operating at 800 nm generated SAWs that were detected by time-delayed probe pulses from the same oscillator via a reflectivity change (ΔR) . In each of the four cases the SAW frequency increased with decreasing pitch, but not in a linear way as had been seen in previous experiments of this sort. By comparing the results with mechanical simulations, we present evidence for the detection of different types of SAWs in each case, including Rayleigh-like waves, Sezawa waves, and leaky or radiative waves. This work was supported by NSF Award DMR1206681.

Excitation energy dependence of excited states dynamics in all- trans-carotenes determined by femtosecond absorption and fluorescence spectroscopy

NASA Astrophysics Data System (ADS)

Kosumi, Daisuke; Yanagi, Kazuhiro; Nishio, Tomohiro; Hashimoto, Hideki; Yoshizawa, Masayuki

2005-06-01

Ultrafast relaxation kinetics in β-carotene and lycopene has been investigated by femtosecond absorption and fluorescence spectroscopies using tunable excitation pulses. The transient signals induced by the photoexcitation with larger excess energy have broader bands and longer lifetimes both in the 11Bu+and21Ag- excited states. The excess vibrational energy remains longer than several picoseconds and slows the relaxation kinetics in carotenoids.

WS2 mode-locked ultrafast fiber laser

PubMed Central

Mao, Dong; Wang, Yadong; Ma, Chaojie; Han, Lei; Jiang, Biqiang; Gan, Xuetao; Hua, Shijia; Zhang, Wending; Mei, Ting; Zhao, Jianlin

2015-01-01

Graphene-like two dimensional materials, such as WS2 and MoS2, are highly anisotropic layered compounds that have attracted growing interest from basic research to practical applications. Similar with MoS2, few-layer WS2 has remarkable physical properties. Here, we demonstrate for the first time that WS2 nanosheets exhibit ultrafast nonlinear saturable absorption property and high optical damage threshold. Soliton mode-locking operations are achieved separately in an erbium-doped fiber laser using two types of WS2-based saturable absorbers, one of which is fabricated by depositing WS2 nanosheets on a D-shaped fiber, while the other is synthesized by mixing WS2 solution with polyvinyl alcohol, and then evaporating them on a substrate. At the maximum pump power of 600 mW, two saturable absorbers can work stably at mode-locking state without damage, indicating that few-layer WS2 is a promising high-power flexible saturable absorber for ultrafast optics. Numerous applications may benefit from the ultrafast nonlinear features of WS2 nanosheets, such as high-power pulsed laser, materials processing, and frequency comb spectroscopy. PMID:25608729

Ultrafast carrier dynamics in GaN/InGaN multiple quantum wells nanorods

NASA Astrophysics Data System (ADS)

Chen, Weijian; Wen, Xiaoming; Latzel, Michael; Yang, Jianfeng; Huang, Shujuan; Shrestha, Santosh; Patterson, Robert; Christiansen, Silke; Conibeer, Gavin

2018-01-01

GaN/InGaN multiple quantum wells (MQW) is a promising material for high-efficiency solid-state lighting. Ultrafast optical pump-probe spectroscopy is an important characterization technique for examining fundamental phenomena in semiconductor nanostructure with sub-picosecond resolution. In this study, ultrafast exciton and charge carrier dynamics in GaN/InGaN MQW planar layer and nanorod are investigated using femtosecond transient absorption (TA) techniques at room temperature. Here nanorods are fabricated by etching the GaN/InGaN MQW planar layers using nanosphere lithography and reactive ion etching. Photoluminescence efficiency of the nanorods have been proved to be much higher than that of the planar layers, but the mechanism of the nanorod structure improvement of PL efficiency is not adequately studied. By comparing the TA profile of the GaN/InGaN MQW planar layers and nanorods, the impact of surface states and nanorods lateral confinement in the ultrafast carrier dynamics of GaN/InGaN MQW is revealed. The nanorod sidewall surface states have a strong influence on the InGaN quantum well carrier dynamics. The ultrafast relaxation processes studied in this GaN/InGaN MQW nanostructure is essential for further optimization of device application.

Phosphorene quantum dot saturable absorbers for ultrafast fiber lasers

PubMed Central

Du, J.; Zhang, M.; Guo, Z.; Chen, J.; Zhu, X.; Hu, G.; Peng, P.; Zheng, Z.; Zhang, H.

2017-01-01

We fabricate ultrasmall phosphorene quantum dots (PQDs) with an average size of 2.6 ± 0.9 nm using a liquid exfoliation method involving ultrasound probe sonication followed by bath sonication. By coupling the as-prepared PQDs with microfiber evanescent light field, the PQD-based saturable absorber (SA) device exhibits ultrafast nonlinear saturable absorption property, with an optical modulation depth of 8.1% at the telecommunication band. With the integration of the all-fiber PQD-based SA, a continuous-wave passively mode-locked erbium-doped (Er-doped) laser cavity delivers stable, self-starting pulses with a pulse duration of 0.88 ps and at the cavity repetition rate of 5.47 MHz. Our results contribute to the growing body of work studying the nonlinear optical properties of ultrasmall PQDs that present new opportunities of this two-dimensional (2D) nanomaterial for future ultrafast photonic technologies. PMID:28211471

Precision machining of pig intestine using ultrafast laser pulses

NASA Astrophysics Data System (ADS)

Beck, Rainer J.; Góra, Wojciech S.; Carter, Richard M.; Gunadi, Sonny; Jayne, David; Hand, Duncan P.; Shephard, Jonathan D.

2015-07-01

Endoluminal surgery for the treatment of early stage colorectal cancer is typically based on electrocautery tools which imply restrictions on precision and the risk of harm through collateral thermal damage to the healthy tissue. As a potential alternative to mitigate these drawbacks we present laser machining of pig intestine by means of picosecond laser pulses. The high intensities of an ultrafast laser enable nonlinear absorption processes and a predominantly nonthermal ablation regime. Laser ablation results of square cavities with comparable thickness to early stage colorectal cancers are presented for a wavelength of 1030 nm using an industrial picosecond laser. The corresponding histology sections exhibit only minimal collateral damage to the surrounding tissue. The depth of the ablation can be controlled precisely by means of the pulse energy. Overall, the application of ultrafast lasers to ablate pig intestine enables significantly improved precision and reduced thermal damage to the surrounding tissue compared to conventional techniques.

Ultrafast switching of valence and generation of coherent acoustic phonons in semiconducting rare-earth monosulfides

NASA Astrophysics Data System (ADS)

Punpongjareorn, Napat; He, Xing; Tang, Zhongjia; Guloy, Arnold M.; Yang, Ding-Shyue

2017-08-01

We report on the ultrafast carrier dynamics and generation of coherent acoustic phonons in YbS, a semiconducting rare-earth monochalcogenide, using two-color pump-probe reflectivity. Compared to the carrier relaxation processes and lifetimes of conventional semiconductors, recombination of photoexcited electrons with holes in localized f orbitals is found to take place rapidly with a density-independent time constant of <500 fs in YbS. Such carrier annihilation signifies the unique and ultrafast nature of valence restoration of ytterbium ions after femtosecond photoexcitation switching. Following transfer of the absorbed energy to the lattice, coherent acoustic phonons emerge on the picosecond timescale as a result of the thermal strain in the photoexcited region. By analyzing the electronic and structural dynamics, we obtain the physical properties of YbS including its two-photon absorption and thermooptic coefficients, the period and decay time of the coherent oscillation, and the sound velocity.

Ultrafast optical pulse convertor caused by oscillations of the energy level structure in the conjugated polymer poly(p-phenylenevinylene).

PubMed

Zhang, Yusong; Chen, Weikang; Lin, Zhe; Li, Sheng; George, Thomas F

2017-08-21

For a conjugated polymer irradiated by two optical pulses, the whole process of excitation, involving lattice oscillations, oscillations of the energy level structure, and evolution of the electron cloud, is investigated. Localization of the electron cloud appears in the first 100 fs of irradiation, which in turn induces vibrations of lattice of the polymer chain as well as oscillations of the band gap. These oscillations filter the absorption of the external optical field inversely and convert the original optical field to an ultrafast light field whose intensity varies with a certain period. Based on the mechanism, oscillations of the energy level structure, induced by the external excitation, can be designed as an ultrafast response optical convertor that is able to change the external optical pulse into a new effective light field with a certain oscillation period. This helps provide new insight into designing nanostructures for polymeric optoelectronics.

Effect of Carrier Thermalization Dynamics on Light Emission and Amplification in Organometal Halide Perovskites.

PubMed

Chen, Kai; Barker, Alex J; Morgan, Francis L C; Halpert, Jonathan E; Hodgkiss, Justin M

2015-01-02

The remarkable rise of organometal halide perovskites as solar photovoltaic materials has been followed by promising developments in light-emitting devices, including lasers. Here we present unique insights into the processes leading to photon emission in these materials. We employ ultrafast broadband photoluminescence (PL) and transient absorption spectroscopies to directly link density dependent ultrafast charge dynamics to PL. We find that exceptionally strong PL at the band edge is preceded by thermalization of free charge carriers. Short-lived PL above the band gap is clear evidence of nonexcitonic emission from hot carriers, and ultrafast PL depolarization confirms that uncorrelated charge pairs are precursors to photon emission. Carrier thermalization has a profound effect on amplified stimulated emission at high fluence; the delayed onset of optical gain we resolve within the first 10 ps and the unusual oscillatory behavior are both consequences of the kinetic interplay between carrier thermalization and optical gain.

Ultrafast hole carrier relaxation dynamics in p-type CuO nanowires

PubMed Central

2011-01-01

Ultrafast hole carrier relaxation dynamics in CuO nanowires have been investigated using transient absorption spectroscopy. Following femtosecond pulse excitation in a non-collinear pump-probe configuration, a combination of non-degenerate transmission and reflection measurements reveal initial ultrafast state filling dynamics independent of the probing photon energy. This behavior is attributed to the occupation of states by photo-generated carriers in the intrinsic hole region of the p-type CuO nanowires located near the top of the valence band. Intensity measurements indicate an upper fluence threshold of 40 μJ/cm2 where carrier relaxation is mainly governed by the hole dynamics. The fast relaxation of the photo-generated carriers was determined to follow a double exponential decay with time constants of 0.4 ps and 2.1 ps. Furthermore, time-correlated single photon counting measurements provide evidence of three exponential relaxation channels on the nanosecond timescale. PMID:22151927

Transient and stationary spectroscopy of cytochrome c: ultrafast internal conversion controls photoreduction.

PubMed

Löwenich, Dennis; Kleinermanns, Karl; Karunakaran, Venugopal; Kovalenko, Sergey Alexander

2008-01-01

Photoreduction of cytochrome c (Cyt c) has been reinvestigated using femtosecond-to-nanosecond transient absorption and stationary spectroscopy. Femtosecond spectra of oxidized Cyt c, recorded in the probe range 270-1000 nm, demonstrate similar evolution upon 266 or 403 nm excitation: an ultrafast 0.3 ps internal conversion followed by a 4 ps vibrational cooling. Late transient spectra after 20 ps, from the cold ground-state chromophore, reveal a small but measurable signal from reduced Cyt c. The yield phi for Fe3+-->Fe2+ photoreduction is measured to be phi(403) = 0.016 and phi(266) = 0.08 for 403 and 266 nm excitation. These yields lead to a guess of the barrier E(f)(A) = 55 kJ mol(-1) for thermal ground-state electron transfer (ET). Nanosecond spectra initially show the typical absorption from reduced Cyt c and then exhibit temperature-dependent sub-microsecond decays (0.5 micros at 297 K), corresponding to a barrier E(A)(b) = 33 kJ mol(-1) for the back ET reaction and a reaction energy DeltaE = 22 kJ mol(-1). The nanosecond transients do not decay to zero on a second time scale, demonstrating the stability of some of the reduced Cyt c. The yields calculated from this stable reduced form agree with quasistationary reduction yields. Modest heating of Cyt c leads to its efficient thermal reduction as demonstrated by differential stationary absorption spectroscopy. In summary, our results point to ultrafast internal conversion of oxidized Cyt c upon UV or visible excitation, followed by Fe-porphyrin reduction due to thermal ground-state ET as the prevailing mechanism.

Effect of Molecular Coupling on Ultrafast Electron-Transfer and Charge-Recombination Dynamics in a Wide-Gap ZnS Nanoaggregate Sensitized by Triphenyl Methane Dyes.

PubMed

Debnath, Tushar; Maity, Partha; Dana, Jayanta; Ghosh, Hirendra N

2016-03-03

Wide-band-gap ZnS nanocrystals (NCs) were synthesized, and after sensitizing the NCs with series of triphenyl methane (TPM) dyes, ultrafast charge-transfer dynamics was demonstrated. HRTEM images of ZnS NCs show the formation of aggregate crystals with a flower-like structure. Exciton absorption and lumimescence, due to quantum confinement of the ZnS NCs, appear at approximately 310 and 340 nm, respectively. Interestingly, all the TPM dyes (pyrogallol red, bromopyrogallol red, and aurin tricarboxylic acid) form charge-transfer complexes with the ZnS NCs, with the appearance of a red-shifted band. Electron injection from the photoexcited TPM dyes into the conduction band of the ZnS NCs is shown to be a thermodynamically viable process, as confirmed by steady-state and time-resolved emission studies. To unravel charge-transfer (both electron injection and charge recombination) dynamics and the effect of molecular coupling, femtosecond transient absorption studies were carried out in TPM-sensitized ZnS NCs. The electron-injection dynamics is pulse-width-limited in all the ZnS/TPM dye systems, however, the back electron transfer differs, depending on the molecular coupling of the sensitizers (TPM dyes). The detailed mechanisms for the above-mentioned processes are discussed. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Slow Organic-to-Inorganic Sub-Lattice Thermalization in Methylammonium Lead Halide Perovskites Observed by Ultrafast Photoluminescence

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chang, Angela Y.; Cho, Yi-Ju; Chen, Kuan-Chen

2016-05-31

Carrier dynamics in methylammonium lead halide (CH3NH3PbI3-xClx) perovskite thin films, of differing crystal morphology, are examined as functions of temperature and excitation wavelength. At room temperature, long-lived (> nanosecond) transient absorption signals indicate negligible carrier trapping. However, in measurements of ultrafast photoluminescence excited at 400 nm, a heretofore unexplained, large amplitude (50%-60%), 45 ps decay process is observed. This feature persists for temperatures down to the orthorhombic phase transition. Varying pump photon energy reveals that the fast, band-edge photoluminescence (PL) decay only appears for excitation >= 2.38 eV (520 nm), with larger amplitudes for higher pump energies. Lower photon-energy excitationmore » yields slow dynamics consistent with negligible carrier trapping. Further, sub-bandgap two-photon pumping yields identical PL dynamics as direct absorption, signifying sensitivity to the total deposited energy and insensitivity to interfacial effects. Together with first principles electronic structure and ab initio molecular dynamics calculations, the results suggest the fast PL decay stems from excitation of high energy phonon modes associated with the organic sub-lattice that temporarily enhance wavefunction overlap within the inorganic component owing to atomic displacement, thereby transiently changing the PL radiative rate during thermalization. Hence, the fast PL decay relates a characteristic organic-to-inorganic sub-lattice equilibration timescale at optoelectronic-relevant excitation energies.« less

Ultrafast Exciton Dissociation and Long-Lived Charge Separation in a Photovoltaic Pentacene-MoS2 van der Waals Heterojunction.

PubMed

Bettis Homan, Stephanie; Sangwan, Vinod K; Balla, Itamar; Bergeron, Hadallia; Weiss, Emily A; Hersam, Mark C

2017-01-11

van der Waals heterojunctions between two-dimensional (2D) layered materials and nanomaterials of different dimensions present unique opportunities for gate-tunable optoelectronic devices. Mixed-dimensional p-n heterojunction diodes, such as p-type pentacene (0D) and n-type monolayer MoS 2 (2D), are especially interesting for photovoltaic applications where the absorption cross-section and charge transfer processes can be tailored by rational selection from the vast library of organic molecules and 2D materials. Here, we study the kinetics of excited carriers in pentacene-MoS 2 p-n type-II heterojunctions by transient absorption spectroscopy. These measurements show that the dissociation of MoS 2 excitons occurs by hole transfer to pentacene on the time scale of 6.7 ps. In addition, the charge-separated state lives for 5.1 ns, up to an order of magnitude longer than the recombination lifetimes from previously reported 2D material heterojunctions. By studying the fractional amplitudes of the MoS 2 decay processes, the hole transfer yield from MoS 2 to pentacene is found to be ∼50%, with the remaining holes undergoing trapping due to surface defects. Overall, the ultrafast charge transfer and long-lived charge-separated state in pentacene-MoS 2 p-n heterojunctions suggest significant promise for mixed-dimensional van der Waals heterostructures in photovoltaics, photodetectors, and related optoelectronic technologies.

Picosecond absorption relaxation measured with nanosecond laser photoacoustics

PubMed Central

Danielli, Amos; Favazza, Christopher P.; Maslov, Konstantin; Wang, Lihong V.

2010-01-01

Picosecond absorption relaxation—central to many disciplines—is typically measured by ultrafast (femtosecond or picosecond) pump-probe techniques, which however are restricted to optically thin and weakly scattering materials or require artificial sample preparation. Here, we developed a reflection-mode relaxation photoacoustic microscope based on a nanosecond laser and measured picosecond absorption relaxation times. The relaxation times of oxygenated and deoxygenated hemoglobin molecules, both possessing extremely low fluorescence quantum yields, were measured at 576 nm. The added advantages in dispersion susceptibility, laser-wavelength availability, reflection sensing, and expense foster the study of natural—including strongly scattering and nonfluorescent—materials. PMID:21079726

Picosecond absorption relaxation measured with nanosecond laser photoacoustics.

PubMed

Danielli, Amos; Favazza, Christopher P; Maslov, Konstantin; Wang, Lihong V

2010-10-18

Picosecond absorption relaxation-central to many disciplines-is typically measured by ultrafast (femtosecond or picosecond) pump-probe techniques, which however are restricted to optically thin and weakly scattering materials or require artificial sample preparation. Here, we developed a reflection-mode relaxation photoacoustic microscope based on a nanosecond laser and measured picosecond absorption relaxation times. The relaxation times of oxygenated and deoxygenated hemoglobin molecules, both possessing extremely low fluorescence quantum yields, were measured at 576 nm. The added advantages in dispersion susceptibility, laser-wavelength availability, reflection sensing, and expense foster the study of natural-including strongly scattering and nonfluorescent-materials.

Change of electric dipole moment in charge transfer transitions of ferrocene oligomers studied by ultrafast two-photon absorption

NASA Astrophysics Data System (ADS)

Mikhaylov, Alexander; Arias, Eduardo; Moggio, Ivana; Ziolo, Ronald; Uudsemaa, Merle; Trummal, Aleksander; Cooper, Thomas; Rebane, Aleksander

2017-02-01

Change of permanent electric dipole moment in the lower-energy charge transfer transitions for a series of symmetrical and non-symmetrical ferrocene-phenyleneethynylene oligomers were studied by measuring the corresponding femtosecond two-photon absorption cross section spectra, and were determined to be in the range Δμ = 3 - 10 D. Quantum-chemical calculations of Δμ for the non-symmetrical oligomers show good quantitative agreement with the experimental results, thus validating two-photon absorption spectroscopy as a viable experimental approach to study electrostatic properties of organometallics and other charge transfer systems.

Ultrafast recovery time and broadband saturable absorption properties of black phosphorus suspension

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang, Yingwei; Huang, Guanghui; Chen, Jiazhang

2015-08-31

As a new type of two-dimensional crystal material, black phosphorus (BP) exhibits excellent electronics and optical performance. Herein, we focus on carrier relaxation dynamics and nonlinear optical properties of BP suspension. Atomic force microscopy, transmission electron microscopy, and optical transmission spectrum are employed to characterize the structure and linear optical properties of the BP. Additionally, pump-probe experiments at wavelength of 1550 nm were carried out to study the carrier dynamics in BP suspension, and ultrafast recovery time was observed (τ{sub s} = 24 ± 2 fs). Furthermore, we demonstrate the saturable absorption signals by open aperture Z-scan experiments at wavelengths of 1550 nm, 532 nm, and 680 nm. Themore » results indicate that BP has broadband saturable absorption properties and the nonlinear absorption coefficients were determined to be β{sub 2} = −0.20 ± 0.08 × 10{sup −3 }cm/GW (532 nm), β{sub 2} = −0.12 ± 0.05 × 10{sup −3 }cm/GW (680 nm), and β{sub 2} = −0.15 ± 0.09 × 10{sup −3 }cm/GW (1550 nm)« less

Pump-probe spectroscopy and imaging of heme proteins: temperature effects and data analysis

NASA Astrophysics Data System (ADS)

Wang, Erkang; Domingue, Scott R.; Bartels, Randy A.; Wilson, Jesse W.

2017-08-01

Ultrafast pump-probe microscopy enables visualization of non-fluorescent materials in biological tissue, such as melanin and hemoglobin. Whereas transient absorption has been primarily a physical chemistry technique, used to gain insight into molecular and electronic structure, pump-probe microscopy represents a paradigm shift in translating transient absorption into an analytical technique, which can clearly resolve pigments with nearly indistinguishable linear absorption spectra. Extending this technique to other important targets, such as mitochondrial respiratory chain hemes, will require new laser sources and new data processing techniques to estimate heme content from the pump-probe response. We will present recent developments on both of these fronts. The laser system we have developed to elicit a pump probe response of respiratory chain hemes is based on an amplified Yb:fiber ultrafast laser that uses modest spectral broadening followed by sum frequency generation to produce a tunable pulse pair in the visible region. Wavelength tuning is accomplished by changing quasi-phase matching conditions. We will present preliminary imaging data in addition to discussing management of sample heating problems that arise from performing transient absorption measurements at the high repetition rates needed for imaging microscopy. In the second part of the talk, we will present the use of regularized and non-negative least squares fitting, along with feature-preserving noise removal to estimate composition of a pixel from its pump-probe response.

Ultrafast time-resolved spectroscopy of the light-harvesting complex 2 (LH2) from the photosynthetic bacterium Thermochromatium tepidum.

PubMed

Niedzwiedzki, Dariusz M; Fuciman, Marcel; Kobayashi, Masayuki; Frank, Harry A; Blankenship, Robert E

2011-10-01

The light-harvesting complex 2 from the thermophilic purple bacterium Thermochromatium tepidum was purified and studied by steady-state absorption and fluorescence, sub-nanosecond-time-resolved fluorescence and femtosecond time-resolved transient absorption spectroscopy. The measurements were performed at room temperature and at 10 K. The combination of both ultrafast and steady-state optical spectroscopy methods at ambient and cryogenic temperatures allowed the detailed study of carotenoid (Car)-to-bacteriochlorophyll (BChl) as well BChl-to-BChl excitation energy transfer in the complex. The studies show that the dominant Cars rhodopin (N=11) and spirilloxanthin (N=13) do not play a significant role as supportive energy donors for BChl a. This is related with their photophysical properties regulated by long π-electron conjugation. On the other hand, such properties favor some of the Cars, particularly spirilloxanthin (N=13) to play the role of the direct quencher of the excited singlet state of BChl. © Springer Science+Business Media B.V. 2011

Ultrafast Absorption Spectroscopy of Aluminum Plasmas Created by LCLS using Betatron X-Ray Radiation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Albert, Felicie

2016-10-12

This document summarizes the goals and accomplishments of a six month-long LDRD project, awarded through the LLNL director Early and Mid Career Recognition (EMCR) program. This project allowed us to support beamtime awarded at the Matter under Extreme Conditions (MEC) end station of the Linac Coherent Light Source (LCLS). The goal of the experiment was to heat metallic samples with the bright x-rays from the LCLS free electron laser. Then, we studied how they relaxed back to equilibrium by probing them with ultrafast x-ray absorption spectroscopy using laser-based betatron radiation. Our work enabled large collaborations between LLNL, SLAC, LBNL, andmore » institutions in France and in the UK, while providing training to undergraduate and graduate students during the experiment. Following this LDRD project, the PI was awarded a 5-year DOE early career research grant to further develop applications of laser-driven x-ray sources for high energy density science experiments and warm dense matter states.« less

Photogenerated Intrinsic Free Carriers in Small-molecule Organic Semiconductors Visualized by Ultrafast Spectroscopy

PubMed Central

He, Xiaochuan; Zhu, Gangbei; Yang, Jianbing; Chang, Hao; Meng, Qingyu; Zhao, Hongwu; Zhou, Xin; Yue, Shuai; Wang, Zhuan; Shi, Jinan; Gu, Lin; Yan, Donghang; Weng, Yuxiang

2015-01-01

Confirmation of direct photogeneration of intrinsic delocalized free carriers in small-molecule organic semiconductors has been a long-sought but unsolved issue, which is of fundamental significance to its application in photo-electric devices. Although the excitonic description of photoexcitation in these materials has been widely accepted, this concept is challenged by recently reported phenomena. Here we report observation of direct delocalized free carrier generation upon interband photoexcitation in highly crystalline zinc phthalocyanine films prepared by the weak epitaxy growth method using ultrafast spectroscopy. Transient absorption spectra spanning the visible to mid-infrared region revealed the existence of short-lived free electrons and holes with a diffusion length estimated to cross at least 11 molecules along the π−π stacking direction that subsequently localize to form charge transfer excitons. The interband transition was evidenced by ultraviolet-visible absorption, photoluminescence and electroluminescence spectroscopy. Our results suggest that delocalized free carriers photogeneration can also be achieved in organic semiconductors when the molecules are packed properly. PMID:26611323

Broadband sensitive pump-probe setup for ultrafast optical switching of photonic nanostructures and semiconductors.

PubMed

Euser, Tijmen G; Harding, Philip J; Vos, Willem L

2009-07-01

We describe an ultrafast time resolved pump-probe spectroscopy setup aimed at studying the switching of nanophotonic structures. Both femtosecond pump and probe pulses can be independently tuned over broad frequency range between 3850 and 21,050 cm(-1). A broad pump scan range allows a large optical penetration depth, while a broad probe scan range is crucial to study strongly photonic crystals. A new data acquisition method allows for sensitive pump-probe measurements, and corrects for fluctuations in probe intensity and pump stray light. We observe a tenfold improvement of the precision of the setup compared to laser fluctuations, allowing a measurement accuracy of better than DeltaR=0.07% in a 1 s measurement time. Demonstrations of the improved technique are presented for a bulk Si wafer, a three-dimensional Si inverse opal photonic bandgap crystal, and z-scan measurements of the two-photon absorption coefficient of Si, GaAs, and the three-photon absorption coefficient of GaP in the infrared wavelength range.

Developing a structure-function model for the cryptophyte phycoerythrin 545 using ultrahigh resolution crystallography and ultrafast laser spectroscopy.

PubMed

Doust, Alexander B; Marai, Christopher N J; Harrop, Stephen J; Wilk, Krystyna E; Curmi, Paul M G; Scholes, Gregory D

2004-11-12

Cryptophyte algae differ from cyanobacteria and red algae in the architecture of their photosynthetic light harvesting systems, even though all three are evolutionarily related. Central to cryptophyte light harvesting is the soluble antenna protein phycoerythrin 545 (PE545). The ultrahigh resolution crystal structure of PE545, isolated from a unicellular cryptophyte Rhodomonas CS24, is reported at both 1.1A and 0.97A resolution, revealing details of the conformation and environments of the chromophores. Absorption, emission and polarized steady state spectroscopy (298K, 77K), as well as ultrafast (20fs time resolution) measurements of population dynamics are reported. Coupled with complementary quantum chemical calculations of electronic transitions of the bilins, these enable assignment of spectral absorption characteristics to each chromophore in the structure. Spectral differences between the tetrapyrrole pigments due to chemical differences between bilins, as well as their binding and interaction with the local protein environment are described. Based on these assignments, and considering customized optical properties such as strong coupling, a model for light harvesting by PE545 is developed which explains the fast, directional harvesting of excitation energy. The excitation energy is funnelled from four peripheral pigments (beta158,beta82) into a central chromophore dimer (beta50/beta61) in approximately 1ps. Those chromophores, in turn, transfer the excitation energy to the red absorbing molecules located at the periphery of the complex in approximately 4ps. A final resonance energy transfer step sensitizes just one of the alpha19 bilins on a time scale of 22ps. Furthermore, it is concluded that binding of PE545 to the thylakoid membrane is not essential for efficient energy transfer to the integral membrane chlorophyll a-containing complexes associated with PS-II.

Population branching in the conical intersection of the retinal chromophore revealed by multipulse ultrafast optical spectroscopy.

PubMed

Zgrablić, Goran; Novello, Anna Maria; Parmigiani, Fulvio

2012-01-18

The branching ratio of the excited-state population at the conical intersection between the S(1) and S(0) energy surfaces (Φ(CI)) of a protonated Schiff base of all-trans retinal in protic and aprotic solvents was studied by multipulse ultrafast transient absorption spectroscopy. In particular, pump-dump-probe experiments allowed to isolate the S(1) reactive state and to measure the photoisomerization time constant with unprecedented precision. Starting from these results, we demonstrate that the polarity of the solvent is the key factor influencing the Φ(CI) and the photoisomerization yield. © 2011 American Chemical Society

Excited state electron and energy relays in supramolecular dinuclear complexes revealed by ultrafast optical and X-ray transient absorption spectroscopy† †Electronic supplementary information (ESI) available: Synthesis schemes, experimental methods, NMR spectra, X-ray crystallographic information, emission spectra, cyclic voltammetry, electronic structure calculations, data analysis and numerical methods, and other additional figures. CCDC 1561879. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7sc04055e

PubMed Central

Kohler, Lars; Hadt, Ryan G.; Zhang, Xiaoyi; Liu, Cunming

2017-01-01

The kinetics of photoinduced electron and energy transfer in a family of tetrapyridophenazine-bridged heteroleptic homo- and heterodinuclear copper(i) bis(phenanthroline)/ruthenium(ii) polypyridyl complexes were studied using ultrafast optical and multi-edge X-ray transient absorption spectroscopies. This work combines the synthesis of heterodinuclear Cu(i)–Ru(ii) analogs of the homodinuclear Cu(i)–Cu(i) targets with spectroscopic analysis and electronic structure calculations to first disentangle the dynamics at individual metal sites by taking advantage of the element and site specificity of X-ray absorption and theoretical methods. The excited state dynamical models developed for the heterodinuclear complexes are then applied to model the more challenging homodinuclear complexes. These results suggest that both intermetallic charge and energy transfer can be observed in an asymmetric dinuclear copper complex in which the ground state redox potentials of the copper sites are offset by only 310 meV. We also demonstrate the ability of several of these complexes to effectively and unidirectionally shuttle energy between different metal centers, a property that could be of great use in the design of broadly absorbing and multifunctional multimetallic photocatalysts. This work provides an important step toward developing both a fundamental conceptual picture and a practical experimental handle with which synthetic chemists, spectroscopists, and theoreticians may collaborate to engineer cheap and efficient photocatalytic materials capable of performing coulombically demanding chemical transformations. PMID:29629153

Nonlinear Saturable and Polarization-induced Absorption of Rhenium Disulfide

PubMed Central

Cui, Yudong; Lu, Feifei; Liu, Xueming

2017-01-01

Monolayer of transition metal dichalcogenides (TMDs), with lamellar structure as that of graphene, has attracted significant attentions in optoelectronics and photonics. Here, we focus on the optical absorption response of a new member TMDs, rhenium disulphide (ReS2) whose monolayer and bulk forms have the nearly identical band structures. The nonlinear saturable and polarization-induced absorption of ReS2 are investigated at near-infrared communication band beyond its bandgap. It is found that the ReS2-covered D-shaped fiber (RDF) displays the remarkable polarization-induced absorption, which indicates the different responses for transverse electric (TE) and transverse magnetic (TM) polarizations relative to ReS2 plane. Nonlinear saturable absorption of RDF exhibits the similar saturable fluence of several tens of μJ/cm2 and modulation depth of about 1% for ultrafast pulses with two orthogonal polarizations. RDF is utilized as a saturable absorber to achieve self-started mode-locking operation in an Er-doped fiber laser. The results broaden the operation wavelength of ReS2 from visible light to around 1550 nm, and numerous applications may benefit from the anisotropic and nonlinear absorption characteristics of ReS2, such as in-line optical polarizers, high-power pulsed lasers, and optical communication system. PMID:28053313

Excited-state dynamics of pentacene derivatives with stable radical substituents.

PubMed

Ito, Akitaka; Shimizu, Akihiro; Kishida, Noriaki; Kawanaka, Yusuke; Kosumi, Daisuke; Hashimoto, Hideki; Teki, Yoshio

2014-06-23

The excited-state dynamics of pentacene derivatives with stable radical substituents were evaluated in detail through transient absorption measurements. The derivatives showed ultrafast formation of triplet excited state(s) in the pentacene moiety from a photoexcited singlet state through the contributions of enhanced intersystem crossing and singlet fission. Detailed kinetic analyses for the transient absorption data were conducted to quantify the excited-state characteristics of the derivatives. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Petawatt laser absorption bounded

PubMed Central

Levy, Matthew C.; Wilks, Scott C.; Tabak, Max; Libby, Stephen B.; Baring, Matthew G.

2014-01-01

The interaction of petawatt (1015 W) lasers with solid matter forms the basis for advanced scientific applications such as table-top particle accelerators, ultrafast imaging systems and laser fusion. Key metrics for these applications relate to absorption, yet conditions in this regime are so nonlinear that it is often impossible to know the fraction of absorbed light f, and even the range of f is unknown. Here using a relativistic Rankine-Hugoniot-like analysis, we show for the first time that f exhibits a theoretical maximum and minimum. These bounds constrain nonlinear absorption mechanisms across the petawatt regime, forbidding high absorption values at low laser power and low absorption values at high laser power. For applications needing to circumvent the absorption bounds, these results will accelerate a shift from solid targets, towards structured and multilayer targets, and lead the development of new materials. PMID:24938656

Laser-driven powerful kHz hard x-ray source

NASA Astrophysics Data System (ADS)

Li, Minghua; Huang, Kai; Chen, Liming; Yan, Wenchao; Tao, Mengze; Zhao, Jiarui; Ma, Yong; Li, Yifei; Zhang, Jie

2017-08-01

A powerful hard x-ray source based on laser plasma interaction is developed. By introducing the kHz, 800 nm pulses onto a rotating molybdenum (Mo) disk target, intense Mo Kα x-rays are emitted with suppressed bremsstrahlung background. Results obtained with different laser intensities suggest that the dominant absorption mechanism responsible for the high conversion efficiency is vacuum heating (VH). The high degree of spatial coherence is verified. With the high average flux and a source size comparable to the laser focus spot, absorption contrast imaging and phase contrast imaging are carried out to test the imaging capability of the source. Not only useful for imaging application, this compact x-ray source is also holding great potential for ultrafast x-ray diffraction (XRD) due to the intrinsic merits such as femtosecond pulse duration and natural synchronization with the driving laser pulses.

Ultrafast photophysics of pi-conjugated polymers for organic light emitting diode applications

NASA Astrophysics Data System (ADS)

Olejnik, Ella

In this work we used the pump-probe photomodulation (PM) spectroscopy technique to measure the transient PM spectrum and decay kinetics in various pi -- conjugated polymers (PCPs) films and blends. Using two ultrafast laser systems, we covered a broad spectral range from 0.25 -- 2.5 eV in the time domain from 200 fs to 1 ns with 150 fs time resolution. We also used continuous wave (CW) photomodulation spectroscopy, photoluminescence (PL), electro-absorption and doping-induced absorption to study the photoexcitations and other optical properties of PCPs and guest/ host blends. In particular we studied two different types of Poly(thienylenevinylene) polymer derivatives. One polymer type is the ordered regio-regular (RR) and regio-random (RRa) -- PTV in which the dark exciton, 2Ag is the lowest excited state. In these polymers the photoexcited exciton shows very fast decay kinetics due to the internal conversion to the dark exciton, which results in weak PL emission; thus these two polymers are non-luminescent. The other PTV derivative is the imide -- PTV which is more luminescent due to the proximity of 1Bu and 2Ag states, that results in longer decay kinetics and a difference between the calculated value of the QEPL (9%) and the measured one (1%). We also demonstrate transient strain spectroscopy in RR -- PTV thin films, where the ultrafast energy release associated with the exciton decay gives rise to substantial static and dynamic strains in the film that dramatically influences the film's transient PM response. We also study the photophysics of poly(dioctyloxy) phenylenevinylene polymer with different isotopes, where we substituted hydrogen (H-polymer) by deuterium (D-polymer), and 12C by 13C isotopes. From the transient decay kinetics measurements we found that the exciton recombination in DOO -- PPV consists of two processes. These are: intrinsic monomolecular, and exciton-exciton annihilation (bimolecular). In the D -- polymer, different probe frequencies of the main exciton photoinduced absorption band (PA1) show a variety of decay kinetics that result from various photoexcitations that contribute to the spectrum. Comparing the transient PM spectrum at 1 ns time delay to the CW PM shows the formation of triplet excitons, which is possible due to singlet fission of mAg (at 2.9 eV) into two triplets (2 X 1.4 eV). In the last part of this thesis we summarize our studies of organic light emitting diodes (OLED) devices based on a host/guest blend of Polyfluorene polymer that is mixed with various percentages of Ir(btp)2acac molecules. In this mixture the PFO (host) shows blue fluorescence, whereas the Ir-complex (guest) has red phosphorescence emission; thus OLED based on this mixture can serve as a `white OLED'. Since the PFO emission spectrum perfectly matches the absorption band of the Ir-complex, it induces an efficient energy transfer from the PFO host to the Ir-complex guest molecules, which we tried to time resolve by the transient PM method.

Femtosecond laser spectroscopy of the rhodopsin photochromic reaction: a concept for ultrafast optical molecular switch creation (ultrafast reversible photoreaction of rhodopsin).

PubMed

Smitienko, Olga; Nadtochenko, Victor; Feldman, Tatiana; Balatskaya, Maria; Shelaev, Ivan; Gostev, Fedor; Sarkisov, Oleg; Ostrovsky, Mikhail

2014-11-11

Ultrafast reverse photoreaction of visual pigment rhodopsin in the femtosecond time range at room temperature is demonstrated. Femtosecond two-pump probe experiments with a time resolution of 25 fs have been performed. The first рump pulse at 500 nm initiated cis-trans photoisomerization of rhodopsin chromophore, 11-cis retinal, which resulted in the formation of the primary ground-state photoproduct within a mere 200 fs. The second pump pulse at 620 nm with a varying delay of 200 to 3750 fs relative to the first рump pulse, initiated the reverse phototransition of the primary photoproduct to rhodopsin. The results of this photoconversion have been observed on the differential spectra obtained after the action of two pump pulses at a time delay of 100 ps. It was found that optical density decreased at 560 nm in the spectral region of bathorhodopsin absorption and increased at 480 nm, where rhodopsin absorbs. Rhodopsin photoswitching efficiency shows oscillations as a function of the time delay between two рump pulses. The quantum yield of reverse photoreaction initiated by the second pump pulse falls within the range 15%±1%. The molecular mechanism of the ultrafast reversible photoreaction of visual pigment rhodopsin may be used as a concept for the development of an ultrafast optical molecular switch.

Two-Dimensional CH3NH3PbI3 Perovskite Nanosheets for Ultrafast Pulsed Fiber Lasers.

PubMed

Li, Pengfei; Chen, Yao; Yang, Tieshan; Wang, Ziyu; Lin, Han; Xu, Yanhua; Li, Lei; Mu, Haoran; Shivananju, Bannur Nanjunda; Zhang, Yupeng; Zhang, Qinglin; Pan, Anlian; Li, Shaojuan; Tang, Dingyuan; Jia, Baohua; Zhang, Han; Bao, Qiaoliang

2017-04-12

Even though the nonlinear optical effects of solution processed organic-inorganic perovskite films have been studied, the nonlinear optical properties in two-dimensional (2D) perovskites, especially their applications for ultrafast photonics, are largely unexplored. In comparison to bulk perovskite films, 2D perovskite nanosheets with small thicknesses of a few unit cells are more suitable for investigating the intrinsic nonlinear optical properties because bulk recombination of photocarriers and the nonlinear scattering are relatively small. In this research, we systematically investigated the nonlinear optical properties of 2D perovskite nanosheets derived from a combined solution process and vapor phase conversion method. It was found that 2D perovskite nanosheets have stronger saturable absorption properties with large modulation depth and very low saturation intensity compared with those of bulk perovskite films. Using an all dry transfer method, we constructed a new type of saturable absorber device based on single piece 2D perovskite nanosheet. Stable soliton state mode-locking was achieved, and ultrafast picosecond pulses were generated at 1064 nm. This work is likely to pave the way for ultrafast photonic and optoelectronic applications based on 2D perovskites.

Ultrafast and nonlinear spectroscopy of brilliant green-based nanoGUMBOS with enhanced near-infrared emission

NASA Astrophysics Data System (ADS)

Karam, Tony E.; Siraj, Noureen; Zhang, Zhenyu; Ezzir, Abdulrahman F.; Warner, Isiah M.; Haber, Louis H.

2017-10-01

The synthesis, characterization, ultrafast dynamics, and nonlinear spectroscopy of 30 nm nanospheres of brilliant green-bis(pentafluoroethylsulfonyl)imide ([BG][BETI]) in water are reported. These thermally stable nanoparticles are derived from a group of uniform materials based on organic salts (nanoGUMBOS) that exhibit enhanced near-infrared emission compared with the molecular dye in water. The examination of ultrafast transient absorption spectroscopy results reveals that the overall excited-state relaxation lifetimes of [BG][BETI] nanoGUMBOS are longer than the brilliant green molecular dye in water due to steric hindrance of the torsional degrees of freedom of the phenyl rings around the central carbon. Furthermore, the second harmonic generation signal of [BG][BETI] nanoGUMBOS is enhanced by approximately 7 times and 23 times as compared with colloidal gold nanoparticles of the same size and the brilliant green molecular dye in water, respectively. A very clear third harmonic generation signal is observed from the [BG][BETI] nanoGUMBOS but not from either the molecular dye or the gold nanoparticles. Overall, these results show that [BG][BETI] nanoGUMBOS exhibit altered ultrafast and nonlinear spectroscopy that is beneficial for various applications including nonlinear imaging probes, biomedical imaging, and molecular sensing.

Enhanced generation and anisotropic Coulomb scattering of hot electrons in an ultra-broadband plasmonic nanopatch metasurface.

PubMed

Sykes, Matthew E; Stewart, Jon W; Akselrod, Gleb M; Kong, Xiang-Tian; Wang, Zhiming; Gosztola, David J; Martinson, Alex B F; Rosenmann, Daniel; Mikkelsen, Maiken H; Govorov, Alexander O; Wiederrecht, Gary P

2017-10-17

The creation of energetic electrons through plasmon excitation of nanostructures before thermalization has been proposed for a wide number of applications in optical energy conversion and ultrafast nanophotonics. However, the use of "nonthermal" electrons is primarily limited by both a low generation efficiency and their ultrafast decay. We report experimental and theoretical results on the use of broadband plasmonic nanopatch metasurfaces comprising a gold substrate coupled to silver nanocubes that produce large concentrations of hot electrons, which we measure using transient absorption spectroscopy. We find evidence for three subpopulations of nonthermal carriers, which we propose arise from anisotropic electron-electron scattering within sp-bands near the Fermi surface. The bimetallic character of the metasurface strongly impacts the physics, with dissipation occurring primarily in the gold, whereas the quantum process of hot electron generation takes place in both components. Our calculations show that the choice of geometry and materials is crucial for producing strong ultrafast nonthermal electron components.The creation of energetic electrons through plasmon excitation has implications in optical energy conversion and ultrafast nanophotonics. Here, the authors find evidence for three subpopulations of nonthermal carriers which arise from anisotropic electron-electron scattering near the Fermi surface.

Ultrafast charge separation dynamics in opaque, operational dye-sensitized solar cells revealed by femtosecond diffuse reflectance spectroscopy

PubMed Central

Ghadiri, Elham; Zakeeruddin, Shaik M.; Hagfeldt, Anders; Grätzel, Michael; Moser, Jacques-E.

2016-01-01

Efficient dye-sensitized solar cells are based on highly diffusive mesoscopic layers that render these devices opaque and unsuitable for ultrafast transient absorption spectroscopy measurements in transmission mode. We developed a novel sub-200 femtosecond time-resolved diffuse reflectance spectroscopy scheme combined with potentiostatic control to study various solar cells in fully operational condition. We studied performance optimized devices based on liquid redox electrolytes and opaque TiO2 films, as well as other morphologies, such as TiO2 fibers and nanotubes. Charge injection from the Z907 dye in all TiO2 morphologies was observed to take place in the sub-200 fs time scale. The kinetics of electron-hole back recombination has features in the picosecond to nanosecond time scale. This observation is significantly different from what was reported in the literature where the electron-hole back recombination for transparent films of small particles is generally accepted to occur on a longer time scale of microseconds. The kinetics of the ultrafast electron injection remained unchanged for voltages between +500 mV and –690 mV, where the injection yield eventually drops steeply. The primary charge separation in Y123 organic dye based devices was clearly slower occurring in two picoseconds and no kinetic component on the shorter femtosecond time scale was recorded. PMID:27095505

High-speed ultrafast laser machining with tertiary beam positioning (Conference Presentation)

NASA Astrophysics Data System (ADS)

Yang, Chuan; Zhang, Haibin

2017-03-01

For an industrial laser application, high process throughput and low average cost of ownership are critical to commercial success. Benefiting from high peak power, nonlinear absorption and small-achievable spot size, ultrafast lasers offer advantages of minimal heat affected zone, great taper and sidewall quality, and small via capability that exceeds the limits of their predecessors in via drilling for electronic packaging. In the past decade, ultrafast lasers have both grown in power and reduced in cost. For example, recently, disk and fiber technology have both shown stable operation in the 50W to 200W range, mostly at high repetition rate (beyond 500 kHz) that helps avoid detrimental nonlinear effects. However, to effectively and efficiently scale the throughput with the fast-growing power capability of the ultrafast lasers while keeping the beneficial laser-material interactions is very challenging, mainly because of the bottleneck imposed by the inertia-related acceleration limit and servo gain bandwidth when only stages and galvanometers are being used. On the other side, inertia-free scanning solutions like acoustic optics and electronic optical deflectors have small scan field, and therefore not suitable for large-panel processing. Our recent system developments combine stages, galvanometers, and AODs into a coordinated tertiary architecture for high bandwidth and meanwhile large field beam positioning. Synchronized three-level movements allow extremely fast local speed and continuous motion over the whole stage travel range. We present the via drilling results from such ultrafast system with up to 3MHz pulse to pulse random access, enabling high quality low cost ultrafast machining with emerging high average power laser sources.

Quantum Nuclear Dynamics Pumped and Probed by Ultrafast Polarization Controlled Steering of a Coherent Electronic State in LiH.

PubMed

Nikodem, Astrid; Levine, R D; Remacle, F

2016-05-19

The quantum wave packet dynamics following a coherent electronic excitation of LiH by an ultrashort, polarized, strong one-cycle infrared optical pulse is computed on several electronic states using a grid method. The coupling to the strong field of the pump and the probe pulses is included in the Hamiltonian used to solve the time-dependent Schrodinger equation. The polarization of the pump pulse allows us to control the localization in time and in space of the nonequilibrium coherent electronic motion and the subsequent nuclear dynamics. We show that transient absorption, resulting from the interaction of the total molecular dipole with the electric fields of the pump and the probe, is a very versatile probe of the different time scales of the vibronic dynamics. It allows probing both the ultrashort, femtosecond time scale of the electronic coherences as well as the longer dozens of femtoseconds time scales of the nuclear motion on the excited electronic states. The ultrafast beatings of the electronic coherences in space and in time are shown to be modulated by the different periods of the nuclear motion.

Polarization-dependent exciton dynamics in tetracene single crystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Bo; Zhang, Chunfeng, E-mail: cfzhang@nju.edu.cn; Xu, Yanqing

2014-12-28

We conduct polarization-dependent ultrafast spectroscopy to study the dynamics of singlet fission (SF) in tetracene single crystals. The spectrotemporal species for singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on probe polarization. By carefully analyzing the polarization dependence, the signals contributed by different transitions related to singlet excitons have been disentangled, which is further applied to construct the correlation between dynamics of singlet and triplet excitons. The anisotropy of exciton dynamics provides an alternative approach to tackle the long-standing challenge in understanding the mechanism of singlet fission in organic semiconductors.

Impact of metal ions in porphyrin-based applied materials for visible-light photocatalysis: key information from ultrafast electronic spectroscopy.

PubMed

Kar, Prasenjit; Sardar, Samim; Alarousu, Erkki; Sun, Jingya; Seddigi, Zaki S; Ahmed, Saleh A; Danish, Ekram Y; Mohammed, Omar F; Pal, Samir Kumar

2014-08-11

Protoporphyrin IX-zinc oxide (PP-ZnO) nanohybrids have been synthesized for applications in photocatalytic devices. High-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and steady-state infrared, absorption, and emission spectroscopies have been used to analyze the structural details and optical properties of these nanohybrids. Time-resolved fluorescence and transient absorption techniques have been applied to study the ultrafast dynamic events that are key to photocatalytic activities. The photocatalytic efficiency under visible-light irradiation in the presence of naturally abundant iron(III) and copper(II) ions has been found to be significantly retarded in the former case, but enhanced in the latter case. More importantly, femtosecond (fs) transient absorption data have clearly demonstrated that the residence of photoexcited electrons from the sensitizer PP in the centrally located iron moiety hinders ground-state bleach recovery of the sensitizer, affecting the overall photocatalytic rate of the nanohybrid. The presence of copper(II) ions, on the other hand, offers additional stability against photobleaching and eventually enhances the efficiency of photocatalysis. In addition, we have also explored the role of UV light in the efficiency of photocatalysis and have rationalized our observations from femtosecond- to picosecond-resolved studies. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ultrafast Transient Absorption Spectroscopy Investigation of Photoinduced Dynamics in Novel Donor-Acceptor Core-Shell Nanostructures for Organic Photovoltaics

NASA Astrophysics Data System (ADS)

Strain, Jacob; Jamhawi, Abdelqader; Abeywickrama, Thulitha M.; Loomis, Wendy; Rathnayake, Hemali; Liu, Jinjun

2016-06-01

Novel donor-acceptor nanostructures were synthesized via covalent synthesis and/or UV cross-linking method. Their photoinduced dynamics were investigated with ultrafast transient absorption (TA) spectroscopy. These new nanostructures are made with the strategy in mind to reduce manufacturing steps in the process of fabricating an organic photovoltaic cell. By imitating the heterojunction interface within a fixed particle domain, several fabrication steps can be bypassed reducing cost and giving more applicability to other film deposition methods. Such applications include aerosol deposition and ink-jet printing. The systems that were studied by TA spectroscopy include PDIB core, PDIB-P3HT core-shell, and PDIB-PANT core-shell which range in size from 60 to 130 nm. Within the experimentally accessible spectra range there resides a region of ground state bleaching, stimulated emission, and excited-state absorption of both neutrals and anions. Control experiments have been carried out to assign these features. At high pump fluences the TA spectra of PDIB core alone also indicate an intramolecular charge separation. The TA spectroscopy results thus far suggest that the core-shells resemble the photoinduced dynamics of a standard film although the particles are dispersed in solution, which indicates the desired outcome of the work.

Effects of heat transfer and energy absorption in the ablation of biological tissues by pulsetrain-burst (>100 MHz) ultrafast laser processing

NASA Astrophysics Data System (ADS)

Forrester, Paul; Bol, Kieran; Lilge, Lothar; Marjoribanks, Robin

2006-09-01

Energy absorption and heat transfer are important factors for regulating the effects of ablation of biological tissues. Heat transfer to surrounding material may be desirable when ablating hard tissue, such as teeth or bone, since melting can produce helpful material modifications. However, when ablating soft tissue it is important to minimize heat transfer to avoid damage to healthy tissue - for example, in eye refractive surgery (e.g., Lasik), nanosecond pulses produce gross absorption and heating in tissue, leading to shockwaves, which kill and thin the non-replicating epithelial cells on the inside of the cornea; ultrafast pulses are recognized to reduce this effect. Using a laser system that delivers 1ps pulses in 10μs pulsetrains at 133MHz we have studied a range of heat- and energy-transfer effects on hard and soft tissue. We describe the ablation of tooth dentin and enamel under various conditions to determine the ablation rate and chemical changes that occur. Furthermore, we characterize the impact of pulsetrain-burst treatment of collagen-based tissue to determine more efficient methods of energy transfer to soft tissues. By studying the optical science of laser tissue interaction we hope to be able to make qualitative improvements to medical treatments using lasers.

Tracking the ultrafast XUV optical properties of x-ray free-electron-laser heated matter with high-order harmonics

NASA Astrophysics Data System (ADS)

Williams, Gareth O.; Künzel, S.; Daboussi, S.; Iwan, B.; Gonzalez, A. I.; Boutu, W.; Hilbert, V.; Zastrau, U.; Lee, H. J.; Nagler, B.; Granados, E.; Galtier, E.; Heimann, P.; Barbrel, B.; Dovillaire, G.; Lee, R. W.; Dunn, J.; Recoules, V.; Blancard, C.; Renaudin, P.; de la Varga, A. G.; Velarde, P.; Audebert, P.; Merdji, H.; Zeitoun, Ph.; Fajardo, M.

2018-02-01

We present measurements of photon absorption by free electrons as a solid is transformed to plasma. A femtosecond x-ray free-electron laser is used to heat a solid, which separates the electron and ion heating time scales. The changes in absorption are measured with an independent probe pulse created through high-order-harmonic generation. We find an increase in electron temperature to have a relatively small impact on absorption, contrary to several predictions, whereas ion heating increases absorption. We compare the data to current theoretical and numerical approaches and find that a smoother electronic structure yields a better fit to the data, suggestive of a temperature-dependent electronic structure in warm dense matter.

Probing ultrafast spin dynamics with high-harmonic magnetic circular dichroism spectroscopy

NASA Astrophysics Data System (ADS)

Willems, F.; Smeenk, C. T. L.; Zhavoronkov, N.; Kornilov, O.; Radu, I.; Schmidbauer, M.; Hanke, M.; von Korff Schmising, C.; Vrakking, M. J. J.; Eisebitt, S.

2015-12-01

Magnetic circular dichroism in the extreme ultraviolet (XUV) spectral range is a powerful technique for element-specific probing of magnetization in multicomponent magnetic alloys and multilayers. We combine a high-harmonic generation source with a λ /4 phase shifter to obtain circularly polarized XUV femtosecond pulses for ultrafast magnetization studies. We report on simultaneously measured resonant magnetic circular dichroism (MCD) of Co and Ni at their respective M2 ,3 edges and of Pt at its O edge, originating from interface magnetism. We present a time-resolved MCD absorption measurement of a thin magnetic Pt/Co/Pt film, showing simultaneous demagnetization of Co and Pt on a femtosecond time scale.

Ultrafast coherence transfer in DNA-templated silver nanoclusters

PubMed Central

Thyrhaug, Erling; Bogh, Sidsel Ammitzbøll; Carro-Temboury, Miguel R; Madsen, Charlotte Stahl; Vosch, Tom; Zigmantas, Donatas

2017-01-01

DNA-templated silver nanoclusters of a few tens of atoms or less have come into prominence over the last several years due to very strong absorption and efficient emission. Applications in microscopy and sensing have already been realized, however little is known about the excited-state structure and dynamics in these clusters. Here we report on a multidimensional spectroscopy investigation of the energy-level structure and the early-time relaxation cascade, which eventually results in the population of an emitting state. We find that the ultrafast intramolecular relaxation is strongly coupled to a specific vibrational mode, resulting in the concerted transfer of population and coherence between excited states on a sub-100 fs timescale. PMID:28548085

Ultrafast Extreme Ultraviolet Spectroscopy of Methylammonium Lead Iodide Perovskite for Carrier Specific Photophysics

NASA Astrophysics Data System (ADS)

Verkamp, Max A.; Lin, Ming-Fu; Ryland, Elizabeth S.; Benke, Kristin; Vura-Weis, Josh

2017-06-01

Methyl ammonium lead iodide (perovskite) is a leading candidate for next-generation solar cell devices. However, the fundamental photophysics responsible for its strong photovoltaic qualities are not fully understood. Ultrafast extreme ultraviolet (XUV) spectroscopy was used to investigate relaxation dynamics in perovskite with carrier specific signals arising from transitions from the common inner-shell level (I 4d) to the valence and conduction bands. Ultrashort (30 fs) pulses of XUV radiation in a broad spectrum (40-70 eV) were obtained using high-harmonic generation in a tabletop instrument. Transient absorption measurements with visible pump and XUV probe directly observed the dynamics of charge carriers after above-band and band-edge excitation.

Ultrafast intersystem-crossing in platinum containing π-conjugated polymers with tunable spin-orbit coupling.

PubMed

Sheng, C-X; Singh, S; Gambetta, A; Drori, T; Tong, M; Tretiak, S; Vardeny, Z V

2013-01-01

The development of efficient organic light-emitting diodes (OLED) and organic photovoltaic cells requires control over the dynamics of spin sensitive excitations. Embedding heavy metal atoms in π-conjugated polymer chains enhances the spin-orbit coupling (SOC), and thus facilitates intersystem crossing (ISC) from the singlet to triplet manifolds. Here we use various nonlinear optical spectroscopies such as two-photon absorption and electroabsorption in conjunction with electronic structure calculations, for studying the energies, emission bands and ultrafast dynamics of spin photoexcitations in two newly synthesized π-conjugated polymers that contain intrachain platinum (Pt) atoms separated by one (Pt-1) or three (Pt-3) organic spacer units. The controllable SOC in these polymers leads to a record ISC time of <~1 ps in Pt-1 and ~6 ps in Pt-3. The tunable ultrafast ISC rate modulates the intensity ratio of the phosphorescence and fluorescence emission bands, with potential applications for white OLEDs.

Ultrafast dynamics of ligand and substrate interaction in endothelial nitric oxide synthase under Soret excitation.

PubMed

Hung, Chih-Chang; Yabushita, Atsushi; Kobayashi, Takayoshi; Chen, Pei-Feng; Liang, Keng S

2016-01-01

Ultrafast transient absorption spectroscopy of endothelial NOS oxygenase domain (eNOS-oxy) was performed to study dynamics of ligand or substrate interaction under Soret band excitation. Photo-excitation dissociates imidazole ligand in <300fs, then followed by vibrational cooling and recombination within 2ps. Such impulsive bond breaking and late rebinding generate proteinquakes, which relaxes in several tens of picoseconds. The photo excited dynamics of eNOS-oxy with L-arginine substrate mainly occurs at the local site of heme, including ultrafast internal conversion within 400fs, vibrational cooling, charge transfer, and complete ground-state recovery within 1.4ps. The eNOS-oxy without additive is partially bound with water molecule, thus its photoexcited dynamics also shows ligand dissociation in <800fs. Then it followed by vibrational cooling coupled with charge transfer in 4.8ps, and recombination of ligand to distal side of heme in 12ps. Copyright © 2016 Elsevier B.V. All rights reserved.

Enhanced generation and anisotropic Coulomb scattering of hot electrons in an ultra-broadband plasmonic nanopatch metasurface

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sykes, Matthew E.; Stewart, Jon W.; Akselrod, Gleb M.

The creation of energetic electrons through plasmon excitation of nanostructures before thermalization has been proposed for a wide number of applications in optical energy conversion and ultrafast nanophotonics. However, the use of "nonthermal" electrons is primarily limited by both a low generation efficiency and their ultrafast decay. We report experimental and theoretical results on the use of broadband plasmonic nanopatch metasurfaces comprising a gold substrate coupled to silver nanocubes that produce large concentrations of hot electrons, which we measure using transient absorption spectroscopy. We find evidence for three subpopulations of nonthermal carriers which we propose arise from anisotropic electron-electron scatteringmore » within sp-bands near the Fermi surface. The bimetallic character of the metasurface strongly impacts the physics, with dissipation occurring primarily in the gold whereas the quantum process of hot electron generation takes place in both components. As a result, our calculations show that the choice of geometry and materials is crucial for producing strong ultrafast nonthermal electron components.« less

Light, Molecules, Action: Using Ultrafast Uv-Visible and X-Ray Spectroscopy to Probe Excited State Dynamics in Photoactive Molecules

NASA Astrophysics Data System (ADS)

Sension, R. J.

2017-06-01

Light provides a versatile energy source capable of precise manipulation of material systems on size scales ranging from molecular to macroscopic. Photochemistry provides the means for transforming light energy from photon to process via movement of charge, a change in shape, a change in size, or the cleavage of a bond. Photochemistry produces action. In the work to be presented here ultrafast UV-Visible pump-probe, and pump-repump-probe methods have been used to probe the excited state dynamics of stilbene-based molecular motors, cyclohexadiene-based switches, and polyene-based photoacids. Both ultrafast UV-Visible and X-ray absorption spectroscopies have been applied to the study of cobalamin (vitamin B_{12}) based compounds. Optical measurements provide precise characterization of spectroscopic signatures of the intermediate species on the S_{1} surface, while time-resolved XANES spectra at the Co K-edge probe the structural changes that accompany these transformations.

Ultrafast structural dynamics of boron nitride nanotubes studied using transmitted electrons.

PubMed

Li, Zhongwen; Sun, Shuaishuai; Li, Zi-An; Zhang, Ming; Cao, Gaolong; Tian, Huanfang; Yang, Huaixin; Li, Jianqi

2017-09-14

We investigate the ultrafast structural dynamics of multi-walled boron nitride nanotubes (BNNTs) upon femtosecond optical excitation using ultrafast electron diffraction in a transmission electron microscope. Analysis of the time-resolved (100) and (002) diffraction profiles reveals highly anisotropic lattice dynamics of BNNTs, which can be attributed to the distinct nature of the chemical bonds in the tubular structure. Moreover, the changes in (002) diffraction positions and intensities suggest that the lattice response of BNNTs to the femtosecond laser excitation involves a fast and a slow lattice dynamic process. The fast process with a time constant of about 8 picoseconds can be understood to be a result of electron-phonon coupling, while the slow process with a time constant of about 100 to 300 picoseconds depending on pump laser fluence is tentatively associated with an Auger recombination effect. In addition, we discuss the power-law relationship of a three-photon absorption process in the BNNT nanoscale system.

Ultrafast X-Ray Spectroscopy of Conical Intersections

NASA Astrophysics Data System (ADS)

Neville, Simon P.; Chergui, Majed; Stolow, Albert; Schuurman, Michael S.

2018-06-01

Ongoing developments in ultrafast x-ray sources offer powerful new means of probing the complex nonadiabatically coupled structural and electronic dynamics of photoexcited molecules. These non-Born-Oppenheimer effects are governed by general electronic degeneracies termed conical intersections, which play a key role, analogous to that of a transition state, in the electronic-nuclear dynamics of excited molecules. Using high-level ab initio quantum dynamics simulations, we studied time-resolved x-ray absorption (TRXAS) and photoelectron spectroscopy (TRXPS) of the prototypical unsaturated organic chromophore, ethylene, following excitation to its S2(π π*) state. The TRXAS, in particular, is highly sensitive to all aspects of the ensuing dynamics. These x-ray spectroscopies provide a clear signature of the wave packet dynamics near conical intersections, related to charge localization effects driven by the nuclear dynamics. Given the ubiquity of charge localization in excited state dynamics, we believe that ultrafast x-ray spectroscopies offer a unique and powerful route to the direct observation of dynamics around conical intersections.

Enhanced generation and anisotropic Coulomb scattering of hot electrons in an ultra-broadband plasmonic nanopatch metasurface

DOE PAGES

Sykes, Matthew E.; Stewart, Jon W.; Akselrod, Gleb M.; ...

2017-10-17

The creation of energetic electrons through plasmon excitation of nanostructures before thermalization has been proposed for a wide number of applications in optical energy conversion and ultrafast nanophotonics. However, the use of "nonthermal" electrons is primarily limited by both a low generation efficiency and their ultrafast decay. We report experimental and theoretical results on the use of broadband plasmonic nanopatch metasurfaces comprising a gold substrate coupled to silver nanocubes that produce large concentrations of hot electrons, which we measure using transient absorption spectroscopy. We find evidence for three subpopulations of nonthermal carriers which we propose arise from anisotropic electron-electron scatteringmore » within sp-bands near the Fermi surface. The bimetallic character of the metasurface strongly impacts the physics, with dissipation occurring primarily in the gold whereas the quantum process of hot electron generation takes place in both components. As a result, our calculations show that the choice of geometry and materials is crucial for producing strong ultrafast nonthermal electron components.« less

Ultrafast re-structuring of the electronic landscape of transparent dielectrics: new material states (Die-Met)

NASA Astrophysics Data System (ADS)

Gamaly, E. G.; Rode, A. V.

2018-03-01

Swift excitation of transparent dielectrics by ultrashort and highly intense laser pulse leads to ultra-fast re-structuring of the electronic landscape and generates many transient material states, which are continuously reshaped in accord with the changing pulse intensity. These unconventional transient material states, which exhibit simultaneously both dielectric and metallic properties, we termed here as the `Die-Met' states. The excited material is transparent and conductive at the same time. The real part of permittivity of the excited material changes from positive to negative values with the increase of excitation, which affects strongly the interaction process during the laser pulse. When the incident field has a component along the permittivity gradient, the amplitude of the field increases resonantly near the point of zero permittivity, which dramatically changes the interaction mode and increases absorption in a way that is similar to the resonant absorption in plasma. The complex 3D structure of the permittivity makes a transparent part of the excited dielectric (at ɛ 0 > ɛ re > 0) optically active. The electro-magnetic wave gets a twisted trajectory and accrues the geometric phase while passing through such a medium. Both the phase and the rotation of the polarisation plane depend on the 3D permittivity structure. Measuring the transmission, polarisation and the phase of the probe beam allows one to quantitatively identify these new transient states. We discuss the revelations of this effect in different experimental situations and their possible applications.

Ultrafast Interlayer Electron Transfer in Incommensurate Transition Metal Dichalcogenide Homobilayers.

PubMed

Li, Yuanyuan; Cui, Qiannan; Ceballos, Frank; Lane, Samuel D; Qi, Zeming; Zhao, Hui

2017-11-08

Two-dimensional materials, such as graphene, transition metal dichalcogenides, and phosphorene, can be used to construct van der Waals multilayer structures. This approach has shown potentials to produce new materials that combine novel properties of the participating individual layers. One key requirement for effectively harnessing emergent properties of these materials is electronic connection of the involved atomic layers through efficient interlayer charge or energy transfer. Recently, ultrafast charge transfer on a time scale shorter than 100 fs has been observed in several van der Waals bilayer heterostructures formed by two different materials. However, information on the transfer between two atomic layers of the same type is rare. Because these homobilayers are essential elements in constructing multilayer structures with desired optoelectronic properties, efficient interlayer transfer is highly desired. Here we show that electron transfer between two monolayers of MoSe 2 occurs on a picosecond time scale. Even faster transfer was observed in homobilayers of WS 2 and WSe 2 . The samples were fabricated by manually stacking two exfoliated monolayer flakes. By adding a graphene layer as a fast carrier recombination channel for one of the two monolayers, the transfer of the photoexcited carriers from the populated to the drained monolayers was time-resolved by femtosecond transient absorption measurements. The observed efficient interlayer carrier transfer indicates that such homobilayers can be used in van der Waals multilayers to enhance their optical absorption without significantly compromising the interlayer transport performance. Our results also provide valuable information for understanding interlayer charge transfer in heterostructures.

Ultrafast X-Ray Absorption Spectroscopy of Isochorically Heated Warm Dense Matter

NASA Astrophysics Data System (ADS)

Engelhorn, Kyle Craig

This dissertation will present a series of new tools, together with new techniques, focused on the understanding of warm and dense matter. We report on the development of a high time resolution and high detection efficiency x-ray camera. The camera is integrated with a short pulse laser and an x-ray beamline at the Advanced Light Source synchrotron. This provides an instrument for single shot, broadband x-ray absorption spectroscopy of warm and dense matter with 2 picosecond time resolution. Warm and dense matter is created by isochorically heating samples of known density with an ultrafast optical laser pulse, and X-ray absorption spectroscopy probes the unoccupied electronic density of states before the onset of hydrodynamic expansion and electron-ion equilibrium is reached. Measured spectra from a variety of materials are compared with first principle molecular dynamics and density functional theory calculations. In heated silicon dioxide spectra, two novel pre-edge features are observed, a peak below the band gap and absorption within the band gap, while a reduction was observed in the features above the edge. From consideration of the calculated spectra, the peak below the gap is attributed to valence electrons that have been promoted to the conduction band, the absorption within the gap is attributed to broken Si-O bonds, and the reduction above the edge is attributed to an elevated ionic temperature. In heated copper spectra, a time-dependent shift and broadening of the absorption edge are observed, consistent with and elevated electron temperature. The temporal evolution of the electronic temperature is accurately determined by fitting the measured spectra with calculated spectra. The electron-ion equilibration is studied with a two-temperature model. In heated nickel spectra, a shift of the absorption edge is observed. This shift is found to be inconsistent with calculated spectra and independent of incident laser fluence. A shift of the chemical potential is applied to the calculated spectra to obtain satisfactory agreement with measured spectra.

Study of nonlinear absorption properties of reduced graphene oxide by Z-scan technique

NASA Astrophysics Data System (ADS)

Sreeja, V. G.; Vinitha, G.; Reshmi, R.; Anila, E. I.; Jayaraj, M. K.

2017-05-01

Graphene has generated enormous research interest during the last decade due to its significant unique properties and wide applications in the field of optoelectronics and photonics. This research studied the structural and nonlinear absorption properties of reduced graphene oxide (rGO) synthesized by Modified Hummer's method. Structural and physiochemical properties of the rGO were explored with the help of Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy (Raman). Nonlinear absorption property in rGO, was investigated by open aperture Z-scan technique by using a continuous wave (CW) laser. The Z-scan results demonstrate saturable absorption property of rGO with a nonlinear absorption coefficient, β, of -2.62 × 10-4 cm/W, making it suitable for applications in Q switching, generation of ultra-fast high energy pulses in laser cavity and mode lockers.

Single-wall carbon nanotubes and graphene oxide-based saturable absorbers for low phase noise mode-locked fiber lasers

PubMed Central

Li, Xiaohui; Wu, Kan; Sun, Zhipei; Meng, Bo; Wang, Yonggang; Wang, Yishan; Yu, Xuechao; Yu, Xia; Zhang, Ying; Shum, Perry Ping; Wang, Qi Jie

2016-01-01

Low phase noise mode-locked fiber laser finds important applications in telecommunication, ultrafast sciences, material science, and biology, etc. In this paper, two types of carbon nano-materials, i.e. single-wall carbon nanotube (SWNT) and graphene oxide (GO), are investigated as efficient saturable absorbers (SAs) to achieve low phase noise mode-locked fiber lasers. Various properties of these wall-paper SAs, such as saturable intensity, optical absorption and degree of purity, are found to be key factors determining the performance of the ultrafast pulses. Reduced-noise femtosecond fiber lasers based on such carbon-based SAs are experimentally demonstrated, for which the phase noise has been reduced by more than 10 dB for SWNT SAs and 8 dB for GO SAs at 10 kHz. To the best of our knowledge, this is the first investigation on the relationship between different carbon material based SAs and the phase noise of mode-locked lasers. This work paves the way to generate high-quality low phase noise ultrashort pulses in passively mode-locked fiber lasers. PMID:27126900

Ultraviolet Absorption Induces Hydrogen-Atom Transfer in G⋅C Watson-Crick DNA Base Pairs in Solution.

PubMed

Röttger, Katharina; Marroux, Hugo J B; Grubb, Michael P; Coulter, Philip M; Böhnke, Hendrik; Henderson, Alexander S; Galan, M Carmen; Temps, Friedrich; Orr-Ewing, Andrew J; Roberts, Gareth M

2015-12-01

Ultrafast deactivation pathways bestow photostability on nucleobases and hence preserve the structural integrity of DNA following absorption of ultraviolet (UV) radiation. One controversial recovery mechanism proposed to account for this photostability involves electron-driven proton transfer (EDPT) in Watson-Crick base pairs. The first direct observation is reported of the EDPT process after UV excitation of individual guanine-cytosine (G⋅C) Watson-Crick base pairs by ultrafast time-resolved UV/visible and mid-infrared spectroscopy. The formation of an intermediate biradical species (G[-H]⋅C[+H]) with a lifetime of 2.9 ps was tracked. The majority of these biradicals return to the original G⋅C Watson-Crick pairs, but up to 10% of the initially excited molecules instead form a stable photoproduct G*⋅C* that has undergone double hydrogen-atom transfer. The observation of these sequential EDPT mechanisms across intermolecular hydrogen bonds confirms an important and long debated pathway for the deactivation of photoexcited base pairs, with possible implications for the UV photochemistry of DNA. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Constraining physical parameters of ultra-fast outflows in PDS 456 with Monte Carlo simulations

NASA Astrophysics Data System (ADS)

Hagino, K.; Odaka, H.; Done, C.; Gandhi, P.; Takahashi, T.

2014-07-01

Deep absorption lines with extremely high velocity of ˜0.3c observed in PDS 456 spectra strongly indicate the existence of ultra-fast outflows (UFOs). However, the launching and acceleration mechanisms of UFOs are still uncertain. One possible way to solve this is to constrain physical parameters as a function of distance from the source. In order to study the spatial dependence of parameters, it is essential to adopt 3-dimensional Monte Carlo simulations that treat radiation transfer in arbitrary geometry. We have developed a new simulation code of X-ray radiation reprocessed in AGN outflow. Our code implements radiative transfer in 3-dimensional biconical disk wind geometry, based on Monte Carlo simulation framework called MONACO (Watanabe et al. 2006, Odaka et al. 2011). Our simulations reproduce FeXXV and FeXXVI absorption features seen in the spectra. Also, broad Fe emission lines, which reflects the geometry and viewing angle, is successfully reproduced. By comparing the simulated spectra with Suzaku data, we obtained constraints on physical parameters. We discuss launching and acceleration mechanisms of UFOs in PDS 456 based on our analysis.

Identification of effective exciton-exciton annihilation in squaraine-squaraine copolymers.

PubMed

Hader, Kilian; May, Volkhard; Lambert, Christoph; Engel, Volker

2016-05-11

Ultrafast time-resolved transient absorption spectroscopy is able to monitor the fate of the excited state population in molecular aggregates or polymers. Due to many competing decay processes, the identification of exciton-exciton annihilation (EEA) is difficult. Here, we use a microscopic model to describe exciton annihilation processes in squaraine-squaraine copolymers. Transient absorption time traces measured at different laser powers exhibit an unusual time-dependence. The analysis points towards dynamics taking place on three time-scales. Immediately after laser-excitation a localization of excitons takes place within the femtosecond time-regime. This is followed by exciton-exciton annihilation which is responsible for a fast decay of the exciton population. At later times, excitations being localized on units which are not directly connected remain so that diffusion dominates the dynamics and leads to a slower decay. We thus provide evidence for EEA tracked by time-resolved spectroscopy which has not been reported that clearly before.

NO binding kinetics in myoglobin investigated by picosecond Fe K-edge absorption spectroscopy

PubMed Central

Silatani, Mahsa; Lima, Frederico A.; Penfold, Thomas J.; Rittmann, Jochen; Reinhard, Marco E.; Rittmann-Frank, Hannelore M.; Borca, Camelia; Grolimund, Daniel; Milne, Christopher J.; Chergui, Majed

2015-01-01

Diatomic ligands in hemoproteins and the way they bind to the active center are central to the protein’s function. Using picosecond Fe K-edge X-ray absorption spectroscopy, we probe the NO-heme recombination kinetics with direct sensitivity to the Fe-NO binding after 532-nm photoexcitation of nitrosylmyoglobin (MbNO) in physiological solutions. The transients at 70 and 300 ps are identical, but they deviate from the difference between the static spectra of deoxymyoglobin and MbNO, showing the formation of an intermediate species. We propose the latter to be a six-coordinated domed species that is populated on a timescale of ∼200 ps by recombination with NO ligands. This work shows the feasibility of ultrafast pump–probe X-ray spectroscopic studies of proteins in physiological media, delivering insight into the electronic and geometric structure of the active center. PMID:26438842

Rapidly variable relatvistic absorption

NASA Astrophysics Data System (ADS)

Parker, M.; Pinto, C.; Fabian, A.; Lohfink, A.; Buisson, D.; Alston, W.; Jiang, J.

2017-10-01

I will present results from the 1.5Ms XMM-Newton observing campaign on the most X-ray variable AGN, IRAS 13224-3809. We find a series of nine absorption lines with a velocity of 0.24c from an ultra-fast outflow. For the first time, we are able to see extremely rapid variability of the UFO features, and can link this to the X-ray variability from the inner accretion disk. We find a clear flux dependence of the outflow features, suggesting that the wind is ionized by increasing X-ray emission.

Note: A flexible light emitting diode-based broadband transient-absorption spectrometer

NASA Astrophysics Data System (ADS)

Gottlieb, Sean M.; Corley, Scott C.; Madsen, Dorte; Larsen, Delmar S.

2012-05-01

This Note presents a simple and flexible ns-to-ms transient absorption spectrometer based on pulsed light emitting diode (LED) technology that can be incorporated into existing ultrafast transient absorption spectrometers or operate as a stand-alone instrument with fixed-wavelength laser sources. The LED probe pulses from this instrument exhibit excellent stability (˜0.5%) and are capable of producing high signal-to-noise long-time (>100 ns) transient absorption signals either in a broadband multiplexed (spanning 250 nm) or in tunable narrowband (20 ns) operation. The utility of the instrument is demonstrated by measuring the photoinduced ns-to-ms photodynamics of the red/green absorbing fourth GMP phosphodiesterase/adenylyl cyclase/FhlA domain of the NpR6012 locus of the nitrogen-fixing cyanobacterium Nostoc punctiforme.

Tracking the ultrafast XUV optical properties of x-ray free-electron-laser heated matter with high-order harmonics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Williams, Gareth O.; Künzel, S.; Daboussi, S.

We present measurements of photon absorption by free electrons as a solid is transformed to plasma. A femtosecond x-ray free-electron laser is used to heat a solid, which separates the electron and ion heating time scales. The changes in absorption are measured with an independent probe pulse created through high-order-harmonic generation. We find an increase in electron temperature to have a relatively small impact on absorption, contrary to several predictions, whereas ion heating increases absorption. Here, we compare the data to current theoretical and numerical approaches and find that a smoother electronic structure yields a better fit to the data,more » suggestive of a temperature-dependent electronic structure in warm dense matter.« less

Tracking the ultrafast XUV optical properties of x-ray free-electron-laser heated matter with high-order harmonics

DOE PAGES

Williams, Gareth O.; Künzel, S.; Daboussi, S.; ...

2018-02-14

We present measurements of photon absorption by free electrons as a solid is transformed to plasma. A femtosecond x-ray free-electron laser is used to heat a solid, which separates the electron and ion heating time scales. The changes in absorption are measured with an independent probe pulse created through high-order-harmonic generation. We find an increase in electron temperature to have a relatively small impact on absorption, contrary to several predictions, whereas ion heating increases absorption. Here, we compare the data to current theoretical and numerical approaches and find that a smoother electronic structure yields a better fit to the data,more » suggestive of a temperature-dependent electronic structure in warm dense matter.« less

Quantum state-resolved probing of strong-field-ionized xenon atoms using femtosecond high-order harmonic transient absorption spectroscopy.

PubMed

Loh, Zhi-Heng; Khalil, Munira; Correa, Raoul E; Santra, Robin; Buth, Christian; Leone, Stephen R

2007-04-06

Femtosecond high-order harmonic transient absorption spectroscopy is used to resolve the complete |j,m quantum state distribution of Xe+ produced by optical strong-field ionization of Xe atoms at 800 nm. Probing at the Xe N4/5 edge yields a population distribution rhoj,|m| of rho3/2,1/2ratiorho1/2,1/2ratiorho3/2,3/2=75+/-6 :12+/-3 :13+/-6%. The result is compared to a tunnel ionization calculation with the inclusion of spin-orbit coupling, revealing nonadiabatic ionization behavior. The sub-50-fs time resolution paves the way for tabletop extreme ultraviolet absorption probing of ultrafast dynamics.

Two-photon absorption in layered transition metal dichalcogenides

NASA Astrophysics Data System (ADS)

Dong, Ningning; Zhang, Saifeng; Li, Yuanxin; Wang, Jun

2018-02-01

Two-dimensional (2D) layered transition metal dichalcogenides (TMDCs) exhibit unique nonlinear optical (NLO) features and have becoming intriguing and promising candidate materials for photonic and optoelectronic devices with high performance and unique functions. Owing to layered geometry and the thickness-dependent bandgap, we studied the ultrafast NLO properties of a range of TMDCs. TMDCs with high-quality layered nanosheets were prepared through chemical vapor deposition (CVD) technique and vapor-phase growth method. Saturable absorption, two photon absorption (TPA) and two photon pumped frequency up-converted luminescence were observed from these 2D nanostructures. The exciting results open up the door to 2D photonic devices, such as passive mode-lockers, Q-switchers, optical limiters, light emitters, etc.

In vivo studies of ultrafast near-infrared laser tissue bonding and wound healing

PubMed Central

Sriramoju, Vidyasagar; Alfano, Robert R.

2015-01-01

Abstract. Femtosecond (fs) pulse lasers in the near-infrared (NIR) range exhibit very distinct properties upon their interaction with biomolecules compared to the corresponding continuous wave (CW) lasers. Ultrafast NIR laser tissue bonding (LTB) was used to fuse edges of two opposing animal tissue segments in vivo using fs laser photoexcitation of the native vibrations of chomophores. The fusion of the incised tissues was achieved in vivo at the molecular level as the result of the energy–matter interactions of NIR laser radiation with water and the structural proteins like collagen in the target tissues. Nonthermal vibrational excitation from the fs laser absorption by water and collagen induced the formation of cross-links between tissue proteins on either sides of the weld line resulting in tissue bonding. No extrinsic agents were used to facilitate tissue bonding in the fs LTB. These studies were pursued for the understanding and evaluation of the role of ultrafast NIR fs laser radiation in the LTB and consequent wound healing. The fs LTB can be used for difficult to suture structures such as blood vessels, nerves, gallbladder, liver, intestines, and other viscera. Ultrafast NIR LTB yields promising outcomes and benefits in terms of wound closure and wound healing under optimal conditions. PMID:26465615

In vivo studies of ultrafast near-infrared laser tissue bonding and wound healing

NASA Astrophysics Data System (ADS)

Sriramoju, Vidyasagar; Alfano, Robert R.

2015-10-01

Femtosecond (fs) pulse lasers in the near-infrared (NIR) range exhibit very distinct properties upon their interaction with biomolecules compared to the corresponding continuous wave (CW) lasers. Ultrafast NIR laser tissue bonding (LTB) was used to fuse edges of two opposing animal tissue segments in vivo using fs laser photoexcitation of the native vibrations of chomophores. The fusion of the incised tissues was achieved in vivo at the molecular level as the result of the energy-matter interactions of NIR laser radiation with water and the structural proteins like collagen in the target tissues. Nonthermal vibrational excitation from the fs laser absorption by water and collagen induced the formation of cross-links between tissue proteins on either sides of the weld line resulting in tissue bonding. No extrinsic agents were used to facilitate tissue bonding in the fs LTB. These studies were pursued for the understanding and evaluation of the role of ultrafast NIR fs laser radiation in the LTB and consequent wound healing. The fs LTB can be used for difficult to suture structures such as blood vessels, nerves, gallbladder, liver, intestines, and other viscera. Ultrafast NIR LTB yields promising outcomes and benefits in terms of wound closure and wound healing under optimal conditions.

Tunable Picosecond Laser Pulses via the Contrast of Two Reverse Saturable Absorption Phases in a Waveguide Platform

PubMed Central

Tan, Yang; Chen, Lianwei; Wang, Dong; Chen, Yanxue; Akhmadaliev, Shavkat; Zhou, Shengqiang; Hong, Minghui; Chen, Feng

2016-01-01

How to enhance the optical nonlinearity of saturable absorption materials is an important question to improve the functionality of various applications ranging from the high power laser to photonic computational devices. We demonstrate the saturable absorption (SA) of VO2 film attributed to the large difference of optical nonlinearities between the two states of the phase-transition materials (VO2). Such VO2 film demonstrated significantly improved performance with saturation intensity higher than other existing ultrathin saturable absorbers by 3 orders due to its unique nonlinear optical mechanisms in the ultrafast phase change process. Owing to this feature, a Q-switched pulsed laser was fabricated in a waveguide platform, which is the first time to achieve picosecond pulse duration and maintain high peak power. Furthermore, the emission of this VO2 waveguide laser can be flexibly switched between the continuous-wave (CW) and pulsed operation regimes by tuning the temperature of the VO2 film, which enables VO2-based miniature laser devices with unique and versatile functions. PMID:27188594

Activation of coherent lattice phonon following ultrafast molecular spin-state photo-switching: A molecule-to-lattice energy transfer

PubMed Central

Marino, A.; Cammarata, M.; Matar, S. F.; Létard, J.-F.; Chastanet, G.; Chollet, M.; Glownia, J. M.; Lemke, H. T.; Collet, E.

2015-01-01

We combine ultrafast optical spectroscopy with femtosecond X-ray absorption to study the photo-switching dynamics of the [Fe(PM-AzA)2(NCS)2] spin-crossover molecular solid. The light-induced excited spin-state trapping process switches the molecules from low spin to high spin (HS) states on the sub-picosecond timescale. The change of the electronic state (<50 fs) induces a structural reorganization of the molecule within 160 fs. This transformation is accompanied by coherent molecular vibrations in the HS potential and especially a rapidly damped Fe-ligand breathing mode. The time-resolved studies evidence a delayed activation of coherent optical phonons of the lattice surrounding the photoexcited molecules. PMID:26798836

Tin Oxide Nanowires: The Influence of Trap States on Ultrafast Carrier Relaxation

PubMed Central

2009-01-01

We have studied the optical properties and carrier dynamics in SnO2nanowires (NWs) with an average radius of 50 nm that were grown via the vapor–liquid solid method. Transient differential absorption measurements have been employed to investigate the ultrafast relaxation dynamics of photogenerated carriers in the SnO2NWs. Steady state transmission measurements revealed that the band gap of these NWs is 3.77 eV and contains two broad absorption bands. The first is located below the band edge (shallow traps) and the second near the center of the band gap (deep traps). Both of these absorption bands seem to play a crucial role in the relaxation of the photogenerated carriers. Time resolved measurements suggest that the photogenerated carriers take a few picoseconds to move into the shallow trap states whereas they take ~70 ps to move from the shallow to the deep trap states. Furthermore the recombination process of electrons in these trap states with holes in the valence band takes ~2 ns. Auger recombination appears to be important at the highest fluence used in this study (500 μJ/cm2); however, it has negligible effect for fluences below 50 μJ/cm2. The Auger coefficient for the SnO2NWs was estimated to be 7.5 ± 2.5 × 10−31 cm6/s. PMID:20596473

Ultrafast carrier dynamics of titanic acid nanotubes investigated by transient absorption spectroscopy.

PubMed

Wang, Li; Zhao, Hui; Pan, Lin Yun; Weng, Yu Xiang; Nakato, Yoshihiro; Tamai, Naoto

2010-12-01

Carrier dynamics of titanic acid nanotubes (phase of H2Ti2O5.H2O) deposited on a quartz plate was examined by visible/near-IR transient absorption spectroscopy with an ultraviolet excitation. The carrier dynamics of titanic acid nanotubes follows the fast trapping process which attributed to the intrinsic tubular structure, the relaxation of shallow trapped carriers and the recombination as a second-order kinetic process. Transient absorption of titanic acid nanotubes was dominated by the absorption of surface-trapped holes in visible region around 500 nm, which was proved by the faster decay dynamics in the presence of polyvinyl alcohol as a hole-scavenger. However, the slow relaxation of free carriers was much more pronounced in the TiO2 single crystals, as compared with the transient absorption spectra of titanic acid nanotubes under the similar excitation.

L-band ultrafast fiber laser mode locked by carbon nanotubes

NASA Astrophysics Data System (ADS)

Sun, Z.; Rozhin, A. G.; Wang, F.; Scardaci, V.; Milne, W. I.; White, I. H.; Hennrich, F.; Ferrari, A. C.

2008-08-01

We fabricate a nanotube-polyvinyl alcohol saturable absorber with a broad absorption at 1.6 μm. We demonstrate a pulsed fiber laser working in the telecommunication L band by using this composite as a mode locker. This gives ˜498±16 fs pulses at 1601 nm with a 26.7 MHz repetition rate.

Kinetic Analysis of Benign and Malignant Breast Lesions With Ultrafast Dynamic Contrast-Enhanced MRI: Comparison With Standard Kinetic Assessment.

PubMed

Abe, Hiroyuki; Mori, Naoko; Tsuchiya, Keiko; Schacht, David V; Pineda, Federico D; Jiang, Yulei; Karczmar, Gregory S

2016-11-01

The purposes of this study were to evaluate diagnostic parameters measured with ultrafast MRI acquisition and with standard acquisition and to compare diagnostic utility for differentiating benign from malignant lesions. Ultrafast acquisition is a high-temporal-resolution (7 seconds) imaging technique for obtaining 3D whole-breast images. The dynamic contrast-enhanced 3-T MRI protocol consists of an unenhanced standard and an ultrafast acquisition that includes eight contrast-enhanced ultrafast images and four standard images. Retrospective assessment was performed for 60 patients with 33 malignant and 29 benign lesions. A computer-aided detection system was used to obtain initial enhancement rate and signal enhancement ratio (SER) by means of identification of a voxel showing the highest signal intensity in the first phase of standard imaging. From the same voxel, the enhancement rate at each time point of the ultrafast acquisition and the AUC of the kinetic curve from zero to each time point of ultrafast imaging were obtained. There was a statistically significant difference between benign and malignant lesions in enhancement rate and kinetic AUC for ultrafast imaging and also in initial enhancement rate and SER for standard imaging. ROC analysis showed no significant differences between enhancement rate in ultrafast imaging and SER or initial enhancement rate in standard imaging. Ultrafast imaging is useful for discriminating benign from malignant lesions. The differential utility of ultrafast imaging is comparable to that of standard kinetic assessment in a shorter study time.

Effects of single atom doping on the ultrafast electron dynamics of M1Au24(SR)18 (M = Pd, Pt) nanoclusters

NASA Astrophysics Data System (ADS)

Zhou, Meng; Qian, Huifeng; Sfeir, Matthew Y.; Nobusada, Katsuyuki; Jin, Rongchao

2016-03-01

Atomically precise, doped metal clusters are receiving wide research interest due to their synergistic properties dependent on the metal composition. To understand the electronic properties of doped clusters, it is highly desirable to probe the excited state behavior. Here, we report the ultrafast relaxation dynamics of doped M1@Au24(SR)18 (M = Pd, Pt; R = CH2CH2Ph) clusters using femtosecond visible and near infrared transient absorption spectroscopy. Three relaxation components are identified for both mono-doped clusters: (1) sub-picosecond relaxation within the M1Au12 core states; (2) core to shell relaxation in a few picoseconds; and (3) relaxation back to the ground state in more than one nanosecond. Despite similar relaxation pathways for the two doped nanoclusters, the coupling between the metal core and surface ligands is accelerated by over 30% in the case of the Pt dopant compared with the Pd dopant. Compared to Pd doping, the case of Pt doping leads to much more drastic changes in the steady state and transient absorption of the clusters, which indicates that the 5d orbitals of the Pt atom are more strongly mixed with Au 5d and 6s orbitals than the 4d orbitals of the Pd dopant. These results demonstrate that a single foreign atom can lead to entirely different excited state spectral features of the whole cluster compared to the parent Au25(SR)18 cluster. The detailed excited state dynamics of atomically precise Pd/Pt doped gold clusters help further understand their properties and benefit the development of energy-related applications.Atomically precise, doped metal clusters are receiving wide research interest due to their synergistic properties dependent on the metal composition. To understand the electronic properties of doped clusters, it is highly desirable to probe the excited state behavior. Here, we report the ultrafast relaxation dynamics of doped M1@Au24(SR)18 (M = Pd, Pt; R = CH2CH2Ph) clusters using femtosecond visible and near infrared transient absorption spectroscopy. Three relaxation components are identified for both mono-doped clusters: (1) sub-picosecond relaxation within the M1Au12 core states; (2) core to shell relaxation in a few picoseconds; and (3) relaxation back to the ground state in more than one nanosecond. Despite similar relaxation pathways for the two doped nanoclusters, the coupling between the metal core and surface ligands is accelerated by over 30% in the case of the Pt dopant compared with the Pd dopant. Compared to Pd doping, the case of Pt doping leads to much more drastic changes in the steady state and transient absorption of the clusters, which indicates that the 5d orbitals of the Pt atom are more strongly mixed with Au 5d and 6s orbitals than the 4d orbitals of the Pd dopant. These results demonstrate that a single foreign atom can lead to entirely different excited state spectral features of the whole cluster compared to the parent Au25(SR)18 cluster. The detailed excited state dynamics of atomically precise Pd/Pt doped gold clusters help further understand their properties and benefit the development of energy-related applications. Electronic supplementary information (ESI) available: The pump dependent transient absorption spectra and the corresponding global analysis results. See DOI: 10.1039/c6nr01008c

Nonadiabatic molecular dynamics simulations: synergies between theory and experiments.

PubMed

Tavernelli, Ivano

2015-03-17

Recent developments in nonadiabatic dynamics enabled ab inito simulations of complex ultrafast processes in the condensed phase. These advances have opened new avenues in the study of many photophysical and photochemical reactions triggered by the absorption of electromagnetic radiation. In particular, theoretical investigations can be combined with the most sophisticated femtosecond experimental techniques to guide the interpretation of measured time-resolved observables. At the same time, the availability of experimental data at high (spatial and time) resolution offers a unique opportunity for the benchmarking and the improvement of those theoretical models used to describe complex molecular systems in their natural environment. The established synergy between theory and experiments can produce a better understanding of new ultrafast physical and chemical processes at atomistic scale resolution. Furthermore, reliable ab inito molecular dynamics simulations can already be successfully employed as predictive tools to guide new experiments as well as the design of novel and better performing materials. In this paper, I will give a concise account on the state of the art of molecular dynamics simulations of complex molecular systems in their excited states. The principal aim of this approach is the description of a given system of interest under the most realistic ambient conditions including all environmental effects that influence experiments, for instance, the interaction with the solvent and with external time-dependent electric fields, temperature, and pressure. To this end, time-dependent density functional theory (TDDFT) is among the most efficient and accurate methods for the representation of the electronic dynamics, while trajectory surface hopping gives a valuable representation of the nuclear quantum dynamics in the excited states (including nonadiabatic effects). Concerning the environment and its effects on the dynamics, the quantum mechanics/molecular mechanics (QM/MM) approach has the advantage of providing an atomistic (even though approximated) description of the solvent molecules, which is crucial for the characterization of all ultrafast relaxation phenomena that depend on the geometrical arrangement at the interface between a molecule and the solvent, for example, the hydrogen bond network. After a short description of the TDDFT-based implementation of Ehrenfest and trajectory surface hopping dynamics, I will present applications in different domains of molecular chemistry and physics: the analysis and the understanding of (time-resolved) X-ray absorption spectra, the interpretation of the ultrafast relaxation dynamics of photoexcited dyes in solution, and the design of specific laser pulses (capable of inducing desired chemical reactions) using local control theory.

Investigation of Optical Cavity Modes and Ultrafast Carrier Dynamics in Zinc Oxide Rods Using Second-Harmonic Generation and Transient Absorption Pump-Probe Microscopy

NASA Astrophysics Data System (ADS)

Mehl, Brian Peter

The polydispersity intrinsic to nanoscale and microscale semiconductor materials poses a major challenge to using individual objects as building blocks for device applications. The ability to manipulate the shape of ZnO structures is enormous, making it an ideal material for studying shape-dependent phenomena. We have built a nonlinear microscope used to directly image optical cavity modes in ZnO rods using second-harmonic generation. Images of second-harmonic generation in needle-shaped ZnO rods obtained from individual structures show areas of enhanced second-harmonic intensity along the longitudinal axis of the rod that are periodically distributed and symmetrically situated relative to the rod midpoint. The spatial modulation is a direct consequence of the fundamental optical field coupling into standing wave resonator modes of the ZnO structure, leading to an enhanced backscattered second-harmonic condition that cannot be achieved in bulk ZnO. A more complicated second-harmonic image is observed when excitation is below the band gap, which is attributed to whispering gallery modes. Additionally, the nonlinear microscope was combined with transient absorption pump-probe to follow the electron-hole recombination dynamics at different points within individual needle-shaped ZnO rods to characterize spatial differences in dynamical behavior. The results from pump-probe experiments are correlated with spatially resolved ultrafast emission measurements, and scanning electron microscopy provides structural details. Dramatically different electron-hole recombination dynamics are observed in the narrow tips compared to the interior, with the ends exhibiting a greater propensity for electron-hole plasma formation and faster recombination of carriers across the band gap that stem from a physical confinement of the charge carriers. In the interior of the rod, a greater fraction of the electron-hole recombination is trap-mediated and occurs on a significantly longer time scale.

Capturing ultrafast photoinduced local structural distortions of BiFeO 3

DOE PAGES

Wen, Haidan; Sassi, Michel JPC; Luo, Zhenlin; ...

2015-10-14

The interaction of light with materials is an intensively studied research forefront, in which the coupling of radiation energy to selective degrees of freedom offers contact-free tuning of functionalities on ultrafast time scales. Capturing the fundamental processes and understanding the mechanism of photoinduced structural rearrangement are essential to applications such as photo-active actuators and efficient photovoltaic devices. Using ultrafast x-ray absorption spectroscopy aided by density functional theory calculations, we reveal the local structural arrangement around the transition metal atom in a unit cell of the photoferroelectric archetype BiFeO 3 film. The out-of-plane elongation of the unit cell is accompanied bymore » the in-plane shrinkage with minimal change of interaxial lattice angles upon photoexcitation. This uniaxial elastic deformation of the unit cell is driven by localized electric field as a result of photoinduced charge separation, in contrast to a global lattice constant increase and lattice angle variations as a result of heating. The finding of a photoinduced elastic unit cell deformation elucidates a microscopic picture of photocarrier-mediated nonequilibrium processes in polar materials.« less

Vibronic coupling explains the ultrafast carotenoid-to-bacteriochlorophyll energy transfer in natural and artificial light harvesters

NASA Astrophysics Data System (ADS)

Perlík, Václav; Seibt, Joachim; Cranston, Laura J.; Cogdell, Richard J.; Lincoln, Craig N.; Savolainen, Janne; Šanda, František; Mančal, Tomáš; Hauer, Jürgen

2015-06-01

The initial energy transfer steps in photosynthesis occur on ultrafast timescales. We analyze the carotenoid to bacteriochlorophyll energy transfer in LH2 Marichromatium purpuratum as well as in an artificial light-harvesting dyad system by using transient grating and two-dimensional electronic spectroscopy with 10 fs time resolution. We find that Förster-type models reproduce the experimentally observed 60 fs transfer times, but overestimate coupling constants, which lead to a disagreement with both linear absorption and electronic 2D-spectra. We show that a vibronic model, which treats carotenoid vibrations on both electronic ground and excited states as part of the system's Hamiltonian, reproduces all measured quantities. Importantly, the vibronic model presented here can explain the fast energy transfer rates with only moderate coupling constants, which are in agreement with structure based calculations. Counterintuitively, the vibrational levels on the carotenoid electronic ground state play the central role in the excited state population transfer to bacteriochlorophyll; resonance between the donor-acceptor energy gap and the vibrational ground state energies is the physical basis of the ultrafast energy transfer rates in these systems.

Towards validated chemistry at extreme conditions: reactive MD simulations of shocked Polyvinyl Nitrate and Nitromethane

NASA Astrophysics Data System (ADS)

Islam, Md Mahbubul; Strachan, Alejandro

A detailed atomistic-level understanding of the ultrafast chemistry of detonation processes of high energy materials is crucial to understand their performance and safety. Recent advances in laser shocks and ultra-fast spectroscopy is yielding the first direct experimental evidence of chemistry at extreme conditions. At the same time, reactive molecular dynamics (MD) in current high-performance computing platforms enable an atomic description of shock-induced chemistry with length and timescales approaching those of experiments. We use MD simulations with the reactive force field ReaxFF to investigate the shock-induced chemical decomposition mechanisms of polyvinyl nitrate (PVN) and nitromethane (NM). The effect of shock pressure on chemical reaction mechanisms and kinetics of both the materials are investigated. For direct comparison of our simulation results with experimentally derived IR absorption data, we performed spectral analysis using atomistic velocity at various shock conditions. The combination of reactive MD simulations and ultrafast spectroscopy enables both the validation of ReaxFF at extreme conditions and contributes to the interpretation of the experimental data relating changes in spectral features to atomic processes. Office of Naval Research MURI program.

Capturing ultrafast photoinduced local structural distortions of BiFeO3

PubMed Central

Wen, Haidan; Sassi, Michel; Luo, Zhenlin; Adamo, Carolina; Schlom, Darrell G.; Rosso, Kevin M.; Zhang, Xiaoyi

2015-01-01

The interaction of light with materials is an intensively studied research forefront, in which the coupling of radiation energy to selective degrees of freedom offers contact-free tuning of functionalities on ultrafast time scales. Capturing the fundamental processes and understanding the mechanism of photoinduced structural rearrangement are essential to applications such as photo-active actuators and efficient photovoltaic devices. Using ultrafast x-ray absorption spectroscopy aided by density functional theory calculations, we reveal the local structural arrangement around the transition metal atom in a unit cell of the photoferroelectric archetype BiFeO3 film. The out-of-plane elongation of the unit cell is accompanied by the in-plane shrinkage with minimal change of interaxial lattice angles upon photoexcitation. This anisotropic elastic deformation of the unit cell is driven by localized electric field as a result of photoinduced charge separation, in contrast to a global lattice constant increase and lattice angle variations as a result of heating. The finding of a photoinduced elastic unit cell deformation elucidates a microscopic picture of photocarrier-mediated non-equilibrium processes in polar materials. PMID:26463128

Capturing ultrafast photoinduced local structural distortions of BiFeO3.

PubMed

Wen, Haidan; Sassi, Michel; Luo, Zhenlin; Adamo, Carolina; Schlom, Darrell G; Rosso, Kevin M; Zhang, Xiaoyi

2015-10-14

The interaction of light with materials is an intensively studied research forefront, in which the coupling of radiation energy to selective degrees of freedom offers contact-free tuning of functionalities on ultrafast time scales. Capturing the fundamental processes and understanding the mechanism of photoinduced structural rearrangement are essential to applications such as photo-active actuators and efficient photovoltaic devices. Using ultrafast x-ray absorption spectroscopy aided by density functional theory calculations, we reveal the local structural arrangement around the transition metal atom in a unit cell of the photoferroelectric archetype BiFeO3 film. The out-of-plane elongation of the unit cell is accompanied by the in-plane shrinkage with minimal change of interaxial lattice angles upon photoexcitation. This anisotropic elastic deformation of the unit cell is driven by localized electric field as a result of photoinduced charge separation, in contrast to a global lattice constant increase and lattice angle variations as a result of heating. The finding of a photoinduced elastic unit cell deformation elucidates a microscopic picture of photocarrier-mediated non-equilibrium processes in polar materials.

Capturing ultrafast photoinduced local structural distortions of BiFeO3

NASA Astrophysics Data System (ADS)

Wen, Haidan; Sassi, Michel; Luo, Zhenlin; Adamo, Carolina; Schlom, Darrell G.; Rosso, Kevin M.; Zhang, Xiaoyi

2015-10-01

The interaction of light with materials is an intensively studied research forefront, in which the coupling of radiation energy to selective degrees of freedom offers contact-free tuning of functionalities on ultrafast time scales. Capturing the fundamental processes and understanding the mechanism of photoinduced structural rearrangement are essential to applications such as photo-active actuators and efficient photovoltaic devices. Using ultrafast x-ray absorption spectroscopy aided by density functional theory calculations, we reveal the local structural arrangement around the transition metal atom in a unit cell of the photoferroelectric archetype BiFeO3 film. The out-of-plane elongation of the unit cell is accompanied by the in-plane shrinkage with minimal change of interaxial lattice angles upon photoexcitation. This anisotropic elastic deformation of the unit cell is driven by localized electric field as a result of photoinduced charge separation, in contrast to a global lattice constant increase and lattice angle variations as a result of heating. The finding of a photoinduced elastic unit cell deformation elucidates a microscopic picture of photocarrier-mediated non-equilibrium processes in polar materials.

Size and structure dependent ultrafast dynamics of plasmonic gold nanosphere heterostructures on poly (ethylene glycol) brushes

NASA Astrophysics Data System (ADS)

Karatay, Ahmet; Küçüköz, Betül; Pekdemir, Sami; Onses, Mustafa Serdar; Elmali, Ayhan

2017-11-01

We have investigated the plasmonic properties of heterostructures that consist of gold nanosphere (NSs) with average diameters of 60 nm, 40 nm and 20 nm on poly (ethylene glycol) (PEG) brushes by using ultrafast pump-probe spectroscopy experiments. Gold NSs start to behave like gold nanorods with increasing number of immobilization cycles due to the close proximity. Gold NSs immobilized by 3 and 5 deposition cycles show longitudinal modes of plasmon bands at long wavelengths which are characteristic behaviors for gold nanorods. Increasing the number of immobilization cycle also increase relaxation times of samples due to the close proximity. Linear absorption spectra and scanning electron microscopy images show that there are close packing assemblies for heterostructures containing 20 nm gold NSs as the small particle. Ultrafast electron transfer (<100 fs) occurs between transverse and longitudinal modes by exciting the samples at both 520 nm and 650 nm. Further, experimental results indicate that, heterostructures with the small particles have faster relaxation times than other heterostructures due to closed packing of 20 nm gold NSs.

WS₂ as a saturable absorber for ultrafast photonic applications of mode-locked and Q-switched lasers.

PubMed

Wu, Kan; Zhang, Xiaoyan; Wang, Jun; Li, Xing; Chen, Jianping

2015-05-04

Two-dimensional (2D) nanomaterials, especially the transition metal sulfide semiconductors, have drawn great interests due to their potential applications in viable photonic and optoelectronic devices. In this work, 2D tungsten disulfide (WS2) based saturable absorber (SA) for ultrafast photonic applications was demonstrated. WS2 nanosheets were prepared using liquid-phase exfoliation method and embedded in polyvinyl alcohol (PVA) thin film for the practical usage. Saturable absorption was discovered in the WS2-PVA SA at the telecommunication wavelength near 1550 nm. By incorporating WS2-PVA SA into a fiber laser cavity, both stable mode locking operation and Q-switching operation were achieved. In the mode locking operation, the laser obtained femtosecond output pulse width and high spectral purity in the radio frequency spectrum. In the Q-switching operation, the laser had tunable repetition rate and output pulse energy of a few tens of nano joule. Our findings suggest that few-layer WS2 nanosheets embedded in PVA thin film are promising nonlinear optical materials for ultrafast photonic applications as a mode locker or Q-switcher.

Ultrafast time scale X-rotation of cold atom storage qubit using Rubidium clock states

NASA Astrophysics Data System (ADS)

Song, Yunheung; Lee, Han-Gyeol; Kim, Hyosub; Jo, Hanlae; Ahn, Jaewook

2017-04-01

Ultrafast-time-scale optical interaction is a local operation on the electronic subspace of an atom, thus leaving its nuclear state intact. However, because atomic clock states are maximally entangled states of the electronic and nuclear degrees of freedom, their entire Hilbert space should be accessible only with local operations and classical communications (LOCC). Therefore, it may be possible to achieve hyperfine qubit gates only with electronic transitions. Here we show an experimental implementation of ultrafast X-rotation of atomic hyperfine qubits, in which an optical Rabi oscillation induces a geometric phase between the constituent fine-structure states, thus bringing about the X-rotation between the two ground hyperfine levels. In experiments, cold atoms in a magneto-optical trap were controlled with a femtosecond laser pulse from a Ti:sapphire laser amplifier. Absorption imaging of the as-controlled atoms initially in the ground hyperfine state manifested polarization dependence, strongly agreeing with the theory. The result indicates that single laser pulse implementations of THz clock speed qubit controls are feasible for atomic storage qubits. Samsung Science and Technology Foundation [SSTF-BA1301-12].

Existence of a new emitting singlet state of proflavine: femtosecond dynamics of the excited state processes and quantum chemical studies in different solvents.

PubMed

Kumar, Karuppannan Senthil; Selvaraju, Chellappan; Malar, Ezekiel Joy Padma; Natarajan, Paramasivam

2012-01-12

Proflavine (3,6-diaminoacridine) shows fluorescence emission with lifetime, 4.6 ± 0.2 ns, in all the solvents irrespective of the solvent polarity. To understand this unusual photophysical property, investigations were carried out using steady state and time-resolved fluorescence spectroscopy in the pico- and femtosecond time domain. Molecular geometries in the ground and low-lying excited states of proflavine were examined by complete structural optimization using ab initio quantum chemical computations at HF/6-311++G** and CIS/6-311++G** levels. Time dependent density functional theory (TDDFT) calculations were performed to study the excitation energies in the low-lying excited states. The steady state absorption and emission spectral details of proflavine are found to be influenced by solvents. The femtosecond fluorescence decay of the proflavine in all the solvents follows triexponential function with two ultrafast decay components (τ(1) and τ(2)) in addition to the nanosecond component. The ultrafast decay component, τ(1), is attributed to the solvation dynamics of the particular solvent used. The second ultrafast decay component, τ(2), is found to vary from 50 to 215 ps depending upon the solvent. The amplitudes of the ultrafast decay components vary with the wavelength and show time dependent spectral shift in the emission maximum. The observation is interpreted that the time dependent spectral shift is not only due to solvation dynamics but also due to the existence of more than one emitting state of proflavine in the solvent used. Time resolved area normalized emission spectral (TRANES) analysis shows an isoemissive point, indicating the presence of two emitting states in homogeneous solution. Detailed femtosecond fluorescence decay analysis allows us to isolate the two independent emitting components of the close lying singlet states. The CIS and TDDFT calculations also support the existence of the close lying emitting states. The near constant lifetime observed for proflavine in different solvents is suggested to be due to the similar dipole moments of the ground and the evolved emitting singlet state of the dye from the Franck-Condon excited state.

First-principles electron dynamics control simulation of diamond under femtosecond laser pulse train irradiation.

PubMed

Wang, Cong; Jiang, Lan; Wang, Feng; Li, Xin; Yuan, Yanping; Xiao, Hai; Tsai, Hai-Lung; Lu, Yongfeng

2012-07-11

A real-time and real-space time-dependent density functional is applied to simulate the nonlinear electron-photon interactions during shaped femtosecond laser pulse train ablation of diamond. Effects of the key pulse train parameters such as the pulse separation, spatial/temporal pulse energy distribution and pulse number per train on the electron excitation and energy absorption are discussed. The calculations show that photon-electron interactions and transient localized electron dynamics can be controlled including photon absorption, electron excitation, electron density, and free electron distribution by the ultrafast laser pulse train.

Plasmon-Exciton Coupling Interaction for Surface Catalytic Reactions.

PubMed

Wang, Jingang; Lin, Weihua; Xu, Xuefeng; Ma, Fengcai; Sun, Mengtao

2018-05-01

In this review, we firstly reveal the physical principle of plasmon-exciton coupling interaction with steady absorption spectroscopy, and ultrafast transition absorption spectroscopy, based on the pump-prop technology. Secondly, we introduce the fabrication of electro-optical device of two-dimensional semiconductor-nanostructure noble metals hybrid, based on the plasmon-exciton coupling interactions. Thirdly, we introduce the applications of plasmon-exciton coupling interaction in the field of surface catalytic reactions. Lastly, the perspective of plasmon-exciton coupling interaction and applications closed this review. © 2018 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Photosensitive function of encapsulated dye in carbon nanotubes.

PubMed

Yanagi, Kazuhiro; Iakoubovskii, Konstantin; Matsui, Hiroyuki; Matsuzaki, Hiroyuki; Okamoto, Hiroshi; Miyata, Yasumitsu; Maniwa, Yutaka; Kazaoui, Said; Minami, Nobutsugu; Kataura, Hiromichi

2007-04-25

Single-wall carbon nanotubes (SWCNTs) exhibit resonant absorption localized in specific spectral regions. To expand the light spectrum that can be utilized by SWCNTs, we have encapsulated squarylium dye into SWCNTs and clarified its microscopic structure and photosensitizing function. X-ray diffraction and polarization-resolved optical absorption measurements revealed that the encapsulated dye molecules are located at an off center position inside the tubes and aligned to the nanotube axis. Efficient energy transfer from the encapsulated dye to SWCNTs was clearly observed in the photoluminescence spectra. Enhancement of transient absorption saturation in the S1 state of the semiconducting SWCNTs was detected after the photoexcitation of the encapsulated dye, which indicates that ultrafast (<190 fs) energy transfer occurred from the dye to the SWCNTs.

Ultrafast Exciton Delocalization, Localization, and Excimer Formation Dynamics in a Highly Defined Perylene Bisimide Quadruple π-Stack.

PubMed

Kaufmann, Christina; Kim, Woojae; Nowak-Król, Agnieszka; Hong, Yongseok; Kim, Dongho; Würthner, Frank

2018-03-28

An adequately designed, bay-tethered perylene bisimide (PBI) dimer Bis-PBI was synthesized by Pd/Cu-catalyzed Glaser-type oxidative homocoupling of the respective PBI building block. This newly synthesized PBI dimer self-assembles exclusively and with high binding constants of up to 10 6 M -1 into a discrete π-stack of four chromophores. Steady-state absorption and emission spectra show the signatures of H-type excitonic coupling among the dye units. Broadband fluorescence upconversion spectroscopy (FLUPS) reveals an ultrafast dynamics in the optically excited state. An initially coherent Frenkel exciton state that is delocalized over the whole quadruple stack rapidly (τ = ∼200 fs) loses its coherence and relaxes into an excimer state. Comparison with Frenkel exciton dynamics in PBI dimeric and oligomeric H-aggregates demonstrates that in the quadruple stack coherent exciton propagation is absent due to its short length of aggregates, thereby it has only one relaxation pathway to the excimer state. Furthermore, the absence of pump-power dependence in transient absorption experiments suggests that multiexciton cannot be generated in the quadruple stack, which is in line with time-resolved fluorescence measurements.

Ultrafast photodynamics of the indoline dye D149 adsorbed to porous ZnO in dye-sensitized solar cells.

PubMed

Rohwer, Egmont; Richter, Christoph; Heming, Nadine; Strauch, Kerstin; Litwinski, Christian; Nyokong, Tebello; Schlettwein, Derck; Schwoerer, Heinrich

2013-01-14

We investigate the ultrafast dynamics of the photoinduced electron transfer between surface-adsorbed indoline D149 dye and porous ZnO as used in the working electrodes of dye-sensitized solar cells. Transient absorption spectroscopy was conducted on the dye in solution, on solid state samples and for the latter in contact to a I(-)/I(3)(-) redox electrolyte typical for dye-sensitized solar cells to elucidate the effect of each component in the observed dynamics. D149 in a solution of 1:1 acetonitrile and tert-butyl alcohol shows excited-state lifetimes of 300±50 ps. This signature is severely quenched when D149 is adsorbed to ZnO, with the fastest component of the decay trace measured at 150±20 fs due to the charge-transfer mechanism. Absorption bands of the oxidized dye molecule were investigated to determine regeneration times which are in excess of 1 ns. The addition of the redox electrolyte to the system results in faster regeneration times, of the order of 1 ns. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Synchronous Measurement of Ultrafast Anisotropy Decay of the B850 in Bacterial LH2 Complex

NASA Astrophysics Data System (ADS)

Wang, Yun-Peng; Du, Lu-Chao; Zhu, Gang-Bei; Wang, Zhuan; Weng, Yu-Xiang

2015-02-01

Ultrafast anisotropic decay is a prominent parameter revealing ultrafast energy and electron transfer; however, it is difficult to be determined reliably owing to the requirement of a simultaneous availability of the parallel and perpendicular polarized decay kinetics. Nowadays, any measurement of anisotropic decay is a kind of approach to the exact simultaneity. Here we report a novel method for a synchronous ultrafast anisotropy decay measurement, which can well determine the anisotropy, even at a very early time, as the rising phase of the excitation laser pulse. The anisotropic decay of the B850 in bacterial light harvesting antenna complex LH2 of Rhodobacter sphaeroides in solution at room temperature with coherent excitation is detected by this method, which shows a polarization response time of 30 fs, and the energy transfer from the initial excitation to the bacteriochlorophylls in B850 ring takes about 70 fs. The anisotropic decay that is probed at the red side of the absorption spectrum, such as 880 nm, has an initial value of 0.4, corresponding to simulated emission, while the blue side with an anisotropy of 0.1 contributes to the ground-state bleaching. Our results show that the coherent excitation covering the whole ring might not be realized owing to the symmetry breaking of LH2: from C9 symmetry in membrane to C2 symmetry in solution.

IR-Driven Ultrafast Transfer of Plasmonic Hot Electrons in Nonmetallic Branched Heterostructures for Enhanced H2 Generation.

PubMed

Zhang, Zhenyi; Jiang, Xiaoyi; Liu, Benkang; Guo, Lijiao; Lu, Na; Wang, Li; Huang, Jindou; Liu, Kuichao; Dong, Bin

2018-03-01

The ultrafast transfer of plasmon-induced hot electrons is considered an effective kinetics process to enhance the photoconversion efficiencies of semiconductors through strong localized surface plasmon resonance (LSPR) of plasmonic nanostructures. Although this classical sensitization approach is widely used in noble-metal-semiconductor systems, it remains unclear in nonmetallic plasmonic heterostructures. Here, by combining ultrafast transient absorption spectroscopy with theoretical simulations, IR-driven transfer of plasmon-induced hot electron in a nonmetallic branched heterostructure is demonstrated, which is fabricated through solvothermal growth of plasmonic W 18 O 49 nanowires (as branches) onto TiO 2 electrospun nanofibers (as backbones). The ultrafast transfer of hot electron from the W 18 O 49 branches to the TiO 2 backbones occurs within a timeframe on the order of 200 fs with very large rate constants ranging from 3.8 × 10 12 to 5.5 × 10 12 s -1 . Upon LSPR excitation by low-energy IR photons, the W 18 O 49 /TiO 2 branched heterostructure exhibits obviously enhanced catalytic H 2 generation from ammonia borane compared with that of W 18 O 49 nanowires. Further investigations by finely controlling experimental conditions unambiguously confirm that this plasmon-enhanced catalytic activity arises from the transfer of hot electron rather than from the photothermal effect. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ultrafast spectroscopic investigation of a fullerene poly(3-hexylthiophene) dyad

NASA Astrophysics Data System (ADS)

Banerji, Natalie; Seifter, Jason; Wang, Mingfeng; Vauthey, Eric; Wudl, Fred; Heeger, Alan J.

2011-08-01

We present the femtosecond spectroscopic investigation of a covalently linked dyad, PCB-P3HT, formed by a segment of the conjugated polymer P3HT (regioregular poly(3-hexylthiophene)) that is end capped with the fullerene derivative PCB ([6,6]-phenyl-C61-butyric acid ester), adapted from PCBM. The fluorescence of the P3HT segment in tetrahydrofuran (THF) solution is reduced by 64% in the dyad compared to a control compound without attached fullerene (P3HT-OH). Fluorescence upconversion measurements reveal that the partial fluorescence quenching of PCB-P3HT in THF is multiphasic and occurs on an average time scale of 100 ps, in parallel to excited-state relaxation processes. Judging from ultrafast transient absorption experiments, the origin of the quenching is excitation energy transfer from the P3HT donor to the PCB acceptor. Due to the much higher solubility of P3HT compared to PCB in THF, the PCB-P3HT dyad molecules self-assemble into micelles. When pure C60 is added to the solution, it is incorporated into the fullerene-rich center of the micelles. This dramatically increases the solubility of C60 but does not lead to significant additional quenching of the P3HT fluorescence by the C60 contained in the micelles. In PCB-P3HT thin films drop-cast from THF, the micelle structure is conserved. In contrast to solution, quantitative and ultrafast (<150 fs) charge separation occurs in the solid-state films and leads to the formation of long-lived mobile charge carriers with characteristic transient absorption signatures similar to those that have been observed in P3HT:PCBM bulk heterojunction blends. While π-stacking interactions between neighboring P3HT chains are weak in the micelles, they are strong in thin films drop-cast from ortho-dichlorobenzene. Here, PCB-P3HT self-assembles into a network of long fibers, clearly seen in atomic force microscopy images. Ultrafast charge separation occurs also for the fibrous morphology, but the transient absorption experiments show fast loss of part of the charge carriers due to intensity-induced recombination and annihilation processes and monomolecular interfacial trap-mediated or geminate recombination. The yield of the long-lived charge carriers in the highly organized fibers is however comparable to that obtained with annealed P3HT:PCBM blends. PCB-P3HT can therefore be considered as an active material in organic photovoltaic devices.

Broadband ultrafast photoprotection by oxybenzone across the UVB and UVC spectral regions.

PubMed

Baker, Lewis A; Horbury, Michael D; Greenough, Simon E; Ashfold, Michael N R; Stavros, Vasilios G

2015-10-01

Recent studies have shed light on the energy dissipation mechanism of oxybenzone, a common ingredient in commercial sunscreens. After UVA photoexcitation, the dissipation mechanism may be understood in terms of an initial ultrafast excited state enol → keto tautomerisation, followed by nonadiabatic transfer to the ground electronic state and subsequent collisional relaxation to the starting enol tautomer. We expand on these studies using femtosecond transient electronic absorption spectroscopy to understand the non-radiative relaxation pathways of oxybenzone in cyclohexane and in methanol after UVB and UVC excitation. We find that the relaxation pathway may be understood in the same way as when exciting in the UVA region, concluding that oxybenzone displays proficient broadband non-radiative photoprotection, and thus photophysically justifying its inclusion in sunscreen mixtures.

Ultrafast dynamics of colloidal semiconductor nanocrystals relevant to solar fuels production

NASA Astrophysics Data System (ADS)

Cogan, Nicole M. B.; Liu, Cunming; Qiu, Fen; Burke, Rebeckah; Krauss, Todd D.

2017-05-01

Artificial conversion of sunlight to chemical fuels has attracted attention for several decades as a potential source of clean, renewable energy. We recently found that CdSe quantum dots (QDs) and simple aqueous Ni2+ salts in the presence of a sacrificial electron donor form a highly efficient, active, and robust system for photochemical reduction of protons to molecular hydrogen. Ultrafast transient absorption spectroscopy studies of electron transfer (ET) processes from the QDs to the Ni catalysts reveal extremely fast ET, and provide a fundamental explanation for the exceptional photocatalytic H2 activity. Additionally, by studying H2 production of the Ni catalyst with CdSe/CdS nanoparticles of various structures, it was determined that surface charge density plays an important role in charge transfer and ultimately H2 production activity.

Redox Conditions Affect Ultrafast Exciton Transport in Photosynthetic Pigment-Protein Complexes.

PubMed

Allodi, Marco A; Otto, John P; Sohail, Sara H; Saer, Rafael G; Wood, Ryan E; Rolczynski, Brian S; Massey, Sara C; Ting, Po-Chieh; Blankenship, Robert E; Engel, Gregory S

2018-01-04

Pigment-protein complexes in photosynthetic antennae can suffer oxidative damage from reactive oxygen species generated during solar light harvesting. How the redox environment of a pigment-protein complex affects energy transport on the ultrafast light-harvesting time scale remains poorly understood. Using two-dimensional electronic spectroscopy, we observe differences in femtosecond energy-transfer processes in the Fenna-Matthews-Olson (FMO) antenna complex under different redox conditions. We attribute these differences in the ultrafast dynamics to changes to the system-bath coupling around specific chromophores, and we identify a highly conserved tyrosine/tryptophan chain near the chromophores showing the largest changes. We discuss how the mechanism of tyrosine/tryptophan chain oxidation may contribute to these differences in ultrafast dynamics that can moderate energy transfer to downstream complexes where reactive oxygen species are formed. These results highlight the importance of redox conditions on the ultrafast transport of energy in photosynthesis. Tailoring the redox environment may enable energy transport engineering in synthetic light-harvesting systems.

Ultrafast-Acting Insulins: State of the Art

PubMed Central

Heinemann, Lutz; Muchmore, Douglas B.

2012-01-01

Optimal coverage of prandial insulin requirements remains an elusive goal. The invention of rapid-acting insulin analogs (RAIAs) was a big step forward in reducing postprandial glycemic excursions in patients with diabetes in comparison with using regular human insulin; however, even with these, the physiological situation cannot be adequately mimicked. Developing ultrafast-acting insulins (UFIs)—showing an even more rapid onset of action and a shorter duration of action after subcutaneous (SC) administration—is another step forward in achieving this goal. The need for UFIs has been gradually recognized over the years, and subsequently, a number of different approaches to cover this need are in clinical development. A rapid increase in circulating insulin levels can be achieved by different measures: modification of the primary structure of insulin molecule (as we know from RAIAs), addition of excipients that enhance the appearance in the monomeric state post-injection, or addition of enzymes that enable more free spreading of the insulin molecules in the SC tissue. Other measures to increase the insulin absorption rate increase the local blood flow nearby the insulin depot in the SC tissue, injecting the insulin intradermally or applying via another route, e.g., the lung. The development of these approaches is in different stages, from quite early stages to nearing market authorization. In time, daily practice will show if the introduction of UFIs will fulfill their clinical promise. In this review, the basic idea for UFIs will be presented and the different approaches will be briefly characterized. PMID:22920797

Exciplex formation in bimolecular photoinduced electron-transfer investigated by ultrafast time-resolved infrared spectroscopy.

PubMed

Koch, Marius; Letrun, Romain; Vauthey, Eric

2014-03-12

The dynamics of bimolecular photoinduced electron-transfer reactions has been investigated with three donor/acceptor (D/A) pairs in tetrahydrofuran (THF) and acetonitrile (ACN) using a combination of ultrafast spectroscopic techniques, including time-resolved infrared absorption. For the D/A pairs with the highest driving force of electron transfer, all transient spectroscopic features can be unambiguously assigned to the excited reactant and the ionic products. For the pair with the lowest driving force, three additional transient infrared bands, more intense in THF than in ACN, with a time dependence that differs from those of the other bands are observed. From their frequency and solvent dependence, these bands can be assigned to an exciplex. Moreover, polarization-resolved measurements point to a relatively well-defined mutual orientation of the constituents and to a slower reorientational time compared to those of the individual reactants. Thanks to the minimal overlap of the infrared signature of all transient species in THF, a detailed reaction scheme including the relevant kinetic and thermodynamic parameters could be deduced for this pair. This analysis reveals that the formation and recombination of the ion pair occur almost exclusively via the exciplex.

Solution Phase Exciton Diffusion Dynamics of a Charge-Transfer Copolymer PTB7 and a Homopolymer P3HT

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cho, Sung; Rolczynski, Brian S.; Xu, Tao

2015-06-18

Using ultrafast polarization-controlled transient absorption (TA) measurements, dynamics of the initial exciton states were investigated on the time scale of tens of femtoseconds to about 80 ps in two different types of conjugated polymers extensively used in active layers of organic photovoltaic devices. These polymers are poly(3-fluorothienothiophenebenzodithiophene) (PTB7) and poly-3-hexylthiophene (P3HT), which are charge-transfer polymers and homopolymers, respectively. In PTB7, the initial excitons with excess vibrational energy display two observable ultrafast time constants, corresponding to coherent exciton diffusion before the vibrational relaxation, and followed by incoherent exciton diffusion processes to a neighboring local state after the vibrational relaxation. In contrast,more » P3HT shows only one exciton diffusion or conformational motion time constant of 34 ps, even though its exciton decay kinetics are multiexponential. Based on the experimental results, an exciton dynamics mechanism is conceived taking into account the excitation energy and structural dependence in coherent and incoherent exciton diffusion processes, as well as other possible deactivation processes including the formation of the pseudo-charge-transfer and charge separate states, as well as interchain exciton hopping or coherent diffusion.« less

Solution Phase Exciton Diffusion Dynamics of a Charge-Transfer Copolymer PTB7 and a Homopolymer P3HT.

PubMed

Cho, Sung; Rolczynski, Brian S; Xu, Tao; Yu, Luping; Chen, Lin X

2015-06-18

Using ultrafast polarization-controlled transient absorption (TA) measurements, dynamics of the initial exciton states were investigated on the time scale of tens of femtoseconds to about 80 ps in two different types of conjugated polymers extensively used in active layers of organic photovoltaic devices. These polymers are poly(3-fluorothienothiophenebenzodithiophene) (PTB7) and poly-3-hexylthiophene (P3HT), which are charge-transfer polymers and homopolymers, respectively. In PTB7, the initial excitons with excess vibrational energy display two observable ultrafast time constants, corresponding to coherent exciton diffusion before the vibrational relaxation, and followed by incoherent exciton diffusion processes to a neighboring local state after the vibrational relaxation. In contrast, P3HT shows only one exciton diffusion or conformational motion time constant of 34 ps, even though its exciton decay kinetics are multiexponential. Based on the experimental results, an exciton dynamics mechanism is conceived taking into account the excitation energy and structural dependence in coherent and incoherent exciton diffusion processes, as well as other possible deactivation processes including the formation of the pseudo-charge-transfer and charge separate states, as well as interchain exciton hopping or coherent diffusion.

Ultrafast photoinduced dynamics of the 3,6-diaminoacridinium derivative ATTO 465 in solution.

PubMed

Arden-Jacob, Jutta; Drexhage, Karl-Heinz; Druzhinin, Sergey I; Ekimova, Maria; Flender, Oliver; Lenzer, Thomas; Oum, Kawon; Scholz, Mirko

2013-02-14

The excited state dynamics of the dye ATTO 465, a well-known fluorescence marker for biological applications, have been characterized in various solvents including THF, ethanol, methanol, water and the highly polar protic ionic liquid 2-hydroxyethylammonium formate (2-OH-EAF) by combining results from time-correlated single-photon counting (TCSPC) and ultrafast pump-supercontinuum probe (PSCP) spectroscopy as well as steady-state absorption and fluorescence. In water, 2-OH-EAF and two fluorinated alcohols, there is a pronounced blue-shift and broadening of the S(0) → S(1) absorption band and also a larger Stokes shift than in the other solvents, indicating a particular influence of hydrogen-bonding interactions. S(1) lifetimes from TCSPC at 25 °C range from 3.3 ns to 5.6 ns. An unusual increase in the S(1) lifetime with temperature is observed for ethanol and methanol, however water behaves in the opposite way. The behavior can be tentatively explained by a solvent- and temperature-dependent "proximity effect", where coupling of the close-lying S(1) and S(2) states influences the intramolecular relaxation rate of the dye. In addition, temperature-dependent complex equilibria of ATTO 465 with solvent molecules may influence the measured lifetimes. Several excited-state absorption (ESA) transitions are identified in the PSCP spectra, which are in good agreement with the position of the UV bands in the steady-state absorption spectra. Small shifts of the stimulated emission and ESA bands are consistent with solvation dynamics in the excited electronic state. An additional ~16 ps component in water, visible over the entire spectral range, is tentatively ascribed to a fast IC channel which is accessed by a fraction of ATTO 465 molecules.

Ultrafast relaxation dynamics of amine-substituted bipyridyl ruthenium(II) complexes

NASA Astrophysics Data System (ADS)

Song, Hongwei; Wang, Xian; Yang, WenWen; He, Guiying; Kuang, Zhuoran; Li, Yang; Xia, Andong; Zhong, Yu-Wu; Kong, Fan'ao

2017-09-01

The excited state properties of a series of ruthenium(II) amine-substituted bipyridyl complexes, [Ru(bpy)n(NNbpy)3-n]2+, were investigated by steady-state and transient absorption spectroscopy, as well as quantum chemical calculations. The steady-state absorption spectra of these complexes in CH3CN show a distinct red-shift of the 1MLCT absorption with increasing numbers of amine substituent, whereas the emission spectra indicate an energy gap order of [Ru(bpy)3]2+ > [Ru(bpy)2(NNbpy)]2+ > [Ru(NNbpy)3]2+ > [Ru(bpy)(NNbpy)2]2+. Nanosecond, femtosecond transient absorption and electrochemical measurements suggest that NNbpy ligand has a strong influence on the electronic and emission properties of these complexes, due to electron-rich amine substituent. We illustrate how the numbers of amine substituent modulate the spectroscopic properties of transition metal complexes, which is related to the design of new electro-active systems with novel photoelectrochemical properties.

Physical mechanisms of SiN{sub x} layer structuring with ultrafast lasers by direct and confined laser ablation

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rapp, S., E-mail: rapp@hm.edu; Erlangen Graduate School in Advanced Optical Technologies; Heinrich, G.

2015-03-14

In the production process of silicon microelectronic devices and high efficiency silicon solar cells, local contact openings in thin dielectric layers are required. Instead of photolithography, these openings can be selectively structured with ultra-short laser pulses by confined laser ablation in a fast and efficient lift off production step. Thereby, the ultrafast laser pulse is transmitted by the dielectric layer and absorbed at the substrate surface leading to a selective layer removal in the nanosecond time domain. Thermal damage in the substrate due to absorption is an unwanted side effect. The aim of this work is to obtain a deepermore » understanding of the physical laser-material interaction with the goal of finding a damage-free ablation mechanism. For this, thin silicon nitride (SiN{sub x}) layers on planar silicon (Si) wafers are processed with infrared fs-laser pulses. Two ablation types can be distinguished: The known confined ablation at fluences below 300 mJ/cm{sup 2} and a combined partial confined and partial direct ablation at higher fluences. The partial direct ablation process is caused by nonlinear absorption in the SiN{sub x} layer in the center of the applied Gaussian shaped laser pulses. Pump-probe investigations of the central area show ultra-fast reflectivity changes typical for direct laser ablation. Transmission electron microscopy results demonstrate that the Si surface under the remaining SiN{sub x} island is not damaged by the laser ablation process. At optimized process parameters, the method of direct laser ablation could be a good candidate for damage-free selective structuring of dielectric layers on absorbing substrates.« less

Ultrafast charge transfer between MoTe2 and MoS2 monolayers

NASA Astrophysics Data System (ADS)

Pan, Shudi; Ceballos, Frank; Bellus, Matthew Z.; Zereshki, Peymon; Zhao, Hui

2017-03-01

High quality and stable electrical contact between metal and two-dimensional materials, such as transition metal dichalcogenides, is a necessary requirement that has yet to be achieved in order to successfully exploit the advantages that these materials offer to electronics and optoelectronics. MoTe2, owing to its phase changing property, can potentially offer a solution. A recent study demonstrated that metallic phase of MoTe2 connects its semiconducting phase with very low resistance. To utilize this property to connect other two-dimensional materials, it is important to achieve efficient charge transfer between MoTe2 and other semiconducting materials. Using MoS2 as an example, we report ultrafast and efficient charge transfer between MoTe2 and MoS2 monolayers. In the transient absorption measurements, an ultrashort pump pulse is used to selectively excite electrons in MoTe2. The appearance of the excited electrons in the conduction band of MoS2 is monitored by using a probe pulse that is tuned to the resonance of MoS2. We found that electrons transfer to MoS2 on a time scale of at most 0.3 ps. The transferred electrons give rise to a large transient absorption signal at both A-exciton and B-exciton resonances due to the screening effect. We also observed ultrafast transfer of holes from MoS2 to MoTe2. Our results suggest the feasibility of using MoTe2 as a bridge material to connect MoS2 and other transition metal dichalcogenides, and demonstrate a new transition metal dichalcogenide heterostructure involving MoTe2, which extends the spectral range of such structures to infrared.

Ultrafast optical switching in three-dimensional photonic crystals

NASA Astrophysics Data System (ADS)

Mazurenko, D. A.

2004-09-01

The rapidly expanding research on photonic crystals is driven by potential applications in all-optical switches, optical computers, low-threshold lasers, and holographic data storage. The performance of such devices might surpass the speed of traditional electronics by several orders of magnitude and may result in a true revolution in nanotechnology. The heart of such devices would likely be an optical switching element. This thesis analyzes different regimes of ultrafast all-optical switching in various three-dimensional photonic crystals, in particular opals filled with silicon or vanadium dioxide and periodic arrays of silica-gold core-shell spherical particles with silica outer shell. In the experiment an ultrashort optical pulse is used to excite a photonic crystal and change its complex effective dielectric constant. The change in the imaginary part of the dielectric constant corresponds to the change in absorption that suppresses interference inside the photonic crystal and modifies the amplitude of the reflectivity, while the change in the real part of the dielectric constant accounts for a shift in a spectral position of the photonic stop band. The first type of switching is shown on an example of an opal filled with silicon. In this crystal, switching is induced by photo-excited carriers in silicon that act as an electron plasma and increase the absorption in silicon. Within 30 fs constructive interference inside the opal vanishes and Bragg reflectivity drops down. Changes in reflectivity reach values as high as 46% at maximum excitation power. The experimental results are in a good agreement with calculations. The second type of switching is demonstrated in opal filled with vanadium dioxide. Here, the optical switching is driven by a photoinduced phase transition of vanadium dioxide. The phase transition takes place on a subpicosecond time scale and changes the effective dielectric constant of the opal. As a result, the spectral position of the photonic stop band shifts to the blue leading to large (up to 35%) changes in the reflectivity. Metallo-dielectric photonic crystals give even more possibilities for the band-tuning, since in addition to the resonance for light they posses surface plasmon resonances. The interplay of these resonances leads to unusual optical phenomena. As an example, reflected light produces an unexpected beaming in the apexes of a hexagon with a divergence angle of 8°, in our sample. This angle is too small to be attributed to a simple diffraction on the periodic lattice of core-shells but can be explained by interference between surface plasmons and propagating surface waves. Time-resolved spectra demonstrate rapid changes immediately after the arrival of the pump pulse. Ultrafast reflection changes are dramatically enhanced by the plasmon resonances, and can reach values as high as 35%. A completely different mechanism for ultrafast switching is explored, based on the excitation of coherent acoustic radial vibrations of the gold spheres. This results in a 4% modulation of the reflectivity on a subnanosecond timescale. The observed oscillation properties of our gold-shell spheres are in excellent agreement with the calculations. The described results show that the demonstrated dynamical changes in the reflectivity of a three-dimensional photonic crystal can be made both large and ultrafast and therefore may prove to be relevant for future applications.

Ultrafast studies of the excited-state dynamics of copper and nickel phthalocyanine tetrasulfonates: potential sensitizers for the two-photon photodynamic therapy of tumors.

PubMed

Fournier, Michel; Pépin, Claude; Houde, Daniel; Ouellet, René; van Lier, Johan E

2004-01-01

In order to evaluate the potential of copper and nickel phthalocyanine tetrasulfonates as sensitizers for two-photon photodynamic therapy, we conducted kinetic femtosecond measurements of transient absorption and bleaching of their excited state dynamics in aqueous solution. Samples were pumped with 620 nm and 310 nm laser light, which allowed us to study relaxation processes from both the first and second singlet (or doublet for the copper phthalocyanine) excited states. A second excitation from the first excited triplet state, approximately 685 and 105 ps after the first excitation for copper and nickel phthalocyanine tetrasulfonate respectively, was the most efficient way to bring the molecules to an upper triplet state. Presumably this highest triplet state can inflict molecular damage on adjacent biomolecules int eh absence of oxygen, resulting in the desired cytotoxic cellular response. Transient absorption spectra at different fixed delays indicate that optimum efficiency would require that the second photon has a wavelength of approximately 750 nm.

Magnetic-field-induced crossover from the inverse Faraday effect to the optical orientation in EuTe

NASA Astrophysics Data System (ADS)

Pavlov, V. V.; Pisarev, R. V.; Nefedov, S. G.; Akimov, I. A.; Yakovlev, D. R.; Bayer, M.; Henriques, A. B.; Rappl, P. H. O.; Abramof, E.

2018-05-01

A time-resolved optical pump-probe technique has been applied for studying the ultrafast dynamics in the magnetic semiconductor EuTe near the absorption band gap. We show that application of external magnetic field up to 6 T results in crossover from the inverse Faraday effect taking place on the femtosecond time scale to the optical orientation phenomenon with an evolution in the picosecond time domain. We propose a model which includes both these processes, possessing different spectral and temporal properties. The circularly polarized optical pumping induces the electronic transition 4 f 7 5 d 0 → 4 f 6 5 d 1 forming the absorption band gap in EuTe. The observed crossover is related to a strong magnetic-field shift of the band gap in EuTe at low temperatures. It was found that manipulation of spin states on intrinsic defect levels takes place on a time scale of 19 ps in the applied magnetic field of 6 T.

Confocal spectroscopic imaging measurements of depth dependent hydration dynamics in human skin in-vivo

NASA Astrophysics Data System (ADS)

Behm, P.; Hashemi, M.; Hoppe, S.; Wessel, S.; Hagens, R.; Jaspers, S.; Wenck, H.; Rübhausen, M.

2017-11-01

We present confocal spectroscopic imaging measurements applied to in-vivo studies to determine the depth dependent hydration profiles of human skin. The observed spectroscopic signal covers the spectral range from 810 nm to 2100 nm allowing to probe relevant absorption signals that can be associated with e.g. lipid and water-absorption bands. We employ a spectrally sensitive autofocus mechanism that allows an ultrafast focusing of the measurement spot on the skin and subsequently probes the evolution of the absorption bands as a function of depth. We determine the change of the water concentration in m%. The water concentration follows a sigmoidal behavior with an increase of the water content of about 70% within 5 μm in a depth of about 14 μm. We have applied our technique to study the hydration dynamics of skin before and after treatment with different concentrations of glycerol indicating that an increase of the glycerol concentration leads to an enhanced water concentration in the stratum corneum. Moreover, in contrast to traditional corneometry we have found that the application of Aluminium Chlorohydrate has no impact to the hydration of skin.

Spin-vibronic quantum dynamics for ultrafast excited-state processes.

PubMed

Eng, Julien; Gourlaouen, Christophe; Gindensperger, Etienne; Daniel, Chantal

2015-03-17

Ultrafast intersystem crossing (ISC) processes coupled to nuclear relaxation and solvation dynamics play a central role in the photophysics and photochemistry of a wide range of transition metal complexes. These phenomena occurring within a few hundred femtoseconds are investigated experimentally by ultrafast picosecond and femtosecond transient absorption or luminescence spectroscopies, and optical laser pump-X-ray probe techniques using picosecond and femtosecond X-ray pulses. The interpretation of ultrafast structural changes, time-resolved spectra, quantum yields, and time scales of elementary processes or transient lifetimes needs robust theoretical tools combining state-of-the-art quantum chemistry and developments in quantum dynamics for solving the electronic and nuclear problems. Multimode molecular dynamics beyond the Born-Oppenheimer approximation has been successfully applied to many small polyatomic systems. Its application to large molecules containing a transition metal atom is still a challenge because of the nuclear dimensionality of the problem, the high density of electronic excited states, and the spin-orbit coupling effects. Rhenium(I) α-diimine carbonyl complexes, [Re(L)(CO)3(N,N)](n+) are thermally and photochemically robust and highly flexible synthetically. Structural variations of the N,N and L ligands affect the spectroscopy, the photophysics, and the photochemistry of these chromophores easily incorporated into a complex environment. Visible light absorption opens the route to a wide range of applications such as sensors, probes, or emissive labels for imaging biomolecules. Halide complexes [Re(X)(CO)3(bpy)] (X = Cl, Br, or I; bpy = 2,2'-bipyridine) exhibit complex electronic structure and large spin-orbit effects that do not correlate with the heavy atom effects. Indeed, the (1)MLCT → (3)MLCT intersystem crossing (ISC) kinetics is slower than in [Ru(bpy)3](2+) or [Fe(bpy)3](2+) despite the presence of a third-row transition metal. Counterintuitively, singlet excited-state lifetime increases on going from Cl (85 fs) to Br (128 fs) and to I (152 fs). Moreover, correlation between the Re-X stretching mode and the rate of ISC is observed. In this Account, we emphasize on the role of spin-vibronic coupling on the mechanism of ultrafast ISC put in evidence in [Re(Br)(CO)3(bpy)]. For this purpose, we have developed a model Hamiltonian for solving an 11 electronic excited states multimode problem including vibronic and SO coupling within the linear vibronic coupling (LVC) approximation and the assumption of harmonic potentials. The presence of a central metal atom coupled to rigid ligands, such as α-diimine, ensures nuclear motion of small amplitudes and a priori justifies the use of the LVC model. The simulation of the ultrafast dynamics by wavepacket propagations using the multiconfiguration time-dependent Hartree (MCTDH) method is based on density functional theory (DFT), and its time-dependent extension to excited states (TD-DFT) electronic structure data. We believe that the interplay between time-resolved experiments and these pioneering simulations covering the first picoseconds and including spin-vibronic coupling will promote a number of quantum dynamical studies that will contribute to a better understanding of ultrafast processes in a wide range of organic and inorganic chromophores easily incorporated in biosystems or supramolecular devices for specific functions.

Ultrafast structural and electronic dynamics of the metallic phase in a layered manganite

PubMed Central

Piazza, L.; Ma, C.; Yang, H. X.; Mann, A.; Zhu, Y.; Li, J. Q.; Carbone, F.

2013-01-01

The transition between different states in manganites can be driven by various external stimuli. Controlling these transitions with light opens the possibility to investigate the microscopic path through which they evolve. We performed femtosecond (fs) transmission electron microscopy on a bi-layered manganite to study its response to ultrafast photoexcitation. We show that a photoinduced temperature jump launches a pressure wave that provokes coherent oscillations of the lattice parameters, detected via ultrafast electron diffraction. Their impact on the electronic structure are monitored via ultrafast electron energy loss spectroscopy, revealing the dynamics of the different orbitals in response to specific structural distortions. PMID:26913564

Ultra-fast Object Recognition from Few Spikes

DTIC Science & Technology

2005-07-06

Computer Science and Artificial Intelligence Laboratory Ultra-fast Object Recognition from Few Spikes Chou Hung, Gabriel Kreiman , Tomaso Poggio...neural code for different kinds of object-related information. *The authors, Chou Hung and Gabriel Kreiman , contributed equally to this work...Supplementary Material is available at http://ramonycajal.mit.edu/ kreiman /resources/ultrafast

Long-range hot-carrier transport in hybrid perovskites visualized by ultrafast microscopy

NASA Astrophysics Data System (ADS)

Guo, Zhi; Wan, Yan; Yang, Mengjin; Snaider, Jordan; Zhu, Kai; Huang, Libai

2017-04-01

The Shockley-Queisser limit for solar cell efficiency can be overcome if hot carriers can be harvested before they thermalize. Recently, carrier cooling time up to 100 picoseconds was observed in hybrid perovskites, but it is unclear whether these long-lived hot carriers can migrate long distance for efficient collection. We report direct visualization of hot-carrier migration in methylammonium lead iodide (CH3NH3PbI3) thin films by ultrafast transient absorption microscopy, demonstrating three distinct transport regimes. Quasiballistic transport was observed to correlate with excess kinetic energy, resulting in up to 230 nanometers transport distance that could overcome grain boundaries. The nonequilibrium transport persisted over tens of picoseconds and ~600 nanometers before reaching the diffusive transport limit. These results suggest potential applications of hot-carrier devices based on hybrid perovskites.

Ultrafast photophysical studies of a multicomponent sunscreen: Oxybenzone-titanium dioxide mixtures

NASA Astrophysics Data System (ADS)

Baker, Lewis A.; Grosvenor, Lucy C.; Ashfold, Michael N. R.; Stavros, Vasilios G.

2016-11-01

Recent studies of the sunscreen constituent oxybenzone have suggested that the dominant mechanism underlying the efficient photoprotection it offers relies on an initial ultrafast enol → keto tautomerisation, followed by nonadiabatic transfer to the ground electronic state. Subsequent collisions with the solvent bath then reform the original enol-tautomer. Utilising femtosecond transient electronic absorption spectroscopy we explore the dissipation of electronic excitation energy in oxybenzone in the presence of titanium dioxide, a widely used, and complementary sunscreen component. We find the relaxation dynamics of this popular organic filter are unaltered by the presence of this favoured inorganic scatterer and the overall dynamics can be described by the additive contribution of the individual constituents. The combination of the two components provides broadband photoprotective properties justifying the widely used organic filter and inorganic scatterer mixtures in commercial sunscreen products.

Theoretical ultra-fast spectroscopy in transition metal dichalcogenides

NASA Astrophysics Data System (ADS)

Molina-Sanchez, Alejandro; Sangalli, Davide; Marini, Andrea; Wirtz, Ludger

Semiconducting 2D-materials like the transition metal dichalcogenides (TMDs) MoS2, MoSe2, WS2, WSe2 are promising alternatives to graphene for designing novel opto-electronic devices. The strong spin-orbit interaction along with the breaking of inversion symmetry in single-layer TMDs allow using the valley-index as a new quantum number. The practical use of valley physics depends on the lifetimes of valley-polarized excitons which are affected by scattering at phonons, impurities and by carrier-carrier interactions. The carrier dynamics can be monitored using ultra-fast spectroscopies such as pump-probe experiments. The carrier dynamics is simulated using non-equilibrium Green's function theory in an ab-initio framework. We include carrier relaxation through electron-phonon interaction. We obtain the transient absorption spectra of single-layer TMD and compare our simulations with recent pump-probe experiments

Ultrafast myoglobin structural dynamics observed with an X-ray free-electron laser

DOE PAGES

Levantino, Matteo; Schirò, Giorgio; Lemke, Henrik Till; ...

2015-04-02

Light absorption can trigger biologically relevant protein conformational changes. The light induced structural rearrangement at the level of a photoexcited chromophore is known to occur in the femtosecond timescale and is expected to propagate through the protein as a quake-like intramolecular motion. Here we report direct experimental evidence of such ‘proteinquake’ observed in myoglobin through femtosecond X-ray solution scattering measurements performed at the Linac Coherent Light Source X-ray free-electron laser. An ultrafast increase of myoglobin radius of gyration occurs within 1 picosecond and is followed by a delayed protein expansion. As the system approaches equilibrium it undergoes damped oscillations withmore » a ~3.6-picosecond time period. Our results unambiguously show how initially localized chemical changes can propagate at the level of the global protein conformation in the picosecond timescale.« less

Influence of optical pumping wavelength on the ultrafast gain and phase recovery acceleration of quantum-dot semiconductor optical amplifiers

NASA Astrophysics Data System (ADS)

Kim, Jungho

2013-10-01

We numerically investigate the influence of the optical pumping wavelength on the ultrafast gain and phase recovery acceleration of quantum-dot (QD) semiconductor optical amplifiers (SOAs) by solving 1088 coupled rate equations. The temporal variations of the gain and phase recovery response at the ground state (GS) of QDs are calculated at various signal wavelengths when the optical pumping wavelengths at the excited state (ES) of QDs are varied. The phase recovery response is fastest when the wavelength of the signal and pumping beams corresponds to the respective emission wavelength of the GS and the ES in the same size of QDs. The absorption efficiency of the optical pumping beam at the ES is determined by the Lorentzian line shape function of the homogeneous broadening.

Ultrafast studies of gold, nickel, and palladium nanorods

NASA Astrophysics Data System (ADS)

Sando, Gerald M.; Berry, Alan D.; Owrutsky, Jeffrey C.

2007-08-01

Steady state and ultrafast transient absorption studies have been carried out for gold, nickel, and palladium high aspect ratio nanorods. For each metal, nanorods were fabricated by electrochemical deposition into ˜6μm thick polycarbonate templates. Two nominal pore diameters(10 and 30nm, resulting in nanorod diameters of about 40 and 60nm, respectively) were used, yielding nanorods with high aspect ratios (>25). Static spectra of nanorods of all three metals reveal both a longitudinal surface plasmon resonance (SPRL) band in the mid-infrared as well as a transverse band in the visible for the gold and larger diameter nickel and palladium nanorods. The appearance of SPRL bands in the infrared for high aspect ratio metal nanorods and the trends in their maxima for the different aspect ratios and metals are consistent with calculations based on the Gans theory. For the gold and nickel samples, time resolved studies were performed with a subpicosecond resolution using 400nm excitation and a wide range of probe wavelengths from the visible to the mid-IR as well as for infrared excitation (near 2000cm-1) probed at 800nm. The dynamics observed for nanorods of both metals and both diameters include transients due to electron-phonon coupling and impulsively excited coherent acoustic breathing mode oscillations, which are similar to those previously reported for spherical and smaller rod-shaped gold nanoparticles. The dynamics we observe are the same within the experimental uncertainty for 400nm and infrared (5μm) excitation probed at 800nm. The transient absorption using 400nm excitation and 800nm probe pulses of the palladium nanorods also reveal coherent acoustic oscillations. The results demonstrate that the dynamics for high aspect ratio metal nanorods are similar to those for smaller nanoparticles.

Effects of single atom doping on the ultrafast electron dynamics of M1Au24(SR)18 (M = Pd, Pt) nanoclusters.

PubMed

Zhou, Meng; Qian, Huifeng; Sfeir, Matthew Y; Nobusada, Katsuyuki; Jin, Rongchao

2016-04-07

Atomically precise, doped metal clusters are receiving wide research interest due to their synergistic properties dependent on the metal composition. To understand the electronic properties of doped clusters, it is highly desirable to probe the excited state behavior. Here, we report the ultrafast relaxation dynamics of doped M1@Au24(SR)18 (M = Pd, Pt; R = CH2CH2Ph) clusters using femtosecond visible and near infrared transient absorption spectroscopy. Three relaxation components are identified for both mono-doped clusters: (1) sub-picosecond relaxation within the M1Au12 core states; (2) core to shell relaxation in a few picoseconds; and (3) relaxation back to the ground state in more than one nanosecond. Despite similar relaxation pathways for the two doped nanoclusters, the coupling between the metal core and surface ligands is accelerated by over 30% in the case of the Pt dopant compared with the Pd dopant. Compared to Pd doping, the case of Pt doping leads to much more drastic changes in the steady state and transient absorption of the clusters, which indicates that the 5d orbitals of the Pt atom are more strongly mixed with Au 5d and 6s orbitals than the 4d orbitals of the Pd dopant. These results demonstrate that a single foreign atom can lead to entirely different excited state spectral features of the whole cluster compared to the parent Au25(SR)18 cluster. The detailed excited state dynamics of atomically precise Pd/Pt doped gold clusters help further understand their properties and benefit the development of energy-related applications.

Effects of single atom doping on the ultrafast electron dynamics of M 1Au 24(SR) 18 (M = Pd, Pt) nanoclusters

DOE PAGES

Zhou, Meng; Qian, Huifeng; Sfeir, Matthew Y.; ...

2016-02-29

Atomically precise, doped metal clusters are receiving wide research interest due to their synergistic properties dependent on the metal composition. To understand the electronic properties of doped clusters, it is highly desirable to probe the excited state behavior. Here, we report the ultrafast relaxation dynamics of doped M 1@Au 24(SR) 18 (M = Pd, Pt; R = CH 2CH 2Ph) clusters using femtosecond visible and near infrared transient absorption spectroscopy. Three relaxation components are identified for both mono-doped clusters: (1) sub-picosecond relaxation within the M 1Au 12 core states; (2) core to shell relaxation in a few picoseconds; and (3)more » relaxation back to the ground state in more than one nanosecond. Despite similar relaxation pathways for the two doped nanoclusters, the coupling between the metal core and surface ligands is accelerated by over 30% in the case of the Pt dopant compared with the Pd dopant. Compared to Pd doping, the case of Pt doping leads to much more drastic changes in the steady state and transient absorption of the clusters, which indicates that the 5d orbitals of the Pt atom are more strongly mixed with Au 5d and 6s orbitals than the 4d orbitals of the Pd dopant. These results demonstrate that a single foreign atom can lead to entirely different excited state spectral features of the whole cluster compared to the parent Au 25(SR) 18 cluster. As a result, the detailed excited state dynamics of atomically precise Pd/Pt doped gold clusters help further understand their properties and benefit the development of energy-related applications.« less

Wavelength-dependent ultrafast charge carrier separation in the WO 3/BiVO 4 coupled system

DOE PAGES

Grigioni, Ivan; Stamplecoskie, Kevin G.; Jara, Danilo H.; ...

2017-05-08

Due to its ~2.4 eV band gap, BiVO 4 is a very promising photoanode material for harvesting the blue portion of the solar light for photoelectrochemical (PEC) water splitting applications. In WO 3/BiVO 4 heterojunction films, the electrons photoexcited in BiVO 4 are injected into WO 3, overcoming the lower charge carriers’ diffusion properties limiting the PEC performance of BiVO 4 photoanodes. Here, we investigate by ultrafast transient absorption spectroscopy the charge carrier interactions occurring at the interface between the two oxides in heterojunction systems to directly unveil their wavelength dependence. Under selective BiVO 4 excitation, a favorable electron transfermore » from photoexcited BiVO 4 to WO 3 occurs immediately after excitation and leads to an increase of the trapped holes’ lifetime in BiVO4. However, a recombination channel opens when both oxides are simultaneously excited, evidenced by a shorter lifetime of trapped holes in BiVO 4. As a result, PEC measurements reveal the implication of these wavelength-dependent ultrafast interactions on the performances of the WO 3/BiVO 4 heterojunction.« less

Vibronic coupling explains the ultrafast carotenoid-to-bacteriochlorophyll energy transfer in natural and artificial light harvesters

DOE Office of Scientific and Technical Information (OSTI.GOV)

Perlík, Václav; Seibt, Joachim; Šanda, František

The initial energy transfer steps in photosynthesis occur on ultrafast timescales. We analyze the carotenoid to bacteriochlorophyll energy transfer in LH2 Marichromatium purpuratum as well as in an artificial light-harvesting dyad system by using transient grating and two-dimensional electronic spectroscopy with 10 fs time resolution. We find that Förster-type models reproduce the experimentally observed 60 fs transfer times, but overestimate coupling constants, which lead to a disagreement with both linear absorption and electronic 2D-spectra. We show that a vibronic model, which treats carotenoid vibrations on both electronic ground and excited states as part of the system’s Hamiltonian, reproduces all measuredmore » quantities. Importantly, the vibronic model presented here can explain the fast energy transfer rates with only moderate coupling constants, which are in agreement with structure based calculations. Counterintuitively, the vibrational levels on the carotenoid electronic ground state play the central role in the excited state population transfer to bacteriochlorophyll; resonance between the donor-acceptor energy gap and the vibrational ground state energies is the physical basis of the ultrafast energy transfer rates in these systems.« less

Ultrafast dynamics of the photo-excited hemes b and cn in the cytochrome b6f complex.

PubMed

Agarwal, Rachna; Chauvet, Adrien A P

2017-01-25

The dynamics of hemes b and c n within the cytochrome b 6 f complex are investigated by means of ultrafast broad-band transient absorption spectroscopy. On the one hand, the data reveal that, subsequent to visible light excitation, part of the b hemes undergoes pulse-limited photo-oxidation, with the liberated electron supposedly being transferred to one of the adjacent aromatic amino acids. Photo-oxidation is followed by charge recombination in about 8.2 ps. Subsequent to charge recombination, heme b is promoted to a vibrationally excited ground state that relaxes in about 4.6 ps. On the other hand, heme c n undergoes ultrafast ground state recovery in about 140 fs. Interestingly, the data also show that, in contrast to previous beliefs, Chl a is involved in the photochemistry of hemes. Indeed, subsequent to heme excitation, Chl a bleaches and recovers to its ground state in 90 fs and 650 fs, respectively. Chl a bleaching allegedly corresponds to the formation of a short lived Chl a anion. Beyond the previously suggested structural role, this study provides unique evidence that Chl a is directly involved in the photochemistry of the hemes.

Wavelength-dependent ultrafast charge carrier separation in the WO 3/BiVO 4 coupled system

DOE Office of Scientific and Technical Information (OSTI.GOV)

Grigioni, Ivan; Stamplecoskie, Kevin G.; Jara, Danilo H.

Due to its ~2.4 eV band gap, BiVO 4 is a very promising photoanode material for harvesting the blue portion of the solar light for photoelectrochemical (PEC) water splitting applications. In WO 3/BiVO 4 heterojunction films, the electrons photoexcited in BiVO 4 are injected into WO 3, overcoming the lower charge carriers’ diffusion properties limiting the PEC performance of BiVO 4 photoanodes. Here, we investigate by ultrafast transient absorption spectroscopy the charge carrier interactions occurring at the interface between the two oxides in heterojunction systems to directly unveil their wavelength dependence. Under selective BiVO 4 excitation, a favorable electron transfermore » from photoexcited BiVO 4 to WO 3 occurs immediately after excitation and leads to an increase of the trapped holes’ lifetime in BiVO4. However, a recombination channel opens when both oxides are simultaneously excited, evidenced by a shorter lifetime of trapped holes in BiVO 4. As a result, PEC measurements reveal the implication of these wavelength-dependent ultrafast interactions on the performances of the WO 3/BiVO 4 heterojunction.« less

Ultrafast nonlinear optical properties of thin-solid DNA film and their application as a saturable absorber in femtosecond mode-locked fiber laser

PubMed Central

Khazaeinezhad, Reza; Hosseinzadeh Kassani, Sahar; Paulson, Bjorn; Jeong, Hwanseong; Gwak, Jiyoon; Rotermund, Fabian; Yeom, Dong-Il; Oh, Kyunghwan

2017-01-01

A new extraordinary application of deoxyribonucleic acid (DNA) thin-solid-film was experimentally explored in the field of ultrafast nonlinear photonics. Optical transmission was investigated in both linear and nonlinear regimes for two types of DNA thin-solid-films made from DNA in aqueous solution and DNA-cetyltrimethylammonium chloride (CTMA) in an organic solvent. Z-scan measurements revealed a high third-order nonlinearity with n2 exceeding 10−9 at a wavelength of 1570 nm, for a nonlinarity about five orders of magnitude larger than that of silica. We also demonstrated ultrafast saturable absorption (SA) with a modulation depth of 0.43%. DNA thin solid films were successfully deposited on a side-polished optical fiber, providing an efficient evanescent wave interaction. We built an organic-inorganic hybrid all-fiber ring laser using DNA film as an ultrafast SA and using Erbium-doped fiber as an efficient optical gain medium. Stable transform-limited femtosecond soliton pulses were generated with full width half maxima of 417 fs for DNA and 323 fs for DNA-CTMA thin-solid-film SAs. The average output power was 4.20 mW for DNA and 5.46 mW for DNA-CTMA. Detailed conditions for DNA solid film preparation, dispersion control in the laser cavity and subsequent characteristics of soliton pulses are discussed, to confirm unique nonlinear optical applications of DNA thin-solid-film. PMID:28128340

Ultrafast Plasmon-Enhanced Hot Electron Generation at Ag Nanocluster/Graphite Heterojunctions.

PubMed

Tan, Shijing; Liu, Liming; Dai, Yanan; Ren, Jindong; Zhao, Jin; Petek, Hrvoje

2017-05-03

Hot electron processes at metallic heterojunctions are central to optical-to-chemical or electrical energy transduction. Ultrafast nonlinear photoexcitation of graphite (Gr) has been shown to create hot thermalized electrons at temperatures corresponding to the solar photosphere in less than 25 fs. Plasmonic resonances in metallic nanoparticles are also known to efficiently generate hot electrons. Here we deposit Ag nanoclusters (NC) on Gr to study the ultrafast hot electron generation and dynamics in their plasmonic heterojunctions by means of time-resolved two-photon photoemission (2PP) spectroscopy. By tuning the wavelength of p-polarized femtosecond excitation pulses, we find an enhancement of 2PP yields by 2 orders of magnitude, which we attribute to excitation of a surface-normal Mie plasmon mode of Ag/Gr heterojunctions at 3.6 eV. The 2PP spectra include contributions from (i) coherent two-photon absorption of an occupied interface state (IFS) 0.2 eV below the Fermi level, which electronic structure calculations assign to chemisorption-induced charge transfer, and (ii) hot electrons in the π*-band of Gr, which are excited through the coherent screening response of the substrate. Ultrafast pump-probe measurements show that the IFS photoemission occurs via virtual intermediate states, whereas the characteristic lifetimes attribute the hot electrons to population of the π*-band of Gr via the plasmon dephasing. Our study directly probes the mechanisms for enhanced hot electron generation and decay in a model plasmonic heterojunction.

Mapping CTTS dynamics of Na- in tetrahydrofurane with ultrafast multichannel pump-probe spectroscopy.

PubMed

Shoshana, O; Pérez Lustres, J L; Ernsting, N P; Ruhman, S

2006-06-14

Using multichannel femtosecond spectroscopy we have followed Na- charge transfer to solvent (CTTS) dynamics in THF solution. Absorption of the primary photoproducts in the visible, resolved here for the first time, consists of an asymmetric triplet centered at 595 nm, which we assign to a metastable incompletely solvated neutral atomic sodium species. Decay of this feature within approximately 1 ps to a broad and structureless solvated neutral is accompanied by broadening and loss of spectral detail. Kinetic analysis shows that both the spectral structure and the decay of this band are independent of the excitation photon frequency in the range 400-800 nm. With different pump-probe polarizations the anisotropy in transient transmission has been charted and its variation with excitation wavelength surveyed. The anisotropies are assigned to the reactant bleach, indicating that due to solvent-induced symmetry breaking, the CTTS absorption band of Na- is made up of discreet orthogonally polarized sub bands. None of the anisotropy in transient absorption could be associated with the photoproduct triplet band even at the earliest measurable time delays. Along with the documented differences in the spatial distribution of ejected electrons across the tested excitation wavelength range, these results lead us to conclude that photoejection is extremely rapid, and that loss of correlations between the departing electron and its neutral core is faster than our time resolution of approximately 60 fs.

Recent Chandra/HETGS and NuSTAR observations of the quasar PDS 456 and its Ultra-Fast Outflow

NASA Astrophysics Data System (ADS)

Boissay Malaquin, Rozenn; Marshall, Herman L.; Nowak, Michael A.

2018-01-01

Evidence is growing that the interaction between outflows from active galactic nuclei (AGN) and their surrounding medium may play an important role in galaxy evolution, i.e. in the regulation of star formation in galaxies, through AGN feedback processes. Indeed, powerful outflows, such as the ultra-fast outflows (UFOs) that can reach mildly relativistic velocities of 0.2-0.4c, could blow away a galaxy’s reservoir of star-forming gas and hence quench the star formation in host galaxies. The low-redshift (z=0.184) radio-quiet quasar PDS 456 has showed the presence of a strong and blueshifted absorption trough in the Fe K band above 7 keV, that has been associated with the signature of such a fast and highly ionized accretion disk wind of a velocity of 0.25-0.3c. This persistent and variable feature has been detected in many observations of PDS 456, in particular by XMM-Newton, Suzaku and NuSTAR, together with other blueshifted absorption lines in the soft energy band (e.g. Nardini et al. 2015, Reeves et al. 2016). I will present here the results of the analysis of recent and contemporaneous high-resolution Chandra/HETGS and NuSTAR observations of PDS 456, and compare them with the previous findings.

Yb:Lu2SiO5 crystal : characterization of the laser emission along the three dielectric axes

NASA Astrophysics Data System (ADS)

Toci, Guido; Pirri, Angela; Beitlerova, Alena; Shoji, Yasuhiro; Yoshikawa, Akira; Hybler, Jiri; Nikl, Martin; Vannini, Matteo

2015-05-01

Yb:doped Lu2SiO5 (Lutetium orthosilicate, LSO) is an optically biaxial crystal with laser emission in the range 1000- 1100 nm. It features different absorption and emission spectra for polarization along its three dielectric axes. In this work we have characterized the laser emission properties of Yb:LSO along all the three dielectric axis, evidencing differences that can be exploited in the design of ultrafast laser sources. The material was tested in a longitudinally pumped laser cavity. The laser emission efficiency was found similar along all the three dielectric axes, with slope efficiencies around 90% in most cases. Regarding the tuning range, for the most favourable polarization direction we obtained a continuously tunable emission between 993 and 1088 nm (i. e. 95 nm) peaked at 1040 nm. The tuning curves along the three dielectric axes spanned similar ranges but with relevant differences in the shape.

The response of relativistic outflowing gas to the inner accretion disk of a black hole.

PubMed

Parker, Michael L; Pinto, Ciro; Fabian, Andrew C; Lohfink, Anne; Buisson, Douglas J K; Alston, William N; Kara, Erin; Cackett, Edward M; Chiang, Chia-Ying; Dauser, Thomas; De Marco, Barbara; Gallo, Luigi C; Garcia, Javier; Harrison, Fiona A; King, Ashley L; Middleton, Matthew J; Miller, Jon M; Miniutti, Giovanni; Reynolds, Christopher S; Uttley, Phil; Vasudevan, Ranjan; Walton, Dominic J; Wilkins, Daniel R; Zoghbi, Abderahmen

2017-03-01

The brightness of an active galactic nucleus is set by the gas falling onto it from the galaxy, and the gas infall rate is regulated by the brightness of the active galactic nucleus; this feedback loop is the process by which supermassive black holes in the centres of galaxies may moderate the growth of their hosts. Gas outflows (in the form of disk winds) release huge quantities of energy into the interstellar medium, potentially clearing the surrounding gas. The most extreme (in terms of speed and energy) of these-the ultrafast outflows-are the subset of X-ray-detected outflows with velocities higher than 10,000 kilometres per second, believed to originate in relativistic (that is, near the speed of light) disk winds a few hundred gravitational radii from the black hole. The absorption features produced by these outflows are variable, but no clear link has been found between the behaviour of the X-ray continuum and the velocity or optical depth of the outflows, owing to the long timescales of quasar variability. Here we report the observation of multiple absorption lines from an extreme ultrafast gas flow in the X-ray spectrum of the active galactic nucleus IRAS 13224-3809, at 0.236 ± 0.006 times the speed of light (71,000 kilometres per second), where the absorption is strongly anti-correlated with the emission of X-rays from the inner regions of the accretion disk. If the gas flow is identified as a genuine outflow then it is in the fastest five per cent of such winds, and its variability is hundreds of times faster than in other variable winds, allowing us to observe in hours what would take months in a quasar. We find X-ray spectral signatures of the wind simultaneously in both low- and high-energy detectors, suggesting a single ionized outflow, linking the low- and high-energy absorption lines. That this disk wind is responding to the emission from the inner accretion disk demonstrates a connection between accretion processes occurring on very different scales: the X-ray emission from within a few gravitational radii of the black hole ionizing the disk wind hundreds of gravitational radii further away as the X-ray flux rises.

The dependence of the ultrafast relaxation kinetics of the S2 and S1 states in β-carotene homologs and lycopene on conjugation length studied by femtosecond time-resolved absorption and Kerr-gate fluorescence spectroscopies

NASA Astrophysics Data System (ADS)

Kosumi, Daisuke; Fujiwara, Masazumi; Fujii, Ritsuko; Cogdell, Richard J.; Hashimoto, Hideki; Yoshizawa, Masayuki

2009-06-01

The ultrafast relaxation kinetics of all-trans-β-carotene homologs with varying numbers of conjugated double bonds n(n =7-15) and lycopene (n =11) has been investigated using femtosecond time-resolved absorption and Kerr-gate fluorescence spectroscopies, both carried out under identical excitation conditions. The nonradiative relaxation rates of the optically allowed S2(1Bu+1) state were precisely determined by the time-resolved fluorescence. The kinetics of the optically forbidden S1(2Ag-1) state were observed by the time-resolved absorption measurements. The dependence of the S1 relaxation rates upon the conjugation length is adequately described by application of the energy gap law. In contrast to this, the nonradiative relaxation rates of S2 have a minimum at n =9 and show a reverse energy gap law dependence for values of n above 11. This anomalous behavior of the S2 relaxation rates can be explained by the presence of an intermediate state (here called the Sx state) located between the S2 and S1 states at large values of n (such as n =11). The presence of such an intermediate state would then result in the following sequential relaxation pathway S2→Sx→S1→S0. A model based on conical intersections between the potential energy curves of these excited singlet states can readily explain the measured relationships between the decay rates and the energy gaps.

Ultrafast Energy Transfer Dynamics Between a Polypyridyl Ru(II) Chromophore and a Covalently Attached Acceptor

NASA Astrophysics Data System (ADS)

Styers-Barnett, David; Gannon, Erika; Papanikolas, John; Meyer, Thomas

2003-03-01

The energy transfer dynamics between the ^3MLCT excited state of a polypyridyl Ru(II) chromophore and a ligand-bound anthracene has been studied using femtosecond transient absorption spectroscopy. Photoexcitation of the metal complex at 450 nm promotes an electron from a d-orbital on the metal to a π* orbital on the bipyridine, forming a metal-to-ligand charge-transfer (MLCT) excited state. Energy transfer to the covalently appended anthracene is followed by the growth of the anthracene excited state absorption at 425 nm, and the simultaneous decay of the ^3MLCT absorption at 380 nm. The observed growth is biexponential, with the fast component attributed to energy transfer (19 ps), and the slow component arising from a combination of interligand electron transfer between the polypyridyl ligands and energy transfer (75 ps).

Simple and ultra-fast recognition and quantitation of compounded monoclonal antibodies: Application to flow injection analysis combined to UV spectroscopy and matching method.

PubMed

Jaccoulet, E; Schweitzer-Chaput, A; Toussaint, B; Prognon, P; Caudron, E

2018-09-01

Compounding of monoclonal antibody (mAbs) constantly increases in hospital. Quality control (QC) of the compounded mAbs based on quantification and identification is required to prevent potential errors and fast method is needed to manage outpatient chemotherapy administration. A simple and ultra-fast (less than 30 s) method using flow injection analysis associated to least square matching method issued from the analyzer software was performed and evaluated for the routine hospital QC of three compounded mAbs: bevacizumab, infliximab and rituximab. The method was evaluated through qualitative and quantitative parameters. Preliminary analysis of the UV absorption and second derivative spectra of the mAbs allowed us to adapt analytical conditions according to the therapeutic range of the mAbs. In terms of quantitative QC, linearity, accuracy and precision were assessed as specified in ICH guidelines. Very satisfactory recovery was achieved and the RSD (%) of the intermediate precision were less than 1.1%. Qualitative analytical parameters were also evaluated in terms of specificity, sensitivity and global precision through a matrix of confusion. Results showed to be concentration and mAbs dependant and excellent (100%) specificity and sensitivity were reached within specific concentration range. Finally, routine application on "real life" samples (n = 209) from different batch of the three mAbs complied with the specifications of the quality control i.e. excellent identification (100%) and ± 15% of targeting concentration belonging to the calibration range. The successful use of the combination of second derivative spectroscopy and partial least square matching method demonstrated the interest of FIA for the ultra-fast QC of mAbs after compounding using matching method. Copyright © 2018 Elsevier B.V. All rights reserved.

Femtosecond dynamics of a non-steroidal anti-inflammatory drug (piroxicam) in solution: The involvement of twisting motion

NASA Astrophysics Data System (ADS)

Gil, Michał; Douhal, Abderrazzak

2008-06-01

In this contribution, we report on fast and ultrafast dynamics of a non-steroidal anti-inflammatory drug, piroxicam (PX), in methyl acetate (MAC) and triacetin (TAC), two solvents of different viscosities. The enol form of PX undergoes a femtosecond (shorter than 100 fs) electronically excited state intramolecular proton-transfer reaction to produce keto tautomers. These structures exhibit an internal twisting motion to generate keto rotamers in ˜2-5 ps, a time being longer in TAC. The transient absorption/emission spectrum is very broad indicating that the potential-energy surface at the electronically excited state is very flat, and reflecting the involvement of several coordinates along which the wavepacket of the fs-produced structures evolve.

The picosecond structure of ultra-fast rogue waves

NASA Astrophysics Data System (ADS)

Klein, Avi; Shahal, Shir; Masri, Gilad; Duadi, Hamootal; Sulimani, Kfir; Lib, Ohad; Steinberg, Hadar; Kolpakov, Stanislav A.; Fridman, Moti

2018-02-01

We investigated ultrafast rogue waves in fiber lasers and found three different patterns of rogue waves: single- peaks, twin-peaks, and triple-peaks. The statistics of the different patterns as a function of the pump power of the laser reveals that the probability for all rogue waves patterns increase close to the laser threshold. We developed a numerical model which prove that the ultrafast rogue waves patterns result from both the polarization mode dispersion in the fiber and the non-instantaneous nature of the saturable absorber. This discovery reveals that there are three different types of rogue waves in fiber lasers: slow, fast, and ultrafast, which relate to three different time-scales and are governed by three different sets of equations: the laser rate equations, the nonlinear Schrodinger equation, and the saturable absorber equations, accordingly. This discovery is highly important for analyzing rogue waves and other extreme events in fiber lasers and can lead to realizing types of rogue waves which were not possible so far such as triangular rogue waves.

Impact and recovery process of mini flash crashes: An empirical study.

PubMed

Braun, Tobias; Fiegen, Jonas A; Wagner, Daniel C; Krause, Sebastian M; Guhr, Thomas

2018-01-01

In an Ultrafast Extreme Event (or Mini Flash Crash), the price of a traded stock increases or decreases strongly within milliseconds. We present a detailed study of Ultrafast Extreme Events in stock market data. In contrast to popular belief, our analysis suggests that most of the Ultrafast Extreme Events are not necessarily due to feedbacks in High Frequency Trading: In at least 60 percent of the observed Ultrafast Extreme Events, the largest fraction of the price change is due to a single market order. In times of financial crisis, large market orders are more likely which leads to a significant increase of Ultrafast Extreme Events occurrences. Furthermore, we analyze the 100 trades following each Ultrafast Extreme Events. While we observe a tendency of the prices to partially recover, less than 40 percent recover completely. On the other hand we find 25 percent of the Ultrafast Extreme Events to be almost recovered after only one trade which differs from the usually found price impact of market orders.

Impact and recovery process of mini flash crashes: An empirical study

PubMed Central

Wagner, Daniel C.; Krause, Sebastian M.; Guhr, Thomas

2018-01-01

In an Ultrafast Extreme Event (or Mini Flash Crash), the price of a traded stock increases or decreases strongly within milliseconds. We present a detailed study of Ultrafast Extreme Events in stock market data. In contrast to popular belief, our analysis suggests that most of the Ultrafast Extreme Events are not necessarily due to feedbacks in High Frequency Trading: In at least 60 percent of the observed Ultrafast Extreme Events, the largest fraction of the price change is due to a single market order. In times of financial crisis, large market orders are more likely which leads to a significant increase of Ultrafast Extreme Events occurrences. Furthermore, we analyze the 100 trades following each Ultrafast Extreme Events. While we observe a tendency of the prices to partially recover, less than 40 percent recover completely. On the other hand we find 25 percent of the Ultrafast Extreme Events to be almost recovered after only one trade which differs from the usually found price impact of market orders. PMID:29782503

Evidence for ultra-fast outflows in radio-quiet AGNs. I. Detection and statistical incidence of Fe K-shell absorption lines

NASA Astrophysics Data System (ADS)

Tombesi, F.; Cappi, M.; Reeves, J. N.; Palumbo, G. G. C.; Yaqoob, T.; Braito, V.; Dadina, M.

2010-10-01

Context. Blue-shifted Fe K absorption lines have been detected in recent years between 7 and 10 keV in the X-ray spectra of several radio-quiet AGNs. The derived blue-shifted velocities of the lines can often reach mildly relativistic values, up to 0.2-0.4c. These findings are important because they suggest the presence of a previously unknown massive and highly ionized absorbing material outflowing from their nuclei, possibly connected with accretion disk winds/outflows. Aims: The scope of the present work is to statistically quantify the parameters and incidence of the blue-shifted Fe K absorption lines through a uniform analysis on a large sample of radio-quiet AGNs. This allows us to assess their global detection significance and to overcome any possible publication bias. Methods: We performed a blind search for narrow absorption features at energies greater than 6.4 keV in a sample of 42 radio-quiet AGNs observed with XMM-Newton. A simple uniform model composed by an absorbed power-law plus Gaussian emission and absorption lines provided a good fit for all the data sets. We derived the absorption lines parameters and calculated their detailed detection significance making use of the classical F-test and extensive Monte Carlo simulations. Results: We detect 36 narrow absorption lines on a total of 101 XMM-Newton EPIC pn observations. The number of absorption lines at rest-frame energies higher than 7 keV is 22. Their global probability to be generated by random fluctuations is very low, less than 3 × 10-8, and their detection have been independently confirmed by a spectral analysis of the MOS data, with associated random probability <10-7. We identify the lines as Fe XXV and Fe XXVI K-shell resonant absorption. They are systematically blue-shifted, with a velocity distribution ranging from zero up to ~0.3c, with a peak and mean value at ~0.1c. We detect variability of the lines on both EWs and blue-shifted velocities among different XMM-Newton observations even on time-scales as short as a few days, possibly suggesting somewhat compact absorbers. Moreover, we find no significant correlation between the cosmological red-shifts of the sources and the lines blue-shifted velocities, ruling out any systematic contamination by local absorption. If we define ultra-fast outflows (UFOs) those highly ionized absorbers with outflow velocities higher than 104 km s-1, then the majority of the lines are consistent with being associated to UFOs and the fraction of objects with detected UFOs in the whole sample is at least ~35%. This fraction is similar for type 1 and type 2 sources. The global covering fraction of the absorbers is consequently estimated to be in the range C ˜ 0.4-0.6, thereby implying large opening angles. Conclusions: From our systematic X-ray spectral analysis on a large sample of radio-quiet AGNs we have been able to clearly assess the global veracity of the blue-shifted Fe K absorption lines at E > 7 keV and to overcome their publication bias. These lines indicate that UFOs are a rather common phenomenon observable in the central regions of these sources and they are probably the direct signature of AGN accretion disk winds/ejecta. The detailed photo-ionization modeling of these absorbers is presented in a companion paper. Appendices are only available in electronic form at http://www.aanda.org

X-ray phase contrast imaging of biological specimens with femtosecond pulses of betatron radiation from a compact laser plasma wakefield accelerator

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kneip, S.; Center for Ultrafast Optical Science, University of Michigan, Ann Arbor 48109; McGuffey, C.

2011-08-29

We show that x-rays from a recently demonstrated table top source of bright, ultrafast, coherent synchrotron radiation [Kneip et al., Nat. Phys. 6, 980 (2010)] can be applied to phase contrast imaging of biological specimens. Our scheme is based on focusing a high power short pulse laser in a tenuous gas jet, setting up a plasma wakefield accelerator that accelerates and wiggles electrons analogously to a conventional synchrotron, but on the centimeter rather than tens of meter scale. We use the scheme to record absorption and phase contrast images of a tetra fish, damselfly and yellow jacket, in particular highlightingmore » the contrast enhancement achievable with the simple propagation technique of phase contrast imaging. Coherence and ultrafast pulse duration will allow for the study of various aspects of biomechanics.« less

Through-Space Ultrafast Photoinduced Electron Transfer Dynamics of a C 70 -Encapsulated Bisporphyrin Covalent Organic Polyhedron in a Low-Dielectric Medium

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ortiz, Michael; Cho, Sung; Niklas, Jens

Ultrafast photoinduced electron transfer (PIET) dynamics of a C 70-encapsulated bisporphyrin covalent organic polyhedron hybrid (C 70@COP-5) is studied in a nonpolar toluene medium with fluorescence and transient absorption spectroscopies. This structurally rigid donor (D)-acceptor (A) molecular hybrid offers a new platform featuring conformationally predetermined cofacial D-A orientation with a fixed edge-to-edge separation, R EE (2.8 Å), without the aid of covalent bonds. Sub-picosecond PIET (T ET ≤ 0.4 ps) and very slow charge recombination (T CR ≈ 600 ps) dynamics are observed. The origin of these dynamics is discussed in terms of enhanced D-A coupling (V = 675 cmmore » -1) and extremely small reorganization energy (λ ≈ 0.18 eV), induced by the intrinsic structural rigidity of the C 70@COP-5 complex.« less

Direct growth of graphene on quartz substrate as saturable absorber for femtosecond solid-state laser

NASA Astrophysics Data System (ADS)

Xu, S. C.; Man, B. Y.; Jiang, S. Z.; Chen, C. S.; Liu, M.; Yang, C.; Gao, S. B.; Feng, D. J.; Hu, G. D.; Huang, Q. J.; Chen, X. F.; Zhang, C.

2014-08-01

We present a novel method for the direct metal-free growth of graphene on quartz substrate. The direct-grown graphene yields excellent nonlinear saturable absorption properties and is demonstrated to be suitable as a saturable absorber (SA) for an ultrafast solid-state laser. Nearly Fourier-limited 367 fs was obtained at a central wavelength of 1048 nm with a repetition rate of 105.7 MHz. At a pump power of 7.95 W, the average output power was 1.93 W and the highest pulse energy reached 18.3 nJ, with a peak power of 49.8 kW. Our work opens an easy route for making a reliable graphene SA with a mode-locking technique and also displays an exciting prospect in making low-cost and ultrafast lasers.

Through-Space Ultrafast Photoinduced Electron Transfer Dynamics of a C 70 -Encapsulated Bisporphyrin Covalent Organic Polyhedron in a Low-Dielectric Medium

DOE PAGES

Ortiz, Michael; Cho, Sung; Niklas, Jens; ...

2017-03-13

Ultrafast photoinduced electron transfer (PIET) dynamics of a C 70-encapsulated bisporphyrin covalent organic polyhedron hybrid (C 70@COP-5) is studied in a nonpolar toluene medium with fluorescence and transient absorption spectroscopies. This structurally rigid donor (D)-acceptor (A) molecular hybrid offers a new platform featuring conformationally predetermined cofacial D-A orientation with a fixed edge-to-edge separation, R EE (2.8 Å), without the aid of covalent bonds. Sub-picosecond PIET (T ET ≤ 0.4 ps) and very slow charge recombination (T CR ≈ 600 ps) dynamics are observed. The origin of these dynamics is discussed in terms of enhanced D-A coupling (V = 675 cmmore » -1) and extremely small reorganization energy (λ ≈ 0.18 eV), induced by the intrinsic structural rigidity of the C 70@COP-5 complex.« less

Through-Space Ultrafast Photoinduced Electron Transfer Dynamics of a C70-Encapsulated Bisporphyrin Covalent Organic Polyhedron in a Low-Dielectric Medium.

PubMed

Ortiz, Michael; Cho, Sung; Niklas, Jens; Kim, Seonah; Poluektov, Oleg G; Zhang, Wei; Rumbles, Garry; Park, Jaehong

2017-03-29

Ultrafast photoinduced electron transfer (PIET) dynamics of a C 70 -encapsulated bisporphyrin covalent organic polyhedron hybrid (C 70 @COP-5) is studied in a nonpolar toluene medium with fluorescence and transient absorption spectroscopies. This structurally rigid donor (D)-acceptor (A) molecular hybrid offers a new platform featuring conformationally predetermined cofacial D-A orientation with a fixed edge-to-edge separation, R EE (2.8 Å), without the aid of covalent bonds. Sub-picosecond PIET (τ ET ≤ 0.4 ps) and very slow charge recombination (τ CR ≈ 600 ps) dynamics are observed. The origin of these dynamics is discussed in terms of enhanced D-A coupling (V = 675 cm -1 ) and extremely small reorganization energy (λ ≈ 0.18 eV), induced by the intrinsic structural rigidity of the C 70 @COP-5 complex.

Long-range hot-carrier transport in hybrid perovskites visualized by ultrafast microscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Guo, Zhi; Wan, Yan; Yang, Mengjin

The Shockley-Queisser limit for solar cell efficiency can be overcome if hot carriers can be harvested before they thermalize. Recently, carrier cooling time up to 100 picoseconds was observed in hybrid perovskites, but it is unclear whether these long-lived hot carriers can migrate long distance for efficient collection. Here, we report direct visualization of hot-carrier migration in methylammonium lead iodide (CH 3NH 3PbI 3) thin films by ultrafast transient absorption microscopy, demonstrating three distinct transport regimes. Quasiballistic transport was observed to correlate with excess kinetic energy, resulting in up to 230 nanometers transport distance that could overcome grain boundaries. Themore » nonequilibrium transport persisted over tens of picoseconds and ~600 nanometers before reaching the diffusive transport limit. Lastly, these results suggest potential applications of hot-carrier devices based on hybrid perovskites.« less

Ultrafast dynamics of multi-exciton state coupled to coherent vibration in zinc chlorin aggregates for artificial photosynthesis.

PubMed

Shi, Tongchao; Liu, Zhengzheng; Miyatake, Tomohiro; Tamiaki, Hitoshi; Kobayashi, Takayoshi; Zhang, Zeyu; Du, Juan; Leng, Yuxin

2017-11-27

Ultrafast vibronic dynamics induced by the interaction of the Frenkel exciton with the coherent molecular vibrations in a layer-structured zinc chlorin aggregates prepared for artificial photosynthesis have been studied by 7.1 fs real-time vibrational spectroscopy with multi-spectrum detection. The fast decay of 100 ± 5fs is ascribed to the relaxation from the higher multi-exciton state (MES) to the one-exciton state, and the slow one of 863 ± 70fs is assigned to the relaxation from Q-exciton state to the dark nonfluorescent charge-transfer (CT) state, respectively. In addition, the wavelength dependences of the exciton-vibration coupling strength are found to follow the zeroth derivative of the transient absorption spectra of the exciton. It could be explained in term of the transition dipole moment modulated by dynamic intensity borrowing between the B transition and the Q transition through the vibronic interactions.

Electron heating and thermal relaxation of gold nanorods revealed by two-dimensional electronic spectroscopy.

PubMed

Lietard, Aude; Hsieh, Cho-Shuen; Rhee, Hanju; Cho, Minhaeng

2018-03-01

To elucidate the complex interplay between the size and shape of gold nanorods and their electronic, photothermal, and optical properties for molecular imaging, photothermal therapy, and optoelectronic devices, it is a prerequisite to characterize ultrafast electron dynamics in gold nanorods. Time-resolved transient absorption (TA) studies of plasmonic electrons in various nanostructures have revealed the time scales for electron heating, lattice vibrational excitation, and phonon relaxation processes in condensed phases. However, because linear spectroscopic and time-resolved TA signals are vulnerable to inhomogeneous line-broadening, pure dephasing and direct electron heating effects are difficult to observe. Here we show that femtosecond two-dimensional electronic spectroscopy, with its unprecedented time resolution and phase sensitivity, can be used to collect direct experimental evidence for ultrafast electron heating, anomalously strong coherent and transient electronic plasmonic responses, and homogenous dephasing processes resulting from electron-vibration couplings even for polydisperse gold nanorods.

Long-range hot-carrier transport in hybrid perovskites visualized by ultrafast microscopy

DOE PAGES

Guo, Zhi; Wan, Yan; Yang, Mengjin; ...

2017-04-07

The Shockley-Queisser limit for solar cell efficiency can be overcome if hot carriers can be harvested before they thermalize. Recently, carrier cooling time up to 100 picoseconds was observed in hybrid perovskites, but it is unclear whether these long-lived hot carriers can migrate long distance for efficient collection. Here, we report direct visualization of hot-carrier migration in methylammonium lead iodide (CH 3NH 3PbI 3) thin films by ultrafast transient absorption microscopy, demonstrating three distinct transport regimes. Quasiballistic transport was observed to correlate with excess kinetic energy, resulting in up to 230 nanometers transport distance that could overcome grain boundaries. Themore » nonequilibrium transport persisted over tens of picoseconds and ~600 nanometers before reaching the diffusive transport limit. Lastly, these results suggest potential applications of hot-carrier devices based on hybrid perovskites.« less

Hybrid Photon-Plasmon Coupling and Ultrafast Control of Nanoantennas on a Silicon Photonic Chip.

PubMed

Chen, Bigeng; Bruck, Roman; Traviss, Daniel; Khokhar, Ali Z; Reynolds, Scott; Thomson, David J; Mashanovich, Goran Z; Reed, Graham T; Muskens, Otto L

2018-01-10

Hybrid integration of nanoplasmonic devices with silicon photonic circuits holds promise for a range of applications in on-chip sensing, field-enhanced and nonlinear spectroscopy, and integrated nanophotonic switches. Here, we demonstrate a new regime of photon-plasmon coupling by combining a silicon photonic resonator with plasmonic nanoantennas. Using principles from coherent perfect absorption, we make use of standing-wave light fields to maximize the photon-plasmon interaction strength. Precise placement of the broadband antennas with respect to the narrowband photonic racetrack modes results in controlled hybridization of only a subset of these modes. By combining antennas into groups of radiating dipoles with opposite phase, far-field scattering is effectively suppressed. We achieve ultrafast tuning of photon-plasmon hybridization including reconfigurable routing of the standing-wave input between two output ports. Hybrid photonic-plasmonic resonators provide conceptually new approaches for on-chip integrated nanophotonic devices.

Real-Time Visualization of Active Species in a Single-Site Metal–Organic Framework Photocatalyst

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang, Sizhuo; Pattengale, Brian; Lee, Sungsik

In this work, we report a new single-site photocatalyst (Co-Ru-UIO- 67(bpy)) based on a metal-organic framework platform with incorporated molecular photosensitizer and catalyst. We show that this catalyst not only demonstrates exceptional activity for light-driven H2 production but also can be recycled without loss of activity. Using the combination of optical transient absorption spectroscopy and in situ X-ray absorption spectroscopy, we not only captured the key CoI intermediate species formed after ultrafast charge transfer from the incorporated photosensitizer but also identified the rate-limiting step in the catalytic cycle, providing insight into the catalysis mechanism of these single-site metal-organic framework photocatalysts.

High-throughput machining using high average power ultrashort pulse lasers and ultrafast polygon scanner

NASA Astrophysics Data System (ADS)

Schille, Joerg; Schneider, Lutz; Streek, André; Kloetzer, Sascha; Loeschner, Udo

2016-03-01

In this paper, high-throughput ultrashort pulse laser machining is investigated on various industrial grade metals (Aluminium, Copper, Stainless steel) and Al2O3 ceramic at unprecedented processing speeds. This is achieved by using a high pulse repetition frequency picosecond laser with maximum average output power of 270 W in conjunction with a unique, in-house developed two-axis polygon scanner. Initially, different concepts of polygon scanners are engineered and tested to find out the optimal architecture for ultrafast and precision laser beam scanning. Remarkable 1,000 m/s scan speed is achieved on the substrate, and thanks to the resulting low pulse overlap, thermal accumulation and plasma absorption effects are avoided at up to 20 MHz pulse repetition frequencies. In order to identify optimum processing conditions for efficient high-average power laser machining, the depths of cavities produced under varied parameter settings are analyzed and, from the results obtained, the characteristic removal values are specified. The maximum removal rate is achieved as high as 27.8 mm3/min for Aluminium, 21.4 mm3/min for Copper, 15.3 mm3/min for Stainless steel and 129.1 mm3/min for Al2O3 when full available laser power is irradiated at optimum pulse repetition frequency.

Femtosecond to nanosecond excited state dynamics of vapor deposited copper phthalocyanine thin films.

PubMed

Caplins, Benjamin W; Mullenbach, Tyler K; Holmes, Russell J; Blank, David A

2016-04-28

Vapor deposited thin films of copper phthalocyanine (CuPc) were investigated using transient absorption spectroscopy. Exciton-exciton annihilation dominated the kinetics at high exciton densities. When annihilation was minimized, the observed lifetime was measured to be 8.6 ± 0.6 ns, which is over an order of magnitude longer than previous reports. In comparison with metal free phthalocyanine (H2Pc), the data show evidence that the presence of copper induces an ultrafast relaxation process taking place on the ca. 500 fs timescale. By comparison to recent time-resolved photoemission studies, this is assigned as ultrafast intersystem crossing. As the intersystem crossing occurs ca. 10(4) times faster than lifetime decay, it is likely that triplets are the dominant excitons in vapor deposited CuPc films. The exciton lifetime of CuPc thin films is ca. 35 times longer than H2Pc thin films, while the diffusion lengths reported in the literature are typically quite similar for the two materials. These findings suggest that despite appearing to be similar materials at first glance, CuPc and H2Pc may transport energy in dramatically different ways. This has important implications on the design and mechanistic understanding of devices where phthalocyanines are used as an excitonic material.

Ultrafast photoconductivity of undoped cuprates

NASA Astrophysics Data System (ADS)

Dodge, J. Steven; Farahani, Amir; Petersen, Jesse; Liang, Ruixing; Bozovic, Ivan

2010-03-01

Using a visible pump-THz probe technique, we studied the ultrafast transient photoconductivity of the insulating cuprate La2CuO4, and compared it with earlier measurements of Sr2CuO2Cl2 and YBa2Cu3O6. In all these compounds, we observe a rapid onset of photoconductivity that is followed by a non-exponential relaxation on a picosecond time scales, the dynamics of which are independent of photocarrier concentration ranging from 0.2 to 1.5 percent. The photoconductivity decay is qualitatively similar to the decay of the photoinduced gap absorption in Sr2CuO2Cl2,footnotetextJ. S. Dodge, arXiv:0910.5048 indicating a common origin for the two effects. Assuming a quantum efficiency of unity, the estimated peak mobility for all three compounds is within 0.1-0.4 cm^2/V.s; this is lower than the Hall mobility in chemically doped systems with similar carrier concentrations,footnotetext Ando et al, Phys. Rev. Lett. 87, 017001 (2001) but orders of magnitude larger than earlier DC photoconductivity results.footnotetextThio et al, Phys. Rev. B 42, 10800 (1990) The similarity of the peak photoconductivity across three different compounds indicates that it is an intrinsic feature of the copper oxide plane.

Ultrafast dynamics in multifunctional Ru(II)-loaded polymers for solar energy conversion.

PubMed

Morseth, Zachary A; Wang, Li; Puodziukynaite, Egle; Leem, Gyu; Gilligan, Alexander T; Meyer, Thomas J; Schanze, Kirk S; Reynolds, John R; Papanikolas, John M

2015-03-17

The use of sunlight to make chemical fuels (i.e., solar fuels) is an attractive approach in the quest to develop sustainable energy sources. Using nature as a guide, assemblies for artificial photosynthesis will need to perform multiple functions. They will need to be able to harvest light across a broad region of the solar spectrum, transport excited-state energy to charge-separation sites, and then transport and store redox equivalents for use in the catalytic reactions that produce chemical fuels. This multifunctional behavior will require the assimilation of multiple components into a single macromolecular system. A wide variety of different architectures including porphyrin arrays, peptides, dendrimers, and polymers have been explored, with each design posing unique challenges. Polymer assemblies are attractive due to their relative ease of production and facile synthetic modification. However, their disordered nature gives rise to stochastic dynamics not present in more ordered assemblies. The rational design of assemblies requires a detailed understanding of the energy and electron transfer events that follow light absorption, which can occur on time scales ranging from femtoseconds to hundreds of microseconds, necessitating the use of sophisticated techniques. We have used a combination of time-resolved absorption and emission spectroscopies with observation times that span 9 orders of magnitude to follow the excited-state evolution within polymer-based molecular assemblies. We complement experimental observations with molecular dynamics simulations to develop a microscopic view of these dynamics. This Account provides an overview of our work on polymers decorated with pendant Ru(II) chromophores, both in solution and on surfaces. We have examined site-to-site energy transport among the Ru(II) complexes, and in systems incorporating π-conjugated polymers, we have observed ultrafast formation of a long-lived charge-separated state. When attached to TiO2, these assemblies exhibit multifunctional behavior in which photon absorption is followed by energy transport to the surface and electron injection to produce an oxidized metal complex. The oxidizing equivalent is then transferred to the conjugated polymer, giving rise to a long-lived charge-separated state.

Broadband photocarrier dynamics and nonlinear absorption of PLD-grown WTe2 semimetal films

NASA Astrophysics Data System (ADS)

Gao, Wenbin; Huang, Lei; Xu, Jinlong; Chen, Yequan; Zhu, Chunhui; Nie, Zhonghui; Li, Yao; Wang, Xuefeng; Xie, Zhenda; Zhu, Shining; Xu, Jun; Wan, Xiangang; Zhang, Chao; Xu, Yongbing; Shi, Yi; Wang, Fengqiu

2018-04-01

WTe2 is a unique material in the family of transition metal dichalcogenides and it has been proposed as a candidate for type-II Weyl semimetals. However, thus far, studies on the optical properties of this emerging material have been significantly hindered by the lack of large-area, high-quality WTe2 materials. Here, we grow a centimeter-scale, highly crystalline WTe2 ultrathin film (˜35 nm) by a pulsed laser deposition technique. Broadband pump-probe spectroscopy (1.2-2.5 μm) reveals a peculiar ultrafast optical response where an initial photo-bleaching signal (lasting ˜3 ps) is followed by a long-lived photoinduced absorption signature. Nonlinear absorption characterization using femtosecond pulses confirms the saturable absorption response of the WTe2 ultrathin films, and we further demonstrated a mode-locked Thulium fiber laser using a WTe2 absorber. Our work provides important insights into linear and nonlinear optical responses of WTe2 thin films.

A new paradigm for use of ultrafast lasers in ophthalmology for enhancement of corneal mechanical properties and permanent correction of refractive errors

NASA Astrophysics Data System (ADS)

Wang, Chao; Fomovsky, Mikhail; Hall, Jamie R.; Paik, David C.; Trokel, Stephen L.; Vukelic, Sinisa

2017-02-01

A new paradigm for strengthening of corneal tissue as well as permanent correction of refractive errors has been proposed. Ultrafast laser irradiation is confined to the levels below optical breakdown such that tissue damage is avoided while creating an ionization field responsible for subsequent photochemical modification of the stroma. The concept was assed using newly developed platform for precise application of a near-IR femtosecond laser irradiation to the cornea in in-vitro experiments. Targeted irradiation with tightly focused ultrafast laser pulses allows spatially resolved crosslinking in the interior of the porcine cornea in the absence of photosensitizers. The formation of intra- or interstromal covalent bonds in collagen matrix locally increases lamellar density. Due to high resolution, treatment is spatially resolved and therefore can be tailored to either enhance structure of corneal stroma or adjust corneal curvature towards correcting refractive errors. As the induced modification is primarily driven by nonlinear absorption, the treatment is essentially wavelength independent, and as such potentially less harmful than current method of choice, joint application of UVA light irradiation in conjunction with riboflavin. Potential applicability of a near-IR femtosecond laser for biomechanical stabilization of cornea and non-invasive refractive eye corrections is discussed.

Probing energy transfer events in the light harvesting complex 2 (LH2) of Rhodobacter sphaeroides with two-dimensional spectroscopy.

PubMed

Fidler, Andrew F; Singh, Ved P; Long, Phillip D; Dahlberg, Peter D; Engel, Gregory S

2013-10-21

Excitation energy transfer events in the photosynthetic light harvesting complex 2 (LH2) of Rhodobacter sphaeroides are investigated with polarization controlled two-dimensional electronic spectroscopy. A spectrally broadened pulse allows simultaneous measurement of the energy transfer within and between the two absorption bands at 800 nm and 850 nm. The phased all-parallel polarization two-dimensional spectra resolve the initial events of energy transfer by separating the intra-band and inter-band relaxation processes across the two-dimensional map. The internal dynamics of the 800 nm region of the spectra are resolved as a cross peak that grows in on an ultrafast time scale, reflecting energy transfer between higher lying excitations of the B850 chromophores into the B800 states. We utilize a polarization sequence designed to highlight the initial excited state dynamics which uncovers an ultrafast transfer component between the two bands that was not observed in the all-parallel polarization data. We attribute the ultrafast transfer component to energy transfer from higher energy exciton states to lower energy states of the strongly coupled B850 chromophores. Connecting the spectroscopic signature to the molecular structure, we reveal multiple relaxation pathways including a cyclic transfer of energy between the two rings of the complex.

Ultrafast band-gap oscillations in iron pyrite

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kolb, B; Kolpak, AM

2013-12-20

With its combination of favorable band gap, high absorption coefficient, material abundance, and low cost, iron pyrite, FeS2, has received a great deal of attention over the past decades as a promising material for photovoltaic applications such as solar cells and photoelectrochemical cells. Devices made from pyrite, however, exhibit open circuit voltages significantly lower than predicted, and despite a recent resurgence of interest in the material, there currently exists no widely accepted explanation for this disappointing behavior. In this paper, we show that phonons, which have been largely overlooked in previous efforts, may play a significant role. Using fully self-consistentmore » GW calculations, we demonstrate that a phonon mode related to the oscillation of the sulfur-sulfur bond distance in the pyrite structure is strongly coupled to the energy of the conduction-band minimum, leading to an ultrafast (approximate to 100 fs) oscillation in the band gap. Depending on the coherency of the phonons, we predict that this effect can cause changes of up to +/- 0.3 eV relative to the accepted FeS2 band gap at room temperature. Harnessing this effect via temperature or irradiation with infrared light could open up numerous possibilities for novel devices such as ultrafast switches and adaptive solar absorbers.« less

Ultrafast Electron Transfer across a Nanocapsular Wall: Coumarins as Donors, Viologen as Acceptor, and Octa Acid Capsule as the Mediator.

PubMed

Chuang, Chi-Hung; Porel, Mintu; Choudhury, Rajib; Burda, Clemens; Ramamurthy, V

2018-01-11

Results of our study on ultrafast electron transfer (eT) dynamics from coumarins (coumarin-1, coumarin-480, and coumarin-153) incarcerated within octa acid (OA) capsules as electron donors to methyl viologen dissolved in water as acceptor are presented. Upon photoexcitation, coumarin inside the OA capsule transfers an electron to the acceptor electrostatically attached to the capsule leading to a long-lived radical-ion pair separated by the OA capsular wall. This charge-separated state returns to the neutral ground state via back electron transfer on the nanosecond time scale. This system allows for ultrafast electron transfer processes through a molecular wall from the apolar capsular interior to the highly polar (aqueous) environment on the femtosecond time scale. Employing femtosecond transient absorption spectroscopy, distinct rates of both forward (1-25 ps) and backward eT (700-1200 ps) processes were measured. Further understanding of the energetics is provided using Rehm-Weller analysis for the investigated photoinduced eT reactions. The results provide the rates of the eT across a molecular wall, akin to an isotropic solution, depending on the standard free energy of the reaction. The insights from this work could be utilized in the future design of efficient electron transfer processes across interfaces separating apolar and polar environments.

Ultrafast magneto-optical spectroscopy of GaMnAs (Invited Paper)

NASA Astrophysics Data System (ADS)

Heroux, Jean Benoit; Kojima, Eiji; Ino, Y.; Hashimoto, Y.; Katsumoto, Shingo; Iye, Yasushiro; Kuwata-Gonokami, Makoto

2005-04-01

Spin dynamics in the III-V dilute magnetic semiconductor GaMnAs is investigated by photo-induced demagnetization. Experimental results obtained from two different time-dependent characterization techniques - "two color-probe" magneto-optical Kerr effect (TR-MOKE) and mid-infrared differential transmittance -- are compared. Upon photo-excitation with a 100 fs, 3.1 eV light pulse, a long demagnetization time in the hundreds of picoseconds timescale is found by TR-MOKE, indicating a spin-dependent band structure in this material. In mid-infrared measurements, a positive increase of the differential transmittance is observed in the same time interval when the sample is cooled below its Currie temperature. It is shown that this mid-infrared absorption feature is directly related to ferromagnetism in this material. The magnetism-related component of the broad DC mid-infrared absorption peak characteristic of this p-type material could be observed with this time-resolved measurement. Experimental results were simulated with a model describing the interaction between three thermal reservoirs (hole, spin and lattice) and taking thermal diffusion into account.

Generation of terahertz from ZnGeP2 crystal and its application to record the time-resolved photoacoustic spectra of nitromethane

NASA Astrophysics Data System (ADS)

Rao, K. S.; Ganesh, D.; Chaudhary, A. K.

2018-07-01

This paper reports the generation of THz radiation through the optical rectification process from type-I cut Zinc germanium phosphate (ZnGeP2or ZGP) crystal using optical parametric amplifier pulses (tunable between 1.15-1.6 μm range, at 60 fs pulse duration and 1 kHz repetition rate) as pump wavelengths at an average power of 50 mW. Also, we have ascertained the conversion efficiency of the generated THz signal, which is of the order of 0.62% at 1.5 THz. Further, the generated radiation is employed for the recording of absorption bands (in terms of PA spectra) of nitromethane and methanol vapor at room temperature using bandpass filters having central frequencies of 0.5 and 1.5 THz. In addition, we have employed the UV-266 nm as an excitation wavelength to record the time domain photoacoustic (PA) spectra of nitromethane (CH3NO2). In case of UV and THz radiations, the excitation mechanisms follow strong electronic (π∗ ← n) and weak vibrational-rotational due to ultrafast transition, respectively. In case of UV-266 nm the characteristic PA spectra has been also presented as a function of vapor pressure and data acquisition time with two different PA cells. The current study reveals the effect of the nanoseconds and the ultrafast pulses on some of the common excited acoustic modes (due to similar functional group), which follow two different types of excitation mechanism.

Synthesis of Photoresponsive Dual NIR Two-Photon Absorptive [60]Fullerene Triads and Tetrads

PubMed Central

Jeon, Seaho; Wang, Min; Tan, Loon-Seng; Cooper, Thomas; Hamblin, Michael R.; Chiang, Long Y.

2013-01-01

Broadband nonlinear optical (NLO) organic nanostructures exhibiting both ultrafast photoresponse and a large cross-section of two-photon absorption throughout a wide NIR spectrum may make them suitable for use as nonlinear biophotonic materials. We report here the synthesis and characterization of two C60-(antenna)x analogous compounds as branched triad C60(>DPAF-C18)(>CPAF-C2M) and tetrad C60(>DPAF-C18)(>CPAF-C2M)2 nanostructures. These compounds showed approximately equal extinction coefficients of optical absorption over 400–550 nm that corresponds to near-IR two-photon based excitation wavelengths at 780–1,100 nm. Accordingly, they may be utilized as potential precursor candidates to the active-core structures of photosensitizing nanodrugs for 2γ-PDT in the biological optical window of 800–1,050 nm. PMID:23941881

Chirped pulse digital holography for measuring the sequence of ultrafast optical wavefronts

NASA Astrophysics Data System (ADS)

Karasawa, Naoki

2018-04-01

Optical setups for measuring the sequence of ultrafast optical wavefronts using a chirped pulse as a reference wave in digital holography are proposed and analyzed. In this method, multiple ultrafast object pulses are used to probe the temporal evolution of ultrafast phenomena and they are interfered with a chirped reference wave to record a digital hologram. Wavefronts at different times can be reconstructed separately from the recorded hologram when the reference pulse can be treated as a quasi-monochromatic wave during the pulse width of each object pulse. The feasibility of this method is demonstrated by numerical simulation.

Ultrafast Primary Reactions in the Photosystems of Oxygen-Evolving Organisms

NASA Astrophysics Data System (ADS)

Holzwarth, A. R.

In oxygen-evolving photosynthetic organisms (plants, green algae, cyanobacteria), the primary steps of photosynthesis occur in two membrane-bound protein supercomplexes, Photosystem I (PS I) and Photosystem II (PS II), located in the thylakoid membrane (c.f. Fig. 7.1) along with two other important protein complexes, the cytochrome b6/f complex and the ATP-synthase [1]. Each of the photosystems consists of a reaction center (RC) where the photoinduced early electron transfer processes occur, of a so-called core antenna consisting of chlorophyll (Chl) protein complexes responsible for light absorption and ultrafast energy transfer to the RC pigments, and additional peripheral antenna complexes of various kinds that increase the absorption cross-section. The peripheral complexes are Chl a/b-protein complexes in higher plants and green algae (LHC I or LHC II for PS I or PS II, respectively) and so-called phycobilisomes in cyanobacteria and red algae [2-4]. The structures and light-harvesting functions of these antenna systems have been extensively reviewed [2, 5-9]. Recently, X-ray structures of both PS I and PS II antenna/RC complexes have been determined, some to atomic resolution. Although many details of the pigment content and organization of the RCs and antenna systems of PS I and PS II have been known before, the high resolution structures of the integral complexes allow us for the first time to try to understand structure/function relationships in detail. This article covers our present understanding of the ultrafast energy transfer and early electron transfer processes occurring in the photosystems of oxygen-evolving organisms. The main emphasis will be on the electron transfer processes. However, in both photosystems the kinetics of the energy transfer processes in the core antennae is intimately interwoven with the kinetics of the electron transfer steps. Since both types of processes occur on a similar time scale, their kinetics cannot be considered separately in any experiment and consequently they have to be discussed together.

Experimental Identification of Ultrafast Reverse Hole Transfer at the Interface of the Photoexcited Methanol/Graphitic Carbon Nitride System.

PubMed

Chen, Zongwei; Zhang, Qun; Luo, Yi

2018-05-04

An experimental scrutiny of the photoexcited hole dynamics in a prototypical system is presented in which hole-scavenging methanol molecules are chemisorbed on a graphitic carbon nitride (g-C 3 N 4 ) substrate. A set of comparison and control experiments by means of femtosecond time-resolved transient absorption (fs-TA) spectroscopy were conducted. The elusive reverse hole transfer (RHT) process was identified, which occurs on a timescale of a few hundred picoseconds. The critical role of interfacially chemisorbed methoxy (instead of methanol) as the dominant species responsible for hole scavenging was confirmed by a control experiment using protonated g-C 3 N 4 as the substrate. A hot-hole transfer effect was revealed by implementing different interband photoexcitation scenarios. The RHT rate is the key factor governing the hole-scavenging ability of different hole scavengers. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Linear and Nonlinear Optical Properties of Photoresponsive [60]Fullerene Hybrid Triads and Tetrads with Dual NIR Two-Photon Absorption Characteristics

PubMed Central

Jeon, Seaho; Haley, Joy; Flikkema, Jonathan; Nalla, Venkatram; Wang, Min; Sfeir, Matthew; Tan, Loon-Seng; Cooper, Thomas; Ji, Wei; Hamblin, Michael R.; Chiang, Long Y.

2013-01-01

Two C60-(antenna)x analogous compounds having branched hybrid triad C60(>DPAF-C18)(>CPAF-C2M) and tetrad C60(>DPAF-C18)(>CPAF-C2M)2 nanostructures were synthesized and characterized. The structural design was intended to facilitate the ultrafast fs intramolecular energy-transfer from photoexcited C60[>1(DPAF)*-C18](>CPAF-C2M)1or2 or C60(>DPAF-C18)[>1(CPAF)*-C2M]1or2 to the C60> cage moiety upon two-photon pumping at either 780 or 980 nm, respectively. The latter nanostructure showed approximately equal extinction coefficients of optical absorption over 400–550 nm that corresponds to near-IR two-photon based excitation wavelengths at 780–1100 nm for broadband nonlinear optical (NLO) applications. Aside from their enhanced two-photon absorption (2PA) activity at 780 nm, we also demonstrated ultrafast photo-responses at 980 nm showing 2PA cross-section (σ2) values of 995–1100 GM for the hybrid tetrad. These σ2 values were correlated to the observed good efficiency in reducing fs light-transmittance down to 35% at the light intensity of 110 GW/cm2. Accordingly, 2PA characteristics of these nanostructures at multiple NIR wavelengths provided support for their suitability in uses as broadband NLO nanomaterials at 600–1100 nm that includes the 2PA ability of two antenna, DPAF (700–850 nm) and CPAF (850–1100 nm), and the fullerene cage at shorter wavelengths (600–700 nm). PMID:24163713

Ultrafast photo-initiated molecular quantum dynamics in the DNA dinucleotide d(ApG) revealed by broadband transient absorption spectroscopy.

PubMed

Stuhldreier, Mayra C; Temps, Friedrich

2013-01-01

The ultrafast photo-initiated quantum dynamics of the adenine-guanine dinucleotide d(ApG) in aqueous solution (pH 7) has been studied by femtosecond time-resolved spectroscopy after excitation at lambda = 260 nm. The results reveal a hierarchy of processes on time scales from tau < 100 fs to tau > 100 ps. Characteristic spectro-temporal signatures are observed indicating the transformation of the molecules in the electronic relaxation from the photo-excited state to a long-lived exciplex. In particular, broadband UV/VIS excited-state absorption (ESA) measurements detected a distinctive absorption by the excited dinucleotide around lambda = 335 nm, approximately 0.5 eV to the blue compared to the maximum of the broad and unstructured ESA spectrum after excitation of an equimolar mixture of the mononucleotides dAMP and dGMP. A similar feature has been identified as signature of the excimer in the dynamics of the adenine dinucleotide d(ApA). The lifetime of the d(ApG) exciplex was found to be tau = 124 +/- 4 ps both from the ESA decay time and from the ground-state recovery time, far longer than the sub-picosecond lifetimes of excited dAMP or dGMP. Fluorescence-time profiles measured by the up-conversion technique indicate that the exciplex state is reached around approximately 6 ps after excitation. Very weak residual fluorescence at longer times red-shifted to the emission from the photo-excited state shows that the exciplex is almost optically dark, but still has enough oscillator strength to give rise to the dual fluorescence of the dinucleotide in the static fluorescence spectrum.

Ultrafast Excited-State Dynamics of Cytosine Aza-Derivative and Analogues.

PubMed

Zhou, Zhongneng; Zhou, Xueyao; Wang, Xueli; Jiang, Bin; Li, Yongle; Chen, Jinquan; Xu, Jianhua

2017-04-13

Excited state dynamics of 5-azacytosine (5-AC), 2,4-diamino-1,3,5-triazine (2,4-DT), and 2-amino-1,3,5-triazine (2-AT) were comprehensively investigated by steady state absorption, fluorescence, and femtosecond transient absorption measurements. Time-dependent density functional theory (TDDFT) calculations were performed to help assign the absorption bands and understand the excited state decay mechanisms. The experimental results of excited singlet state dynamics for 5-AC, 2,4-DT, and 2-AT with femtosecond time resolution were reported for the first time. Two distinct decay pathways, with ∼1 ps and tens of picosecond lifetimes, were observed in 5-AC. Only one decay pathway with 17 ps lifetime was observed in 2,4-DT while an emissive state was found in 2-AT. TDDFT calculations suggest that 5-AC has a dark nπ* (S 1 ) state below the first allowed ππ* (S 2 ) state, which leads to the ultrafast decay of the ππ* state. In 2,4-DT, there is no dark nπ* state below the bright ππ* (S 1 ) state and the 17 ps lifetime is assigned to the relaxation from the ππ* (S 1 ) state to ground state. Two dark nπ* states (S 1 and S 2 ) were found in 2-AT, which exhibits much more complex excited state dynamics compared with the other two. Photoluminescence in 2-AT has been confirmed to be fluorescence emission from its bright ππ* (S 3 ) state. Our results strongly suggest that electronic structures are very sensitive to the substitution on the triazine ring and that the photophysical properties of nucleic acid analogues depend highly on their molecular structures.

N-H stretching vibrations of guanosine-cytidine base pairs in solution: ultrafast dynamics, couplings, and line shapes.

PubMed

Fidder, Henk; Yang, Ming; Nibbering, Erik T J; Elsaesser, Thomas; Röttger, Katharina; Temps, Friedrich

2013-02-07

Dynamics and couplings of N-H stretching vibrations of chemically modified guanosine-cytidine (G·C) base pairs in chloroform are investigated with linear infrared spectroscopy and ultrafast two-dimensional infrared (2D-IR) spectroscopy. Comparison of G·C absorption spectra before and after H/D exchange reveals significant N-H stretching absorption in the region from 2500 up to 3300 cm(-1). Both of the local stretching modes ν(C)(NH(2))(b) of the hydrogen-bonded N-H moiety of the cytidine NH(2) group and ν(G)(NH) of the guanosine N-H group contribute to this broad absorption band. Its complex line shape is attributed to Fermi resonances of the N-H stretching modes with combination and overtones of fingerprint vibrations and anharmonic couplings to low-frequency modes. Cross-peaks in the nonlinear 2D spectra between the 3491 cm(-1) free N-H oscillator band and the bands centered at 3145 and 3303 cm(-1) imply N-H···O═C hydrogen bond character for both of these transitions. Time evolution illustrates that the 3303 cm(-1) band is composed of a nearly homogeneous band absorbing at 3301 cm(-1), ascribed to ν(G)(NH(2))(b), and a broad inhomogeneous band peaking at 3380 cm(-1) with mainly guanosine carbonyl overtone character. Kinetics and signal strengths indicate a <0.2 ps virtually complete population transfer from the excited ν(G)(NH(2))(b) mode to the ν(G)(NH) mode at 3145 cm(-1), suggesting lifetime broadening as the dominant source for the homogeneous line shape of the 3301 cm(-1) transition. For the 3145 cm(-1) band, a 0.3 ps population lifetime was obtained.

Ultrafast excited-state dynamics in shape- and composition-controlled gold–silver bimetallic nanostructures

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zarick, Holly F.; Boulesbaa, Abdelaziz; Talbert, Eric M.

In this paper, we have examined the ultrafast dynamics of shape- and composition-controlled bimetallic Au/Ag core/shell nanostructures with transient absorption spectroscopy (TAS) as a function of Ag layer thickness (0–15 nm) and pump excitation fluence (50–500 nJ/pulse). Our synthesis approach generated both bimetallic nanocubes and nanopyramids with distinct dipolar plasmon resonances and plasmon dephasing behavior at the resonance. Lifetimes obtained from TAS at low powers (50 nJ/pulse) demonstrated minimal dependence on the Ag layer thickness, whereas at high power (500 nJ/pulse) a rise in electron–phonon coupling lifetime (τ 1) was observed with increasing Ag shell thickness for both nanocubes andmore » nanopyramids. This is attributable to the stronger absorption of the 400 nm pump pulse with higher Ag content, which induced higher electron temperatures. The phonon–phonon scattering lifetime (τ 2) also rises with increasing Ag layer, contributed both by the increasing size of the Au/Ag nanostructures as well as by surface chemistry effects. Further, we observed that even the thinnest, 2 nm, Ag shell strongly impacts both τ 1 and τ 2 at high power despite minimal change in overall size, indicating that the nanostructure composition also strongly impacts the thermalization temperature following absorption of 400 nm light. We also observed a shape-dependent trend at high power, where τ 2 increased for the nanopyramids with increasing Ag shell thickness and nanostructure size, but bimetallic nanocubes demonstrated an unexpected decrease in τ 2 for the thickest, 15 nm, Ag shell. This was attributed to the larger number of corners and edges in the nanocubes relative to the nanopyramids.« less

Ultrafast responses of two V-shaped compounds with a reverse conjugated structural configuration: an investigation of the reason for the enhanced two-photon absorption cross-section

NASA Astrophysics Data System (ADS)

Wang, Yaochuan; Jiang, Yihua; Liu, Dajun; Wang, Yizhuo; Wang, Guiqiu; Hua, Jianli

2018-06-01

To study the effect of the conjugated structural configuration on the two-photon absorption (TPA) properties of V-shaped compounds, two 1,3,5-triazine-based compounds with the same electron donor (D) and acceptor (A) connected in a reverse-conjugated structural configuration ( T02: D-π-A-π-D; R02: A-π-D-π-A) were systematically investigated using steady-state and transient absorption spectroscopy, open-aperture Z-scan measurements, and two-photon fluorescence measurements. The TPA cross-section of compound R02 connected in a A-π-D-π-A-conjugated structural configuration with triphenylamine as the central core was 203 GM, which showed a 2.3-fold enhancement compared with compound T02 connected in a reverse D-π-A-π-D-conjugated structural configuration (90 GM, with 1,3,5-triazine as the central core). This result indicates that the conjugated structural configuration plays an important role in the TPA properties. A two-color pump-probe experiment was used to investigate the effect of the conjugated structural configuration on the excited state and intra-molecular charge transfer (ICT) properties of these V-shaped compounds. The formation and relaxation lifetimes of the ICT state were determined. The results indicate that the electron-donating/accepting strength of the central group, which serves as a communal group for two D-π-A subunits, was confirmed to be a key role to the overall effect of the ICT for V-shaped compounds. These ultrafast dynamic results are in agreement with the TPA properties.

Ultrafast Spectroscopic Studies of Two-Photon States in Third Order Optical Processes of Dye Chromophores.

NASA Astrophysics Data System (ADS)

Yu, Yi-Zhong

1995-01-01

Conjugated organic and polymeric materials usually have large, nonresonant third order optical nonlinearity due to correlations of their delocalized pi -electrons. Most materials studied so far show positive values of third order nonlinear susceptibility when all frequencies that generate the third order effect are below any optical transition. A new class of organic molecules, namely indole squarylium (ISQ) and anilinium squarylium (BSQ), exhibit negative < gamma(-omega_4;omega_1, omega_2,omega_3)> when all three frequencies, omega_1, omega_2 and omega_3, lie below the first electronic transition. Although quantum many-electron calculations based on multiple-excitation configuration interaction have shown that the negative third order coefficient is essentially due to the contribution from high-lying two-photon states, the field of experimental studies exploring the microscopic origins of the negative squaraines remains uncultivated. The work presented in this thesis involves extensive experimental investigation of squaraines using techniques such as time-resolved transit absorption spectroscopy and saturable absorption. Theoretical simulations studying nonlinear absorption behavior of a simplified two-level system with ultrashort pulses are also presented. Part of the thesis is dedicated to the development, fabrication and characterization of our ultrafast laser system which offers tunable femtosecond pulses at wavelengths from UV to IR and served as a major tool in the experimental measurements. The dynamics of the population inversion between the ground state and the first excited state was also investigated through time-resolved experiments. The experiment results agree well with the theoretical predictions. Strong couplings between the gateway state and high-lying two-photon states were observed in BSQ squarylium molecules, which suggested a complete quantum calculation with multiple energy levels is required to correctly describe the negative third order effect.

Suzaku Discovery of Ultra-fast Outflows in Radio-loud AGN

NASA Astrophysics Data System (ADS)

Sambruna, Rita M.; Tombesi, F.; Reeves, J.; Braito, V.; Gofford, J.; Cappi, M.

2010-03-01

We present the results of an analysis of the 3.5--10.5 keV spectra of five bright Broad-Line Radio Galaxies (BLRGs) using proprietary and archival Suzaku observations. In three sources -- 3C 111, 3C 120, and 3C 390.3 -- we find evidence, for the first time in a radio-loud AGN, for absorption features at observed energies 7 keV and 8--9 keV, with high significance according to both the F-test and extensive Monte Carlo simulations (99% or larger). In the remaining two BLRGs, 3C 382 and 3C 445, there is no evidence for such absorption features in the XIS spectra. If interpreted as due to Fe XXV and/or Fe XXVI K-shell resonance lines, the absorption features in 3C 111, 3C 120, and 3C 390.3 imply an origin from an ionized gas outflowing with velocities in the range v 0.04-0.15c, reminiscent of Ultra-Fast Outflows (UFOs) previously observed in radio-quiet Seyfert galaxies. A fit with specific photoionization models gives ionization parameters log ξ 4--5.6 erg s-1 cm and column densities of NH 1022-23 cm-2, similar to the values observed in Seyferts. Based on light travel time arguments, we estimate that the UFOs in the three BLRGs are located within 20--500 gravitational radii from the central black hole, and thus most likely are connected to disk winds/outflows. Our estimates show that the UFOs mass outflow rate is comparable to the accretion rate and their kinetic energy a significant fraction of the AGN bolometric luminosity, making these outflows significant for the global energetic of these systems, in particular for mechanisms of jet formation.

Magnetically Driven Accretion Disk Winds and Ultra-fast Outflows in PG 1211+143

NASA Astrophysics Data System (ADS)

Fukumura, Keigo; Tombesi, Francesco; Kazanas, Demosthenes; Shrader, Chris; Behar, Ehud; Contopoulos, Ioannis

2015-05-01

We present a study of X-ray ionization of MHD accretion-disk winds in an effort to constrain the physics underlying the highly ionized ultra-fast outflows (UFOs) inferred by X-ray absorbers often detected in various sub classes of Seyfert active galactic nuclei (AGNs). Our primary focus is to show that magnetically driven outflows are indeed physically plausible candidates for the observed outflows accounting for the AGN absorption properties of the present X-ray spectroscopic observations. Employing a stratified MHD wind launched across the entire AGN accretion disk, we calculate its X-ray ionization and the ensuing X-ray absorption-line spectra. Assuming an appropriate ionizing AGN spectrum, we apply our MHD winds to model the absorption features in an XMM-Newton/EPIC spectrum of the narrow-line Seyfert, PG 1211+143. We find, through identifying the detected features with Fe Kα transitions, that the absorber has a characteristic ionization parameter of log (ξc[erg cm s-1]) ≃ 5-6 and a column density on the order of NH ≃ 1023 cm-2 outflowing at a characteristic velocity of vc/c ≃ 0.1-0.2 (where c is the speed of light). The best-fit model favors its radial location at rc ≃ 200 Ro (Ro is the black hole’s innermost stable circular orbit), with an inner wind truncation radius at Rt ≃ 30 Ro. The overall K-shell feature in the data is suggested to be dominated by Fe xxv with very little contribution from Fe xxvi and weakly ionized iron, which is in good agreement with a series of earlier analyses of the UFOs in various AGNs, including PG 1211+143.

Tuning optical absorption and photoexcited recombination dynamics in La1-xSrxFeO3-δ through A-site substitution and oxygen vacancies

NASA Astrophysics Data System (ADS)

Smolin, Sergey; Scafetta, Mark; Choquette, Amber; Sfeir, Matthew; Baxter, Jason; May, Steven

We study optical absorption and recombination dynamics in La1-xSrxFeO3-δ thin films, uncovering the effects of tuning nominal Fe valence via A-site substitution and oxygen stoichiometry. Variable angle spectroscopic ellipsometry was used to measure static optical properties, revealing a linear increase in absorption coefficient at 1.25 eV and a red-shifting of the optical absorption edge with increasing Sr fraction. The absorption spectra can be similarly tuned through the introduction of oxygen vacancies, indicating the critical role that nominal Fe valence plays in optical absorption. Dynamic optoelectronic properties were studied with ultrafast transient reflectance spectroscopy, revealing similar nanosecond photoexcited carrier lifetimes for oxygen deficient and stoichiometric films with the same nominal Fe valence. These results demonstrate that while the static optical absorption is strongly dependent on Fe valence tuned through cation or anion stoichiometry, oxygen vacancies do not appear to play a significantly detrimental role in the recombination kinetics. Nsf: ECCS-1201957, MRI DMR-0922929, MRI DMR-1040166. This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

Observing and understanding the ultrafast photochemistry in small molecules: applications to sunscreens.

PubMed

Baker, Lewis A; Stavros, Vasilios G

2016-09-01

In this review, we discuss the importance of biological and artificial photoprotection against overexposure to harmful ultraviolet radiation. Transient electronic and transient vibrational absorption spectroscopies are highlighted as important tools in understanding the energy transfer in small molecules, with a focus on the application to commercial sunscreens with representative examples given. Oxybenzone, a common ingredient in commercial sunscreens and sinapoyl malate, a biological sunscreen in plant leaves are presented as case studies.

Application of Advanced Laser Diagnostics to High-Impact Technologies: Science and Applications of Ultrafast, Ultraintense Lasers

DTIC Science & Technology

2013-11-01

ultrashort - pulse lasers because of the very large photon density. As the intensity increases, the electric field of the light can modify the Coulomb ...absorption studies of argon clusters in intense laser pulses ,” Physics of Plasmas 16(4), 043301-1 – 043301-5. Lu, W., Nicoul, M., Shymanovich, U... intensity of ultrashort - pulse lasers , possess unique and advantageous capabilities for use in a wide variety of applications and are

Detailed transient heme structures of Mb-CO in solution after CO dissociation: an X-ray transient absorption spectroscopic study.

PubMed

Stickrath, Andrew B; Mara, Michael W; Lockard, Jenny V; Harpham, Michael R; Huang, Jier; Zhang, Xiaoyi; Attenkofer, Klaus; Chen, Lin X

2013-04-25

Although understanding the structural dynamics associated with ligand photodissociation is necessary in order to correlate structure and function in biological systems, few techniques are capable of measuring the ultrafast dynamics of these systems in solution-phase at room temperature. We present here a detailed X-ray transient absorption (XTA) study of the photodissociation of CO-bound myoglobin (Fe(II)CO-Mb) in room-temperature aqueous buffer solution with a time resolution of 80 ps, along with a general procedure for handling biological samples under the harsh experimental conditions that transient X-ray experiments entail. The XTA spectra of (Fe(II)CO-Mb) exhibit significant XANES and XAFS alterations following 527 nm excitation, which remain unchanged for >47 μs. These spectral changes indicate loss of the CO ligand, resulting in a five-coordinate, domed heme, and significant energetic reorganization of the 3d orbitals of the Fe center. With the current experimental setup, each X-ray pulse in the pulse train, separated by ~153 ns, can be separately discriminated, yielding snapshots of the myoglobin evolution over time. These methods can be easily applied to other biological systems, allowing for simultaneous structural and electronic measurements of any biological system with both ultrafast and slow time resolutions, effectively mapping out all of the samples' relevant physiological processes.

Measurement of Nanoplasmonic Field Enhancement with Ultrafast Photoemission.

PubMed

Rácz, Péter; Pápa, Zsuzsanna; Márton, István; Budai, Judit; Wróbel, Piotr; Stefaniuk, Tomasz; Prietl, Christine; Krenn, Joachim R; Dombi, Péter

2017-02-08

Probing nanooptical near-fields is a major challenge in plasmonics. Here, we demonstrate an experimental method utilizing ultrafast photoemission from plasmonic nanostructures that is capable of probing the maximum nanoplasmonic field enhancement in any metallic surface environment. Directly measured field enhancement values for various samples are in good agreement with detailed finite-difference time-domain simulations. These results establish ultrafast plasmonic photoelectrons as versatile probes for nanoplasmonic near-fields.

Ultrafast all-optical imaging technique using low-temperature grown GaAs/AlxGa1 - xAs multiple-quantum-well semiconductor

NASA Astrophysics Data System (ADS)

Gao, Guilong; Tian, Jinshou; Wang, Tao; He, Kai; Zhang, Chunmin; Zhang, Jun; Chen, Shaorong; Jia, Hui; Yuan, Fenfang; Liang, Lingliang; Yan, Xin; Li, Shaohui; Wang, Chao; Yin, Fei

2017-11-01

We report and experimentally demonstrate an ultrafast all-optical imaging technique capable of single-shot ultrafast recording with a picosecond-scale temporal resolution and a micron-order two-dimensional spatial resolution. A GaAs/AlxGa1 - xAs multiple-quantum-well (MQW) semiconductor with a picosecond response time, grown using molecular beam epitaxy (MBE) at a low temperature (LT), is used for the first time in ultrafast imaging technology. The semiconductor transforms the signal beam information to the probe beam, the birefringent delay crystal time-serializes the input probe beam, and the beam displacer maps different polarization probe beams onto different detector locations, resulting in two frames with an approximately 9 ps temporal separation and approximately 25 lp/mm spatial resolution in the visible range.

Interpreting Quasi-Thermal Effects in Ultrafast Spectroscopy of Hydrogen-Bonded Systems.

PubMed

Stingel, Ashley M; Petersen, Poul B

2018-03-15

Vibrational excitation of molecules in the condensed phase relaxes through vibrational modes of decreasing energy to ultimately generate an equilibrium state in which the energy is distributed among low-frequency modes. In ultrafast vibrational spectroscopy, changes in the vibrational features of hydrogen-bonded NH and OH stretch modes are typically observed to persist long after these high-frequency vibrations have relaxed. Due to the resemblance to the spectral changes caused by heating the sample, these features are typically described as arising from a hot ground state. However, these spectral features appear on ultrafast time scales that are much too fast to result from a true thermal state, and significant differences between the thermal difference spectrum and the induced quasi-thermal changes in ultrafast spectroscopy are often observed. Here, we examine and directly compare the thermal and quasi-thermal responses of the hydrogen-bonded homodimer of 7-azaindole with temperature-dependent FTIR spectroscopy and ultrafast mid-IR continuum spectroscopy. We find that the thermal difference spectra contain contributions from both dissociation of the hydrogen bonds and from frequency shifts due to changes in the thermal population of low-frequency modes. The transient spectra in ultrafast vibrational spectroscopy are also found to contain two contributions: initial frequency shifts over 2.3 ± 0.11 ps associated with equilibration of the initial excitation, and frequency shifts associated with the excitation of several fingerprint modes, which decay over 21.8 ± 0.11 ps, giving rise to a quasi-thermal response caused by a distribution of fingerprint modes being excited within the sample ensemble. This resembles the thermal frequency shifts due to population changes of low-frequency modes, but not the overall thermal spectrum, which is dominated by features caused by dimer dissociation. These findings provide insight into the changes in the vibrational spectrum from different origins and are important for assigning, analyzing, and comparing features in thermal and ultrafast vibrational spectroscopy of hydrogen-bonded complexes.

Nonlinear THz absorption and cyclotron resonance in InSb

NASA Astrophysics Data System (ADS)

Heffernan, Kate; Yu, Shukai; Talbayev, Diyar

The emergence of coherent high-field terahertz (THz) sources in the past decade has allowed the exploration of nonlinear light-matter interaction at THz frequencies. Nonlinear THz response of free electrons in semiconductors has received a great deal of attention. Such nonlinear phenomena as saturable absorption and self-phase modulation have been reported. InSb is a narrow-gap (bandgap 0.17 eV) semiconductor with a very low electron effective mass and high electron mobility. Previous high-field THz work on InSb reported the observation of ultrafast electron cascades via impact ionization. We study the transmission of an intense THz electric field pulse by an InSb wafer at different incident THz amplitudes and 10 K temperature. Contrary to previous reports, we observe an increased transmission at higher THz field. Our observation appears similar to the saturable THz absorption reported in other semiconductors. Along with the increased absorption, we observe a strong modulation of the THz phase at high incident fields, most likely due to the self-phase modulation of the THz pulse. We also study the dependence of the cyclotron resonance on the incident THz field amplitude. The cyclotron resonance exhibits a lower strength and frequency at the higher incident THz field. The work at Tulane was supported by the Louisiana Board of Regents through the Board of Regents Support Fund Contract No. LEQSF(2012-15)-RD-A-23 and through the Pilot Funding for New Research (PFund) Contract No. LEQSF-EPS(2014)-PFUND-378.

Ultrafast transformation of graphite to diamond: an ab initio study of graphite under shock compression.

PubMed

Mundy, Christopher J; Curioni, Alessandro; Goldman, Nir; Will Kuo, I-F; Reed, Evan J; Fried, Laurence E; Ianuzzi, Marcella

2008-05-14

We report herein ab initio molecular dynamics simulations of graphite under shock compression in conjunction with the multiscale shock technique. Our simulations reveal that a novel short-lived layered diamond intermediate is formed within a few hundred of femtoseconds upon shock loading at a shock velocity of 12 kms (longitudinal stress>130 GPa), followed by formation of cubic diamond. The layered diamond state differs from the experimentally observed hexagonal diamond intermediate found at lower pressures and previous hydrostatic calculations in that a rapid buckling of the graphitic planes produces a mixture of hexagonal and cubic diamond (layered diamond). Direct calculation of the x-ray absorption spectra in our simulations reveals that the electronic structure of the final state closely resembles that of compressed cubic diamond.

All-optical reservoir computer based on saturation of absorption.

PubMed

Dejonckheere, Antoine; Duport, François; Smerieri, Anteo; Fang, Li; Oudar, Jean-Louis; Haelterman, Marc; Massar, Serge

2014-05-05

Reservoir computing is a new bio-inspired computation paradigm. It exploits a dynamical system driven by a time-dependent input to carry out computation. For efficient information processing, only a few parameters of the reservoir needs to be tuned, which makes it a promising framework for hardware implementation. Recently, electronic, opto-electronic and all-optical experimental reservoir computers were reported. In those implementations, the nonlinear response of the reservoir is provided by active devices such as optoelectronic modulators or optical amplifiers. By contrast, we propose here the first reservoir computer based on a fully passive nonlinearity, namely the saturable absorption of a semiconductor mirror. Our experimental setup constitutes an important step towards the development of ultrafast low-consumption analog computers.

Monomolecular layer of squarylium dye J aggregates exhibiting a femtosecond optical response of delocalized excitons

NASA Astrophysics Data System (ADS)

Furuki, Makoto; Pu, Lyong Sun; Sasaki, Fumio; Kobayashi, Shyunsuke; Tani, Toshiro

1998-05-01

We report on the demonstration of the femtosecond nonlinear optical response from a two-dimensional monomolecular layer of squarylium dye J aggregate at 5 °C. The formation of a monomolecular layer Langmuir film was achieved by spreading squarylium dye modified by two propyl and two hexyl groups at the air-water interface, which resulted in a very strong J band (o.d.=0.3) at 777 nm. The transient absorption spectra in a resonant pump-probe measurement showed a low absorption saturation power (9.7×106W/cm2) and an ultrafast response (300 fs), which are indicative of exciton delocalization over 18 molecules in this J aggregate, even at 5 °C.

Ultrafast control and monitoring of material properties using terahertz pulses

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bowlan, Pamela Renee

These are a set of slides on ultrafast control and monitoring of material properties using terahertz pulses. A few of the topics covered in these slides are: How fast is a femtosecond (fs), Different frequencies probe different properties of molecules or solids, What can a THz pulse do to a material, Ultrafast spectroscopy, Generating and measuring ultrashort THz pulses, Tracking ultrafast spin dynamics in antiferromagnets through spin wave resonances, Coherent two-dimensional THz spectroscopy, and Probing vibrational dynamics at a surface. Conclusions are: Coherent two-dimensional THz spectroscopy: a powerful approach for studying coherence and dynamics of low energy resonances. Applying thismore » to graphene we investigated the very strong THz light mater interaction which dominates over scattering. Useful for studying coupled excitations in multiferroics and monitoring chemical reactions. Also, THz-pump, SHG-probe spectoscopy: an ultrafast, surface sensitive probe of atomic-scale symmetry changes and nonlinear phonon dymanics. We are using this in Bi 2Se 3 to investigate the nonlinear surface phonon dynamics. This is potentially very useful for studying catalysis.« less

Ultrafast Solvation Dynamics and Vibrational Coherences of Halogenated Boron-Dipyrromethene Derivatives Revealed through Two-Dimensional Electronic Spectroscopy.

PubMed

Lee, Yumin; Das, Saptaparna; Malamakal, Roy M; Meloni, Stephen; Chenoweth, David M; Anna, Jessica M

2017-10-18

Boron-dipyrromethene (BODIPY) chromophores have a wide range of applications, spanning areas from biological imaging to solar energy conversion. Understanding the ultrafast dynamics of electronically excited BODIPY chromophores could lead to further advances in these areas. In this work, we characterize and compare the ultrafast dynamics of halogenated BODIPY chromophores through applying two-dimensional electronic spectroscopy (2DES). Through our studies, we demonstrate a new data analysis procedure for extracting the dynamic Stokes shift from 2DES spectra revealing an ultrafast solvent relaxation. In addition, we extract the frequency of the vibrational modes that are strongly coupled to the electronic excitation, and compare the results of structurally different BODIPY chromophores. We interpret our results with the aid of DFT calculations, finding that structural modifications lead to changes in the frequency, identity, and magnitude of Franck-Condon active vibrational modes. We attribute these changes to differences in the electron density of the electronic states of the structurally different BODIPY chromophores.

Ultrafast laser ablation for targeted atherosclerotic plaque removal

NASA Astrophysics Data System (ADS)

Lanvin, Thomas; Conkey, Donald B.; Descloux, Laurent; Frobert, Aurelien; Valentin, Jeremy; Goy, Jean-Jacques; Cook, Stéphane; Giraud, Marie-Noelle; Psaltis, Demetri

2015-07-01

Coronary artery disease, the main cause of heart disease, develops as immune cells and lipids accumulate into plaques within the coronary arterial wall. As a plaque grows, the tissue layer (fibrous cap) separating it from the blood flow becomes thinner and increasingly susceptible to rupturing and causing a potentially lethal thrombosis. The stabilization and/or treatment of atherosclerotic plaque is required to prevent rupturing and remains an unsolved medical problem. Here we show for the first time targeted, subsurface ablation of atherosclerotic plaque using ultrafast laser pulses. Excised atherosclerotic mouse aortas were ablated with ultrafast near-infrared (NIR) laser pulses. The physical damage was characterized with histological sections of the ablated atherosclerotic arteries from six different mice. The ultrafast ablation system was integrated with optical coherence tomography (OCT) imaging for plaque-specific targeting and monitoring of the resulting ablation volume. We find that ultrafast ablation of plaque just below the surface is possible without causing damage to the fibrous cap, which indicates the potential use of ultrafast ablation for subsurface atherosclerotic plaque removal. We further demonstrate ex vivo subsurface ablation of a plaque volume through a catheter device with the high-energy ultrafast pulse delivered via hollow-core photonic crystal fiber.

The Ultrafast Wolff Rearrangement in the Gas Phase

NASA Astrophysics Data System (ADS)

Steinbacher, Andreas; Roeding, Sebastian; Brixner, Tobias; Nuernberger, Patrick

The Wolff rearrangement of gas-phase 5-diazo Meldrum's acid is disclosed with femtosecond ion spectroscopy. Distinct differences are found for 267 nm and 200 nm excitation, the latter leading to even two ultrafast rearrangement reactions.

Ultrafast Multi-Dimentional Infrared Vibrational Echo Spectroscopy of Molecular Dynamics on Surfaces and in Bulk Systems

DTIC Science & Technology

2012-02-28

dimethylsulfoxide ( DMSO ). When chloroform is dissolved in a mixed solvent consisting of acetone and DMSO , both types of hydrogen bonded complexes exist. The...transition (negative) in the 2D IR spectrum. Also, line shape distortions caused by solvent background absorption and finite pulse durations do not affect...conditions as  = 7  1 ps. This is the first direct measurement of hydrogen bond exchange. b. Solute- Solvent Complex Switching Dynamics3 Hydrogen

Ultrafast Multi-Dimensional Infrared Vibrational Echo Spectroscopy of Molecular Dynamics on Surfaces and in Bulk Systems

DTIC Science & Technology

2012-02-28

dimethylsulfoxide ( DMSO ). When chloroform is dissolved in a mixed solvent consisting of acetone and DMSO , both types of hydrogen bonded complexes exist. The...transition (negative) in the 2D IR spectrum. Also, line shape distortions caused by solvent background absorption and finite pulse durations do not affect...conditions as  = 7  1 ps. This is the first direct measurement of hydrogen bond exchange. b. Solute- Solvent Complex Switching Dynamics3 Hydrogen

Perspective: Ultrafast magnetism and THz spintronics

NASA Astrophysics Data System (ADS)

Walowski, Jakob; Münzenberg, Markus

2016-10-01

This year the discovery of femtosecond demagnetization by laser pulses is 20 years old. For the first time, this milestone work by Bigot and coworkers gave insight directly into the time scales of microscopic interactions that connect the spin and electron system. While intense discussions in the field were fueled by the complexity of the processes in the past, it now became evident that it is a puzzle of many different parts. Rather than providing an overview that has been presented in previous reviews on ultrafast processes in ferromagnets, this perspective will show that with our current depth of knowledge the first applications are developed: THz spintronics and all-optical spin manipulation are becoming more and more feasible. The aim of this perspective is to point out where we can connect the different puzzle pieces of understanding gathered over 20 years to develop novel applications. Based on many observations in a large number of experiments. Differences in the theoretical models arise from the localized and delocalized nature of ferromagnetism. Transport effects are intrinsically non-local in spintronic devices and at interfaces. We review the need for multiscale modeling to address the processes starting from electronic excitation of the spin system on the picometer length scale and sub-femtosecond time scale, to spin wave generation, and towards the modeling of ultrafast phase transitions that altogether determine the response time of the ferromagnetic system. Today, our current understanding gives rise to the first usage of ultrafast spin physics for ultrafast magnetism control: THz spintronic devices. This makes the field of ultrafast spin-dynamics an emerging topic open for many researchers right now.

Ultrafast photophysics of transition metal complexes.

PubMed

Chergui, Majed

2015-03-17

The properties of transition metal complexes are interesting not only for their potential applications in solar energy conversion, OLEDs, molecular electronics, biology, photochemistry, etc. but also for their fascinating photophysical properties that call for a rethinking of fundamental concepts. With the advent of ultrafast spectroscopy over 25 years ago and, more particularly, with improvements in the past 10-15 years, a new area of study was opened that has led to insightful observations of the intramolecular relaxation processes such as internal conversion (IC), intersystem crossing (ISC), and intramolecular vibrational redistribution (IVR). Indeed, ultrafast optical spectroscopic tools, such as fluorescence up-conversion, show that in many cases, intramolecular relaxation processes can be extremely fast and even shorter than time scales of vibrations. In addition, more and more examples are appearing showing that ultrafast ISC rates do not scale with the magnitude of the metal spin-orbit coupling constant, that is, that there is no heavy-atom effect on ultrafast time scales. It appears that the structural dynamics of the system and the density of states play a crucial role therein. While optical spectroscopy delivers an insightful picture of electronic relaxation processes involving valence orbitals, the photophysics of metal complexes involves excitations that may be centered on the metal (called metal-centered or MC) or the ligand (called ligand-centered or LC) or involve a transition from one to the other or vice versa (called MLCT or LMCT). These excitations call for an element-specific probe of the photophysics, which is achieved by X-ray absorption spectroscopy. In this case, transitions from core orbitals to valence orbitals or higher allow probing the electronic structure changes induced by the optical excitation of the valence orbitals, while also delivering information about the geometrical rearrangement of the neighbor atoms around the atom of interest. With the emergence of new instruments such as X-ray free electron lasers (XFELs), it is now possible to perform ultrafast laser pump/X-ray emission probe experiments. In this case, one probes the density of occupied states. These core-level spectroscopies and other emerging ones, such as photoelectron spectroscopy of solutions, are delivering a hitherto unseen degree of detail into the photophysics of metal-based molecular complexes. In this Account, we will give examples of applications of the various methods listed above to address specific photophysical processes.

Direct Imaging of Frenkel Exciton Transport by Ultrafast Microscopy.

PubMed

Zhu, Tong; Wan, Yan; Huang, Libai

2017-07-18

Long-range transport of Frenkel excitons is crucial for achieving efficient molecular-based solar energy harvesting. Understanding of exciton transport mechanisms is important for designing materials for solar energy applications. One major bottleneck in unraveling of exciton transport mechanisms is the lack of direct measurements to provide information in both spatial and temporal domains, imposed by the combination of fast energy transfer (typically ≤1 ps) and short exciton diffusion lengths (typically ≤100 nm). This challenge requires developing experimental tools to directly characterize excitation energy transport, and thus facilitate the elucidation of mechanisms. To address this challenge, we have employed ultrafast transient absorption microscopy (TAM) as a means to directly image exciton transport with ∼200 fs time resolution and ∼50 nm spatial precision. By mapping population in spatial and temporal domains, such approach has unraveled otherwise obscured information and provided important parameters for testing exciton transport models. In this Account, we discuss the recent progress in imaging Frenkel exciton migration in molecular crystals and aggregates by ultrafast microscopy. First, we establish the validity of the TAM methods by imaging singlet and triplet exciton transport in a series of polyacene single crystals that undergo singlet fission. A new singlet-mediated triplet transport pathway has been revealed by TAM, resulting from the equilibrium between triplet and singlet exciton populations. Such enhancement of triplet exciton transport enables triplet excitons to migrate as singlet excitons and leads to orders of magnitude faster apparent triplet exciton diffusion rate in the picosecond and nanosecond time scales, favorable for solar cell applications. Next we discuss how information obtained by ultrafast microscopy can evaluate coherent effects in exciton transport. We use tubular molecular aggregates that could support large exciton delocalization sizes as a model system. The initial experiments measure exciton diffusion constants of 3-6 cm 2 s -1 , 3-5 times higher than the incoherent limit predicted by theory, suggesting that coherent effects play a role. In summary, combining ultrafast spectroscopic methods with microscopic techniques provides a direct approach for obtaining important parameters to unravel the underlying exciton transport mechanisms in molecular solids. We discuss future directions to bridge the gap in understanding of fundamental energy transfer theories to include coherent and incoherent effects. We are still in the infancy of ultrafast microscopy, and the vast potential is not limited to the systems discussed in this Account.

Acceptor and Excitation Density Dependence of the Ultrafast Polaron Absorption Signal in Donor-Acceptor Organic Solar Cell Blends.

PubMed

Zarrabi, Nasim; Burn, Paul L; Meredith, Paul; Shaw, Paul E

2016-07-21

Transient absorption spectroscopy on organic semiconductor blends for solar cells typically shows efficient charge generation within ∼100 fs, accounting for the majority of the charge carriers. In this Letter, we show using transient absorption spectroscopy on blends containing a broad range of acceptor content (0.01-50% by weight) that the rise of the polaron signal is dependent on the acceptor concentration. For low acceptor content (<10% by weight), the polaron signal rises gradually over ∼1 ps with most polarons generated after 200 fs, while for higher acceptor concentrations (>10%) most polarons are generated within 200 fs. The rise time in blends with low acceptor content was also found to be sensitive to the pump fluence, decreasing with increasing excitation density. These results indicate that the sub-100 fs rise of the polaron signal is a natural consequence of both the high acceptor concentrations in many donor-acceptor blends and the high excitation densities needed for transient absorption spectroscopy, which results in a short average distance between the exciton and the donor-acceptor interface.

Two-Photon Absorption in Organometallic Bromide Perovskites.

PubMed

Walters, Grant; Sutherland, Brandon R; Hoogland, Sjoerd; Shi, Dong; Comin, Riccardo; Sellan, Daniel P; Bakr, Osman M; Sargent, Edward H

2015-09-22

Organometallic trihalide perovskites are solution-processed semiconductors that have made great strides in third-generation thin film light-harvesting and light-emitting optoelectronic devices. Recently, it has been demonstrated that large, high-purity single crystals of these perovskites can be synthesized from the solution phase. These crystals' large dimensions, clean bandgap, and solid-state order have provided us with a suitable medium to observe and quantify two-photon absorption in perovskites. When CH3NH3PbBr3 single crystals are pumped with intense 800 nm light, we observe band-to-band photoluminescence at 572 nm, indicative of two-photon absorption. We report the nonlinear absorption coefficient of CH3NH3PbBr3 perovskites to be 8.6 cm GW(-1) at 800 nm, comparable to epitaxial single-crystal semiconductors of similar bandgap. We have leveraged this nonlinear process to electrically autocorrelate a 100 fs pulsed laser using a two-photon perovskite photodetector. This work demonstrates the viability of organometallic trihalide perovskites as a convenient and low-cost nonlinear absorber for applications in ultrafast photonics.

Near- and Extended-Edge X-Ray-Absorption Fine-Structure Spectroscopy Using Ultrafast Coherent High-Order Harmonic Supercontinua

NASA Astrophysics Data System (ADS)

Popmintchev, Dimitar; Galloway, Benjamin R.; Chen, Ming-Chang; Dollar, Franklin; Mancuso, Christopher A.; Hankla, Amelia; Miaja-Avila, Luis; O'Neil, Galen; Shaw, Justin M.; Fan, Guangyu; Ališauskas, Skirmantas; Andriukaitis, Giedrius; Balčiunas, Tadas; Mücke, Oliver D.; Pugzlys, Audrius; Baltuška, Andrius; Kapteyn, Henry C.; Popmintchev, Tenio; Murnane, Margaret M.

2018-03-01

Recent advances in high-order harmonic generation have made it possible to use a tabletop-scale setup to produce spatially and temporally coherent beams of light with bandwidth spanning 12 octaves, from the ultraviolet up to x-ray photon energies >1.6 keV . Here we demonstrate the use of this light for x-ray-absorption spectroscopy at the K - and L -absorption edges of solids at photon energies near 1 keV. We also report x-ray-absorption spectroscopy in the water window spectral region (284-543 eV) using a high flux high-order harmonic generation x-ray supercontinuum with 109 photons/s in 1% bandwidth, 3 orders of magnitude larger than has previously been possible using tabletop sources. Since this x-ray radiation emerges as a single attosecond-to-femtosecond pulse with peak brightness exceeding 1026 photons/s /mrad2/mm2/1 % bandwidth, these novel coherent x-ray sources are ideal for probing the fastest molecular and materials processes on femtosecond-to-attosecond time scales and picometer length scales.

Simulation of X-ray transient absorption for following vibrations in coherently ionized F2 molecules

NASA Astrophysics Data System (ADS)

Dutoi, Anthony D.; Leone, Stephen R.

2017-01-01

Femtosecond and attosecond X-ray transient absorption experiments are becoming increasingly sophisticated tools for probing nuclear dynamics. In this work, we explore and develop theoretical tools needed for interpretation of such spectra,in order to characterize the vibrational coherences that result from ionizing a molecule in a strong IR field. Ab initio data for F2 is combined with simulations of nuclear dynamics, in order to simulate time-resolved X-ray absorption spectra for vibrational wavepackets after coherent ionization at 0 K and at finite temperature. Dihalogens pose rather difficult electronic structure problems, and the issues encountered in this work will be reflective of those encountered with any core-valence excitation simulation when a bond is breaking. The simulations reveal a strong dependence of the X-ray absorption maximum on the locations of the vibrational wave packets. A Fourier transform of the simulated signal shows features at the overtone frequencies of both the neutral and the cation, which reflect spatial interferences of the vibrational eigenstates. This provides a direct path for implementing ultrafast X-ray spectroscopic methods to visualize coherent nuclear dynamics.

Correlating Nanoscopic Energy Transfer and Far-Field Emission to Unravel Lasing Dynamics in Plasmonic Nanocavity Arrays

DOE PAGES

Deeb, Claire; Guo, Zhi; Yang, Ankun; ...

2018-01-25

Excited-state interactions between nanoscale cavities and photoactive molecules are critical in plasmonic nanolasing, although the underlying details are less-resolved. This paper reports direct visualization of the energy-transfer dynamics between two-dimensional arrays of plasmonic gold bowtie nanocavities and dye molecules. Transient absorption microscopy measurements of single bowties within the array surrounded by gain molecules showed fast excited-state quenching (2.6 ± 1 ps) characteristic of individual nanocavities. Upon optical pumping at powers above threshold, lasing action emerged depending on the spacing of the array. By correlating ultrafast microscopy and far-field light emission characteristics, we found that bowtie nanoparticles acted as isolated cavitiesmore » when the diffractive modes of the array did not couple to the plasmonic gap mode. These results demonstrate how ultrafast microscopy can provide insight into energy relaxation pathways and, specifically, how nanocavities in arrays can show single-unit nanolaser properties.« less

Ultrafast Relaxation Dynamics of Photoexcited Zinc-Porphyrin: Electronic-Vibrational Coupling

DOE PAGES

Abraham, Baxter; Nieto-Pescador, Jesus; Gundlach, Lars

2016-08-02

Cyclic tetrapyrroles are the active core of compounds with crucial roles in living systems, such as hemoglobin and chlorophyll, and in technology as photocatalysts and light absorbers for solar energy conversion. Zinc-tetraphenylporphyrin (Zn-TPP) is a prototypical cyclic tetrapyrrole that has been intensely studied in past decades. Because of its importance for photochemical processes the optical properties are of particular interest, and, accordingly, numerous studies have focused on light absorption and excited-state dynamics of Zn-TPP. Relaxation after photoexcitation in the Soret band involves internal conversion that is preceded by an ultrafast process. This relaxation process has been observed by several groups.more » Until now, it has not been established if it involves a higher lying ”dark” state or vibrational relaxation in the excited S 2 state. Here we combine high time resolution electronic and vibrational spectroscopy to show that this process constitutes vibrational relaxation in the anharmonic 2 potential.« less

Ultrafast Relaxation Dynamics of Photoexcited Zinc-Porphyrin: Electronic-Vibrational Coupling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Abraham, Baxter; Nieto-Pescador, Jesus; Gundlach, Lars

Cyclic tetrapyrroles are the active core of compounds with crucial roles in living systems, such as hemoglobin and chlorophyll, and in technology as photocatalysts and light absorbers for solar energy conversion. Zinc-tetraphenylporphyrin (Zn-TPP) is a prototypical cyclic tetrapyrrole that has been intensely studied in past decades. Because of its importance for photochemical processes the optical properties are of particular interest, and, accordingly, numerous studies have focused on light absorption and excited-state dynamics of Zn-TPP. Relaxation after photoexcitation in the Soret band involves internal conversion that is preceded by an ultrafast process. This relaxation process has been observed by several groups.more » Until now, it has not been established if it involves a higher lying ”dark” state or vibrational relaxation in the excited S 2 state. Here we combine high time resolution electronic and vibrational spectroscopy to show that this process constitutes vibrational relaxation in the anharmonic 2 potential.« less

Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses

PubMed Central

Canton, Sophie E.; Kjær, Kasper S.; Vankó, György; van Driel, Tim B.; Adachi, Shin-ichi; Bordage, Amélie; Bressler, Christian; Chabera, Pavel; Christensen, Morten; Dohn, Asmus O.; Galler, Andreas; Gawelda, Wojciech; Gosztola, David; Haldrup, Kristoffer; Harlang, Tobias; Liu, Yizhu; Møller, Klaus B.; Németh, Zoltán; Nozawa, Shunsuke; Pápai, Mátyás; Sato, Tokushi; Sato, Takahiro; Suarez-Alcantara, Karina; Togashi, Tadashi; Tono, Kensuke; Uhlig, Jens; Vithanage, Dimali A.; Wärnmark, Kenneth; Yabashi, Makina; Zhang, Jianxin; Sundström, Villy; Nielsen, Martin M.

2015-01-01

Ultrafast photoinduced electron transfer preceding energy equilibration still poses many experimental and conceptual challenges to the optimization of photoconversion since an atomic-scale description has so far been beyond reach. Here we combine femtosecond transient optical absorption spectroscopy with ultrafast X-ray emission spectroscopy and diffuse X-ray scattering at the SACLA facility to track the non-equilibrated electronic and structural dynamics within a bimetallic donor–acceptor complex that contains an optically dark centre. Exploiting the 100-fold increase in temporal resolution as compared with storage ring facilities, these measurements constitute the first X-ray-based visualization of a non-equilibrated intramolecular electron transfer process over large interatomic distances. Experimental and theoretical results establish that mediation through electronically excited molecular states is a key mechanistic feature. The present study demonstrates the extensive potential of femtosecond X-ray techniques as diagnostics of non-adiabatic electron transfer processes in synthetic and biological systems, and some directions for future studies, are outlined. PMID:25727920

Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses

DOE PAGES

Canton, Sophie E.; Kjær, Kasper S.; Vankó, György; ...

2015-03-02

Ultrafast photoinduced electron transfer preceding energy equilibration still poses many experimental and conceptual challenges to the optimization of photoconversion since an atomic-scale description has so far been beyond reach. Here we combine femtosecond transient optical absorption spectroscopy with ultrafast X-ray emission spectroscopy and diffuse X-ray scattering at the SACLA facility to track the non-equilibrated electronic and structural dynamics within a bimetallic donor–acceptor complex that contains an optically dark centre. Exploiting the 100-fold increase in temporal resolution as compared with storage ring facilities, these measurements constitute the first X-ray-based visualization of a non-equilibrated intramolecular electron transfer process over large interatomic distances.more » Thus experimental and theoretical results establish that mediation through electronically excited molecular states is a key mechanistic feature. The present study demonstrates the extensive potential of femtosecond X-ray techniques as diagnostics of non-adiabatic electron transfer processes in synthetic and biological systems, and some directions for future studies, are outlined.« less

Chromatic effect in a novel THz generation scheme

NASA Astrophysics Data System (ADS)

Li, Bin; Zhang, Wenyan; Liu, Xiaoqing; Deng, Haixiao; Lan, Taihe; Liu, Bo; Liu, Jia; Wang, Xingtao; Zeng, Zhinan; Zhang, Lijian

2017-11-01

Deriving single or few cycle terahertz (THz) pulse by an intense femtosecond laser through cascaded optical rectification is a crucial technique in cutting-edge time-resolved spectroscopy to characterize micro-scale structures and ultrafast dynamics. Due to the broadband nature of the ultrafast driving laser, the chromatic effect limits the THz conversion efficiency in optical rectification crystals, especially for those implementing the pulse-front tilt scheme, e.g. lithium niobate (LN) crystal, has been prevalently used in the past decade. In this research we developed a brand new type of LN crystal utilizing Brewster coupling, and conducted systematically experimental and simulative investigation for the chromatic effect and multi-dimensionally entangled parameters in THz generation, predicting that an extreme conversion efficiency of ˜10% would be potentially achievable at the THz absorption coefficient of ˜0.5 cm-1. Moreover, we first discovered that the chirp of the driving laser plays a decisive role in the pulse-front tilt scheme, and the THz generation efficiency could be enhanced tremendously by applying an appropriate chirp.

Selective ultrafast probing of transient hot chemisorbed and precursor states of CO on Ru(0001).

PubMed

Beye, M; Anniyev, T; Coffee, R; Dell'Angela, M; Föhlisch, A; Gladh, J; Katayama, T; Kaya, S; Krupin, O; Møgelhøj, A; Nilsson, A; Nordlund, D; Nørskov, J K; Öberg, H; Ogasawara, H; Pettersson, L G M; Schlotter, W F; Sellberg, J A; Sorgenfrei, F; Turner, J J; Wolf, M; Wurth, W; Oström, H

2013-05-03

We have studied the femtosecond dynamics following optical laser excitation of CO adsorbed on a Ru surface by monitoring changes in the occupied and unoccupied electronic structure using ultrafast soft x-ray absorption and emission. We recently reported [M. Dell'Angela et al. Science 339, 1302 (2013)] a phonon-mediated transition into a weakly adsorbed precursor state occurring on a time scale of >2 ps prior to desorption. Here we focus on processes within the first picosecond after laser excitation and show that the metal-adsorbate coordination is initially increased due to hot-electron-driven vibrational excitations. This process is faster than, but occurs in parallel with, the transition into the precursor state. With resonant x-ray emission spectroscopy, we probe each of these states selectively and determine the respective transient populations depending on optical laser fluence. Ab initio molecular dynamics simulations of CO adsorbed on Ru(0001) were performed at 1500 and 3000 K providing insight into the desorption process.

Lead Telluride Quantum Dot Solar Cells Displaying External Quantum Efficiencies Exceeding 120%

PubMed Central

2015-01-01

Multiple exciton generation (MEG) in semiconducting quantum dots is a process that produces multiple charge-carrier pairs from a single excitation. MEG is a possible route to bypass the Shockley-Queisser limit in single-junction solar cells but it remains challenging to harvest charge-carrier pairs generated by MEG in working photovoltaic devices. Initial yields of additional carrier pairs may be reduced due to ultrafast intraband relaxation processes that compete with MEG at early times. Quantum dots of materials that display reduced carrier cooling rates (e.g., PbTe) are therefore promising candidates to increase the impact of MEG in photovoltaic devices. Here we demonstrate PbTe quantum dot-based solar cells, which produce extractable charge carrier pairs with an external quantum efficiency above 120%, and we estimate an internal quantum efficiency exceeding 150%. Resolving the charge carrier kinetics on the ultrafast time scale with pump–probe transient absorption and pump–push–photocurrent measurements, we identify a delayed cooling effect above the threshold energy for MEG. PMID:26488847

Correlating Nanoscopic Energy Transfer and Far-Field Emission to Unravel Lasing Dynamics in Plasmonic Nanocavity Arrays

DOE Office of Scientific and Technical Information (OSTI.GOV)

Deeb, Claire; Guo, Zhi; Yang, Ankun

Excited-state interactions between nanoscale cavities and photoactive molecules are critical in plasmonic nanolasing, although the underlying details are less-resolved. This paper reports direct visualization of the energy-transfer dynamics between two-dimensional arrays of plasmonic gold bowtie nanocavities and dye molecules. Transient absorption microscopy measurements of single bowties within the array surrounded by gain molecules showed fast excited-state quenching (2.6 ± 1 ps) characteristic of individual nanocavities. Upon optical pumping at powers above threshold, lasing action emerged depending on the spacing of the array. By correlating ultrafast microscopy and far-field light emission characteristics, we found that bowtie nanoparticles acted as isolated cavitiesmore » when the diffractive modes of the array did not couple to the plasmonic gap mode. These results demonstrate how ultrafast microscopy can provide insight into energy relaxation pathways and, specifically, how nanocavities in arrays can show single-unit nanolaser properties.« less

Ultrafast dynamics of exciton fission in polycrystalline pentacene.

PubMed

Wilson, Mark W B; Rao, Akshay; Clark, Jenny; Kumar, R Sai Santosh; Brida, Daniele; Cerullo, Giulio; Friend, Richard H

2011-08-10

We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ∼80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells. © 2011 American Chemical Society

Ultrafast Photoinduced Symmetry-Breaking Charge Separation and Electron Sharing in Perylenediimide Molecular Triangles.

PubMed

Wu, Yilei; Young, Ryan M; Frasconi, Marco; Schneebeli, Severin T; Spenst, Peter; Gardner, Daniel M; Brown, Kristen E; Würthner, Frank; Stoddart, J Fraser; Wasielewski, Michael R

2015-10-21

We report on a visible-light-absorbing chiral molecular triangle composed of three covalently linked 1,6,7,12-tetra(phenoxy)perylene-3,4:9,10-bis(dicarboximide) (PDI) units. The rigid triangular architecture reduces the electronic coupling between the PDIs, so ultrafast symmetry-breaking charge separation is kinetically favored over intramolecular excimer formation, as revealed by femtosecond transient absorption spectroscopy. Photoexcitation of the PDI triangle dissolved in CH2Cl2 gives PDI(+•)-PDI(-•) in τCS = 12.0 ± 0.2 ps. Fast subsequent intramolecular electron/hole hopping can equilibrate the six possible energetically degenerate ion-pair states, as suggested by electron paramagnetic resonance/electron-nuclear double resonance spectroscopy, which shows that one-electron reduction of the PDI triangle results in complete electron sharing among the three PDIs. Charge recombination of PDI(+•)-PDI(-•) to the ground state occurs in τCR = 1.12 ± 0.01 ns with no evidence of triplet excited state formation.

Direct Observation of Photoinduced Charge Separation in Ruthenium Complex/Ni(OH) 2 Nanoparticle Hybrid

DOE PAGES

Tang, Yu; Pattengale, Brian A.; Ludwig, John M.; ...

2015-12-17

We report that Ni(OH) 2 have emerged as important functional materials for solar fuel conversion because of their potential as cost-effective bifunctional catalysts for both hydrogen and oxygen evolution reactions. However, their roles as photocatalysts in the photoinduced charge separation (CS) reactions remain unexplored. In this paper, we investigate the CS dynamics of a newly designed hybrid catalyst by integrating a Ru complex with Ni(OH) 2 nanoparticles (NPs). Using time resolved X-ray absorption spectroscopy (XTA), we directly observed the formation of the reduced Ni metal site (~60 ps), unambiguously demonstrating CS process in the hybrid through ultrafast electron transfer frommore » Ru complex to Ni(OH) 2 NPs. Compared to the ultrafast CS process, the charge recombination in the hybrid is ultraslow (>>50 ns). These results not only suggest the possibility of developing Ni(OH) 2 as solar fuel catalysts, but also represent the first time direct observation of efficient CS in a hybrid catalyst using XTA.« less

Anisotropic ultrafast response of MoS2 on rippled substrates

NASA Astrophysics Data System (ADS)

Cinquanta, Eugenio; Camellini, Andrea; Martella, Christian; Mennucci, Carlo; Lamperti, Alessio; Della Valle, Giuseppe; Zavelani Rossi, Margherita; Buatier de Mongeot, Francesco; Molle, Alessandro; Stagira, Salvatore

TMDs represent one of the most promising option for new devices characterized by high performances for opto- and nanoelectronics applications. Top-down schemes have been fruitfully exploited for the tuning of TMDs physics by stain engineering in exfoliated flakes. We propose an original bottom-up strategy based on the CVD growth of MoS2 on anisotropic substrates and its characterization by means of pump-probe spectroscopy. The ultrafast response of the rippled MoS2 reveals strongly anisotropic. While the transient absorption emerges as independent from the orientation of the pump beam polarization, the angle between the probe beam polarization and the ripples induces remarkable effects. Within an orthogonal geometry, both the overall intensity of the transient spectrum and the el-ph scattering decay time are halved while the photo-bleaching at 450 nm is blueshifted with respect to the parallel orientation case. Our results demonstrate that the coupling of TMDs with anisotropic substrates is a promising way for the integration of TMDs photonics devices.

Carbon monoxide recombination dynamics in truncated hemoglobins studied with visible-pump midIR-probe spectroscopy.

PubMed

Lapini, Andrea; Di Donato, Mariangela; Patrizi, Barbara; Marcelli, Agnese; Lima, Manuela; Righini, Roberto; Foggi, Paolo; Sciamanna, Natascia; Boffi, Alberto

2012-08-02

Carbon monoxide recombination dynamics upon photodissociation with visible light has been characterized by means of ultrafast visible-pump/MidIR probe spectroscopy for the truncated hemoglobins from Thermobifida fusca and Bacillus subtilis. Photodissociation has been induced by exciting the sample at two different wavelengths: 400 nm, corresponding to the heme absorption in the B-band, and 550 nm, in the Q-bands. The bleached iron-CO coordination band located at 1850-1950 cm(-1) and the free CO absorption band in the region 2050-2200 cm(-1) have been observed by probe pulses tuned in the appropriate infrared region. The kinetic traces measured at 1850-1950 cm(-1) reveal multiexponential subnanosecond dynamics that have been interpreted as arising from fast geminate recombination of the photolyzed CO. A compared analysis of the crystal structure of the two proteins reveals a similar structure of their distal heme pocket, which contains conserved polar and aromatic amino acid residues closely interacting with the iron ligand. Although fast geminate recombination is observed in both proteins, several kinetic differences can be evidenced, which can be interpreted in terms of a different structural flexibility of the corresponding heme distal pockets. The analysis of the free CO band-shape and of its dynamic evolution brings out novel features about the nature of the docking site inside the protein cavity.

Exciton exciton annihilation dynamics in chromophore complexes. II. Intensity dependent transient absorption of the LH2 antenna system.

PubMed

Bruggemann, B; May, V

2004-02-01

Using the multiexciton density matrix theory of excitation energy transfer in chromophore complexes developed in a foregoing paper [J. Chem. Phys. 118, 746 (2003)], the computation of ultrafast transient absorption spectra is presented. Beside static disorder and standard mechanisms of excitation energy dissipation the theory incorporates exciton exciton annihilation (EEA) processes. To elucidate signatures of EEA in intensity dependent transient absorption data the approach is applied to the B850 ring of the LH2 found in rhodobacter sphaeroides. As main indications for two-exciton population and resulting EEA we found (i) a weakening of the dominant single-exciton bleaching structure in the transient absorption, and (ii) an intermediate suppression of long-wavelength and short-wavelength shoulders around the bleaching structure. The suppression is caused by stimulated emission from the two-exciton to the one-exciton state and the return of the shoulders follows from a depletion of two-exciton population according to EEA. The EEA-signature survives as a short-wavelength shoulder in the transient absorption if orientational and energetic disorder are taken into account. Therefore, the observation of the EEA-signatures should be possible when doing frequency resolved transient absorption experiments with a sufficiently strongly varying pump-pulse intensity. Copyright 2004 American Institute of Physics

Observation of Reverse Saturable Absorption of an X-ray Laser

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cho, B. I.; Cho, M. S.; Kim, M.

A nonlinear absorber in which the excited state absorption is larger than the ground state can undergo a process called reverse saturable absorption. It is a well-known phenomenon in laser physics in the optical regime, but is more difficult to generate in the x-ray regime, where fast nonradiative core electron transitions typically dominate the population kinetics during light matter interactions. Here, we report the first observation of decreasing x-ray transmission in a solid target pumped by intense x-ray free electron laser pulses. The measurement has been made below the K-absorption edge of aluminum, and the x-ray intensity ranges are 10more » 16 –10 17 W=cm 2. It has been confirmed by collisional radiative population kinetic calculations, underscoring the fast spectral modulation of the x-ray pulses and charge states relevant to the absorption and transmission of x-ray photons. The processes shown through detailed simulations are consistent with reverse saturable absorption, which would be the first observation of this phenomena in the x-ray regime. These light matter interactions provide a unique opportunity to investigate optical transport properties in the extreme state of matters, as well as affording the potential to regulate ultrafast x-ray freeelectron laser pulses.« less

Observation of Reverse Saturable Absorption of an X-ray Laser

DOE PAGES

Cho, B. I.; Cho, M. S.; Kim, M.; ...

2017-08-16

A nonlinear absorber in which the excited state absorption is larger than the ground state can undergo a process called reverse saturable absorption. It is a well-known phenomenon in laser physics in the optical regime, but is more difficult to generate in the x-ray regime, where fast nonradiative core electron transitions typically dominate the population kinetics during light matter interactions. Here, we report the first observation of decreasing x-ray transmission in a solid target pumped by intense x-ray free electron laser pulses. The measurement has been made below the K-absorption edge of aluminum, and the x-ray intensity ranges are 10more » 16 –10 17 W=cm 2. It has been confirmed by collisional radiative population kinetic calculations, underscoring the fast spectral modulation of the x-ray pulses and charge states relevant to the absorption and transmission of x-ray photons. The processes shown through detailed simulations are consistent with reverse saturable absorption, which would be the first observation of this phenomena in the x-ray regime. These light matter interactions provide a unique opportunity to investigate optical transport properties in the extreme state of matters, as well as affording the potential to regulate ultrafast x-ray freeelectron laser pulses.« less

Nonthermal ultrafast optical control of the magnetization in garnet films

NASA Astrophysics Data System (ADS)

Hansteen, Fredrik; Kimel, Alexey; Kirilyuk, Andrei; Rasing, Theo

2006-01-01

We demonstrate coherent optical control of the magnetization in ferrimagnetic garnet films on the femtosecond time scale through a combination of two different ultrafast and nonthermal photomagnetic effects and by employing multiple pump pulses. Linearly polarized laser pulses are shown to create a long-lived modification of the magnetocrystalline anisotropy via optically induced electron transfer between nonequivalent ion sites while circularly polarized pulses additionally act as strong transient magnetic field pulses originating from the nonabsorptive inverse Faraday effect. Due to the slow phonon-magnon interaction in these dielectrics, thermal effects of the laser excitation are clearly distinguished from the ultrafast nonthermal effects and can be seen only on the time scale of nanoseconds for sample temperatures near the Curie point. The reported effects open exciting possibilities for ultrafast manipulation of spins by light, and provide insight into the physics of magnetism on ultrafast time scales.

Carotenoid-to-bacteriochlorophyll energy transfer through vibronic coupling in LH2 from Phaeosprillum molischianum.

PubMed

Thyrhaug, Erling; Lincoln, Craig N; Branchi, Federico; Cerullo, Giulio; Perlík, Václav; Šanda, František; Lokstein, Heiko; Hauer, Jürgen

2018-03-01

The peripheral light-harvesting antenna complex (LH2) of purple photosynthetic bacteria is an ideal testing ground for models of structure-function relationships due to its well-determined molecular structure and ultrafast energy deactivation. It has been the target for numerous studies in both theory and ultrafast spectroscopy; nevertheless, certain aspects of the convoluted relaxation network of LH2 lack a satisfactory explanation by conventional theories. For example, the initial carotenoid-to-bacteriochlorophyll energy transfer step necessary on visible light excitation was long considered to follow the Förster mechanism, even though transfer times as short as 40 femtoseconds (fs) have been observed. Such transfer times are hard to accommodate by Förster theory, as the moderate coupling strengths found in LH2 suggest much slower transfer within this framework. In this study, we investigate LH2 from Phaeospirillum (Ph.) molischianum in two types of transient absorption experiments-with narrowband pump and white-light probe resulting in 100 fs time resolution, and with degenerate broadband 10 fs pump and probe pulses. With regard to the split Q x band in this system, we show that vibronically mediated transfer explains both the ultrafast carotenoid-to-B850 transfer, and the almost complete lack of transfer to B800. These results are beyond Förster theory, which predicts an almost equal partition between the two channels.

High-yield, ultrafast, surface plasmon-enhanced, Au nanorod optical field electron emitter arrays.

PubMed

Hobbs, Richard G; Yang, Yujia; Fallahi, Arya; Keathley, Philip D; De Leo, Eva; Kärtner, Franz X; Graves, William S; Berggren, Karl K

2014-11-25

Here we demonstrate the design, fabrication, and characterization of ultrafast, surface-plasmon enhanced Au nanorod optical field emitter arrays. We present a quantitative study of electron emission from Au nanorod arrays fabricated by high-resolution electron-beam lithography and excited by 35 fs pulses of 800 nm light. We present accurate models for both the optical field enhancement of Au nanorods within high-density arrays, and electron emission from those nanorods. We have also studied the effects of surface plasmon damping induced by metallic interface layers at the substrate/nanorod interface on near-field enhancement and electron emission. We have identified the peak optical field at which the electron emission mechanism transitions from a 3-photon absorption mechanism to strong-field tunneling emission. Moreover, we have investigated the effects of nanorod array density on nanorod charge yield, including measurement of space-charge effects. The Au nanorod photocathodes presented in this work display 100-1000 times higher conversion efficiency relative to previously reported UV triggered emission from planar Au photocathodes. Consequently, the Au nanorod arrays triggered by ultrafast pulses of 800 nm light in this work may outperform equivalent UV-triggered Au photocathodes, while also offering nanostructuring of the electron pulse produced from such a cathode, which is of interest for X-ray free-electron laser (XFEL) development where nanostructured electron pulses may facilitate more efficient and brighter XFEL radiation.

UV laser photoactivation of hexachloroplatinate bound to individual nucleobases in vacuo as molecular level probes of a model photopharmaceutical.

PubMed

Matthews, Edward; Sen, Ananya; Yoshikawa, Naruo; Bergström, Ed; Dessent, Caroline E H

2016-06-01

Isolated molecular clusters of adenine, cytosine, thymine and uracil bound to hexachloroplatinate, PtCl6(2-), have been studied using laser electronic photodissociation spectroscopy to investigate photoactivation of a platinum complex in the vicinity of a nucleobase. These metal complex-nucleobase clusters represent model systems for identifying the fundamental photochemical processes occurring in photodynamic platinum drug therapies that target DNA. This is the first study to explore the specific role of a strongly photoactive platinum compound in the aggregate complex. Each of the clusters studied displays a broadly similar absorption spectra, with a strong λmax ∼ 4.6 eV absorption band and a subsequent increase in the absorption intensity towards higher spectral-energy. The absorption bands are traced to ligand-to-metal-charge-transfer excitations on the PtCl6(2-) moiety within the cluster, and result in Cl(-)·nucleobase and PtCl5(-) as primary photofragments. These results demonstrate how selective photoexcitation can drive distinctive photodecay channels for a model photo-pharmaceutical. In addition, cluster absorption due to excitation of nucleobase-centred chromophores is observed in the region around 5 eV. For the uracil cluster, photofragments consistent with ultrafast decay of the excited state and vibrational predissociation on the ground-state surface are observed. However, this decay channel becomes successively weaker on going from thymine to cytosine to adenine, due to differential coupling of the excited states to the electron detachment continuum. These effects demonstrate the distinctive photophysical characteristics of the different nucleobases, and are discussed in the context of the recently recorded photoelectron spectra of theses clusters.

Ultrafast absorption of intense x rays by nitrogen molecules

NASA Astrophysics Data System (ADS)

Buth, Christian; Liu, Ji-Cai; Chen, Mau Hsiung; Cryan, James P.; Fang, Li; Glownia, James M.; Hoener, Matthias; Coffee, Ryan N.; Berrah, Nora

2012-06-01

We devise a theoretical description for the response of nitrogen molecules (N2) to ultrashort and intense x rays from the free electron laser Linac Coherent Light Source (LCLS). We set out from a rate-equation description for the x-ray absorption by a nitrogen atom. The equations are formulated using all one-x-ray-photon absorption cross sections and the Auger and radiative decay widths of multiply-ionized nitrogen atoms. Cross sections are obtained with a one-electron theory and decay widths are determined from ab initio computations using the Dirac-Hartree-Slater (DHS) method. We also calculate all binding and transition energies of nitrogen atoms in all charge states with the DHS method as the difference of two self-consistent field (SCF) calculations (ΔSCF method). To describe the interaction with N2, a detailed investigation of intense x-ray-induced ionization and molecular fragmentation are carried out. As a figure of merit, we calculate ion yields and the average charge state measured in recent experiments at the LCLS. We use a series of phenomenological models of increasing sophistication to unravel the mechanisms of the interaction of x rays with N2: a single atom, a symmetric-sharing model, and a fragmentation-matrix model are developed. The role of the formation and decay of single and double core holes, the metastable states of N_2^{2+}, and molecular fragmentation are explained.

Nonlinear Optical Spectroscopy in the Time Domain: Studies of Ultrafast Molecular Processes in the Condensed Phase.

NASA Astrophysics Data System (ADS)

Joo, Taiha

Ultrafast molecular processes in the condensed phase at room temperature are studied in the time domain by four wave mixing spectroscopy. The structure/dynamics of various quantum states can be studied by varying the time ordering of the incident fields, their polarization, their colors, etc. In one, time-resolved coherent Stokes Raman spectroscopy of benzene is investigated at room temperature. The reorientational correlation time of benzene as well as the T_2 time of the nu _1 ring-breathing mode have been measured by using two different polarization geometries. Bohr frequency difference beats have also been resolved between the nu_1 modes of ^ {12}C_6H_6 and ^{12}C_5^{13 }CH_6.. The dephasing dynamics of the nu _1 ring-breathing mode of neat benzene is studied by time-resolved coherent anti-Stokes Raman scattering. Ultrafast time resolution reveals deviation from the conventional exponential decay. The correlation time, tau _{rm c}, and the rms magnitude, Delta, of the Bohr frequency modulation are determined for the process responsible for the vibrational dephasing by Kubo dephasing function analysis. The electronic dephasing of two oxazine dyes in ethylene glycol at room temperature is investigated by photon echo experiments. It was found that at least two stochastic processes are responsible for the observed electronic dephasing. Both fast (homogeneous) and slow (inhomogeneous) dynamics are recovered using Kubo line shape analysis. Moreover, the slow dynamics is found to spectrally diffuse over the inhomogeneous distribution on the time scale around a picosecond. Time-resolved degenerate four wave mixing signal of dyes in a population measurement geometry is reported. The vibrational coherences both in the ground and excited electronic states produced strong oscillations in the signal together with the usual population decay from the excited electronic state. Absolute frequencies and their dephasing times of the vibrational modes at ~590 cm^{-1} are obtained. Finally, a new inverse transform procedure is presented that calculates the absorption band (ABS) from an experimental Raman excitation profile (REP). An iterative solution is sought for an integral Hilbert transform relation. An exact ABS is recovered regardless of the starting ABS when sufficient iterations are performed.

Active cooling of pulse compression diffraction gratings for high energy, high average power ultrafast lasers

DOE Office of Scientific and Technical Information (OSTI.GOV)

Alessi, David A.; Rosso, Paul A.; Nguyen, Hoang T.

Laser energy absorption and subsequent heat removal from diffraction gratings in chirped pulse compressors poses a significant challenge in high repetition rate, high peak power laser development. In order to understand the average power limitations, we have modeled the time-resolved thermo-mechanical properties of current and advanced diffraction gratings. We have also developed and demonstrated a technique of actively cooling Petawatt scale, gold compressor gratings to operate at 600W of average power - a 15x increase over the highest average power petawatt laser currently in operation. As a result, combining this technique with low absorption multilayer dielectric gratings developed in ourmore » group would enable pulse compressors for petawatt peak power lasers operating at average powers well above 40kW.« less

Ultrafast laser control of autoionizing resonances observed in attosecond transient absorption

NASA Astrophysics Data System (ADS)

Liao, Chen-Ting; Harkema, Nathan; Sandhu, Arvinder

2017-04-01

Attosecond and femtosecond extreme ultraviolet (XUV) pulses can be used to probe electron dynamics in high-lying excited states that autoionize on a femtosecond timescale, thus providing information on the process of Auger decay and its interference with the continua. Here we utilize XUV pulses in connection with infrared (IR) pulses to perform attosecond transient absorption spectroscopy of the impulsive response of argon autoionizing Rydberg states in the vicinity of the 3s-1 4 p resonance. We show that by tuning the time delay and field polarization of IR pulse, it is possible to control the dipolar coupling between neighboring states and hence the spectral line shape of the resonance, such as the transition between Breit-Wigner to Beutler-Fano profiles. NSF Grant No. PHY-1505556.

Optical and electrical measurement of energy transfer between nanocrystalline quantum dots and photosystem I.

PubMed

Jung, Hyeson; Gulis, Galina; Gupta, Subhadra; Redding, Kevin; Gosztola, David J; Wiederrecht, Gary P; Stroscio, Michael A; Dutta, Mitra

2010-11-18

In the natural photosynthesis process, light harvesting complexes (LHCs) absorb light and pass excitation energy to photosystem I (PSI) and photosystem II (PSII). In this study, we have used nanocrystalline quantum dots (NQDs) as an artificial LHC by integrating them with PSI to extend their spectral range. We have performed photoluminescence (PL) and ultrafast time-resolved absorption measurements to investigate this process. Our PL experiments showed that emission from the NQDs is quenched, and the fluorescence from PSI is enhanced. Transient absorption and bleaching results can be explained by fluorescence resonance energy transfer (FRET) from the NQDs to the PSI. This nonradiative energy transfer occurs in ∼6 ps. Current-voltage (I-V) measurements on the composite NQD-PSI samples demonstrate a clear photoresponse.

Active cooling of pulse compression diffraction gratings for high energy, high average power ultrafast lasers

DOE PAGES

Alessi, David A.; Rosso, Paul A.; Nguyen, Hoang T.; ...

2016-12-26

Laser energy absorption and subsequent heat removal from diffraction gratings in chirped pulse compressors poses a significant challenge in high repetition rate, high peak power laser development. In order to understand the average power limitations, we have modeled the time-resolved thermo-mechanical properties of current and advanced diffraction gratings. We have also developed and demonstrated a technique of actively cooling Petawatt scale, gold compressor gratings to operate at 600W of average power - a 15x increase over the highest average power petawatt laser currently in operation. As a result, combining this technique with low absorption multilayer dielectric gratings developed in ourmore » group would enable pulse compressors for petawatt peak power lasers operating at average powers well above 40kW.« less

Discrete decoding based ultrafast multidimensional nuclear magnetic resonance spectroscopy

NASA Astrophysics Data System (ADS)

Wei, Zhiliang; Lin, Liangjie; Ye, Qimiao; Li, Jing; Cai, Shuhui; Chen, Zhong

2015-07-01

The three-dimensional (3D) nuclear magnetic resonance (NMR) spectroscopy constitutes an important and powerful tool in analyzing chemical and biological systems. However, the abundant 3D information arrives at the expense of long acquisition times lasting hours or even days. Therefore, there has been a continuous interest in developing techniques to accelerate recordings of 3D NMR spectra, among which the ultrafast spatiotemporal encoding technique supplies impressive acquisition speed by compressing a multidimensional spectrum in a single scan. However, it tends to suffer from tradeoffs among spectral widths in different dimensions, which deteriorates in cases of NMR spectroscopy with more dimensions. In this study, the discrete decoding is proposed to liberate the ultrafast technique from tradeoffs among spectral widths in different dimensions by focusing decoding on signal-bearing sites. For verifying its feasibility and effectiveness, we utilized the method to generate two different types of 3D spectra. The proposed method is also applicable to cases with more than three dimensions, which, based on the experimental results, may widen applications of the ultrafast technique.

Ultrafast dynamics and excited state spectra of open-chain carotenoids at room and low temperatures.

PubMed

Niedzwiedzki, Dariusz; Koscielecki, Jeremy F; Cong, Hong; Sullivan, James O; Gibson, George N; Birge, Robert R; Frank, Harry A

2007-05-31

Many of the spectroscopic features and photophysical properties of carotenoids are explained using a three-state model in which the strong visible absorption of the molecules is associated with an S0 (1(1)Ag-) --> S2 (1(1)Bu+) transition, and the lowest lying singlet state, S1 (2(1)Ag-), is a state into which absorption from the ground state is forbidden by symmetry. However, semiempirical and ab initio quantum calculations have suggested additional excited singlet states may lie either between or in the vicinity of S1 (2(1)Ag-) and S2 (1(1)Bu+), and some ultrafast spectroscopic studies have reported evidence for these states. One such state, denoted S*, has been implicated as an intermediate in the depopulation of S2 (1(1)Bu+) and as a pathway for the formation of carotenoid triplet states in light-harvesting complexes. In this work, we present the results of an ultrafast, time-resolved spectroscopic investigation of a series of open-chain carotenoids derived from photosynthetic bacteria and systematically increasing in their number of pi-electron carbon-carbon double bonds (n). The molecules are neurosporene (n = 9), spheroidene (n = 10), rhodopin glucoside (n = 11), rhodovibrin (n = 12), and spirilloxanthin (n = 13). The molecules were studied in acetone and CS2 solvents at room temperature. These experiments explore the effect of solvent polarity and polarizability on the spectroscopic and kinetic behavior of the molecules. The molecules were also studied in ether/isopentane/ethanol (EPA) glasses at 77 K, in which the spectral resolution is greatly enhanced. Analysis of the data using global fitting techniques has revealed the ultrafast dynamics of the excited states and spectral changes associated with their decay, including spectroscopic features not previously reported. The data are consistent with S* being identified with a twisted conformational structure, the yield of which is increased in molecules having longer pi-electron conjugations. In particular, for the longest molecule in the series, spirilloxanthin, the experiments and a detailed quantum computational analysis reveal the presence of two S* states associated with relaxed S1 (2(1)Ag-) conformations involving nearly planar 6-s-cis and 6-s-trans geometries. We propose that in polar solvents, the ground state of spirilloxanthin takes on a corkscrew conformation that generates a net solute dipole moment while decreasing the cavity formation energy. Upon excitation and relaxation into the S1 (2(1)Ag-) state, the polyene unravels and flattens into a more planar geometry with comparable populations of 6-s-trans and 6-s-cis conformations.

Ultrafast propagation of Schroedinger waves in absorbing media

DOE Office of Scientific and Technical Information (OSTI.GOV)

Delgado, F.; Muga, J.G.; Ruschhaupt, A.

2004-02-01

We show that the temporal peak of a quantum wave may arrive at different locations simultaneously in an absorbing medium. The arrival occurs at the lifetime of the particle in the medium from the instant when a point source with a sharp onset is turned on. We also identify other characteristic times. In particular, the 'traversal' or 'Buettiker-Landauer' time (which grows linearly with the distance to the source) for the Hermitian, non-absorbing case is substituted by several characteristic quantities in the absorbing case. The simultaneous arrival due to absorption, unlike the Hartman effect, occurs for carrier frequencies under or abovemore » the cutoff, and for arbitrarily large distances. It holds also in a relativistic generalization but limited by causality. A possible physical realization is proposed by illuminating a two-level atom with a detuned laser.« less

Nanosystems in ultrafast and superstrong fields: attosecond phenomena (Conference Presentation)

NASA Astrophysics Data System (ADS)

Stockman, Mark I.

2017-02-01

We present our latest results for a new class of phenomena in condensed matter nanooptics when a strong optical field ˜1-3 V/Å changes a solid within optical cycle [1-8]. Such a pulse drives ampere-scale currents in dielectrics and adiabatically controls their properties, including optical absorption and reflection, extreme UV absorption, and generation of high harmonics [9] in a non-perturbative manner on a 100-as temporal scale. Applied to a metal, such a pulse causes an instantaneous and, potentially, reversible change from the metallic to semimetallic properties. We will also discuss our latest theoretical results on graphene that in a strong ultrashort pulse field exhibits unique behavior [10-12]. New phenomena are predicted for buckled two-dimensional solids, silicene and germanene [13]. These are fastest phenomena in optics unfolding within half period of light. They offer potential for petahertz-bandwidth signal processing, generation of high harmonics on a nanometer spatial scale, etc. References 1. M. Durach, A. Rusina, M. F. Kling, and M. I. Stockman, Metallization of Nanofilms in Strong Adiabatic Electric Fields, Phys. Rev. Lett. 105, 086803-1-4 (2010). 2. M. Durach, A. Rusina, M. F. Kling, and M. I. Stockman, Predicted Ultrafast Dynamic Metallization of Dielectric Nanofilms by Strong Single-Cycle Optical Fields, Phys. Rev. Lett. 107, 086602-1-5 (2011). 3. A. Schiffrin, T. Paasch-Colberg, N. Karpowicz, V. Apalkov, D. Gerster, S. Muhlbrandt, M. Korbman, J. Reichert, M. Schultze, S. Holzner, J. V. Barth, R. Kienberger, R. Ernstorfer, V. S. Yakovlev, M. I. Stockman, and F. Krausz, Optical-Field-Induced Current in Dielectrics, Nature 493, 70-74 (2013). 4. M. Schultze, E. M. Bothschafter, A. Sommer, S. Holzner, W. Schweinberger, M. Fiess, M. Hofstetter, R. Kienberger, V. Apalkov, V. S. Yakovlev, M. I. Stockman, and F. Krausz, Controlling Dielectrics with the Electric Field of Light, Nature 493, 75-78 (2013). 5. V. Apalkov and M. I. Stockman, Metal Nanofilm in Strong Ultrafast Optical Fields, Phys. Rev. B 88, 245438-1-7 (2013). 6. V. Apalkov and M. I. Stockman, Theory of Dielectric Nanofilms in Strong Ultrafast Optical Fields, Phys. Rev. B 86, 165118-1-13 (2012). 7. F. Krausz and M. I. Stockman, Attosecond Metrology: From Electron Capture to Future Signal Processing, Nat. Phot. 8, 205-213 (2014). 8. O. Kwon, T. Paasch-Colberg, V. Apalkov, B.-K. Kim, J.-J. Kim, M. I. Stockman, and D. E. Kim, Semimetallization of Dielectrics in Strong Optical Fields, Sci. Rep, 6, 21272-1-9 (2016). 9. T. Higuchi, M. I. Stockman, and P. Hommelhoff, Strong-Field Perspective on High-Harmonic Radiation from Bulk Solids, Phys. Rev. Lett. 113, 213901-1-5 (2014). 10. H. K. Kelardeh, V. Apalkov, and M. I. Stockman, Wannier-Stark States of Graphene in Strong Electric Field, Phys. Rev. B 90, 085313-1-11 (2014). 11. H. K. Kelardeh, V. Apalkov, and M. I. Stockman, Graphene in Ultrafast and Superstrong Laser Fields, Phys. Rev. B 91, 0454391-8 (2015). 12. H. K. Kelardeh, V. Apalkov, and M. I. Stockman, Attosecond Strong-Field Interferometry in Graphene: Chirality, Singularity, and Berry Phase, Phys. Rev. B 93, 155434-1-7 (2016). 13. H. K. Kelardeh, V. Apalkov, and M. I. Stockman, Ultrafast Field Control of Symmetry, Reciprocity, and Reversibility in Buckled Graphene-Like Materials, Phys. Rev. B 92, 045413-1-9 (2015).

Ultrafast formation of the benzoic acid triplet upon ultraviolet photolysis and its sequential photodissociation in solution

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yang Chunfan; Su Hongmei; Sun Xuezhong

2012-05-28

Time-resolved infrared (TR-IR) absorption spectroscopy in both the femtosecond and nanosecond time domain has been applied to examine the photolysis of benzoic acid in acetonitrile solution following either 267 nm or 193 nm excitation. By combining the ultrafast and nanosecond TR-IR measurements, both the excited states and the photofragments have been detected and key mechanistic insights were obtained. We show that the solvent interaction modifies the excited state relaxation pathways and thus the population dynamics, leading to different photolysis behavior in solution from that observed in the gas phase. Vibrational energy transfer to solvents dissipates excitation energy efficiently, suppressing themore » photodissociation and depopulating the excited S{sub 2} or S{sub 3} state molecules to the lowest T{sub 1} state with a rate of {approx}2.5 ps after a delayed onset of {approx}3.7 ps. Photolysis of benzoic acid using 267 nm excitation is dominated by the formation of the T{sub 1} excited state and no photofragments could be detected. The results from TR-IR experiments using higher energy of 193 nm indicate that photodissociation proceeds more rapidly than the vibrational energy transfer to solvents and C-C bond fission becomes the dominant relaxation pathway in these experiments as featured by the prominent observation of the COOH photofragments and negligible yield of the T{sub 1} excited state. The measured ultrafast formation of T{sub 1} excited state supports the existence of the surface intersections of S{sub 2}/S{sub 1}, S{sub 2}/T{sub 2}, and S{sub 1}/T{sub 1}/T{sub 2}, and the large T{sub 1} quantum yield of {approx}0.65 indicates the importance of the excited state depopulation to triplet manifold as the key factor affecting the photophysical and photochemical behavior of the monomeric benzoic acid.« less

Charge and spin control of ultrafast electron and hole dynamics in single CdSe/ZnSe quantum dots

NASA Astrophysics Data System (ADS)

Hinz, C.; Gumbsheimer, P.; Traum, C.; Holtkemper, M.; Bauer, B.; Haase, J.; Mahapatra, S.; Frey, A.; Brunner, K.; Reiter, D. E.; Kuhn, T.; Seletskiy, D. V.; Leitenstorfer, A.

2018-01-01

We study the dynamics of photoexcited electrons and holes in single negatively charged CdSe/ZnSe quantum dots with two-color femtosecond pump-probe spectroscopy. An initial characterization of the energy level structure is performed at low temperatures and magnetic fields of up to 5 T. Emission and absorption resonances are assigned to specific transitions between few-fermion states by a theoretical model based on a configuration interaction approach. To analyze the dynamics of individual charge carriers, we initialize the quantum system into excited trion states with defined energy and spin. Subsequently, the time-dependent occupation of the trion ground state is monitored by spectrally resolved differential transmission measurements. We observe subpicosecond dynamics for a hole excited to the D shell. The energy dependence of this D -to-S shell intraband transition is investigated in quantum dots of varying size. Excitation of an electron-hole pair in the respective p shells leads to the formation of singlet and triplet spin configurations. Relaxation of the p -shell singlet is observed to occur on a time scale of a few picoseconds. Pumping of p -shell triplet transitions opens up two pathways with distinctly different scattering times. These processes are shown to be governed by the mixing of singlet and triplet states due to exchange interactions enabling simultaneous electron and hole spin flips. To isolate the relaxation channels, we align the spin of the residual electron by a magnetic field and employ laser pulses of defined helicity. This step provides ultrafast preparation of a fully inverted trion ground state of the quantum dot with near unity probability, enabling deterministic addition of a single photon to the probe pulse. Therefore our experiments represent a significant step towards using single quantum emitters with well-controled inversion to manipulate the photon statistics of ultrafast light pulses.

Quantitative detection of melamine based on terahertz time-domain spectroscopy

NASA Astrophysics Data System (ADS)

Zhao, Xiaojing; Wang, Cuicui; Liu, Shangjian; Zuo, Jian; Zhou, Zihan; Zhang, Cunlin

2018-01-01

Melamine is an organic base and a trimer of cyanamide, with a 1, 3, 5-triazine skeleton. It is usually used for the production of plastics, glue and flame retardants. Melamine combines with acid and related compounds to form melamine cyanurate and related crystal structures, which have been implicated as contaminants or biomarkers in protein adulterations by lawbreakers, especially in milk powder. This paper is focused on developing an available method for quantitative detection of melamine in the fields of security inspection and nondestructive testing based on THz-TDS. Terahertz (THz) technology has promising applications for the detection and identification of materials because it exhibits the properties of spectroscopy, good penetration and safety. Terahertz time-domain spectroscopy (THz-TDS) is a key technique that is applied to spectroscopic measurement of materials based on ultrafast femtosecond laser. In this study, the melamine and its mixture with polyethylene powder in different consistence are measured using the transmission THz-TDS. And we obtained the refractive index spectra and the absorption spectrum of different concentrations of melamine on 0.2-2.8THz. In the refractive index spectra, it is obvious to see that decline trend with the decrease of concentration; and in the absorption spectrum, two peaks of melamine at 1.98THz and 2.28THz can be obtained. Based on the experimental result, the absorption coefficient and the consistence of the melamine in the mixture are determined. Finally, methods for quantitative detection of materials in the fields of nondestructive testing and quality control based on THz-TDS have been studied.

Ultrafast multi-pulse transient absorption spectroscopy of fucoxanthin chlorophyll a protein from Phaeodactylum tricornutum.

PubMed

West, Robert G; Bína, David; Fuciman, Marcel; Kuznetsova, Valentyna; Litvín, Radek; Polívka, Tomáš

2018-05-01

We have applied femtosecond transient absorption spectroscopy in pump-probe and pump-dump-probe regimes to study energy transfer between fucoxanthin and Chl a in fucoxanthin-Chl a complex from the pennate diatom Phaeodactylum tricornutum. Experiments were carried out at room temperature and 77 K to reveal temperature dependence of energy transfer. At both temperatures, the ultrafast (<100 fs) energy transfer channel from the fucoxanthin S 2 state is active and is complemented by the second pathway via the combined S 1 /ICT state. The S 1 /ICT-Chl a pathway has two channels, the fast one characterized by sub-picosecond energy transfer, and slow having time constants of 4.5 ps at room temperature and 6.6 ps at 77 K. The overall energy transfer via the S 1 /ICT is faster at 77 K, because the fast component gains amplitude upon lowering the temperature. The pump-dump-probe regime, with the dump pulse centered in the spectral region of ICT stimulated emission at 950 nm and applied at 2 ps after excitation, proved that the S 1 and ICT states of fucoxanthin in FCP are individual, yet coupled entities. Analysis of the pump-dump-probe data suggested that the main energy donor in the slow S 1 /ICT-Chl a route is the S 1 part of the S 1 /ICT potential surface. Copyright © 2018 Elsevier B.V. All rights reserved.

Direct measurement of the static and transient magneto-optical permittivity of cobalt across the entire M -edge in reflection geometry by use of polarization scanning

NASA Astrophysics Data System (ADS)

Zusin, Dmitriy; Tengdin, Phoebe M.; Gopalakrishnan, Maithreyi; Gentry, Christian; Blonsky, Adam; Gerrity, Michael; Legut, Dominik; Shaw, Justin M.; Nembach, Hans T.; Silva, T. J.; Oppeneer, Peter M.; Kapteyn, Henry C.; Murnane, Margaret M.

2018-01-01

The microscopic state of a magnetic material is characterized by its resonant magneto-optical response through the off-diagonal dielectric tensor component ɛx y. However, the measurement of the full complex ɛx y in the extreme ultraviolet spectral region covering the M absorption edges of 3 d ferromagnets is challenging due to the need for either a careful polarization analysis, which is complicated by a lack of efficient polarization analyzers, or scanning the angle of incidence in fine steps. Here, we propose and demonstrate a technique to extract the complex resonant permittivity ɛx y simply by scanning the polarization angle of linearly polarized high harmonics to measure the magneto-optical asymmetry in reflection geometry. Because this technique is more practical and faster to experimentally implement than previous approaches, we can directly measure the full time evolution of ɛx y(t ) during laser-induced demagnetization across the entire M2 ,3 absorption edge of cobalt with femtosecond time resolution. We find that for polycrystalline Co films on an insulating substrate, the changes in ɛx y are uniform throughout the spectrum, to within our experimental precision. This result suggests that, in the regime of strong demagnetization, the ultrafast demagnetization response is primarily dominated by magnon generation. We estimate the contribution of exchange-splitting reduction to the ultrafast demagnetization process to be no more than 25%.

Ultrafast Transient Absorption Spectroscopy of Polymer-Based Organophotoredox Catalysts Mimicking Transition-Metal Complexes

NASA Astrophysics Data System (ADS)

Jamhawi, Abdelqader; Paul, Anam C.; Smith, Justin D.; Handa, Sachin; Liu, Jinjun

2017-06-01

Transition-metal complexes of rare earth metals including ruthenium and iridium are most commonly employed as visible-light photocatalysts. Despite their highly important and broad applications, they have many disadvantages including high cost associated with low abundance in earth crust, potential toxicity, requirement of specialized ligands for desired activity, and difficulty in recycling of metal contents as well as associated ligands. Polymer-based organophotoredox catalysts are promising alternatives and possess unique advantages such as easier synthesis from inexpensive starting material, longer excited state life time, broad range of activity, sustainability, and recyclability. In this research talk, time-resolved photoluminescence and femtosecond transient absorption (TA) spectroscopy measurements of three novel polymer-based organophotoredox catalysts will be presented. By our synthetic team, their catalytic activity has been proven in some highly valuable chemical transformations, that otherwise require transition metal complexes. Time-resolved spectroscopic investigations have demonstrated that photoinduced processes in these catalysts are similar to the transition metal complexes. Especially, intramolecular vibrational relaxation, internal conversion, and intersystem crossing from the S1 state to the T1 state all occur on a sub-picosecond timescale. The long lifetime of the T1 state ( 2-3 microsecond) renders these polymers potent oxidizing and reducing agents. A spectroscopic and kinetic model has been developed for global fitting of TA spectra in both the frequency and time domains. Implication of the current ultrafast spectroscopy studies of these novel molecules to their roles in photocatalysis will be discussed.

X-ray evidence for ultra-fast outflows in AGNs

NASA Astrophysics Data System (ADS)

Tombesi, Francesco; Sambruna, Rita; Braito, Valentina; Reeves, James; Reynolds, Christopher; Cappi, Massimo

2012-07-01

X-ray evidence for massive, highly ionized, ultra-fast outflows (UFOs) has been recently reported in a number of AGNs through the detection of blue-shifted Fe XXV/XXVI absorption lines. We present the results of a comprehensive spectral analysis of a large sample of 42 local Seyferts and 5 radio galaxies observed with XMM-Newton and Suzaku. We assessed the global detection significance of the absorption lines and performed a detailed photo-ionization modeling. We find that UFOs are common phenomena, being present in >40% of the sources. Their outflow velocity distribution is in the range ˜0.03--0.3c, with mean value of ˜0.14c. The ionization parameter is very high, in the range logξ˜3--6 erg~s^{-1}~cm, and the associated column densities are also large, in the range ˜10^{22}--10^{24} cm^{-2}. Their location is constrained at ˜0.0003--0.03pc (˜10^2--10^4 r_s) from the central black hole, consistent with what is expected for accretion disk winds/outflows. The mass outflow rates are in the interval ˜0.01--1M_{⊙}~yr^{-1} and the associated mechanical power is high, in the range ˜10^{43}--10^{45} erg/s. Therefore, UFOs are capable to provide a significant contribution to the AGN cosmological feedback and their study can provide important clues on the connection between accretion disks, winds and jets.

Plasmonic field confinement for separate absorption-multiplication in InGaAs nanopillar avalanche photodiodes

PubMed Central

Farrell, Alan C.; Senanayake, Pradeep; Hung, Chung-Hong; El-Howayek, Georges; Rajagopal, Abhejit; Currie, Marc; Hayat, Majeed M.; Huffaker, Diana L.

2015-01-01

Avalanche photodiodes (APDs) are essential components in quantum key distribution systems and active imaging systems requiring both ultrafast response time to measure photon time of flight and high gain to detect low photon flux. The internal gain of an APD can improve system signal-to-noise ratio (SNR). Excess noise is typically kept low through the selection of material with intrinsically low excess noise, using separate-absorption-multiplication (SAM) heterostructures, or taking advantage of the dead-space effect using thin multiplication regions. In this work we demonstrate the first measurement of excess noise and gain-bandwidth product in III–V nanopillars exhibiting substantially lower excess noise factors compared to bulk and gain-bandwidth products greater than 200 GHz. The nanopillar optical antenna avalanche detector (NOAAD) architecture is utilized for spatially separating the absorption region from the avalanche region via the NOA resulting in single carrier injection without the use of a traditional SAM heterostructure. PMID:26627932

Femtosecond time-resolved X-ray absorption spectroscopy of anatase TiO2 nanoparticles using XFEL

PubMed Central

Obara, Yuki; Ito, Hironori; Ito, Terumasa; Kurahashi, Naoya; Thürmer, Stephan; Tanaka, Hiroki; Katayama, Tetsuo; Togashi, Tadashi; Owada, Shigeki; Yamamoto, Yo-ichi; Karashima, Shutaro; Nishitani, Junichi; Yabashi, Makina; Suzuki, Toshinori; Misawa, Kazuhiko

2017-01-01

The charge-carrier dynamics of anatase TiO2 nanoparticles in an aqueous solution were studied by femtosecond time-resolved X-ray absorption spectroscopy using an X-ray free electron laser in combination with a synchronized ultraviolet femtosecond laser (268 nm). Using an arrival time monitor for the X-ray pulses, we obtained a temporal resolution of 170 fs. The transient X-ray absorption spectra revealed an ultrafast Ti K-edge shift and a subsequent growth of a pre-edge structure. The edge shift occurred in ca. 100 fs and is ascribed to reduction of Ti by localization of generated conduction band electrons into shallow traps of self-trapped polarons or deep traps at penta-coordinate Ti sites. Growth of the pre-edge feature and reduction of the above-edge peak intensity occur with similar time constants of 300–400 fs, which we assign to the structural distortion dynamics near the surface. PMID:28713842

Imaging at an x-ray absorption edge using free electron laser pulses for interface dynamics in high energy density systems [Resonant phase contrast imaging for interface physics

DOE PAGES

Beckwith, M. A.; Jiang, S.; Schropp, A.; ...

2017-05-01

Tuning the energy of an x-ray probe to an absorption line or edge can provide material-specific measurements that are particularly useful for interfaces. Simulated hard x-ray images above the Fe K-edge are presented to examine ion diffusion across an interface between Fe 2O 3 and SiO 2 aerogel foam materials. The simulations demonstrate the feasibility of such a technique for measurements of density scale lengths near the interface with submicron spatial resolution. A proof-of-principle experiment is designed and performed at the Linac coherent light source facility. Preliminary data show the change of the interface after shock compression and heating withmore » simultaneous fluorescence spectra for temperature determination. Here, the results provide the first demonstration of using x-ray imaging at an absorption edge as a diagnostic to detect ultrafast phenomena for interface physics in high-energy-density systems.« less

Multiphoton-gated cycloreversion reaction of a fluorescent diarylethene derivative as revealed by transient absorption spectroscopy.

PubMed

Nagasaka, Tatsuhiro; Kunishi, Tomohiro; Sotome, Hikaru; Koga, Masafumi; Morimoto, Masakazu; Irie, Masahiro; Miyasaka, Hiroshi

2018-06-07

The one- and two-photon cycloreversion reactions of a fluorescent diarylethene derivative with oxidized benzothiophene moieties were investigated by means of ultrafast laser spectroscopy. Femtosecond transient absorption spectroscopy under the one-photon excitation condition revealed that the excited closed-ring isomer is simply deactivated into the initial ground state with a time constant of 2.6 ns without remarkable cycloreversion, the results of which are consistent with the very low cycloreversion reaction yield (<10-5) under steady-state light irradiation. On the other hand, an efficient cycloreversion reaction was observed under irradiation with a picosecond laser pulse at 532 nm. The excitation intensity dependence of the cycloreversion reaction indicates that a highly excited state attained by the stepwise two-photon absorption is responsible for the marked increase of the cycloreversion reaction, and the quantum yield at the highly excited state was estimated to be 0.018 from quantitative analysis, indicating that the reaction is enhanced by a factor of >1800.

Time-resolved x-ray absorption spectroscopy: Watching atoms dance

NASA Astrophysics Data System (ADS)

Milne, Chris J.; Pham, Van-Thai; Gawelda, Wojciech; van der Veen, Renske M.; El Nahhas, Amal; Johnson, Steven L.; Beaud, Paul; Ingold, Gerhard; Lima, Frederico; Vithanage, Dimali A.; Benfatto, Maurizio; Grolimund, Daniel; Borca, Camelia; Kaiser, Maik; Hauser, Andreas; Abela, Rafael; Bressler, Christian; Chergui, Majed

2009-11-01

The introduction of pump-probe techniques to the field of x-ray absorption spectroscopy (XAS) has allowed the monitoring of both structural and electronic dynamics of disordered systems in the condensed phase with unprecedented accuracy, both in time and in space. We present results on the electronically excited high-spin state structure of an Fe(II) molecular species, [FeII(bpy)3]2+, in aqueous solution, resolving the Fe-N bond distance elongation as 0.2 Å. In addition an analysis technique using the reduced χ2 goodness of fit between FEFF EXAFS simulations and the experimental transient absorption signal in energy space has been successfully tested as a function of excited state population and chemical shift, demonstrating its applicability in situations where the fractional excited state population cannot be determined through other measurements. Finally by using a novel ultrafast hard x-ray 'slicing' source the question of how the molecule relaxes after optical excitation has been successfully resolved using femtosecond XANES.

Perspective: Ultrafast magnetism and THz spintronics

DOE Office of Scientific and Technical Information (OSTI.GOV)

Walowski, Jakob; Münzenberg, Markus

This year the discovery of femtosecond demagnetization by laser pulses is 20 years old. For the first time, this milestone work by Bigot and coworkers gave insight directly into the time scales of microscopic interactions that connect the spin and electron system. While intense discussions in the field were fueled by the complexity of the processes in the past, it now became evident that it is a puzzle of many different parts. Rather than providing an overview that has been presented in previous reviews on ultrafast processes in ferromagnets, this perspective will show that with our current depth of knowledgemore » the first applications are developed: THz spintronics and all-optical spin manipulation are becoming more and more feasible. The aim of this perspective is to point out where we can connect the different puzzle pieces of understanding gathered over 20 years to develop novel applications. Based on many observations in a large number of experiments. Differences in the theoretical models arise from the localized and delocalized nature of ferromagnetism. Transport effects are intrinsically non-local in spintronic devices and at interfaces. We review the need for multiscale modeling to address the processes starting from electronic excitation of the spin system on the picometer length scale and sub-femtosecond time scale, to spin wave generation, and towards the modeling of ultrafast phase transitions that altogether determine the response time of the ferromagnetic system. Today, our current understanding gives rise to the first usage of ultrafast spin physics for ultrafast magnetism control: THz spintronic devices. This makes the field of ultrafast spin-dynamics an emerging topic open for many researchers right now.« less

Pump-probe microscopy of respiratory chain pigments: towards non-fluorescent label-free metabolic imaging

NASA Astrophysics Data System (ADS)

Domingue, Scott R.; Chicco, Adam J.; Bartels, Randy A.; Wilson, Jesse W.

2017-02-01

Current label-free metabolic microscopy techniques are limited to obtaining contrast from fluorescent molecules NAD(P)H and FAD+, and are unable to determine redox state along the mitochondrial respiratory chain itself. The respiratory chain electron carriers do not fluoresce, but some are heme proteins that have redox-dependent absorption spectra. The most prominent of these, cytochrome c, has been extensively characterized by transient absorption spectroscopy, which suggests that pump-probe measurements in the vicinity of 450 - 600 nm can provide strong contrast between its redox states. Motivated by the success of pump-probe microscopy targeting another heme protein, hemoglobin, we seek to extend the technique to the cytochromes, with the ultimate goal of dissecting respiratory chain function of individual cells in live tissue. To that end, we have developed a new optical system producing ultrafast, visible, independently-tunable pulse pairs via sum-frequency generation of nonlinearly broadened pulses in periodically-poled lithium niobate. The system is pumped by a homebuilt fiber-based oscillator/amplifier emitting 1060 nm pulses at 1.3 W (63 MHz repetition rate), and produces tunable pulses in the vicinity of 488 and 532 nm. Pump-probe spectroscopy of cytochrome c with this source reveals differences in excited-state absorption relaxation times between redox states. Though redox contrast is weak with this setup, we argue that this can be improved with a resonant galvo-scanning microscope. Moreover, pump-probe images were acquired of brown adipose tissue (which contains dense mitochondria), demonstrating label-free contrast from excited-state absorption in respiratory chain hemes.

New Ligand Design Provides Delocalization and Promotes Strong Absorption throughout the Visible Region in a Ru(II) Complex

DOE PAGES

Whittemore, Tyler J.; White, Travis A.; Turro, Claudia

2017-12-20

The new Ru(II)–anthraquinone complex [Ru(bpy) 2(qdpq)](PF 6) 2 (Ru-qdpq; bpy = 2,2'-bipyridine; qdpq = 2,3-di(2-pyridyl)naphtho[2,3-f]quinoxaline-7,12-quinone) possesses a strong 1MLCT Ru → qdpq absorption with a maximum at 546 nm that tails into the near-IR and is significantly red-shifted relative to that of the related complex [Ru(bpy) 2(qdppz)](PF 6) 2 (Ru-qdppz; qdppz = naphtho[2,3-a]dipyrido[3,2-h:2',3'-f]phenazine-5,18-dione), with λ max = 450 nm. Ru-qdppz possesses electronically isolated proximal and distal qdppz-based excited states; the former is initially generated and decays to the latter, which repopulates the ground state with τ = 362 ps. In contrast, excitation of Ru-qdpq results in the population of amore » relatively long-lived (τ = 19 ns) Ru(dπ) → qdpq(π*) 3MLCT excited state where the promoted electron is delocalized throughout the qdpq ligand. Ultrafast spectroscopy, used together with steady-state absorption, electrochemistry, and DFT calculations, indicates that the unique coordination modes of the qdpq and qdppz ligands impart substantially different electronic communication throughout the quinone-containing ligand, affecting the excited state and electron transfer properties of these molecules. As a result, these observations create a pathway to synthesize complexes with red-shifted absorptions that possess long-lived, redox-active excited states that are useful for various applications, including solar energy conversion and photochemotherapy.« less

New Ligand Design Provides Delocalization and Promotes Strong Absorption throughout the Visible Region in a Ru(II) Complex

DOE Office of Scientific and Technical Information (OSTI.GOV)

Whittemore, Tyler J.; White, Travis A.; Turro, Claudia

The new Ru(II)–anthraquinone complex [Ru(bpy) 2(qdpq)](PF 6) 2 (Ru-qdpq; bpy = 2,2'-bipyridine; qdpq = 2,3-di(2-pyridyl)naphtho[2,3-f]quinoxaline-7,12-quinone) possesses a strong 1MLCT Ru → qdpq absorption with a maximum at 546 nm that tails into the near-IR and is significantly red-shifted relative to that of the related complex [Ru(bpy) 2(qdppz)](PF 6) 2 (Ru-qdppz; qdppz = naphtho[2,3-a]dipyrido[3,2-h:2',3'-f]phenazine-5,18-dione), with λ max = 450 nm. Ru-qdppz possesses electronically isolated proximal and distal qdppz-based excited states; the former is initially generated and decays to the latter, which repopulates the ground state with τ = 362 ps. In contrast, excitation of Ru-qdpq results in the population of amore » relatively long-lived (τ = 19 ns) Ru(dπ) → qdpq(π*) 3MLCT excited state where the promoted electron is delocalized throughout the qdpq ligand. Ultrafast spectroscopy, used together with steady-state absorption, electrochemistry, and DFT calculations, indicates that the unique coordination modes of the qdpq and qdppz ligands impart substantially different electronic communication throughout the quinone-containing ligand, affecting the excited state and electron transfer properties of these molecules. As a result, these observations create a pathway to synthesize complexes with red-shifted absorptions that possess long-lived, redox-active excited states that are useful for various applications, including solar energy conversion and photochemotherapy.« less

Ultrafast Intramolecular Electron and Proton Transfer in Bis(imino)isoindole Derivatives.

PubMed

Driscoll, Eric; Sorenson, Shayne; Dawlaty, Jahan M

2015-06-04

Concerted motion of electrons and protons in the excited state is pertinent to a wide range of chemical phenomena, including those relevant for solar-to-fuel light harvesting. The excited state dynamics of small proton-bearing molecules are expected to serve as models for better understanding such phenomena. In particular, for designing the next generation of multielectron and multiproton redox catalysts, understanding the dynamics of more than one proton in the excited state is important. Toward this goal, we have measured the ultrafast dynamics of intramolecular excited state proton transfer in a recently synthesized dye with two equivalent transferable protons. We have used a visible ultrafast pump to initiate the proton transfer in the excited state, and have probed the transient absorption of the molecule over a wide bandwidth in the visible range. The measurement shows that the signal which is characteristic of proton transfer emerges within ∼710 fs. To identify whether both protons were transferred in the excited state, we have measured the ultrafast dynamics of a related derivative, where only a single proton was available for transfer. The measured proton transfer time in that molecule was ∼427 fs. The observed dynamics in both cases were reasonably fit with single exponentials. Supported by the ultrafast observations, steady-state fluorescence, and preliminary computations of the relaxed excited states, we argue that the doubly protonated derivative most likely transfers only one of its two protons in the excited state. We have performed calculations of the frontier molecular orbitals in the Franck-Condon region. The calculations show that in both derivatives, the excitation is primarily from the HOMO to LUMO causing a large rearrangement of the electronic charge density immediately after photoexcitation. In particular, charge density is shifted away from the phenolic protons and toward the proton acceptor nitrogens. The proton transfer is hypothesized to occur both due to enhanced acidity of the phenolic proton and enhanced basicity of the nitrogen in the excited state. We hope this study can provide insight for better understanding of the general class of excited state concerted electron-proton dynamics.

TDDFT investigation of excitation of water tetramer under femtosecond laser pulse irradiation

NASA Astrophysics Data System (ADS)

Wang, Zhiping; Xu, Xuefen; Zhang, Fengshou; Qian, Chaoyi

2018-04-01

We study the static properties of water tetramer in ground state, the optical absorption spectra and ultrafast nonadiabatic dynamical response of water tetramer to short and intense laser pulses with different intensities by a real-space, real-time implementation of time-dependent density functional theory coupled to molecular dynamics (TDDFT-MD) nonadiabatically. The calculated results are in good agreement with available values in literature. Four typical irradiated scenarios of water tetramer in laser field, which are “normal vibration,” “break and reorganization,” “fragmentation and new formation” and “pure fragmentation”, are explored by discussing the ionization, the bond lengths of OH bonds and hydrogen bonds and the kinetic energy of ions. The dynamic simulation shows that the reaction channel of water tetramer can really be controlled by choosing appropriate laser parameters referring to the optical absorption spectra and hydrogen ions play an important role in the reaction channel. Furthermore, it is found that the laser intensity affects the kinetic energy of ejected protons more than that of the remaining fragments and all dynamic processes are somehow directly related to the velocity of departing protons.

Solvent Dependent Dynamics of Salicylidene Aniline in Binary Mixtures of Supercritical CO2 with 1-Propanol or Cyclohexane.

PubMed

Kieda, Ryan D; Dunkelberger, Adam D; Case, Amanda S; Crim, F Fleming

2017-02-02

The role of different solvent environments in determining the behavior of molecules in solution is a fundamental aspect of chemical reactivity. We present an approach for exploring the influence of solvent properties on condensed-phase dynamics using ultrafast transient absorption spectroscopy in supercritical CO 2 . Using supercritical CO 2 permits adjustment of the density, by varying the temperature and pressure, whereas varying the concentration or identity of a second solvent, the cosolvent, in a binary mixture allows for adjustments of the degree of interaction between the solute and the solvent. Salicylidene aniline, a prototypical excited-state intramolecular proton-transfer system, is the subject of this study. In this system, the decay rate of the transient absorption signal decreases as the fraction of the cosolvent (for both 1-propanol and cyclohexane) increases. The decay rate also decreases with an increase in the viscosity of the mixture, but the effect is much larger for the 1-propanol cosolvent than for cyclohexane. These observations illustrate that the decay rate of the photoexcited salicylidene aniline depends on more than just the solvent viscosity, suggesting that properties such as polarity also play a role in the dynamics.

A Combination of Chemometrics and Quantum Mechanics Methods Applied to Analysis of Femtosecond Transient Absorption Spectrum of Ortho-Nitroaniline

PubMed Central

Yi, Jing; Xiong, Ying; Cheng, Kemei; Li, Menglong; Chu, Genbai; Pu, Xuemei; Xu, Tao

2016-01-01

A combination of the advanced chemometrics method with quantum mechanics calculation was for the first time applied to explore a facile yet efficient analysis strategy to thoroughly resolve femtosecond transient absorption spectroscopy of ortho-nitroaniline (ONA), served as a model compound of important nitroaromatics and explosives. The result revealed that the ONA molecule is primarily excited to S3 excited state from the ground state and then ultrafast relaxes to S2 state. The internal conversion from S2 to S1 occurs within 0.9 ps. One intermediate state S* was identified in the intersystem crossing (ISC) process, which is different from the specific upper triplet receiver state proposed in some other nitroaromatics systems. The S1 state decays to the S* one within 6.4 ps and then intersystem crossing to the lowest triplet state within 19.6 ps. T1 was estimated to have a lifetime up to 2 ns. The relatively long S* state and very long-lived T1 one should play a vital role as precursors to various nitroaromatic and explosive photoproducts. PMID:26781083

Time-Resolved Transient Optical Absorption Study of Bis(terpyridyl)oligothiophenes and Their Metallo-Supramolecular Polymers with Zn(II) Ion Couplers.

PubMed

Rais, David; Menšík, Miroslav; Štenclová-Bláhová, Pavla; Svoboda, Jan; Vohlídal, Jiří; Pfleger, Jiří

2015-06-18

α,ω-Bis(terpyridyl)oligothiophenes spontaneously assemble with Zn(II) ions giving conjugated constitutional dynamic polymers (dynamers) of the metallo-supramolecular class, which potentially might be utilized in optoelectronics. Their photophysical properties, which are of great importance in this field of application, are strongly influenced by the dynamic morphology. It was assessed in this study by using ultrafast pump-probe optical absorption spectroscopy. We identified and characterized relaxation processes running in photoexcited molecules of these oligomers and dynamers and show impacts of disturbed coplanarity of adjacent rings (twisting the thiophene-thiophene and thiophene-terpyridyl bonds by attached hexyl side groups) and Zn(II) ion couplers on these processes. Major effects are seen in the time constants of rotational relaxation, intersystem crossing, and de-excitation lifetimes. The photoexcited states formed on different repeating units within the same dynamer chain do not interact with each other even at very high excitation density. The method is presented that allows determining the equilibrium fraction of unbound oligothiophene species in a dynamer solution, from which otherwise hardly accessible values of the average degree of polymerization of constitutionally dynamic chains in solution can be estimated.

Photophysical Study of Polymer-Based Solar Cells with an Organo-Boron Molecule in the Active Layer

PubMed Central

Romero-Servin, Sergio; de Anda Villa, Manuel; Carriles, R.; Ramos-Ortíz, Gabriel; Maldonado, José-Luis; Rodríguez, Mario; Güizado-Rodríguez, M.

2015-01-01

Our group previously reported the synthesis of four polythiophene derivatives (P1–P4) used for solar cells. The cells were prepared under room conditions by spin coating, leading to low efficiencies. However, after the addition of 6-nitro-3-(E)-3-(4-dimethylaminophenyl)allylidene)-2,3-dihydrobenzo[d]-[1,3,2] oxazaborole (M1) to their active layers, the efficiencies of the cells showed approximately a two-fold improvement. In this paper, we study this enhancement mechanism by performing ultrafast transient absorption (TA) experiments on the active layer of the different cells. Our samples consisted of thin films of a mixture of PC61BM with the polythiophenes derivatives P1–P4. We prepared two versions of each sample, one including the molecule M1 and another without it. The TA data suggests that the efficiency improvement after addition of M1 is due not only to an extended absorption spectrum towards the infrared region causing a larger population of excitons but also to the possible creation of additional channels for transport of excitons and/or electrons to the PC61BM interface. PMID:28793438

Measurements of ultrafast spin-profiles and spin-diffusion properties in the domain wall area at a metal/ferromagnetic film interface.

PubMed

Sant, T; Ksenzov, D; Capotondi, F; Pedersoli, E; Manfredda, M; Kiskinova, M; Zabel, H; Kläui, M; Lüning, J; Pietsch, U; Gutt, C

2017-11-08

Exciting a ferromagnetic material with an ultrashort IR laser pulse is known to induce spin dynamics by heating the spin system and by ultrafast spin diffusion processes. Here, we report on measurements of spin-profiles and spin diffusion properties in the vicinity of domain walls in the interface region between a metallic Al layer and a ferromagnetic Co/Pd thin film upon IR excitation. We followed the ultrafast temporal evolution by means of an ultrafast resonant magnetic scattering experiment in surface scattering geometry, which enables us to exploit the evolution of the domain network within a 1/e distance of 3 nm to 5 nm from the Al/FM film interface. We observe a magnetization-reversal close to the domain wall boundaries that becomes more pronounced closer to the Al/FM film interface. This magnetization-reversal is driven by the different transport properties of majority and minority carriers through a magnetically disordered domain network. Its finite lateral extension has allowed us to measure the ultrafast spin-diffusion coefficients and ultrafast spin velocities for majority and minority carriers upon IR excitation.

Distributed ultrafast fibre laser

PubMed Central

Liu, Xueming; Cui, Yudong; Han, Dongdong; Yao, Xiankun; Sun, Zhipei

2015-01-01

A traditional ultrafast fibre laser has a constant cavity length that is independent of the pulse wavelength. The investigation of distributed ultrafast (DUF) lasers is conceptually and technically challenging and of great interest because the laser cavity length and fundamental cavity frequency are changeable based on the wavelength. Here, we propose and demonstrate a DUF fibre laser based on a linearly chirped fibre Bragg grating, where the total cavity length is linearly changeable as a function of the pulse wavelength. The spectral sidebands in DUF lasers are enhanced greatly, including the continuous-wave (CW) and pulse components. We observe that all sidebands of the pulse experience the same round-trip time although they have different round-trip distances and refractive indices. The pulse-shaping of the DUF laser is dominated by the dissipative processes in addition to the phase modulations, which makes our ultrafast laser simple and stable. This laser provides a simple, stable, low-cost, ultrafast-pulsed source with controllable and changeable cavity frequency. PMID:25765454

Ultrafast Surface-Enhanced Raman Probing of the Role of Hot Electrons in Plasmon-Driven Chemistry.

PubMed

Brandt, Nathaniel C; Keller, Emily L; Frontiera, Renee R

2016-08-18

Hot electrons generated through plasmonic excitations in metal nanostructures show great promise for efficiently driving chemical reactions with light. However, the lifetime, yield, and mechanism of action of plasmon-generated hot electrons involved in a given photocatalytic process are not well understood. Here, we develop ultrafast surface-enhanced Raman scattering (SERS) as a direct probe of plasmon-molecule interactions in the plasmon-catalyzed dimerization of 4-nitrobenzenethiol to p,p'-dimercaptoazobenzene. Ultrafast SERS probing of these molecular reporters in plasmonic hot spots reveals transient Fano resonances, which we attribute to near-field coupling of Stokes-shifted photons to hot electron-driven metal photoluminescence. Surprisingly, we find that hot spots that yield more photoluminescence are much more likely to drive the reaction, which indirectly proves that plasmon-generated hot electrons induce the photochemistry. These ultrafast SERS results provide insight into the relative reactivity of different plasmonic hot spot environments and quantify the ultrafast lifetime of hot electrons involved in plasmon-driven chemistry.

4D multiple-cathode ultrafast electron microscopy

PubMed Central

Baskin, John Spencer; Liu, Haihua; Zewail, Ahmed H.

2014-01-01

Four-dimensional multiple-cathode ultrafast electron microscopy is developed to enable the capture of multiple images at ultrashort time intervals for a single microscopic dynamic process. The dynamic process is initiated in the specimen by one femtosecond light pulse and probed by multiple packets of electrons generated by one UV laser pulse impinging on multiple, spatially distinct, cathode surfaces. Each packet is distinctly recorded, with timing and detector location controlled by the cathode configuration. In the first demonstration, two packets of electrons on each image frame (of the CCD) probe different times, separated by 19 picoseconds, in the evolution of the diffraction of a gold film following femtosecond heating. Future elaborations of this concept to extend its capabilities and expand the range of applications of 4D ultrafast electron microscopy are discussed. The proof-of-principle demonstration reported here provides a path toward the imaging of irreversible ultrafast phenomena of materials, and opens the door to studies involving the single-frame capture of ultrafast dynamics using single-pump/multiple-probe, embedded stroboscopic imaging. PMID:25006261

4D multiple-cathode ultrafast electron microscopy.

PubMed

Baskin, John Spencer; Liu, Haihua; Zewail, Ahmed H

2014-07-22

Four-dimensional multiple-cathode ultrafast electron microscopy is developed to enable the capture of multiple images at ultrashort time intervals for a single microscopic dynamic process. The dynamic process is initiated in the specimen by one femtosecond light pulse and probed by multiple packets of electrons generated by one UV laser pulse impinging on multiple, spatially distinct, cathode surfaces. Each packet is distinctly recorded, with timing and detector location controlled by the cathode configuration. In the first demonstration, two packets of electrons on each image frame (of the CCD) probe different times, separated by 19 picoseconds, in the evolution of the diffraction of a gold film following femtosecond heating. Future elaborations of this concept to extend its capabilities and expand the range of applications of 4D ultrafast electron microscopy are discussed. The proof-of-principle demonstration reported here provides a path toward the imaging of irreversible ultrafast phenomena of materials, and opens the door to studies involving the single-frame capture of ultrafast dynamics using single-pump/multiple-probe, embedded stroboscopic imaging.

Measurement of carotid pulse wave velocity using ultrafast ultrasound imaging in hypertensive patients.

PubMed

Li, Xiaopeng; Jiang, Jue; Zhang, Hong; Wang, Hua; Han, Donggang; Zhou, Qi; Gao, Ya; Yu, Shanshan; Qi, Yanhua

2017-04-01

The study aimed to assess the utility of ultrafast ultrasound imaging for evaluation of carotid pulse wave velocity (PWV) in newly diagnosed hypertension patients. This prospective non-randomized study enrolled 90 hypertensive patients in our hospital from September to December 2013 as a hypertension group. An age- and sex-matched cohort of 50 healthy adults in our hospital from September to December 2013 was also included in the study as a control group. Carotid PWV at the beginning and at the end of systole (PWV-BS and PWV-ES, respectively) and intima-media thickness (IMT) were measured by ultrafast ultrasound imaging technology. The associations of PWV-BS, PWV-ES, and IMT with hypertension stage were evaluated by Spearman correlation analysis. PWV-BS and PWV-ES in the hypertension group were significantly elevated compared with those in control group. Different hypertension stages significantly differed in PWV-BS and PWV-ES. PWV-BS and PWV-ES appeared to increase with the hypertension stage. Moreover, IMT, PWV-BS, and PWV-ES were positively correlated with the hypertension stage in hypertensive patients. Ultrafast ultrasound imaging was a valid and convenient method for the measurement of carotid PWV in hypertensive patients. Ultrafast ultrasound imaging might be recommended as a promising alternative method for early detection of arterial abnormality in clinical practice.

Influence of Processing Additives on Charge-Transfer Time Scales and Sound Velocity in Organic Bulk Heterojunction Films.

PubMed

Kaake, Loren G; Welch, Gregory C; Moses, Daniel; Bazan, Guillermo C; Heeger, Alan J

2012-05-17

The role of processing additives in organic bulk heterojunction thin films was investigated by means of transient absorption spectroscopy. The rate of ultrafast charge transfer was found to increase when a small amount of diiodooctane was used during film formation. In addition, coherent acoustic phonons were observed, and their velocity was determined. A strong correlation between the sound velocity and the charge-transfer time scale was observed, both of which could be explained by a subtle increase in thin film density.

Observation of rapid exciton-exciton annihilation in monolayer molybdenum disulfide.

PubMed

Sun, Dezheng; Rao, Yi; Reider, Georg A; Chen, Gugang; You, Yumeng; Brézin, Louis; Harutyunyan, Avetik R; Heinz, Tony F

2014-10-08

Monolayer MoS2 is a direct-gap two-dimensional semiconductor that exhibits strong electron-hole interactions, leading to the formation of stable excitons and trions. Here we report the existence of efficient exciton-exciton annihilation, a four-body interaction, in this material. Exciton-exciton annihilation was identified experimentally in ultrafast transient absorption measurements through the emergence of a decay channel varying quadratically with exciton density. The rate of exciton-exciton annihilation was determined to be (4.3 ± 1.1) × 10(-2) cm(2)/s at room temperature.

Interplay of coupling and superradiant emission in the optical response of a double quantum dot

NASA Astrophysics Data System (ADS)

Sitek, Anna; Machnikowski, Paweł

2009-09-01

We study theoretically the optical response of a double quantum dot structure to an ultrafast optical excitation. We show that the interplay of a specific type of coupling between the dots and their collective interaction with the radiative environment leads to very characteristic features in the time-resolved luminescence as well as in the absorption spectrum of the system. For a sufficiently strong coupling, these effects survive even if the transition energy mismatch between the two dots exceeds by far the emission linewidth.

The origin of efficient triplet state population in sulfur-substituted nucleobases

PubMed Central

Mai, Sebastian; Pollum, Marvin; Martínez-Fernández, Lara; Dunn, Nicholas; Marquetand, Philipp; Corral, Inés; Crespo-Hernández, Carlos E.; González, Leticia

2016-01-01

Elucidating the photophysical mechanisms in sulfur-substituted nucleobases (thiobases) is essential for designing prospective drugs for photo- and chemotherapeutic applications. Although it has long been established that the phototherapeutic activity of thiobases is intimately linked to efficient intersystem crossing into reactive triplet states, the molecular factors underlying this efficiency are poorly understood. Herein we combine femtosecond transient absorption experiments with quantum chemistry and nonadiabatic dynamics simulations to investigate 2-thiocytosine as a necessary step to unravel the electronic and structural elements that lead to ultrafast and near-unity triplet-state population in thiobases in general. We show that different parts of the potential energy surfaces are stabilized to different extents via thionation, quenching the intrinsic photostability of canonical DNA and RNA nucleobases. These findings satisfactorily explain why thiobases exhibit the fastest intersystem crossing lifetimes measured to date among bio-organic molecules and have near-unity triplet yields, whereas the triplet yields of canonical nucleobases are nearly zero. PMID:27703148

Insights into the Mechanism of a Covalently Linked Organic Dye–Cobaloxime Catalyst System for Dye‐Sensitized Solar Fuel Devices

PubMed Central

Pati, Palas Baran; Zhang, Lei; Philippe, Bertrand; Fernández‐Terán, Ricardo; Ahmadi, Sareh; Tian, Lei; Rensmo, Håkan; Hammarström, Leif

2017-01-01

Abstract A covalently linked organic dye–cobaloxime catalyst system based on mesoporous NiO is synthesized by a facile click reaction for mechanistic studies and application in a dye‐sensitized solar fuel device. The system is systematically investigated by photoelectrochemical measurements, density functional theory, time‐resolved fluorescence, transient absorption spectroscopy, and photoelectron spectroscopy. The results show that irradiation of the dye–catalyst on NiO leads to ultrafast hole injection into NiO from the excited dye, followed by a fast electron transfer process to reduce the catalyst. Moreover, the dye adopts different structures with different excited state energies, and excitation energy transfer occurs between neighboring molecules on the semiconductor surface. The photoelectrochemical experiments also show hydrogen production by this system. The axial chloride ligands of the catalyst are released during photocatalysis to create the active sites for proton reduction. A working mechanism of the dye–catalyst system on the photocathode is proposed on the basis of this study. PMID:28338295

Non-radiative relaxation of photoexcited chlorophylls: Theoretical and experimental study

DOE PAGES

Bricker, William P.; Shenai, Prathamesh M.; Ghosh, Avishek; ...

2015-09-08

Nonradiative relaxation of high-energy excited states to the lowest excited state in chlorophylls marks the first step in the process of photosynthesis. We perform ultrafast transient absorption spectroscopy measurements, that reveal this internal conversion dynamics to be slightly slower in chlorophyll B than in chlorophyll A. With modeling this process, non-adiabatic excited state molecular dynamics simulations uncovers a critical role played by the different side groups in the two molecules in governing the intramolecular redistribution of excited state wavefunction, leading, in turn, to different time-scales. Even given smaller electron-vibrational couplings compared to common organic conjugated chromophores, these molecules are ablemore » to efficiently dissipate about 1 eV of electronic energy into heat on the timescale of around 200 fs. This is achieved via selective participation of specific atomic groups and complex global migration of the wavefunction from the outer to inner ring, which may have important implications for biological light-harvesting function.« less

Enhanced Absorption in 2D Materials Via Fano- Resonant Photonic Crystals

DOE PAGES

Wang, Wenyi; Klotz, Andrey; Yang, Yuanmu; ...

2015-05-01

The use of two-dimensional (2D) materials in optoelectronics has attracted much attention due to their fascinating optical and electrical properties. For instance, graphenebased devices have been employed for applications such as ultrafast and broadband photodetectors and modulators while transition metal dichalcogenide (TMDC) based photodetectors can be used for ultrasensitive photodetection. However, the low optical absorption of 2D materials arising from their atomic thickness limits the maximum attainable external quantum efficiency. For example, in the visible and NIR regimes monolayer MoS 2 and graphene absorb only ~10% and 2.3% of incoming light, respectively. Here, we experimentally demonstrate the use of Fano-resonantmore » photonic crystals to significantly boost absorption in atomically thin materials. Using graphene as a test bed, we demonstrate that absorption in the monolayer thick material can be enhanced to 77% within the telecommunications band, the highest value reported to date. We also show that the absorption in the Fano-resonant structure is non-local, with light propagating up to 16 μm within the structure. This property is particularly beneficial in harvesting light from large areas in field-effect-transistor based graphene photodetectors in which separation of photo-generated carriers only occurs ~0.2 μm adjacent to the graphene/electrode interface.« less

Ultrafast Microscopy of Energy and Charge Transport

NASA Astrophysics Data System (ADS)

Huang, Libai

The frontier in solar energy research now lies in learning how to integrate functional entities across multiple length scales to create optimal devices. Advancing the field requires transformative experimental tools that probe energy transfer processes from the nano to the meso lengthscales. To address this challenge, we aim to understand multi-scale energy transport across both multiple length and time scales, coupling simultaneous high spatial, structural, and temporal resolution. In my talk, I will focus on our recent progress on visualization of exciton and charge transport in solar energy harvesting materials from the nano to mesoscale employing ultrafast optical nanoscopy. With approaches that combine spatial and temporal resolutions, we have recently revealed a new singlet-mediated triplet transport mechanism in certain singlet fission materials. This work demonstrates a new triplet exciton transport mechanism leading to favorable long-range triplet exciton diffusion on the picosecond and nanosecond timescales for solar cell applications. We have also performed a direct measurement of carrier transport in space and in time by mapping carrier density with simultaneous ultrafast time resolution and 50 nm spatial precision in perovskite thin films using transient absorption microscopy. These results directly visualize long-range carrier transport of 220nm in 2 ns for solution-processed polycrystalline CH3NH3PbI3 thin films. The spatially and temporally resolved measurements reported here underscore the importance of the local morphology and establish an important first step towards discerning the underlying transport properties of perovskite materials.

Ultrafast Adiabatic Photodehydration of 2-Hydroxymethylphenol and the Formation of Quinone Methide.

PubMed

Škalamera, Đani; Antol, Ivana; Mlinarić-Majerski, Kata; Vančik, Hrvoj; Phillips, David Lee; Ma, Jiani; Basarić, Nikola

2018-04-20

The photochemical reactivity of 2-hydroxymethylphenol (1) was investigated experimentally by photochemistry under cryogenic conditions, by detecting reactive intermediates by IR spectroscopy, and by using nanosecond and femtosecond transient absorption spectroscopic methods in solution at room temperature. In addition, theoretical studies were performed to facilitate the interpretation of the experimental results and also to simulate the reaction pathway to obtain a better understanding of the reaction mechanism. The main finding of this work is that photodehydration of 1 takes place in an ultrafast adiabatic photochemical reaction without any clear intermediate, delivering quinone methide (QM) in the excited state. Upon photoexcitation to a higher vibrational level of the singlet excited state, 1 undergoes vibrational relaxation leading to two photochemical pathways, one by which synchronous elimination of H 2 O gives QM 2 in its S 1 state and the other by which homolytic cleavage of the phenolic O-H bond produces a phenoxyl radical (S 0 ). Both are ultrafast processes that occur within a picosecond. The excited state of QM 2 (S 1 ) probably deactivates to S 0 through a conical intersection to give QM 2 (S 0 ), which subsequently delivers benzoxete 4. Elucidation of the reaction mechanisms for the photodehydration of phenols by which QMs are formed is important to tune the reactivity of QMs with DNA and proteins for the potential application of QMs in medicine as therapeutic agents. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

An ultra-fast optical shutter exploiting total light absorption in a phase change material

NASA Astrophysics Data System (ADS)

Jafari, Mohsen; Guo, L. Jay; Rais-Zadeh, Mina

2017-02-01

In this paper, we present an ultra-fast and high-contrast optical shutter with applications in atomic clock assemblies, integrated photonic systems, communication hardware, etc. The shutter design exploits the total light absorption phenomenon in a thin phase change (PC) material placed over a metal layer. The shutter switches between ON and OFF states by changing PC material phase and thus its refractive index. The PC material used in this work is Germanium Telluride (GeTe), a group IV-VI chalcogenide compound, which exhibits good optical contrast when switching from amorphous to crystalline state and vice versa. The stable phase changing behavior and reliability of GeTe and GeSbTe (GST) have been verified in optical memories and RF switches. Here, GeTe is used as it has a lower extinction coefficient in near-IR regions compared to GST. GeTe can be thermally transitioned between two phases by applying electrical pulses to an integrated heater. The memory behavior of GeTe results in zero static power consumption which is useful in applications requiring long time periods between switching activities. We previously demonstrated a meta-surface employing GeTe in sub-wavelength slits with >14 dB isolation at 1.5 μm by exciting the surface plasmon polariton and localized slit resonances. In this work, strong interference effects in a thin layer of GeTe over a gold mirror result in near total light absorption of up to 40 dB (21 dB measured) in the amorphous phase of the shutter at 780 nm with much less fabrication complexity. The optical loss at the shutter ON state is less than 1.5 dB. A nickel chrome (NiCr) heater provides the Joule heating energy required to achieve the crystallographic phase change. The measured switching speed is 2 μs.

Evidence for Ultra-Fast Outflows in Radio-Quiet AGNs. 2; Detailed Photoionization Modeling of Fe K-Shell Absorption Lines

NASA Technical Reports Server (NTRS)

Tombesi, Francesco; Clapp, M.; Reeves, J. N.; Palumbo, G. G. C.; Braito, V.; Dadina, M.

2011-01-01

X-ray absorption line spectroscopy has recently shown evidence for previously unknown Ultra-fast Outflows (UFOs) in radio-quiet AGNs. In the previous paper of this series we defined UFOs as those absorbers with an outflow velocity higher than 10,000km/s and assessed the statistical significance of the associated blue shifted FeK absorption lines in a large sample of 42 local radio-quiet AGNs observed with XMM-Newton. In the present paper we report a detailed curve of growth analysis and directly model the FeK absorbers with the Xstar photo-ionization code. We confirm that the frequency of sources in the radio-quiet sample showing UFOs is >35%. The outflow velocity distribution spans from \\sim10,000km/s (\\sim0.03c) up to \\siml00,000kmis (\\sim0.3c), with a peak and mean value of\\sim42,000km/s (\\sim0.14c). The ionization parameter is very high and in the range log\\xi 3-6 erg s/cm, with a mean value of log\\xi 4.2 erg s/cm. The associated column densities are also large, in the range N_H\\siml0(exp 22)-10(exp 24)/sq cm, with a mean value of N_H\\siml0(exp23)/sq cm. We discuss and estimate how selection effects, such as those related to the limited instrumental sensitivity at energies above 7keV, may hamper the detection of even higher velocities and higher ionization absorbers. We argue that, overall, these results point to the presence of extremely ionized and possibly almost Compton thick outflowing material in the innermost regions of AGNs. This also suggests that UFOs may potentially play a significant role in the expected cosmological feedback from AGNs and their study can provide important clues on the connection between accretion disks, winds and jets.

Ultrafast outflows in radio-loud active galactic nuclei

NASA Astrophysics Data System (ADS)

Tombesi, F.; Tazaki, F.; Mushotzky, R. F.; Ueda, Y.; Cappi, M.; Gofford, J.; Reeves, J. N.; Guainazzi, M.

2014-09-01

Recent X-ray observations show absorbing winds with velocities up to mildly relativistic values of the order of ˜0.1c in a limited sample of six broad-line radio galaxies. They are observed as blueshifted Fe XXV-XXVI K-shell absorption lines, similarly to the ultrafast outflows (UFOs) reported in Seyferts and quasars. In this work we extend the search for such Fe K absorption lines to a larger sample of 26 radio-loud active galactic nuclei (AGN) observed with XMM-Newton and Suzaku. The sample is drawn from the Swift Burst Alert Telescope 58-month catalogue and blazars are excluded. X-ray bright Fanaroff-Riley Class II radio galaxies constitute the majority of the sources. Combining the results of this analysis with those in the literature we find that UFOs are detected in >27 per cent of the sources. However, correcting for the number of spectra with insufficient signal-to-noise ratio, we can estimate that the incidence of UFOs is this sample of radio-loud AGN is likely in the range f ≃ (50 ± 20) per cent. A photoionization modelling of the absorption lines with XSTAR allows us to estimate the distribution of their main parameters. The observed outflow velocities are broadly distributed between vout ≲ 1000 km s-1 and vout ≃ 0.4c, with mean and median values of vout ≃ 0.133c and vout ≃ 0.117c, respectively. The material is highly ionized, with an average ionization parameter of logξ ≃ 4.5 erg s-1 cm, and the column densities are larger than NH > 1022 cm-2. Overall, these characteristics are consistent with the presence of complex accretion disc winds in a significant fraction of radio-loud AGN and demonstrate that the presence of relativistic jets does not preclude the existence of winds, in accordance with several theoretical models.

Photophysics of detection of explosive vapours via luminescence quenching of thin films: impact of inter-molecular interactions.

PubMed

Shoaee, Safa; Fan, Shengqiang; Burn, Paul L; Shaw, Paul E

2016-09-21

Fluorescence-based detection of explosive analytes requires an understanding of the nature of the excited state responsible for the luminescence response of a sensing material. Many measurements are carried out to elucidate the fundamental photophysical properties of an emissive material in solution. However, simple transfer of the understanding gained from the solution measurements to the solid-state can lead to errors. This is in part due to the absence of inter-molecular interactions of the chromophores in solution, which are present in the solid-state. To understand the role of inter-molecular interactions on the detection of explosive analytes we have chosen dendrimers from two different families, D1 and D2, which allow facile control of the inter-molecular interactions through the choice of dendrons and emissive chromophores. Using ultrafast transient absorption spectroscopy we find that the solution photoinduced absorption (PA) for both materials can be explained in terms of the generation of singlet excitons, which decay to the ground state, or intersystem cross (ISC) to form a triplet exciton. In neat films however, we observe different photophysical behaviours; first, ISC to the triplet state does not occur, and second, depending on the chromophore, charge transfer and charge separated states are formed. Furthermore, we find that when either dendrimer is interfaced with analyte vapour, the singlet state is strongly quenched, generating a charge transfer state that undergoes geminate recombination.

Exploring exciton relaxation and multiexciton generation in PbSe nanocrystals using hyperspectral near-IR probing.

PubMed

Gdor, Itay; Sachs, Hanan; Roitblat, Avishy; Strasfeld, David B; Bawendi, Moungi G; Ruhman, Sanford

2012-04-24

Hyperspectral femtosecond transient absorption spectroscopy is employed to record exciton relaxation and recombination in colloidal lead selenide (PbSe) nanocrystals in unprecedented detail. Results obtained with different pump wavelengths and fluences are scrutinized with regard to three issues: (1) early subpicosecond spectral features due to "hot" excitons are analyzed in terms of suggested underlying mechanisms; (2) global kinetic analysis facilitates separation of the transient difference spectra into single, double, and triple exciton state contributions, from which individual band assignments can be tested; and (3) the transient spectra are screened for signatures of multiexciton generation (MEG) by comparing experiments with excitation pulses both below and well above the theoretical threshold for multiplication. For the latter, a recently devised ultrafast pump-probe spectroscopic approach is employed. Scaling sample concentrations and pump pulse intensities inversely with the extinction coefficient at each excitation wavelength overcomes ambiguities due to direct multiphoton excitation, uncertainties of absolute absorption cross sections, and low signal levels. As observed in a recent application of this method to InAs core/shell/shell nanodots, no sign of MEG was detected in this sample up to photon energy 3.7 times the band gap. Accordingly, numerous reports of efficient MEG in other samples of PbSe suggest that the efficiency of this process varies from sample to sample and depends on factors yet to be determined.

Broadband ultrafast transient absorption of multiple exciton dynamics in lead sulfide nanocrystals

NASA Astrophysics Data System (ADS)

Gesuele, Felice; Wong, Chee Wei; Sfeir, Matthew; Misewich, James; Koh, Weonkyu; Murray, Christopher

2011-03-01

Multiple exciton generation (MEG) is under intense investigation as potential third-generation solar photovoltaics with efficiencies beyond the Shockley-Queisser limit. We examine PbS nanocrystals, dispersed and vigorously stirred in TCE solution, by means of supercontinuum femtosecond transient absorption (TA). TA spectra show the presence of first and second order bleaches for the 1Sh-Se and 1Ph-Pe excitonic transition while photoinduced absorption for the 1Sh,e-Ph,e transitions. We found evidence of carrier multiplication (MEG for single absorbed photon) from the analysis of the first and second order bleaches, in the limit of low number of absorbed photons (Nabs ~ 0.01), for energy three times and four times the Energy gap. The MEG efficiency, derived from the ratio between early-time to long-time TA signal, presents a strongly dispersive behavior with maximum red shifted respect the first absorption peak. Analysis of population dynamics shows that in presence of biexciton, the 1Sh-Se bleach peak is red-shifted indicating a positive binding energy. MEG efficiency estimation will be discussed with regards to spectral integration, correlated higher-order and first excitonic transitions, as well as the nanocrystal morphologies.

Ultrafast carrier dynamics in a p-type GaN wafer under different carrier distributions

NASA Astrophysics Data System (ADS)

Fang, Yu; Yang, Junyi; Yang, Yong; Wu, Xingzhi; Xiao, Zhengguo; Zhou, Feng; Song, Yinglin

2016-02-01

The dependence of the carrier distribution on photoexcited carrier dynamics in a p-type Mg-doped GaN (GaN:Mg) wafer were systematically measured by femtosecond transient absorption (TA) spectroscopy. The homogeneity of the carrier distribution was modified by tuning the wavelength of the UV pulse excitation around the band gap of GaN:Mg. The TA kinetics appeared to be biexponential for all carrier distributions, and only the slower component decayed faster as the inhomogeneity of the carrier distribution increased. It was concluded that the faster component (50-70 ps) corresponded to the trap process of holes by the Mg acceptors, and the slower component (150-600 ps) corresponded to the combination of non-radiative surface recombination and intrinsic carrier recombination via dislocations. Moreover, the slower component increased gradually with the incident fluence due to the saturation of surface states.

A transferable model for singlet-fission kinetics.

PubMed

Yost, Shane R; Lee, Jiye; Wilson, Mark W B; Wu, Tony; McMahon, David P; Parkhurst, Rebecca R; Thompson, Nicholas J; Congreve, Daniel N; Rao, Akshay; Johnson, Kerr; Sfeir, Matthew Y; Bawendi, Moungi G; Swager, Timothy M; Friend, Richard H; Baldo, Marc A; Van Voorhis, Troy

2014-06-01

Exciton fission is a process that occurs in certain organic materials whereby one singlet exciton splits into two independent triplets. In photovoltaic devices these two triplet excitons can each generate an electron, producing quantum yields per photon of >100% and potentially enabling single-junction power efficiencies above 40%. Here, we measure fission dynamics using ultrafast photoinduced absorption and present a first-principles expression that successfully reproduces the fission rate in materials with vastly different structures. Fission is non-adiabatic and Marcus-like in weakly interacting systems, becoming adiabatic and coupling-independent at larger interaction strengths. In neat films, we demonstrate fission yields near unity even when monomers are separated by >5 Å. For efficient solar cells, however, we show that fission must outcompete charge generation from the singlet exciton. This work lays the foundation for tailoring molecular properties like solubility and energy level alignment while maintaining the high fission yield required for photovoltaic applications.

Practical witness for electronic coherences

DOE Office of Scientific and Technical Information (OSTI.GOV)

Johnson, Allan S.; Department of Physics, Imperial College London, London; Yuen-Zhou, Joel

2014-12-28

The origin of the coherences in two-dimensional spectroscopy of photosynthetic complexes remains disputed. Recently, it has been shown that in the ultrashort-pulse limit, oscillations in a frequency-integrated pump-probe signal correspond exclusively to electronic coherences, and thus such experiments can be used to form a test for electronic vs. vibrational oscillations in such systems. Here, we demonstrate a method for practically implementing such a test, whereby pump-probe signals are taken at several different pulse durations and used to extrapolate to the ultrashort-pulse limit. We present analytic and numerical results determining requirements for pulse durations and the optimal choice of pulse centralmore » frequency, which can be determined from an absorption spectrum. Our results suggest that for numerous systems, the required experiment could be implemented by many ultrafast spectroscopy laboratories using pulses of tens of femtoseconds in duration. Such experiments could resolve the standing debate over the nature of coherences in photosynthetic complexes.« less

Fabrication of demultiplexer for T bps optical signals by using spincoated squarylium dye J-aggregates exhibiting femtosecond optical response

NASA Astrophysics Data System (ADS)

Iwasa, Izumi; Furuki, Makoto; Tian, Minquan; Sato, Yasuhiro; Pu, Lyong S.; Tatsuura, Setoshi; Wada, Osamu

2001-06-01

We fabricated spincoated films of squarylium dye (SQ) J- aggregates exhibiting femtosecond optical response at room temperature. Optical dynamics measurements revealed that the saturable absorption of the SQ J-aggregates film exhibited a decay time of less than 100 fs at a pump energy of 80 fJ/micrometer2. With this ultrafast SQ optical film, four- output demultiplex operation for T bps pulses was demonstrated. A series of 4 optical pulses with 100 fs duration and 1 ps interval (corresponding to 1 T bps signals) were irradiated onto the SQ film synchronized with a 100 fs gate pulse at a finite angle. Four demultiplexed signals were clearly observed at different areas on the CCD camera. Multi- output serial-to-parallel demultiplexer for T bps optical signals can be formed using the SQ J-aggregates film.

Practical witness for electronic coherences.

PubMed

Johnson, Allan S; Yuen-Zhou, Joel; Aspuru-Guzik, Alán; Krich, Jacob J

2014-12-28

The origin of the coherences in two-dimensional spectroscopy of photosynthetic complexes remains disputed. Recently, it has been shown that in the ultrashort-pulse limit, oscillations in a frequency-integrated pump-probe signal correspond exclusively to electronic coherences, and thus such experiments can be used to form a test for electronic vs. vibrational oscillations in such systems. Here, we demonstrate a method for practically implementing such a test, whereby pump-probe signals are taken at several different pulse durations and used to extrapolate to the ultrashort-pulse limit. We present analytic and numerical results determining requirements for pulse durations and the optimal choice of pulse central frequency, which can be determined from an absorption spectrum. Our results suggest that for numerous systems, the required experiment could be implemented by many ultrafast spectroscopy laboratories using pulses of tens of femtoseconds in duration. Such experiments could resolve the standing debate over the nature of coherences in photosynthetic complexes.

Cooperative Subunit Refolding of a Light-Harvesting Protein through a Self-Chaperone Mechanism.

PubMed

Laos, Alistair J; Dean, Jacob C; Toa, Zi S D; Wilk, Krystyna E; Scholes, Gregory D; Curmi, Paul M G; Thordarson, Pall

2017-07-10

The fold of a protein is encoded by its amino acid sequence, but how complex multimeric proteins fold and assemble into functional quaternary structures remains unclear. Here we show that two structurally different phycobiliproteins refold and reassemble in a cooperative manner from their unfolded polypeptide subunits, without biological chaperones. Refolding was confirmed by ultrafast broadband transient absorption and two-dimensional electronic spectroscopy to probe internal chromophores as a marker of quaternary structure. Our results demonstrate a cooperative, self-chaperone refolding mechanism, whereby the β-subunits independently refold, thereby templating the folding of the α-subunits, which then chaperone the assembly of the native complex, quantitatively returning all coherences. Our results indicate that subunit self-chaperoning is a robust mechanism for heteromeric protein folding and assembly that could also be applied in self-assembled synthetic hierarchical systems. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

X-Ray Evidence for the Accretion Disc-Outflow Connection in 3C 111

NASA Technical Reports Server (NTRS)

Tombesi, Frank; Sambruna, R. M.; Reeves, J. N.; Reynolds, C. S.; Braito, V.

2011-01-01

We present the spectral analysis of three Suzaku X-ray Imaging Spectrometer observations of 3C III requested to monitor the predicted variability of its ultrafast outflow on approximately 7 d time-scales. We detect an ionized iron emission line in the first observation and a blueshifted absorption line in the second, when the flux is approximately 30 per cent higher. The location of the material is constrained at less than 0.006 pc from the variability. Detailed modelling supports an identification with ionized reflection off the accretion disc at approximately 20-100rg from the black hole and a highly ionized and massive ultrafast outflow with velocity approximately 0.1c, respectively. The outflow is most probably accelerated by radiation pressure, but additional magnetic thrust cannot be excluded. The measured high outflow rate and mechanical energy support the claims that disc outflows may have a significant feedback role. This work provides the first direct evidence for an accretion disc-outflow connection in a radio-loud active galactic nucleus, possibly linked also to the jet activity.

Asymmetric-detection time-stretch optical microscopy (ATOM) for ultrafast high-contrast cellular imaging in flow

PubMed Central

Wong, Terence T. W.; Lau, Andy K. S.; Ho, Kenneth K. Y.; Tang, Matthew Y. H.; Robles, Joseph D. F.; Wei, Xiaoming; Chan, Antony C. S.; Tang, Anson H. L.; Lam, Edmund Y.; Wong, Kenneth K. Y.; Chan, Godfrey C. F.; Shum, Ho Cheung; Tsia, Kevin K.

2014-01-01

Accelerating imaging speed in optical microscopy is often realized at the expense of image contrast, image resolution, and detection sensitivity – a common predicament for advancing high-speed and high-throughput cellular imaging. We here demonstrate a new imaging approach, called asymmetric-detection time-stretch optical microscopy (ATOM), which can deliver ultrafast label-free high-contrast flow imaging with well delineated cellular morphological resolution and in-line optical image amplification to overcome the compromised imaging sensitivity at high speed. We show that ATOM can separately reveal the enhanced phase-gradient and absorption contrast in microfluidic live-cell imaging at a flow speed as high as ~10 m/s, corresponding to an imaging throughput of ~100,000 cells/sec. ATOM could thus be the enabling platform to meet the pressing need for intercalating optical microscopy in cellular assay, e.g. imaging flow cytometry – permitting high-throughput access to the morphological information of the individual cells simultaneously with a multitude of parameters obtained in the standard assay. PMID:24413677

THz acoustic phonon spectroscopy and nanoscopy by using piezoelectric semiconductor heterostructures.

PubMed

Mante, Pierre-Adrien; Huang, Yu-Ru; Yang, Szu-Chi; Liu, Tzu-Ming; Maznev, Alexei A; Sheu, Jinn-Kong; Sun, Chi-Kuang

2015-02-01

Thanks to ultrafast acoustics, a better understanding of acoustic dynamics on a short time scale has been obtained and new characterization methods at the nanoscale have been developed. Among the materials that were studied during the development of ultrafast acoustics, nitride based heterostructures play a particular role due to their piezoelectric properties and the possibility to generate phonons with over-THz frequency and bandwidth. Here, we review some of the work performed using this type of structure, with a focus on THz phonon spectroscopy and nanoscopy. First, we present a brief description of the theory of coherent acoustic phonon generation by piezoelectric heterostructure. Then the first experimental observation of coherent acoustic phonon generated by the absorption of ultrashort light pulses in piezoelectric heterostructures is presented. From this starting point, we then present some methods developed to realize customizable phonon generation. Finally we review some more recent applications of these structures, including imaging with a nanometer resolution, broadband attenuation measurements with a frequency up to 1THz and phononic bandgap characterization. Copyright © 2014 Elsevier B.V. All rights reserved.

Ultrafast X-ray Imaging of Fuel Sprays

NASA Astrophysics Data System (ADS)

Wang, Jin

2007-01-01

Detailed analysis of fuel sprays has been well recognized as an important step for optimizing the operation of internal combustion engines to improve efficiency and reduce emissions. Ultrafast radiographic and tomographic techniques have been developed for probing the fuel distribution close to the nozzles of direct-injection diesel and gasoline injectors. The measurement was made using x-ray absorption of monochromatic synchrotron-generated radiation, allowing quantitative determination of the fuel distribution in this optically impenetrable region with a time resolution on the order of 1 μs. Furthermore, an accurate 3-dimensional fuel-density distribution, in the form of fuel volume fraction, was obtained by the time-resolved computed tomography. These quantitative measurements constitute the most detailed near-nozzle study of a fuel spray to date. With high-energy and high-brilliance x-ray beams available at the Advanced Photon Source, propagation-based phase-enhanced imaging was developed as a unique metrology technique to visualize the interior of an injection nozzle through a 3-mm-thick steel with a 10-μs temporal resolution, which is virtually impossible by any other means.

Effects of Ultra-Fast Cooling After Hot Rolling and Intercritical Treatment on Microstructure and Cryogenic Toughness of 3.5%Ni Steel

NASA Astrophysics Data System (ADS)

Wang, Meng; Liu, Zhenyu

2017-07-01

A novel process comprised of ultra-fast cooling after control rolling, intercritical quenching and tempering (UFC-LT) was applied to 3.5%Ni steel. In addition, quenching and tempering (QT) treatment was conducted in comparison. The present study focuses on the relationship between the microstructure and cryogenic toughness of 3.5%Ni steel. Results show that the microstructure of steel treated by UFC-LT consisted of tempered martensite, intercritical ferrite and two types of reversed austenite (RA) (needle shape and blocky). Compared to the QT sample, the UFC-LT sample's ultimate tensile strength decreased slightly, while its elongation increased from 32.3 to 35.7%, and its Charpy absorption energy at -135 °C increased from 112 to 237 J. The ductile-brittle transition temperature of UFC-LT sample was lower than that of the QT sample by 18 °C. The superior cryogenic toughness after UFC-LT compared to QT treatment can be attributed to the dissolution of cementite, approximately 3.0% increase in RA and the decrease in effective grain size.

Fugitive methane leak detection using mid-infrared hollow-core photonic crystal fiber containing ultrafast laser drilled side-holes

NASA Astrophysics Data System (ADS)

Karp, Jason; Challener, William; Kasten, Matthias; Choudhury, Niloy; Palit, Sabarni; Pickrell, Gary; Homa, Daniel; Floyd, Adam; Cheng, Yujie; Yu, Fei; Knight, Jonathan

2016-05-01

The increase in domestic natural gas production has brought attention to the environmental impacts of persistent gas leakages. The desire to identify fugitive gas emission, specifically for methane, presents new sensing challenges within the production and distribution supply chain. A spectroscopic gas sensing solution would ideally combine a long optical path length for high sensitivity and distributed detection over large areas. Specialty micro-structured fiber with a hollow core can exhibit a relatively low attenuation at mid-infrared wavelengths where methane has strong absorption lines. Methane diffusion into the hollow core is enabled by machining side-holes along the fiber length through ultrafast laser drilling methods. The complete system provides hundreds of meters of optical path for routing along well pads and pipelines while being interrogated by a single laser and detector. This work will present transmission and methane detection capabilities of mid-infrared photonic crystal fibers. Side-hole drilling techniques for methane diffusion will be highlighted as a means to convert hollow-core fibers into applicable gas sensors.

Measurement of high-dynamic range x-ray Thomson scattering spectra for the characterization of nano-plasmas at LCLS

DOE PAGES

MacDonald, M. J.; Gorkhover, T.; Bachmann, B.; ...

2016-08-08

Atomic clusters can serve as ideal model systems for exploring ultrafast (~100 fs) laser-driven ionization dynamics of dense matter on the nanometer scale. Resonant absorption of optical laser pulses enables heating to temperatures on the order of 1 keV at near solid density conditions. To date, direct probing of transient states of such nano plasmas was limited to coherent x-ray imaging. Here we present the first measurement of spectrally-resolved incoherent x-ray scattering from clusters, enabling measurements of transient temperature, densities and ionization. Single shot x-ray Thomson scatterings signals were recorded at 120 Hz using a crystal spectrometer in combination withmore » a single-photon counting and energy-dispersive pnCCD. A precise pump laser collimation scheme enabled recording near background-free scattering spectra from Ar clusters with an unprecedented dynamic range of more than 3 orders of magnitude. As a result, such measurements are important for understanding collective effects in laser-matter interactions on femtosecond timescales, opening new routes for the development of schemes for their ultrafast control.« less

Ultrafast carrier dynamics and optical pumping of lasing from Ar-plasma treated ZnO nanoribbons

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sarkar, Ketaki; Mukherjee, Souvik; Wiederrecht, Gary

We report that it is a well-known fact that ZnO has been one of the most studied wide bandgap II-VI materials by the scientific community specifically due to its potential for being used as exciton-related optical devices. Hence, realizing ways to increase the efficiency of these devices is important. We discuss a plasma treatment technique to enhance the near-band-edge (NBE) excitonic emission from ZnO based nanoribbons. We observed an enhancement of the NBE peak and simultaneous quenching of the visible emission peak resulting from the removal of surface traps on these ZnO nanoribbons. More importantly, we report here the associatedmore » ultrafast carrier dynamics resulting from this surface treatment. Femtosecond transient absorption spectroscopy was performed using pump-probe differential transmission measurements shedding new light on these improved dynamics with faster relaxation times. The knowledge obtained is important for improving the application of ZnO based optoelectronic devices. Finally, we also observed how these improved carrier dynamics have a direct effect on the threshold and efficiency of random lasing from the material.« less

Ultrafast hydrogen bond dynamics and partial electron transfer after photoexcitation of diethyl ester of 7-(diethylamino)-coumarin-3-phosphonic acid and its benzoxaphosphorin analog.

PubMed

Wagner, M S; Ilieva, E D; Petkov, P St; Nikolova, R D; Kienberger, R; Iglev, H

2015-04-21

The solvation dynamics after optical excitation of two phosphono-substituted coumarin derivatives dissolved in various solutions are studied by fluorescence up-conversion spectroscopy and quantum chemical simulations. The Kamlet-Taft analysis of the conventional absorption and emission spectra suggests weakening of the solvent-solute H-bonds upon optical excitation, which is in contrast to the results gained by the quantum simulations and earlier studies reported for coumarin derivatives without phosphono groups. The simulations give evidence that the solvent reorganisation around the excited fluorophore leads to partial electron transfer to the first solvation shell. The process occurs on a timescale between 1 and 10 ps depending on the solvent polarity and leads to a fast decay of the time-resolved emission signal. Using the ultrafast spectral shift of the time-dependent fluorescence we estimated the relaxation time of the H-bonds in the electronically excited state to be about 0.6 ps in water, 1.5 ps in ethanol and 2.8 ps in formamide.

Ultrafast carrier dynamics and optical pumping of lasing from Ar-plasma treated ZnO nanoribbons

DOE PAGES

Sarkar, Ketaki; Mukherjee, Souvik; Wiederrecht, Gary; ...

2018-01-04

We report that it is a well-known fact that ZnO has been one of the most studied wide bandgap II-VI materials by the scientific community specifically due to its potential for being used as exciton-related optical devices. Hence, realizing ways to increase the efficiency of these devices is important. We discuss a plasma treatment technique to enhance the near-band-edge (NBE) excitonic emission from ZnO based nanoribbons. We observed an enhancement of the NBE peak and simultaneous quenching of the visible emission peak resulting from the removal of surface traps on these ZnO nanoribbons. More importantly, we report here the associatedmore » ultrafast carrier dynamics resulting from this surface treatment. Femtosecond transient absorption spectroscopy was performed using pump-probe differential transmission measurements shedding new light on these improved dynamics with faster relaxation times. The knowledge obtained is important for improving the application of ZnO based optoelectronic devices. Finally, we also observed how these improved carrier dynamics have a direct effect on the threshold and efficiency of random lasing from the material.« less

Measurement of high-dynamic range x-ray Thomson scattering spectra for the characterization of nano-plasmas at LCLS

DOE Office of Scientific and Technical Information (OSTI.GOV)

MacDonald, M. J., E-mail: macdonm@umich.edu; SLAC National Accelerator Laboratory, Menlo Park, California 94025; Gorkhover, T.

2016-11-15

Atomic clusters can serve as ideal model systems for exploring ultrafast (∼100 fs) laser-driven ionization dynamics of dense matter on the nanometer scale. Resonant absorption of optical laser pulses enables heating to temperatures on the order of 1 keV at near solid density conditions. To date, direct probing of transient states of such nano-plasmas was limited to coherent x-ray imaging. Here we present the first measurement of spectrally resolved incoherent x-ray scattering from clusters, enabling measurements of transient temperature, densities, and ionization. Single shot x-ray Thomson scattering signals were recorded at 120 Hz using a crystal spectrometer in combination withmore » a single-photon counting and energy-dispersive pnCCD. A precise pump laser collimation scheme enabled recording near background-free scattering spectra from Ar clusters with an unprecedented dynamic range of more than 3 orders of magnitude. Such measurements are important for understanding collective effects in laser-matter interactions on femtosecond time scales, opening new routes for the development of schemes for their ultrafast control.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wen, Haidan; Sassi, Michel; Luo, Zhenlin

The interaction of light with materials is an intensively studied research forefront, in which the coupling of radiation energy to selective degrees of freedom offers contact-free tuning of functionalities on ultrafast time scales. Capturing the fundamental processes and understanding the mechanism of photoinduced structural rearrangement are essential to applications such as photo-active actuators and efficient photovoltaic devices. Using ultrafast x-ray absorption spectroscopy aided by density functional theory calculations, we reveal the local structural arrangement around the transition metal atom in a unit cell of the photoferroelectric archetype BiFeO 3 film. The out-of-plane elongation of the unit cell is accompanied bymore » the in-plane shrinkage with minimal change of interaxial lattice angles upon photoexcitation. This anisotropic elastic deformation of the unit cell is driven by localized electric field as a result of photoinduced charge separation, in contrast to a global lattice constant increase and lattice angle variations as a result of heating. The finding of a photoinduced elastic unit cell deformation elucidates a microscopic picture of photocarrier-mediated non-equilibrium processes in polar materials.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wen, Haidan; Sassi, Michel JPC; Luo, Zhenlin

The interaction of light with materials is an intensively studied research forefront, in which the coupling of radiation energy to selective degrees of freedom offers contact-free tuning of functionalities on ultrafast time scales. Capturing the fundamental processes and understanding the mechanism of photoinduced structural rearrangement are essential to applications such as photo-active actuators and efficient photovoltaic devices. Using ultrafast x-ray absorption spectroscopy aided by density functional theory calculations, we reveal the local structural arrangement around the transition metal atom in a unit cell of the photoferroelectric archetype BiFeO 3 film. The out-of-plane elongation of the unit cell is accompanied bymore » the in-plane shrinkage with minimal change of interaxial lattice angles upon photoexcitation. This uniaxial elastic deformation of the unit cell is driven by localized electric field as a result of photoinduced charge separation, in contrast to a global lattice constant increase and lattice angle variations as a result of heating. The finding of a photoinduced elastic unit cell deformation elucidates a microscopic picture of photocarrier-mediated nonequilibrium processes in polar materials.« less

Vibrational energy flow in photoactive yellow protein revealed by infrared pump-visible probe spectroscopy.

PubMed

Nakamura, Ryosuke; Hamada, Norio

2015-05-14

Vibrational energy flow in the electronic ground state of photoactive yellow protein (PYP) is studied by ultrafast infrared (IR) pump-visible probe spectroscopy. Vibrational modes of the chromophore and the surrounding protein are excited with a femtosecond IR pump pulse, and the subsequent vibrational dynamics in the chromophore are selectively probed with a visible probe pulse through changes in the absorption spectrum of the chromophore. We thus obtain the vibrational energy flow with four characteristic time constants. The vibrational excitation with an IR pulse at 1340, 1420, 1500, or 1670 cm(-1) results in ultrafast intramolecular vibrational redistribution (IVR) with a time constant of 0.2 ps. The vibrational modes excited through the IVR process relax to the initial ground state with a time constant of 6-8 ps in parallel with vibrational cooling with a time constant of 14 ps. In addition, upon excitation with an IR pulse at 1670 cm(-1), we observe the energy flow from the protein backbone to the chromophore that occurs with a time constant of 4.2 ps.

Concerted electron-proton transfer in the optical excitation of hydrogen-bonded dyes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Westlake, Brittany C.; Brennaman, Kyle M.; Concepcion, Javier J.

2011-05-24

The simultaneous, concerted transfer of electrons and protons—electron-proton transfer (EPT)—is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following photoexcitation and thermal relaxation. Here we report application of ultrafast excitation with absorption and Raman monitoring to detect a photochemically driven EPT process (photo-EPT). In this process, both electrons and protons are transferred during the absorption of a photon. Photo-EPT is induced by intramolecular charge-transfer (ICT) excitation of hydrogen-bonded-base adducts with either a coumarin dye or 4-nitro-4'-biphenylphenol. Femtosecond transient absorption spectralmore » measurements following ICT excitation reveal the appearance of two spectroscopically distinct states having different dynamical signatures. One of these states corresponds to a conventional ICT excited state in which the transferring H⁺ is initially associated with the proton donor. Proton transfer to the base (B) then occurs on the picosecond time scale. The other state is an ICT-EPT photoproduct. Upon excitation it forms initially in the nuclear configuration of the ground state by application of the Franck–Condon principle. However, due to the change in electronic configuration induced by the transition, excitation is accompanied by proton transfer with the protonated base formed with a highly elongated ⁺H–B bond. Coherent Raman spectroscopy confirms the presence of a vibrational mode corresponding to the protonated base in the optically prepared state.« less

Concerted electron-proton transfer in the optical excitation of hydrogen-bonded dyes.

PubMed

Westlake, Brittany C; Brennaman, M Kyle; Concepcion, Javier J; Paul, Jared J; Bettis, Stephanie E; Hampton, Shaun D; Miller, Stephen A; Lebedeva, Natalia V; Forbes, Malcolm D E; Moran, Andrew M; Meyer, Thomas J; Papanikolas, John M

2011-05-24

The simultaneous, concerted transfer of electrons and protons--electron-proton transfer (EPT)--is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following photoexcitation and thermal relaxation. Here we report application of ultrafast excitation with absorption and Raman monitoring to detect a photochemically driven EPT process (photo-EPT). In this process, both electrons and protons are transferred during the absorption of a photon. Photo-EPT is induced by intramolecular charge-transfer (ICT) excitation of hydrogen-bonded-base adducts with either a coumarin dye or 4-nitro-4'-biphenylphenol. Femtosecond transient absorption spectral measurements following ICT excitation reveal the appearance of two spectroscopically distinct states having different dynamical signatures. One of these states corresponds to a conventional ICT excited state in which the transferring H(+) is initially associated with the proton donor. Proton transfer to the base (B) then occurs on the picosecond time scale. The other state is an ICT-EPT photoproduct. Upon excitation it forms initially in the nuclear configuration of the ground state by application of the Franck-Condon principle. However, due to the change in electronic configuration induced by the transition, excitation is accompanied by proton transfer with the protonated base formed with a highly elongated (+)H ─ B bond. Coherent Raman spectroscopy confirms the presence of a vibrational mode corresponding to the protonated base in the optically prepared state.

Portable semiconductor disk laser for in vivo tissue monitoring: a platform for the development of clinical applications

NASA Astrophysics Data System (ADS)

Aviles-Espinosa, Rodrigo; Filippidis, George; Hamilton, Craig; Malcolm, Graeme; Weingarten, Kurt J.; Südmeyer, Thomas; Barbarin, Yohan; Keller, Ursula; Artigas, David; Loza-Alvarez, Pablo

2011-07-01

Long term in vivo observations at large penetration depths and minimum sample disturbance are some of the key factors that have enabled the study of different cellular and tissue mechanisms. The continuous optimization of these aspects is the main driving force for the development of advanced microscopy techniques such as those based on nonlinear effects. Its wide implementation for general biomedical applications is however, limited as the currently used nonlinear microscopes are based on bulky, maintenance-intensive and expensive excitation sources such as Ti:sapphire ultrafast lasers. We present the suitability of a portable (140x240x70 mm) ultrafast semiconductor disk laser (SDL) source, to be used in nonlinear microscopy. The SDL is modelocked by a quantum-dot semiconductor saturable absorber mirror (SESAM). This enables the source to deliver an average output power of 287 mW with 1.5 ps pulses at 500 MHz, corresponding to a peak power of 0.4 kW. The laser center wavelength (965 nm) virtually matches the two-photon absorption cross-section of the widely used Green Fluorescent Protein (GFP). This property greatly relaxes the required peak powers, thus maximizing sample viability. This is demonstrated by presenting two-photon excited fluorescence images of GFP labeled neurons and second-harmonic generation images of pharyngeal muscles in living C. elegans nematodes. Our results also demonstrate that this compact laser is well suited for efficiently exciting different biological dyes. Importantly this non expensive, turn-key, compact laser system could be used as a platform to develop portable nonlinear bio-imaging devices, facilitating its widespread adoption in biomedical applications.

Modifying ultrafast optical response of sputtered VOX nanostructures in a broad spectral range by altering post annealing atmosphere

NASA Astrophysics Data System (ADS)

Kürüm, U.; Yaglioglu, H. G.; Küçüköz, B.; Oksuzoglu, R. M.; Yıldırım, M.; Yağcı, A. M.; Yavru, C.; Özgün, S.; Tıraş, T.; Elmali, A.

2015-01-01

Nanostructured VOX thin films were grown in a dc magnetron sputter system under two different Ar:O2 gas flow ratios. The films were annealed under vacuum and various ratios of O2/N2 atmospheres. The insulator-to-metal transition properties of the thin films were investigated by temperature dependent resistance measurement. Photo induced insulator-to-metal transition properties were investigated by Z-scan and ultrafast white light continuum pump probe spectroscopy measurements. Experiments showed that not only insulator-to-metal transition, but also wavelength dependence (from NIR to VIS) and time scale (from ns to ultrafast) of nonlinear optical response of the VOX thin films could be fine tuned by carefully adjusting post annealing atmosphere despite different initial oxygen content in the production. Fabricated VO2 thin films showed reflection change in the visible region due to photo induced phase transition. The results have general implications for easy and more effective fabrication of the nanostructured oxide systems with controllable electrical, optical, and ultrafast optical responses.

Filter-Based Dispersion-Managed Versatile Ultrafast Fibre Laser

PubMed Central

Peng, Junsong; Boscolo, Sonia

2016-01-01

We present the operation of an ultrafast passively mode-locked fibre laser, in which flexible control of the pulse formation mechanism is readily realised by an in-cavity programmable filter the dispersion and bandwidth of which can be software configured. We show that conventional soliton, dispersion-managed (DM) soliton (stretched-pulse) and dissipative soliton mode-locking regimes can be reliably targeted by changing the filter’s dispersion and bandwidth only, while no changes are made to the physical layout of the laser cavity. Numerical simulations are presented which confirm the different nonlinear pulse evolutions inside the laser cavity. The proposed technique holds great potential for achieving a high degree of control over the dynamics and output of ultrafast fibre lasers, in contrast to the traditional method to control the pulse formation mechanism in a DM fibre laser, which involves manual optimisation of the relative length of fibres with opposite-sign dispersion in the cavity. Our versatile ultrafast fibre laser will be attractive for applications requiring different pulse profiles such as in optical signal processing and optical communications. PMID:27183882

Tuning ultrafast electron injection dynamics at organic-graphene/metal interfaces.

PubMed

Ravikumar, Abhilash; Kladnik, Gregor; Müller, Moritz; Cossaro, Albano; Bavdek, Gregor; Patera, Laerte L; Sánchez-Portal, Daniel; Venkataraman, Latha; Morgante, Alberto; Brivio, Gian Paolo; Cvetko, Dean; Fratesi, Guido

2018-05-03

We compare the ultrafast charge transfer dynamics of molecules on epitaxial graphene and bilayer graphene grown on Ni(111) interfaces through first principles calculations and X-ray resonant photoemission spectroscopy. We use 4,4'-bipyridine as a prototypical molecule for these explorations as the energy level alignment of core-excited molecular orbitals allows ultrafast injection of electrons from a substrate to a molecule on a femtosecond timescale. We show that the ultrafast injection of electrons from the substrate to the molecule is ∼4 times slower on weakly coupled bilayer graphene than on epitaxial graphene. Through our experiments and calculations, we can attribute this to a difference in the density of states close to the Fermi level between graphene and bilayer graphene. We therefore show how graphene coupling with the substrate influences charge transfer dynamics between organic molecules and graphene interfaces.

Energy cascades, excited state dynamics, and photochemistry in cob(III)alamins and ferric porphyrins.

PubMed

Rury, Aaron S; Wiley, Theodore E; Sension, Roseanne J

2015-03-17

Porphyrins and the related chlorins and corrins contain a cyclic tetrapyrrole with the ability to coordinate an active metal center and to perform a variety of functions exploiting the oxidation state, reactivity, and axial ligation of the metal center. These compounds are used in optically activated applications ranging from light harvesting and energy conversion to medical therapeutics and photodynamic therapy to molecular electronics, spintronics, optoelectronic thin films, and optomagnetics. Cobalt containing corrin rings extend the range of applications through photolytic cleavage of a unique axial carbon-cobalt bond, permitting spatiotemporal control of drug delivery. The photochemistry and photophysics of cyclic tetrapyrroles are controlled by electronic relaxation dynamics including internal conversion and intersystem crossing. Typically the electronic excitation cascades through ring centered ππ* states, ligand to metal charge transfer (LMCT) states, metal to ligand charge transfer (MLCT) states, and metal centered states. Ultrafast transient absorption spectroscopy provides a powerful tool for the investigation of the electronic state dynamics in metal containing tetrapyrroles. The UV-visible spectrum is sensitive to the oxidation state, electronic configuration, spin state, and axial ligation of the central metal atom. Ultrashort broadband white light probes spanning the range from 270 to 800 nm, combined with tunable excitation pulses, permit the detailed unravelling of the time scales involved in the electronic energy cascade. State-of-the-art theoretical calculations provide additional insight required for precise assignment of the states. In this Account, we focus on recent ultrafast transient absorption studies of ferric porphyrins and corrin containing cob(III)alamins elucidating the electronic states responsible for ultrafast energy cascades, excited state dynamics, and the resulting photoreactivity or photostability of these compounds. Iron tetraphenyl porphyrin chloride (Fe((III))TPPCl) exhibits picosecond decay to a metal centered d → d* (4)T state. This state decays on a ca. 16 ps time scale in room temperature solution but persists for much longer in a cryogenic glass. The photoreactivity of the (4)T state may lead to novel future applications for these compounds. In contrast, the nonplanar cob(III)alamins contain two axial ligands to the central cobalt atom. The upper axial ligand can be an alkyl group as in the two biologically active coenzymes or a nonalkyl ligand such as -CN in cyanocobalamin (vitamin B12) or -OH in hydroxocobalamin. The electronic structure, energy cascade, and bond cleavage of these compounds is sensitive to the details of the axial ligand. Nonalkylcobalamins exhibit ultrafast internal conversion to a low-lying state of metal to ligand or ligand to metal charge transfer character. The compounds are generally photostable with ground state recovery complete on a time scale of 2-7 ps in room temperature aqueous solution. Alkylcobalamins exhibit ultrafast internal conversion to an S1 state of d/π → π* character. Most compounds undergo bond cleavage from this state with near unit quantum yield within ∼100 ps. Recent theoretical calculations provide a potential energy surface accounting for these observations. Conformation dependent mixing of the corrin π and cobalt d orbitals plays a significant role in the observed photochemistry and photophysics.

Linear and ultrafast nonlinear plasmonics of single nano-objects

NASA Astrophysics Data System (ADS)

Crut, Aurélien; Maioli, Paolo; Vallée, Fabrice; Del Fatti, Natalia

2017-03-01

Single-particle optical investigations have greatly improved our understanding of the fundamental properties of nano-objects, avoiding the spurious inhomogeneous effects that affect ensemble experiments. Correlation with high-resolution imaging techniques providing morphological information (e.g. electron microscopy) allows a quantitative interpretation of the optical measurements by means of analytical models and numerical simulations. In this topical review, we first briefly recall the principles underlying some of the most commonly used single-particle optical techniques: near-field, dark-field, spatial modulation and photothermal microscopies/spectroscopies. We then focus on the quantitative investigation of the surface plasmon resonance (SPR) of metallic nano-objects using linear and ultrafast optical techniques. While measured SPR positions and spectral areas are found in good agreement with predictions based on Maxwell’s equations, SPR widths are strongly influenced by quantum confinement (or, from a classical standpoint, surface-induced electron scattering) and, for small nano-objects, cannot be reproduced using the dielectric functions of bulk materials. Linear measurements on single nano-objects (silver nanospheres and gold nanorods) allow a quantification of the size and geometry dependences of these effects in confined metals. Addressing the ultrafast response of an individual nano-object is also a powerful tool to elucidate the physical mechanisms at the origin of their optical nonlinearities, and their electronic, vibrational and thermal relaxation processes. Experimental investigations of the dynamical response of gold nanorods are shown to be quantitatively modeled in terms of modifications of the metal dielectric function enhanced by plasmonic effects. Ultrafast spectroscopy can also be exploited to unveil hidden physical properties of more complex nanosystems. In this context, two-color femtosecond pump-probe experiments performed on individual bimetallic heterodimers are discussed in the last part of the review, demonstrating the existence of Fano interferences in the optical absorption of a gold nanoparticle under the influence of a nearby silver one.

Imaging ultrafast excited state pathways in transition metal complexes by X-ray transient absorption and scattering using X-ray free electron laser source

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chen, Lin X.; Shelby, Megan L.; Lestrange, Patrick J.

2016-01-01

This report will describe our recent studies of transition metal complex structural dynamics on the fs and ps time scales using an X-ray free electron laser source, Linac Coherent Light Source (LCLS). Ultrafast XANES spectra at the Ni K-edge of nickel(II) tetramesitylporphyrin (NiTMP) were successfully measured for optically excited state at a timescale from 100 fs to 50 ps, providing insight into its sub-ps electronic and structural relaxation processes. Importantly, a transient reduced state Ni(I) (π, 3dx2-y2) electronic state is captured through the interpretation of a short-lived excited state absorption on the low-energy shoulder of the edge, which is aidedmore » by the computation of X-ray transitions for postulated excited electronic states. The observed and computed inner shell to valence orbital transition energies demonstrate and quantify the influence of electronic configuration on specific metal orbital energies. A strong influence of the valence orbital occupation on the inner shell orbital energies indicates that one should not use the transition energy from 1s to other orbitals to draw conclusions about the d-orbital energies. For photocatalysis, a transient electronic configuration could influence d-orbital energies up to a few eV and any attempt to steer the reaction pathway should account for this to ensure that external energies can be used optimally in driving desirable processes. NiTMP structural evolution and the influence of the porphyrin macrocycle conformation on relaxation kinetics can be likewise inferred from this study.« less

Ultrafast carrier thermalization and trapping in silicon-germanium alloy probed by extreme ultraviolet transient absorption spectroscopy

PubMed Central

Zürch, Michael; Chang, Hung-Tzu; Kraus, Peter M.; Cushing, Scott K.; Borja, Lauren J.; Gandman, Andrey; Kaplan, Christopher J.; Oh, Myoung Hwan; Prell, James S.; Prendergast, David; Pemmaraju, Chaitanya D.; Neumark, Daniel M.; Leone, Stephen R.

2017-01-01

Semiconductor alloys containing silicon and germanium are of growing importance for compact and highly efficient photonic devices due to their favorable properties for direct integration into silicon platforms and wide tunability of optical parameters. Here, we report the simultaneous direct and energy-resolved probing of ultrafast electron and hole dynamics in a silicon-germanium alloy with the stoichiometry Si0.25Ge0.75 by extreme ultraviolet transient absorption spectroscopy. Probing the photoinduced dynamics of charge carriers at the germanium M4,5-edge (∼30 eV) allows the germanium atoms to be used as reporter atoms for carrier dynamics in the alloy. The photoexcitation of electrons across the direct and indirect band gap into conduction band (CB) valleys and their subsequent hot carrier relaxation are observed and compared to pure germanium, where the Ge direct (ΔEgap,Ge,direct=0.8 eV) and Si0.25Ge0.75 indirect gaps (ΔEgap,Si0.25Ge0.75,indirect=0.95 eV) are comparable in energy. In the alloy, comparable carrier lifetimes are observed for the X, L, and Γ valleys in the conduction band. A midgap feature associated with electrons accumulating in trap states near the CB edge following intraband thermalization is observed in the Si0.25Ge0.75 alloy. The successful implementation of the reporter atom concept for capturing the dynamics of the electronic bands by site-specific probing in solids opens a route to study carrier dynamics in more complex materials with femtosecond and sub-femtosecond temporal resolution. PMID:28653020

Ultrafast forward and backward electron transfer dynamics of coumarin 337 in hydrogen-bonded anilines as studied with femtosecond UV-pump/IR-probe spectroscopy.

PubMed

Ghosh, Hirendra N; Verma, Sandeep; Nibbering, Erik T J

2011-02-10

Femtosecond infrared spectroscopy is used to study both forward and backward electron transfer (ET) dynamics between coumarin 337 (C337) and the aromatic amine solvents aniline (AN), N-methylaniline (MAN), and N,N-dimethylaniline (DMAN), where all the aniline solvents can donate an electron but only AN and MAN can form hydrogen bonds with C337. The formation of a hydrogen bond with AN and MAN is confirmed with steady state FT-IR spectroscopy, where the C═O stretching vibration is a direct marker mode for hydrogen bond formation. Transient IR absorption measurements in all solvents show an absorption band at 2166 cm(-1), which has been attributed to the C≡N stretching vibration of the C337 radical anion formed after ET. Forward electron transfer dynamics is found to be biexponential with time constants τ(ET)(1) = 500 fs, τ(ET)(2) = 7 ps in all solvents. Despite the presence of hydrogen bonds of C337 with the solvents AN and MAN, no effect has been found on the forward electron transfer step. Because of the absence of an H/D isotope effect on the forward electron transfer reaction of C337 in AN, hydrogen bonds are understood to play a minor role in mediating electron transfer. In contrast, direct π-orbital overlap between C337 and the aromatic amine solvents causes ultrafast forward electron transfer dynamics. Backward electron transfer dynamics, in contrast, is dependent on the solvent used. Standard Marcus theory explains the observed backward electron transfer rates.

Ultrafast strong broadband light source generated in nanoscale plasmonic Au-AAO-Al structures

NASA Astrophysics Data System (ADS)

Han, Junbo; Yao, Linhua; Ma, Zongwei

we demonstrate an ultrafast strong broadband photoluminescence (PL) from Au-AAO-Al composite under low excitation power intensity of 3.8 34.5 GW /cm2. The emission wavelength is in the range of 450-1050 nm and the lifetime is under sub-nanosecond. Comparative studies of PL in Au-AAO-Al with different Au rod length and Au-AAO without Al coupling layer, together with the finite difference time domain (FDTD) calculations, present that the fast PL originates from the surface plasmon enhanced supercontinuum generation (SCG) in AAO membrane. The observations indicate that strong SCG could be realized in nanoscale plasmonic structures, which have promise applications in the minimization and integration of ultrafast lighting sources in photonic devices. National Natural Scientific Foundation of China (11404124).

Ultrafast all-optical control of the magnetization in magnetic dielectrics

NASA Astrophysics Data System (ADS)

Kirilyuk, Andrei; Kimel, Alexey; Hansteen, Fredrik; Rasing, Theo; Pisarev, Roman V.

2006-08-01

The purpose of this review is to summarize the recent progress on laser-induced magnetization dynamics in magnetic dielectrics. Due to the slow phonon-magnon interaction in these materials, direct thermal effects of the laser excitation can only be seen on the time scale of almost a nanosecond and thus are clearly distinguished from the ultrafast nonthermal effects. However, laser pulses are shown to indirectly modify the magnetic anisotropy in rare-earth orthoferrites via the crystal field, and to bring about spin reorientation within a few picoseconds. More interesting, however, are the direct nonthermal effects of light on spin systems. We demonstrate coherent optical control of the magnetization in ferrimagnetic garnet films on a femtosecond time scale through a combination of two different ultrafast and nonthermal photomagnetic effects and by employing multiple pump pulses. Linearly polarized laser pulses are shown to create a long-lived modification of the magnetocrystalline anisotropy via optically induced electron transfer between nonequivalent ion sites. In addition, circularly polarized pulses are shown to act as strong transient magnetic field pulses originating from the nonabsorptive inverse Faraday effect. An all-optical scheme of excitation and detection of different antiferromagnetic resonance modes with frequencies of up to 500GHz will be discussed as well. The reported effects open new and exciting possibilities for ultrafast manipulation of spins by light and provide new insight into the physics of magnetism on ultrafast time scales.

Ultrafast Three-Dimensional X-ray Imaging of Deformation Modes in ZnO Nanocrystals.

PubMed

Cherukara, Mathew J; Sasikumar, Kiran; Cha, Wonsuk; Narayanan, Badri; Leake, Steven J; Dufresne, Eric M; Peterka, Tom; McNulty, Ian; Wen, Haidan; Sankaranarayanan, Subramanian K R S; Harder, Ross J

2017-02-08

Imaging the dynamical response of materials following ultrafast excitation can reveal energy transduction mechanisms and their dissipation pathways, as well as material stability under conditions far from equilibrium. Such dynamical behavior is challenging to characterize, especially operando at nanoscopic spatiotemporal scales. In this letter, we use X-ray coherent diffractive imaging to show that ultrafast laser excitation of a ZnO nanocrystal induces a rich set of deformation dynamics including characteristic "hard" or inhomogeneous and "soft" or homogeneous modes at different time scales, corresponding respectively to the propagation of acoustic phonons and resonant oscillation of the crystal. By integrating the 3D nanocrystal structure obtained from the ultrafast X-ray measurements with a continuum thermo-electro-mechanical finite element model, we elucidate the deformation mechanisms following laser excitation, in particular, a torsional mode that generates a 50% greater electric potential gradient than that resulting from the flexural mode. Understanding of the time-dependence of these mechanisms on ultrafast scales has significant implications for development of new materials for nanoscale power generation.

Ultrafast Three-Dimensional X-ray Imaging of Deformation Modes in ZnO Nanocrystals

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cherukara, Mathew J.; Sasikumar, Kiran; Cha, Wonsuk

Imaging the dynamical response of materials following ultrafast excitation can reveal energy transduction mechanisms and their dissipation pathways, as well as material stability under conditions far from equilibrium. Such dynamical behaviour is challenging to characterize, especially operando at nanoscopic spatiotemporal scales. In this letter, we use x-ray coherent diffractive imaging to show that ultrafast laser excitation of a ZnO nanocrystal induces a rich set of deformation dynamics including characteristic ‘hard’ or inhomogeneous and ‘soft’ or homogeneous modes at different time scales, corresponding respectively to the propagation of acoustic phonons and resonant oscillation of the crystal. By integrating the 3D nanocrystalmore » structure obtained from the ultrafast x-ray measurements with a continuum thermo-electro-mechanical finite element model, we elucidate the deformation mechanisms following laser excitation, in particular, a torsional mode that generates a 50% greater electric potential gradient than that resulting from the flexural mode. Furthermore, understanding of the time-dependence of these mechanisms on ultrafast scales has significant implications for development of new materials for nanoscale power generation.« less

Ultrafast Three-Dimensional X-ray Imaging of Deformation Modes in ZnO Nanocrystals

DOE PAGES

Cherukara, Mathew J.; Sasikumar, Kiran; Cha, Wonsuk; ...

2016-12-27

Imaging the dynamical response of materials following ultrafast excitation can reveal energy transduction mechanisms and their dissipation pathways, as well as material stability under conditions far from equilibrium. Such dynamical behaviour is challenging to characterize, especially operando at nanoscopic spatiotemporal scales. In this letter, we use x-ray coherent diffractive imaging to show that ultrafast laser excitation of a ZnO nanocrystal induces a rich set of deformation dynamics including characteristic ‘hard’ or inhomogeneous and ‘soft’ or homogeneous modes at different time scales, corresponding respectively to the propagation of acoustic phonons and resonant oscillation of the crystal. By integrating the 3D nanocrystalmore » structure obtained from the ultrafast x-ray measurements with a continuum thermo-electro-mechanical finite element model, we elucidate the deformation mechanisms following laser excitation, in particular, a torsional mode that generates a 50% greater electric potential gradient than that resulting from the flexural mode. Furthermore, understanding of the time-dependence of these mechanisms on ultrafast scales has significant implications for development of new materials for nanoscale power generation.« less

Probing ultrafast ππ*/nπ* internal conversion in organic chromophores via K-edge resonant absorption

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wolf, T. J. A.; Myhre, R. H.; Cryan, J. P.

Many photoinduced processes including photosynthesis and human vision happen in organic molecules and involve coupled femtosecond dynamics of nuclei and electrons. Organic molecules with heteroatoms often possess an important excited-state relaxation channel from an optically allowed ππ* to a dark nπ* state. The ππ*/nπ* internal conversion is difficult to investigate, as most spectroscopic methods are not exclusively sensitive to changes in the excited-state electronic structure. Here, we report achieving the required sensitivity by exploiting the element and site specificity of near-edge soft X-ray absorption spectroscopy. As a hole forms in the n orbital during ππ*/nπ* internal conversion, the absorption spectrummore » at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept using the nucleobase thymine at the oxygen K-edge, and unambiguously show that ππ*/nπ* internal conversion takes place within (60 ± 30) fs. Furthermore, high-level-coupled cluster calculations confirm the method’s impressive electronic structure sensitivity for excited-state investigations.« less

Universal ultrafast signatures of photoexcitations in conjugated polymers: excitons and charge-transfer polarons

NASA Astrophysics Data System (ADS)

McBranch, Duncan W.; Kraabel, Brett; Xu, Su; Wang, Hsing-Lin; Klimov, Victor I.

1999-12-01

Using subpicosecond transient absorption spectroscopy, we have investigated the primary photoexcitations in thin films and solution of several phenylene-based conjugated polymers and an oligomer. We identify two features in the transient absorption spectra and dynamics that are common to all of the materials which we have studied from this family. The first spectral feature is a photoinduced absorption (PA) band peaking near 1 eV which has intensity-dependent dynamics which match the stimulated emission dynamics exactly over two orders of magnitude in excitation density. This band is associated with singlet intrachain excitons. The second spectral feature (observed only in thin films and aggregated solutions) is a PA band peaking near 1.8 eV, which is longer-lived than the 1 eV exciton PA band, and which has dynamics that are independent (or weakly-dependent) on excitation density. This feature is attributed to charge separated (interchain) excitations. These excitations are generated through a bimolecular process. By comparing to samples in which charged excitations are created deliberately by doping with C6O, we assign these secondary species as bound polarons.

Wavelength-versatile graphene-gold film saturable absorber mirror for ultra-broadband mode-locking of bulk lasers.

PubMed

Ma, Jie; Xie, Guoqiang; Lv, Peng; Gao, Wenlan; Yuan, Peng; Qian, Liejia; Griebner, Uwe; Petrov, Valentin; Yu, Haohai; Zhang, Huaijin; Wang, Jiyang

2014-05-23

An ultra-broadband graphene-gold film saturable absorber mirror (GG-SAM) with a spectral coverage exceeding 1300 nm is experimentally demonstrated for mode-locking of bulk solid-state lasers. Owing to the p-type doping effect caused by graphene-gold film interaction, the graphene on gold-film substrate shows a remarkably lower light absorption relative to pristine graphene, which is very helpful to achieve continuous-wave mode-locking in low-gain bulk lasers. Using the GG-SAM sample, stable mode-locking is realized in a Yb:YCOB bulk laser near 1 μm, a Tm:CLNGG bulk laser near 2 μm and a Cr:ZnSe bulk laser near 2.4 μm. The saturable absorption is characterised at an intermediate wavelength of 1.56 μm by pump-probe measurements. The as-fabricated GG-SAM with ultra-broad bandwidth, ultrafast recovery time, low absorption, and low cost has great potential as a universal saturable absorber mirror for mode-locking of various bulk lasers with unprecedented spectral coverage.

Probing ultrafast ππ*/nπ* internal conversion in organic chromophores via K-edge resonant absorption

DOE PAGES

Wolf, T. J. A.; Myhre, R. H.; Cryan, J. P.; ...

2017-06-22

Many photoinduced processes including photosynthesis and human vision happen in organic molecules and involve coupled femtosecond dynamics of nuclei and electrons. Organic molecules with heteroatoms often possess an important excited-state relaxation channel from an optically allowed ππ* to a dark nπ* state. The ππ*/nπ* internal conversion is difficult to investigate, as most spectroscopic methods are not exclusively sensitive to changes in the excited-state electronic structure. Here, we report achieving the required sensitivity by exploiting the element and site specificity of near-edge soft X-ray absorption spectroscopy. As a hole forms in the n orbital during ππ*/nπ* internal conversion, the absorption spectrummore » at the heteroatom K-edge exhibits an additional resonance. We demonstrate the concept using the nucleobase thymine at the oxygen K-edge, and unambiguously show that ππ*/nπ* internal conversion takes place within (60 ± 30) fs. Furthermore, high-level-coupled cluster calculations confirm the method’s impressive electronic structure sensitivity for excited-state investigations.« less

Super-hydrophilicity of hydroxy modified poly(m-phenylenediamine) aerogel for separation of oil/water and biocompatibility

NASA Astrophysics Data System (ADS)

Wang, Gang; Liu, Zhiduo; Zhang, Nan; Li, Jiurong; Xu, Anli; Xiang, Pengcheng; Hu, Xurui; Guo, Qinglei; Chen, Da

2018-04-01

We demonstrate the ultra-light weight and super-hydrophilic hydroxyl modified poly (m-phenylenediamine) (Hy-PmPD) aerogel by utilizing simple oxygen plasma treatment. The average pore size and specific surface area are obtained as 5.21 nm and 671 m2 g‑1, respectively. Due to the large amount of oxygen-containing groups (e.g., C–OH and N–OH), the contact angle of Hy-PmPD for water is about 7.2°, which indicates the super-hydrophilic ability of Hy-PmPD. The large surface area and super-hydrophilic nature of ultra- light weight Hy-PmPD aerogel conclusively certify that high absorption capacities and ultrafast absorption rate for water. As a result, the Hy-PmPD aerogel enables to separate crude oil and water. Additionally, the Hy-PmPD aerogel indicates good biocompatibility that can be implanted as the bio-platform for monitoring the cell culture behavior. This work may provide a facile and effective strategy for the applications in the absorption or removal of organics, particularly in environmental protection, pollution control, as well as noninvasive to the microflora.

Wavelength-Versatile Graphene-Gold Film Saturable Absorber Mirror for Ultra-Broadband Mode-Locking of Bulk Lasers

PubMed Central

Ma, Jie; Xie, Guoqiang; Lv, Peng; Gao, Wenlan; Yuan, Peng; Qian, Liejia; Griebner, Uwe; Petrov, Valentin; Yu, Haohai; Zhang, Huaijin; Wang, Jiyang

2014-01-01

An ultra-broadband graphene-gold film saturable absorber mirror (GG-SAM) with a spectral coverage exceeding 1300 nm is experimentally demonstrated for mode-locking of bulk solid-state lasers. Owing to the p-type doping effect caused by graphene-gold film interaction, the graphene on gold-film substrate shows a remarkably lower light absorption relative to pristine graphene, which is very helpful to achieve continuous-wave mode-locking in low-gain bulk lasers. Using the GG-SAM sample, stable mode-locking is realized in a Yb:YCOB bulk laser near 1 μm, a Tm:CLNGG bulk laser near 2 μm and a Cr:ZnSe bulk laser near 2.4 μm. The saturable absorption is characterised at an intermediate wavelength of 1.56 μm by pump-probe measurements. The as-fabricated GG-SAM with ultra-broad bandwidth, ultrafast recovery time, low absorption, and low cost has great potential as a universal saturable absorber mirror for mode-locking of various bulk lasers with unprecedented spectral coverage. PMID:24853072

Terahertz emission from ultrafast spin and charge currents at a Rashba interface

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhang, Qi; Jungfleisch, Matthias B.; Zhang, Wei

2017-01-01

Abstract: We demonstrate the efficient single-cycle THz emission from a Rashba interface, i.e., Ag/Bi, in a spintronic heterostructure. Different from the previously reported inversed spin Hall effect mechanism in bulk systems, the observed ultrafast spin-to-charge conversion in a 2D Rashba interface is due to the inversed Rashba-Edelstein effect.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Grayson, Katie J.; Faries, Kaitlyn M.; Huang, Xia

Photosynthesis uses a limited range of the solar spectrum, so enhancing spectral coverage could improve the efficiency of light capture. Here, we show that a hybrid reaction centre (RC)/yellow fluorescent protein (YFP) complex accelerates photosynthetic growth in the bacterium Rhodobacter sphaeroides. The structure of the RC/YFP-light-harvesting 1 (LH1) complex shows the position of YFP attachment to the RC-H subunit, on the cytoplasmic side of the RC complex. Fluorescence lifetime microscopy of whole cells and ultrafast transient absorption spectroscopy of purified RC/YFP complexes show that the YFP–RC intermolecular distance and spectral overlap between the emission of YFP and the visible-region (Qmore » X) absorption bands of the RC allow energy transfer via a Fo¨rster mechanism, with an efficiency of 40±10%. Finally, this proof-of-principle study demonstrates the feasibility of increasing spectral coverage for harvesting light using non-native genetically-encoded light-absorbers, thereby augmenting energy transfer and trapping in photosynthesis.« less

Selective Two-Photon Absorptive Resonance Femtosecond-Laser Electronic-Excitation Tagging (STARFLEET) Velocimetry in Flow and Combustion Diagnostics

NASA Technical Reports Server (NTRS)

Jiang, Naibo; Halls, Benjamin R.; Stauffer, Hans U.; Roy, Sukesh; Danehy, Paul M.; Gord, James R.

2016-01-01

Selective Two-Photon Absorptive Resonance Femtosecond-Laser Electronic-Excitation Tagging (STARFLEET), a non-seeded ultrafast-laser-based velocimetry technique, is demonstrated in reactive and non-reactive flows. STARFLEET is pumped via a two-photon resonance in N2 using 202.25-nm 100-fs light. STARFLEET greatly reduces the per-pulse energy required (30 µJ/pulse) to generate the signature FLEET emission compared to the conventional FLEET technique (1.1 mJ/pulse). This reduction in laser energy results in less energy deposited in the flow, which allows for reduced flow perturbations (reactive and non-reactive), increased thermometric accuracy, and less severe damage to materials. Velocity measurements conducted in a free jet of N2 and in a premixed flame show good agreement with theoretical velocities and further demonstrate the significantly less-intrusive nature of STARFLEET.

Yb- and Er-doped fiber laser Q-switched with an optically uniform, broadband WS2 saturable absorber

PubMed Central

Zhang, M.; Hu, Guohua; Hu, Guoqing; Howe, R. C. T.; Chen, L.; Zheng, Z.; Hasan, T.

2015-01-01

We demonstrate a ytterbium (Yb) and an erbium (Er)-doped fiber laser Q-switched by a solution processed, optically uniform, few-layer tungsten disulfide saturable absorber (WS2-SA). Nonlinear optical absorption of the WS2-SA in the sub-bandgap region, attributed to the edge-induced states, is characterized by 3.1% and 4.9% modulation depths with 1.38 and 3.83 MW/cm2 saturation intensities at 1030 and 1558 nm, respectively. By integrating the optically uniform WS2-SA in the Yb- and Er-doped laser cavities, we obtain self-starting Q-switched pulses with microsecond duration and kilohertz repetition rates at 1030 and 1558 nm. Our work demonstrates broadband sub-bandgap saturable absorption of a single, solution processed WS2-SA, providing new potential efficacy for WS2 in ultrafast photonic applications. PMID:26657601

High-speed and high-efficiency travelling wave single-photon detectors embedded in nanophotonic circuits

PubMed Central

Pernice, W.H.P.; Schuck, C.; Minaeva, O.; Li, M.; Goltsman, G.N.; Sergienko, A.V.; Tang, H.X.

2012-01-01

Ultrafast, high-efficiency single-photon detectors are among the most sought-after elements in modern quantum optics and quantum communication. However, imperfect modal matching and finite photon absorption rates have usually limited their maximum attainable detection efficiency. Here we demonstrate superconducting nanowire detectors atop nanophotonic waveguides, which enable a drastic increase of the absorption length for incoming photons. This allows us to achieve high on-chip single-photon detection efficiency up to 91% at telecom wavelengths, repeatable across several fabricated chips. We also observe remarkably low dark count rates without significant compromise of the on-chip detection efficiency. The detectors are fully embedded in scalable silicon photonic circuits and provide ultrashort timing jitter of 18 ps. Exploiting this high temporal resolution, we demonstrate ballistic photon transport in silicon ring resonators. Our direct implementation of a high-performance single-photon detector on chip overcomes a major barrier in integrated quantum photonics. PMID:23271658

Ultrafast third-order nonlinear optical response of pyrene derivatives

NASA Astrophysics Data System (ADS)

Shi, Yufang; Li, Zhongguo; Fang, Yu; Sun, Jinyu; Zhao, Minggen; Song, Yinglin

2017-05-01

Two mono-substituted pyrene derivatives with delocalized electron system 1-(pyren-1-yl)-3-(4-Methyl thiophene-2-yl) acrylic ketone (13#) and 1-(pyren-1-yl)-3-(4-bromo thiophene-2-yl) acrylic ketone (15#) were successfully synthesized. The resultant compounds were characterized by nuclear magnetic resonance (NMR), infrared spectroscopy (IR), high resolution mass spectrum (HR-MS), and UV-vis spectra. The third-order nonlinear optical properties of the compounds were investigated using Z-scan technique with femtosecond laser pulses at 500 nm and 700 nm, respectively. Both of the compounds showed a decrease in transmittance about the focus, which are typical of two-photon absorption. It was found that the two-photon absorption behavior of the pyrene derivatives were modified by substituents on thiophene ring. These results indicate that both compounds can be promising candidates for future optoelectronic and bio-imaging applications.

Simplification of femtosecond transient absorption microscopy data from CH3NH3PbI3 perovskite thin films into decay associated amplitude maps

NASA Astrophysics Data System (ADS)

Doughty, Benjamin; Simpson, Mary Jane; Yang, Bin; Xiao, Kai; Ma, Ying-Zhong

2016-03-01

This work aims to simplify multi-dimensional femtosecond transient absorption microscopy (TAM) data into decay associated amplitude maps (DAAMs) that describe the spatial distributions of dynamical processes occurring on various characteristic timescales. Application of this method to TAM data obtained from a model methyl-ammonium lead iodide (CH3NH3PbI3) perovskite thin film allows us to simplify the data set comprising 68 time-resolved images into four DAAMs. These maps offer a simple means to visualize the complex electronic excited-state dynamics in this system by separating distinct dynamical processes evolving on characteristic timescales into individual spatial images. This approach provides new insight into subtle aspects of ultrafast relaxation dynamics associated with excitons and charge carriers in the perovskite thin film, which have recently been found to coexist at spatially distinct locations.

0.5-keV Soft X-ray attosecond continua

PubMed Central

Teichmann, S. M.; Silva, F.; Cousin, S. L.; Hemmer, M.; Biegert, J.

2016-01-01

Attosecond light pulses in the extreme ultraviolet have drawn a great deal of attention due to their ability to interrogate electronic dynamics in real time. Nevertheless, to follow charge dynamics and excitations in materials, element selectivity is a prerequisite, which demands such pulses in the soft X-ray region, above 200 eV, to simultaneously cover several fundamental absorption edges of the constituents of the materials. Here, we experimentally demonstrate the exploitation of a transient phase matching regime to generate carrier envelope controlled soft X-ray supercontinua with pulse energies up to 2.9±0.1 pJ and a flux of (7.3±0.1) × 107 photons per second across the entire water window and attosecond pulses with 13 as transform limit. Our results herald attosecond science at the fundamental absorption edges of matter by bridging the gap between ultrafast temporal resolution and element specific probing. PMID:27167525

Synthesis, Spectra, and Theoretical Investigations of 1,3,5-Triazines Compounds as Ultraviolet Rays Absorber Based on Time-Dependent Density Functional Calculations and three-Dimensional Quantitative Structure-Property Relationship.

PubMed

Wang, Xueding; Xu, Yilian; Yang, Lu; Lu, Xiang; Zou, Hao; Yang, Weiqing; Zhang, Yuanyuan; Li, Zicheng; Ma, Menglin

2018-03-01

A series of 1,3,5-triazines were synthesized and their UV absorption properties were tested. The computational chemistry methods were used to construct quantitative structure-property relationship (QSPR), which was used to computer aided design of new 1,3,5-triazines ultraviolet rays absorber compounds. The experimental UV absorption data are in good agreement with those predicted data using the Time-dependent density functional theory (TD-DFT) [B3LYP/6-311 + G(d,p)]. A suitable forecasting model (R > 0.8, P < 0.0001) was revealed. Predictive three-dimensional quantitative structure-property relationship (3D-QSPR) model was established using multifit molecular alignment rule of Sybyl program, which conclusion is consistent with the TD-DFT calculation. The exceptional photostability mechanism of such ultraviolet rays absorber compounds was studied and confirmed as principally banked upon their ability to undergo excited-state deactivation via an ultrafast excited-state proton transfer (ESIPT). The intramolecular hydrogen bond (IMHB) of 1,3,5-triazines compounds is the basis for the excited state proton transfer, which was explored by IR spectroscopy, UV spectra, structural and energetic aspects of different conformers and frontier molecular orbitals analysis.

Direct Correlation of Excitonics with Efficiency in a Core-Shell Quantum Dot Solar Cell.

PubMed

Dana, Jayanta; Maiti, Sourav; Tripathi, Vaidehi S; Ghosh, Hirendra N

2018-02-16

Shell thickness dependent band-gap engineering of quasi type II core-shell material with higher carrier cooling time, lower interfacial defect states, and longer charge carrier recombination time can be a promising candidate for both photocatalysis and solar cell. In the present investigation, colloidal CdSe@CdS core-shells with different shell thickness (2, 4 and 6 monolayer CdS) were synthesized through hot injection method and have been characterized by high resolution transmission electron microscope (HRTEM) followed by steady state absorption and luminescence techniques. Ultrafast transient absorption (TA) studies suggest longer carrier cooling, lower interfacial surface states, and slower carrier recombination time in CdSe@CdS core-shell with increasing shell thickness. By TA spectroscopy, the role of CdS shell in power conversion efficiency (PCE) has been explained in detail. The measured PCE was found to initially increase and then decrease with increasing shell thickness. Shell thickness has been optimized to maximize the efficiency after correlating the shell controlled carrier cooling and recombination with PCE values and a maximum PCE of 3.88 % was obtained with 4 monolayers of CdS shell, which is found to be 57 % higher than compared to bare CdSe QDs. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Ultra-fast all-optical plasmon induced transparency in a metal–insulator–metal waveguide containing two Kerr nonlinear ring resonators

NASA Astrophysics Data System (ADS)

Nurmohammadi, Tofiq; Abbasian, Karim; Yadipour, Reza

2018-05-01

In this work, an ultra-fast all-optical plasmon induced transparency based on a metal–insulator–metal nanoplasmonic waveguide with two Kerr nonlinear ring resonators is studied. Two-dimensional simulations utilizing the finite-difference time-domain method are used to show an obvious optical bistability and significant switching mechanisms of the signal light by varying the pump-light intensity. The proposed all-optical switching based on plasmon induced transparency demonstrates femtosecond-scale feedback time (90 fs), meaning ultra-fast switching can be achieved. The presented all-optical switch may have potential significant applications in integrated optical circuits.

Quantum Entanglement Molecular Absorption Spectrum Simulator

NASA Technical Reports Server (NTRS)

Nguyen, Quang-Viet; Kojima, Jun

2006-01-01

Quantum Entanglement Molecular Absorption Spectrum Simulator (QE-MASS) is a computer program for simulating two photon molecular-absorption spectroscopy using quantum-entangled photons. More specifically, QE-MASS simulates the molecular absorption of two quantum-entangled photons generated by the spontaneous parametric down-conversion (SPDC) of a fixed-frequency photon from a laser. The two-photon absorption process is modeled via a combination of rovibrational and electronic single-photon transitions, using a wave-function formalism. A two-photon absorption cross section as a function of the entanglement delay time between the two photons is computed, then subjected to a fast Fourier transform to produce an energy spectrum. The program then detects peaks in the Fourier spectrum and displays the energy levels of very short-lived intermediate quantum states (or virtual states) of the molecule. Such virtual states were only previously accessible using ultra-fast (femtosecond) laser systems. However, with the use of a single-frequency continuous wave laser to produce SPDC photons, and QEMASS program, these short-lived molecular states can now be studied using much simpler laser systems. QE-MASS can also show the dependence of the Fourier spectrum on the tuning range of the entanglement time of any externally introduced optical-path delay time. QE-MASS can be extended to any molecule for which an appropriate spectroscopic database is available. It is a means of performing an a priori parametric analysis of entangled photon spectroscopy for development and implementation of emerging quantum-spectroscopic sensing techniques. QE-MASS is currently implemented using the Mathcad software package.

Modeling ultrafast solvated electronic dynamics using time-dependent density functional theory and polarizable continuum model.

PubMed

Liang, Wenkel; Chapman, Craig T; Ding, Feizhi; Li, Xiaosong

2012-03-01

A first-principles solvated electronic dynamics method is introduced. Solvent electronic degrees of freedom are coupled to the time-dependent electronic density of a solute molecule by means of the implicit reaction field method, and the entire electronic system is propagated in time. This real-time time-dependent approach, incorporating the polarizable continuum solvation model, is shown to be very effective in describing the dynamical solvation effect in the charge transfer process and yields a consistent absorption spectrum in comparison to the conventional linear response results in solution. © 2012 American Chemical Society

Applications of terahertz spectroscopy and imaging

NASA Astrophysics Data System (ADS)

Zhang, Cunlin; Mu, Kaijun

2009-07-01

We have examined application feasibility of THz time-domain spectroscopy (THz-TDS) to inspect 30 kinds of illicit drugs, 20 kinds of amino acid and 10 kinds of explosives and related compounds (ERCs). We also have got their fingerprints, established the corresponding database, and propose the reference-free methods to extract the absorption or reflection spectra, respectively. We also use optical pump THz probe to research the ultrafast dynamics of semiconductor. While, we also present some new THz imaging techniques, such as, focal-plane multiwavelength phase imaging, reference-free phase imaging, polarization imaging, and continuous-wave (CW) standoff distance imaging.

Octave spanning supercontinuum in an As₂S₃ taper using ultralow pump pulse energy.

PubMed

Hudson, Darren D; Dekker, Stephen A; Mägi, Eric C; Judge, Alexander C; Jackson, Stuart D; Li, Enbang; Sanghera, J S; Shaw, L B; Aggarwal, I D; Eggleton, Benjamin J

2011-04-01

An octave spanning spectrum is generated in an As₂S₃ taper via 77 pJ pulses from an ultrafast fiber laser. Using a previously developed tapering method, we construct a 1.3 μm taper that has a zero-dispersion wavelength around 1.4 μm. The low two-photon absorption of sulfide-based chalcogenide fiber allows for higher input powers than previous efforts in selenium-based chalcogenide tapered fibers. This higher power handling capability combined with input pulse chirp compensation allows an octave spanning spectrum to be generated directly from the taper using the unamplified laser output.

Femtosecond optical switches by squarylium dye J-aggregates films

NASA Astrophysics Data System (ADS)

Pu, Lyong Sun

2003-01-01

Formation of Langmuir and spincoated films containing squarylium dye (SQ) J-aggregates and their femtosecond optical response are described. Pump-probe measurements show that saturable absorption of SQJ-films reveal ultrafast decay with time constants of 100-300 fs. We have applied SQJ-film to new femtosecond time-to-space conversion. A pump (gate) pulse and a train of four probe (signal) pulses were illuminated on the same area of the film in the direction of oblique and normal to the film-plane respectively. Demultiplexing operation for T bps optical signals was demonstrated with femtosecond nonlinear optical response of the SQJ-film.

Broadband transient absorption spectroscopy with 1- and 2-photon excitations: Relaxation paths and cross sections of a triphenylamine dye in solution

DOE Office of Scientific and Technical Information (OSTI.GOV)

Moreno, J.; Dobryakov, A. L.; Hecht, S., E-mail: sh@chemie.hu-berlin.de, E-mail: skovale@chemie.hu-berlin.de

2015-07-14

1-photon (382 nm) and 2-photon (752 nm) excitations to the S{sub 1} state are applied to record and compare transient absorption spectra of a push-pull triphenylamine (TrP) dye in solution. After 1-photon excitation, ultrafast vibrational and structural molecular relaxations are detected on a 0.1 ps time scale in nonpolar hexane, while in polar acetonitrile, the spectral evolution is dominated by dipolar solvation. Upon 2-photon excitation, transient spectra in hexane reveal an unexpected growth of stimulated emission (SE) and excited-state absorption (ESA) bands. The behavior is explained by strong population transfer S{sub 1} → S{sub n} due to resonant absorption ofmore » a third pump photon. Subsequent S{sub n} → S{sub 1} internal conversion (with τ{sub 1} = 1 ps) prepares a very hot S{sub 1} state which cools down with τ{sub 2} = 13 ps. The pump pulse energy dependence proves the 2-photon origin of the bleach signal. At the same time, SE and ESA are strongly affected by higher-order pump absorptions that should be taken into account in nonlinear fluorescence applications. The 2-photon excitation cross sections σ{sup (2)} = 32 ⋅ 10{sup −50} cm{sup 4} s at 752 nm are evaluated from the bleach signal.« less

Static and dynamic optical properties of La 1-xSr xFeO 3-δ: The effects of A-site and oxygen stoichiometry

DOE PAGES

Sergey Y. Smolin; Sfeir, Matthew Y.; Scafetta, Mark D.; ...

2015-12-09

Perovskite oxides are a promising material class for photovoltaic and photocatalytic applications due to their visible band gaps, nanosecond recombination lifetimes, and great chemical diversity. However, there is limited understanding of the link between composition and static and dynamic optical properties, despite the critical role these properties play in the design of light-harvesting devices. To clarify these relationships, we systemically studied the optoelectronic properties in La 1-xSr xFeO 3-δ epitaxial films, uncovering the effects of A-site cation substitution and oxygen stoichiometry. Variable-angle spectroscopic ellipsometry was used to measure static optical properties, revealing a linear increase in absorption coefficient at 1.25more » eV and a red-shifting of the optical absorption edge with increasing Sr fraction. The absorption spectra can be similarly tuned through the introduction of oxygen vacancies, indicating the critical role that nominal Fe valence plays in optical absorption. Dynamic optoelectronic properties were studied with ultrafast transient reflectance spectroscopy, revealing similar nanosecond photoexcited carrier lifetimes for oxygen deficient and stoichiometric films with the same nominal Fe valence. Furthermore, these results demonstrate that while the static optical absorption is strongly dependent on nominal Fe valence tuned through cation or anion stoichiometry, oxygen vacancies do not appear to play a significantly detrimental role in the recombination kinetics.« less

Development of novel two-photon absorbing chromophores

NASA Astrophysics Data System (ADS)

Rogers, Joy E.; Slagle, Jonathan E.; McLean, Daniel G.; Sutherland, Richard L.; Krein, Douglas M.; Cooper, Thomas M.; Brant, Mark; Heinrichs, James; Kannan, Ramamurthi; Tan, Loon-Seng; Urbas, Augustine M.; Fleitz, Paul A.

2006-08-01

There has been much interest in the development of two-photon absorbing materials and many efforts to understand the nonlinear absorption properties of these dyes but this area is still not well understood. A computational model has been developed in our lab to understand the nanosecond nonlinear absorption properties that incorporate all of the measured one-photon photophysical parameters of a class of materials called AFX. We have investigated the nonlinear and photophysical properties of the AFX chromophores including the two-photon absorption cross-section, the excited state cross-section, the intersystem crossing quantum yield, and the singlet and triplet excited state lifetimes using a variety of experimental techniques that include UV-visible, fluorescence and phosphorescence spectroscopy, time correlated single photon counting, ultrafast transient absorption, and nanosecond laser flash photolysis. The model accurately predicts the nanosecond nonlinear transmittance data using experimentally measured parameters. Much of the strong nonlinear absorption has been shown to be due to excited state absorption from both the singlet and triplet excited states. Based on this understanding of the nonlinear absorption and the importance of singlet and triplet excited states we have begun to develop new two-photon absorbing molecules within the AFX class as well as linked to other classes of nonlinear absorbing molecules. This opens up the possibilities of new materials with unique and interesting properties. Specifically we have been working on a new class of two-photon absorbing molecules linked to platinum poly-ynes. In the platinum poly-yne chromophores the photophysics are more complicated and we have just started to understand what drives both the linear and non-linear photophysical properties.

Ultrafast Phenomena XIV

NASA Astrophysics Data System (ADS)

Kobayashi, Takayoshi; Okada, Tadashi; Kobayashi, Tetsuro; Nelson, Keith A.; de Silvestri, Sandro

Ultrafast Phenomena XIV presents the latest advances in ultrafast science, including ultrafast laser and measurement technology as well as studies of ultrafast phenomena. Pico-, femto-, and atosecond processes relevant in physics, chemistry, biology, and engineering are presented. Ultrafast technology is now having a profound impact within a wide range of applications, among them imaging, material diagnostics, and transformation and high-speed optoelectronics . This book summarizes results presented at the 14th Ultrafast Phenomena Conference and reviews the state of the art in this important and rapidly advancing field.

Ultrafast Synthetic Transmit Aperture Imaging Using Hadamard-Encoded Virtual Sources With Overlapping Sub-Apertures.

PubMed

Ping Gong; Pengfei Song; Shigao Chen

2017-06-01

The development of ultrafast ultrasound imaging offers great opportunities to improve imaging technologies, such as shear wave elastography and ultrafast Doppler imaging. In ultrafast imaging, there are tradeoffs among image signal-to-noise ratio (SNR), resolution, and post-compounded frame rate. Various approaches have been proposed to solve this tradeoff, such as multiplane wave imaging or the attempts of implementing synthetic transmit aperture imaging. In this paper, we propose an ultrafast synthetic transmit aperture (USTA) imaging technique using Hadamard-encoded virtual sources with overlapping sub-apertures to enhance both image SNR and resolution without sacrificing frame rate. This method includes three steps: 1) create virtual sources using sub-apertures; 2) encode virtual sources using Hadamard matrix; and 3) add short time intervals (a few microseconds) between transmissions of different virtual sources to allow overlapping sub-apertures. The USTA was tested experimentally with a point target, a B-mode phantom, and in vivo human kidney micro-vessel imaging. Compared with standard coherent diverging wave compounding with the same frame rate, improvements on image SNR, lateral resolution (+33%, with B-mode phantom imaging), and contrast ratio (+3.8 dB, with in vivo human kidney micro-vessel imaging) have been achieved. The f-number of virtual sources, the number of virtual sources used, and the number of elements used in each sub-aperture can be flexibly adjusted to enhance resolution and SNR. This allows very flexible optimization of USTA for different applications.

Time-resolved single-shot terahertz time-domain spectroscopy for ultrafast irreversible processes

NASA Astrophysics Data System (ADS)

Zhai, Zhao-Hui; Zhong, Sen-Cheng; Li, Jun; Zhu, Li-Guo; Meng, Kun; Li, Jiang; Liu, Qiao; Peng, Qi-Xian; Li, Ze-Ren; Zhao, Jian-Heng

2016-09-01

Pulsed terahertz spectroscopy is suitable for spectroscopic diagnostics of ultrafast events. However, the study of irreversible or single shot ultrafast events requires ability to record transient properties at multiple time delays, i.e., time resolved at single shot level, which is not available currently. Here by angular multiplexing use of femtosecond laser pulses, we developed and demonstrated a time resolved, transient terahertz time domain spectroscopy technique, where burst mode THz pulses were generated and then detected in a single shot measurement manner. The burst mode THz pulses contain 2 sub-THz pulses, and the time gap between them is adjustable up to 1 ns with picosecond accuracy, thus it can be used to probe the single shot event at two different time delays. The system can detect the sub-THz pulses at 0.1 THz-2.5 THz range with signal to noise ratio (SNR) of ˜400 and spectrum resolution of 0.05 THz. System design was described here, and optimizations of single shot measurement of THz pulses were discussed in detail. Methods to improve SNR were also discussed in detail. A system application was demonstrated where pulsed THz signals at different time delays of the ultrafast process were successfully acquired within single shot measurement. This time resolved transient terahertz time domain spectroscopy technique provides a new diagnostic tool for irreversible or single shot ultrafast events where dynamic information can be extracted at terahertz range within one-shot experiment.

Time-resolved single-shot terahertz time-domain spectroscopy for ultrafast irreversible processes.

PubMed

Zhai, Zhao-Hui; Zhong, Sen-Cheng; Li, Jun; Zhu, Li-Guo; Meng, Kun; Li, Jiang; Liu, Qiao; Peng, Qi-Xian; Li, Ze-Ren; Zhao, Jian-Heng

2016-09-01

Pulsed terahertz spectroscopy is suitable for spectroscopic diagnostics of ultrafast events. However, the study of irreversible or single shot ultrafast events requires ability to record transient properties at multiple time delays, i.e., time resolved at single shot level, which is not available currently. Here by angular multiplexing use of femtosecond laser pulses, we developed and demonstrated a time resolved, transient terahertz time domain spectroscopy technique, where burst mode THz pulses were generated and then detected in a single shot measurement manner. The burst mode THz pulses contain 2 sub-THz pulses, and the time gap between them is adjustable up to 1 ns with picosecond accuracy, thus it can be used to probe the single shot event at two different time delays. The system can detect the sub-THz pulses at 0.1 THz-2.5 THz range with signal to noise ratio (SNR) of ∼400 and spectrum resolution of 0.05 THz. System design was described here, and optimizations of single shot measurement of THz pulses were discussed in detail. Methods to improve SNR were also discussed in detail. A system application was demonstrated where pulsed THz signals at different time delays of the ultrafast process were successfully acquired within single shot measurement. This time resolved transient terahertz time domain spectroscopy technique provides a new diagnostic tool for irreversible or single shot ultrafast events where dynamic information can be extracted at terahertz range within one-shot experiment.

Polarization-controlled optimal scatter suppression in transient absorption spectroscopy

PubMed Central

Malý, Pavel; Ravensbergen, Janneke; Kennis, John T. M.; van Grondelle, Rienk; Croce, Roberta; Mančal, Tomáš; van Oort, Bart

2017-01-01

Ultrafast transient absorption spectroscopy is a powerful technique to study fast photo-induced processes, such as electron, proton and energy transfer, isomerization and molecular dynamics, in a diverse range of samples, including solid state materials and proteins. Many such experiments suffer from signal distortion by scattered excitation light, in particular close to the excitation (pump) frequency. Scattered light can be effectively suppressed by a polarizer oriented perpendicular to the excitation polarization and positioned behind the sample in the optical path of the probe beam. However, this introduces anisotropic polarization contributions into the recorded signal. We present an approach based on setting specific polarizations of the pump and probe pulses, combined with a polarizer behind the sample. Together, this controls the signal-to-scatter ratio (SSR), while maintaining isotropic signal. We present SSR for the full range of polarizations and analytically derive the optimal configuration at angles of 40.5° between probe and pump and of 66.9° between polarizer and pump polarizations. This improves SSR by (or compared to polarizer parallel to probe). The calculations are validated by transient absorption experiments on the common fluorescent dye Rhodamine B. This approach provides a simple method to considerably improve the SSR in transient absorption spectroscopy. PMID:28262765

Matched-filtering line search methods applied to Suzaku data

NASA Astrophysics Data System (ADS)

Miyazaki, Naoto; Yamada, Shin'ya; Enoto, Teruaki; Axelsson, Magnus; Ohashi, Takaya

2016-12-01

A detailed search for emission and absorption lines and an assessment of their upper limits are performed for Suzaku data. The method utilizes a matched-filtering approach to maximize the signal-to-noise ratio for a given energy resolution, which could be applicable to many types of line search. We first applied it to well-known active galactic nuclei spectra that have been reported to have ultra-fast outflows, and find that our results are consistent with previous findings at the ˜3σ level. We proceeded to search for emission and absorption features in two bright magnetars 4U 0142+61 and 1RXS J1708-4009, applying the filtering method to Suzaku data. We found that neither source showed any significant indication of line features, even using long-term Suzaku observations or dividing their spectra into spin phases. The upper limits on the equivalent width of emission/absorption lines are constrained to be a few eV at ˜1 keV and a few hundreds of eV at ˜10 keV. This strengthens previous reports that persistently bright magnetars do not show proton cyclotron absorption features in soft X-rays and, even if they exist, they would be broadened or much weaker than below the detection limit of X-ray CCD.

The Effect of Varying Ultrafast Pulse Laser Energies on the Electrical Properties of Reduced Graphene Oxide Sheets in Solution

NASA Astrophysics Data System (ADS)

Ibrahim, Khaled H.; Irannejad, Mehrdad; Wales, Benjamin; Sanderson, Joseph; Musselman, Kevin P.; Yavuz, Mustafa

2018-02-01

Laser treatment of graphene oxide solution among other techniques is a well-established technique for producing reduced graphene sheets. However, production of high-quality ultra-low sheet resistance reduced graphene oxide (rGO) sheets in solution has been a challenge due to their high degree of randomness, defect-rich medium, and lack of controlability. Recent studies lack an in-depth analytic comparison of laser treatment parameters that yield the highest quality rGO sheets with a low defect ratio. Hence, in this study, we implement a comprehensive comparison of laser treatment parameters and their effect on the yielded rGO sheets from an electronic and physical standpoint. Ultra-low sheet resistance graphene oxide sheets were fabricated using ultrafast laser irradiation with different laser pulse energies in the range of 0.25-2 mJ. Laser treatment for 10 min using a pulse energy of 1 mJ resulted in an increase in the defect spacing, accompanied by a large red shift in the optical absorption of the C=C bond, indicating significant restoration of the s p 2 carbon bonds. These enhancements resulted in a significant reduction in the electrical resistance of the rGO flakes (up to 2 orders of magnitude), raising the electron mobility of the films produced using the irradiated graphene oxide a step closer to that of pristine graphene films. From this study, we can also deduce which exposure regimes result in the fabrication of quantum dots and continuous defect-free films.

Multiplane wave imaging increases signal-to-noise ratio in ultrafast ultrasound imaging.

PubMed

Tiran, Elodie; Deffieux, Thomas; Correia, Mafalda; Maresca, David; Osmanski, Bruno-Felix; Sieu, Lim-Anna; Bergel, Antoine; Cohen, Ivan; Pernot, Mathieu; Tanter, Mickael

2015-11-07

Ultrafast imaging using plane or diverging waves has recently enabled new ultrasound imaging modes with improved sensitivity and very high frame rates. Some of these new imaging modalities include shear wave elastography, ultrafast Doppler, ultrafast contrast-enhanced imaging and functional ultrasound imaging. Even though ultrafast imaging already encounters clinical success, increasing even more its penetration depth and signal-to-noise ratio for dedicated applications would be valuable. Ultrafast imaging relies on the coherent compounding of backscattered echoes resulting from successive tilted plane waves emissions; this produces high-resolution ultrasound images with a trade-off between final frame rate, contrast and resolution. In this work, we introduce multiplane wave imaging, a new method that strongly improves ultrafast images signal-to-noise ratio by virtually increasing the emission signal amplitude without compromising the frame rate. This method relies on the successive transmissions of multiple plane waves with differently coded amplitudes and emission angles in a single transmit event. Data from each single plane wave of increased amplitude can then be obtained, by recombining the received data of successive events with the proper coefficients. The benefits of multiplane wave for B-mode, shear wave elastography and ultrafast Doppler imaging are experimentally demonstrated. Multiplane wave with 4 plane waves emissions yields a 5.8  ±  0.5 dB increase in signal-to-noise ratio and approximately 10 mm in penetration in a calibrated ultrasound phantom (0.7 d MHz(-1) cm(-1)). In shear wave elastography, the same multiplane wave configuration yields a 2.07  ±  0.05 fold reduction of the particle velocity standard deviation and a two-fold reduction of the shear wave velocity maps standard deviation. In functional ultrasound imaging, the mapping of cerebral blood volume results in a 3 to 6 dB increase of the contrast-to-noise ratio in deep structures of the rodent brain.

Photoelectron diffraction from single oriented molecules: Towards ultrafast structure determination of molecules using x-ray free-electron lasers

NASA Astrophysics Data System (ADS)

Kazama, Misato; Fujikawa, Takashi; Kishimoto, Naoki; Mizuno, Tomoya; Adachi, Jun-ichi; Yagishita, Akira

2013-06-01

We provide a molecular structure determination method, based on multiple-scattering x-ray photoelectron diffraction (XPD) calculations. This method is applied to our XPD data on several molecules having different equilibrium geometries. Then it is confirmed that, by our method, bond lengths and bond angles can be determined with a resolution of less than 0.1 Å and 10∘, respectively. Differently from any other scenario of ultrafast structure determination, we measure the two- or three-dimensional XPD of aligned or oriented molecules in the energy range from 100 to 200 eV with a 4π detection velocity map imaging spectrometer. Thanks to the intense and ultrashort pulse properties of x-ray free-electron lasers, our approach exhibits the most probable method for obtaining ultrafast real-time structural information on small to medium-sized molecules consisting of light elements, i.e., a “molecular movie.”

Ultrafast chemical interface scattering as an additional decay channel for nascent nonthermal electrons in small metal nanoparticles.

PubMed

Bauer, Christophe; Abid, Jean-Pierre; Fermin, David; Girault, Hubert H

2004-05-15

The use of 4.2 nm gold nanoparticles wrapped in an adsorbates shell and embedded in a TiO2 metal oxide matrix gives the opportunity to investigate ultrafast electron-electron scattering dynamics in combination with electronic surface phenomena via the surface plasmon lifetimes. These gold nanoparticles (NPs) exhibit a large nonclassical broadening of the surface plasmon band, which is attributed to a chemical interface damping. The acceleration of the loss of surface plasmon phase coherence indicates that the energy and the momentum of the collective electrons can be dissipated into electronic affinity levels of adsorbates. As a result of the preparation process, gold NPs are wrapped in a shell of sulfate compounds that gives rise to a large density of interfacial molecules confined between Au and TiO2, as revealed by Fourier-transform-infrared spectroscopy. A detailed analysis of the transient absorption spectra obtained by broadband femtosecond transient absorption spectroscopy allows separating electron-electron and electron-phonon interaction. Internal thermalization times (electron-electron scattering) are determined by probing the decay of nascent nonthermal electrons (NNEs) and the build-up of the Fermi-Dirac electron distribution, giving time constants of 540 to 760 fs at 0.42 and 0.34 eV from the Fermi level, respectively. Comparison with literature data reveals that lifetimes of NNEs measured for these small gold NPs are more than four times longer than for silver NPs with similar sizes. The surprisingly long internal thermalization time is attributed to an additional decay mechanism (besides the classical e-e scattering) for the energy loss of NNEs, identified as the ultrafast chemical interface scattering process. NNEs experience an inelastic resonant scattering process into unoccupied electronic states of adsorbates, that directly act as an efficient heat bath, via the excitation of molecular vibrational modes. The two-temperature model is no longer valid for this system because of (i) the temporal overlap between the internal and external thermalization process is very important; (ii) a part of the photonic energy is directly transferred toward the adsorbates (not among "cold" conduction band electrons). These findings have important consequence for femtochemistry on metal surfaces since they show that reactions can be initiated by nascent nonthermal electrons (as photoexcited, out of a Fermi-Dirac distribution) besides of the hot electron gas.

Femtochemistry of confined water

NASA Astrophysics Data System (ADS)

Douhal, A.; Carranza, M. A.; Sanz, M.; Organero, J. A.; Santos, L.

In this contribution, we applied ultrafast spectroscopy to study the H-bond network of water confined in nanostructures (Cyclodextrins and Micelles). We examine the effect of caging on ultrafast reaction dynamics and discuss the related processes under different experimental conditions. The results show an ultrafast dynamic giving birth to intermediates of the probe, which show femtosecond and picosecond dynamics leading to the final structure at the excited state. The results show the high sensitivity of the used technique in detecting small of water. This work was supported by the Ministry of Science and Technology (MCYT, Spain) and ``Conserjería de Ciencia y Tecnologia de la JCCM, Spain'' through projects MAT2002-01829 and PAI-02-004.

Nonlinear Optical Spectroscopy of Two-Dimensional Materials

NASA Astrophysics Data System (ADS)

Cui, Qiannan

Nonlinear optical properties of two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs), graphene, black phosphorus, and so on, play a key role of understanding nanoscale light-matter interactions, as well as developing nanophotonics applications from solar cells to quantum computation. With ultrafast lasers, we experimentally study nonlinear optical properties of 2D materials. Employing transient absorption microscopy, we study several members of 2D materials, such as WSe2, TiS3 and ReS2. The dynamical saturable absorption process of 2D excitons is spatiotemporally resolved. Intrinsic parameters of these 2D materials, such as exciton lifetime, exciton diffusion coefficient, and exciton mobility, are effectively measured. Especially, in-plane anisotropy of transient absorption and diffusive transport is observed for 2D excitons in monolayer ReS2, demonstrating the in-plane degree of freedom. Furthermore, with quantum interference and control nanoscopy, we all-optically inject, detect and manipulate nanoscale ballistic charge currents in a ReS2 thin film. By tuning the phase difference between one photon absorption and two photon absorption transition paths, sub-picosecond timescale of ballistic currents is coherently controlled for the first time in TMDs. In addition, the spatial resolution is two-order of magnitude smaller than optical diffraction limit. The second-order optical nonlinearity of 2D monolayers is resolved by second harmonic generation (SHG) microscopy. We measure the second-order susceptibility of monolayer MoS 2. The angular dependence of SHG in monolayer MoS2 shows strong symmetry dependence on its crystal lattice structure. Hence, second harmonic generation microscopy can serve as a powerful tool to noninvasively determine the crystalline directions of 2D monolayers. The real and imaginary parts of third-order optical nonlinearity of 2D monolayers are resolved by third harmonic generation (THG) microscopy and two-photon transient absorption microscopy, respectively. With third harmonic generation microscopy, we observe strong and anisotropic THG in monolayer and multilayer ReS2. Comparing with 2D materials with hexagonal lattice, such as MoS2, the third-order susceptibility is higher by one order of magnitude in ReS2 with a distorted 1T structure. The in-plane anisotropy of THG is attributed to the lattice distortion in ReS2 after comparing with a symmetry analysis. With two-photon transient absorption microscopy, we observe a giant two-photon absorption coefficient of monolayer WS2.

Ultrafast carrier thermalization and trapping in silicon-germanium alloy probed by extreme ultraviolet transient absorption spectroscopy

DOE PAGES

Zürch, Michael; Chang, Hung-Tzu; Kraus, Peter M.; ...

2017-06-06

Semiconductor alloys containing silicon and germanium are of growing importance for compact and highly efficient photonic devices due to their favorable properties for direct integration into silicon platforms and wide tunability of optical parameters. Here, we report the simultaneous direct and energy-resolved probing of ultrafast electron and hole dynamics in a silicon-germanium alloy with the stoichiometry Si 0.25Ge 0.75 by extreme ultraviolet transient absorption spectroscopy. Probing the photoinduced dynamics of charge carriers at the germanium M 4,5-edge (~30 eV) allows the germanium atoms to be used as reporter atoms for carrier dynamics in the alloy. The photoexcitation of electrons acrossmore » the direct and indirect band gap into conduction band (CB) valleys and their subsequent hot carrier relaxation are observed and compared to pure germanium, where the Ge direct (ΔE gap,Ge,direct = 0.8 eV) and Si 0.25Ge 0.75 indirect gaps (ΔE gap,Si0.25Ge0.75,indirect = 0.95 eV) are comparable in energy. In the alloy, comparable carrier lifetimes are observed for the X, L, and Γ valleys in the conduction band. A midgap feature associated with electrons accumulating in trap states near the CB edge following intraband thermalization is observed in the Si 0.25Ge 0.75 alloy. The successful implementation of the reporter atom concept for capturing the dynamics of the electronic bands by site-specific probing in solids opens a route to study carrier dynamics in more complex materials with femtosecond and sub-femtosecond temporal resolution.« less

The Ultra-fast Outflow of the Quasar PG 1211+143 as Viewed by Time-averaged Chandra Grating Spectroscopy

NASA Astrophysics Data System (ADS)

Danehkar, Ashkbiz; Nowak, Michael A.; Lee, Julia C.; Kriss, Gerard A.; Young, Andrew J.; Hardcastle, Martin J.; Chakravorty, Susmita; Fang, Taotao; Neilsen, Joseph; Rahoui, Farid; Smith, Randall K.

2018-02-01

We present a detailed X-ray spectral study of the quasar PG 1211+143 based on Chandra High Energy Transmission Grating Spectrometer (HETGS) observations collected in a multi-wavelength campaign with UV data using the Hubble Space Telescope Cosmic Origins Spectrograph (HST-COS) and radio bands using the Jansky Very Large Array (VLA). We constructed a multi-wavelength ionizing spectral energy distribution using these observations and archival infrared data to create XSTAR photoionization models specific to the PG 1211+143 flux behavior during the epoch of our observations. Our analysis of the Chandra-HETGS spectra yields complex absorption lines from H-like and He-like ions of Ne, Mg, and Si, which confirm the presence of an ultra-fast outflow (UFO) with a velocity of approximately ‑17,300 km s‑1 (outflow redshift z out ∼ ‑0.0561) in the rest frame of PG 1211+143. This absorber is well described by an ionization parameter {log}ξ ∼ 2.9 {erg} {{{s}}}-1 {cm} and column density {log}{N}{{H}}∼ 21.5 {{cm}}-2. This corresponds to a stable region of the absorber’s thermal stability curve, and furthermore its implied neutral hydrogen column is broadly consistent with a broad Lyα absorption line at a mean outflow velocity of approximately ‑16,980 km s‑1 detected by our HST-COS observations. Our findings represent the first simultaneous detection of a UFO in both X-ray and UV observations. Our VLA observations provide evidence for an active jet in PG 1211+143, which may be connected to the X-ray and UV outflows; this possibility can be evaluated using very-long-baseline interferometric observations.

Ultrafast carrier thermalization and trapping in silicon-germanium alloy probed by extreme ultraviolet transient absorption spectroscopy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zürch, Michael; Chang, Hung-Tzu; Kraus, Peter M.

Semiconductor alloys containing silicon and germanium are of growing importance for compact and highly efficient photonic devices due to their favorable properties for direct integration into silicon platforms and wide tunability of optical parameters. Here, we report the simultaneous direct and energy-resolved probing of ultrafast electron and hole dynamics in a silicon-germanium alloy with the stoichiometry Si 0.25Ge 0.75 by extreme ultraviolet transient absorption spectroscopy. Probing the photoinduced dynamics of charge carriers at the germanium M 4,5-edge (~30 eV) allows the germanium atoms to be used as reporter atoms for carrier dynamics in the alloy. The photoexcitation of electrons acrossmore » the direct and indirect band gap into conduction band (CB) valleys and their subsequent hot carrier relaxation are observed and compared to pure germanium, where the Ge direct (ΔE gap,Ge,direct = 0.8 eV) and Si 0.25Ge 0.75 indirect gaps (ΔE gap,Si0.25Ge0.75,indirect = 0.95 eV) are comparable in energy. In the alloy, comparable carrier lifetimes are observed for the X, L, and Γ valleys in the conduction band. A midgap feature associated with electrons accumulating in trap states near the CB edge following intraband thermalization is observed in the Si 0.25Ge 0.75 alloy. The successful implementation of the reporter atom concept for capturing the dynamics of the electronic bands by site-specific probing in solids opens a route to study carrier dynamics in more complex materials with femtosecond and sub-femtosecond temporal resolution.« less

Discovery of Ultra-fast Outflows in a Sample of Broad-line Radio Galaxies Observed with Suzaku

NASA Astrophysics Data System (ADS)

Tombesi, F.; Sambruna, R. M.; Reeves, J. N.; Braito, V.; Ballo, L.; Gofford, J.; Cappi, M.; Mushotzky, R. F.

2010-08-01

We present the results of a uniform and systematic search for blueshifted Fe K absorption lines in the X-ray spectra of five bright broad-line radio galaxies observed with Suzaku. We detect, for the first time in radio-loud active galactic nuclei (AGNs) at X-rays, several absorption lines at energies greater than 7 keV in three out of five sources, namely, 3C 111, 3C 120, and 3C 390.3. The lines are detected with high significance according to both the F-test and extensive Monte Carlo simulations. Their likely interpretation as blueshifted Fe XXV and Fe XXVI K-shell resonance lines implies an origin from highly ionized gas outflowing with mildly relativistic velocities, in the range v ~= 0.04-0.15c. A fit with specific photoionization models gives ionization parameters in the range log ξ ~= 4-5.6 erg s-1 cm and column densities of N H ~= 1022-1023 cm-2. These characteristics are very similar to those of the ultra-fast outflows (UFOs) previously observed in radio-quiet AGNs. Their estimated location within ~0.01-0.3 pc of the central super-massive black hole suggests a likely origin related with accretion disk winds/outflows. Depending on the absorber covering fraction, the mass outflow rate of these UFOs can be comparable to the accretion rate and their kinetic power can correspond to a significant fraction of the bolometric luminosity and is comparable to their typical jet power. Therefore, these UFOs can play a significant role in the expected feedback from the AGN to the surrounding environment and can give us further clues on the relation between the accretion disk and the formation of winds/jets in both radio-quiet and radio-loud AGNs.

Promising roadmap alternatives for the SpaceLiner

NASA Astrophysics Data System (ADS)

Sippel, Martin

2010-06-01

The paper describes the vision and potential roadmap alternatives of an ultrafast intercontinental passenger transport based on a rocket powered two-stage reusable vehicle. An operational scenario and the latest technical lay-out of the configuration's preliminary design including flight performance are described. The question of how the revolutionary ultrafast transport can be realized is addressed by an assessment of the different technological and programmatic roadmap alternatives.

Time-resolved molecular imaging

NASA Astrophysics Data System (ADS)

Xu, Junliang; Blaga, Cosmin I.; Agostini, Pierre; DiMauro, Louis F.

2016-06-01

Time-resolved molecular imaging is a frontier of ultrafast optical science and physical chemistry. In this article, we review present and future key spectroscopic and microscopic techniques for ultrafast imaging of molecular dynamics and show their differences and connections. The advent of femtosecond lasers and free electron x-ray lasers bring us closer to this goal, which eventually will extend our knowledge about molecular dynamics to the attosecond time domain.

Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices.

PubMed

Pati, Palas Baran; Zhang, Lei; Philippe, Bertrand; Fernández-Terán, Ricardo; Ahmadi, Sareh; Tian, Lei; Rensmo, Håkan; Hammarström, Leif; Tian, Haining

2017-06-09

A covalently linked organic dye-cobaloxime catalyst system based on mesoporous NiO is synthesized by a facile click reaction for mechanistic studies and application in a dye-sensitized solar fuel device. The system is systematically investigated by photoelectrochemical measurements, density functional theory, time-resolved fluorescence, transient absorption spectroscopy, and photoelectron spectroscopy. The results show that irradiation of the dye-catalyst on NiO leads to ultrafast hole injection into NiO from the excited dye, followed by a fast electron transfer process to reduce the catalyst. Moreover, the dye adopts different structures with different excited state energies, and excitation energy transfer occurs between neighboring molecules on the semiconductor surface. The photoelectrochemical experiments also show hydrogen production by this system. The axial chloride ligands of the catalyst are released during photocatalysis to create the active sites for proton reduction. A working mechanism of the dye-catalyst system on the photocathode is proposed on the basis of this study. © 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

Stimulated Emission of Terahertz Radiation from Internal ExcitonTransitions in Cu2O

NASA Astrophysics Data System (ADS)

Schmid, B. A.; Huber, R.; Shen, Y. R.; Kaindl, R. A.; Chemla, D. S.

2006-03-01

Excitons are among the most fundamental optical excitation modes in semiconductors. Resonant infrared pulses have been used to sensitively probe absorptive transitions between hydrogen-like bound pair states [1,2]. We report the first observation of the reverse quantum process: stimulated emission of electromagnetic radiation from intra-excitonic transitions [3]. Broadband terahertz pulses monitor the far-infrared electromagnetic response of Cu2O after ultrafast resonant photogeneration of 3p excitons. Stimulated emission from the 3p to the energetically lower 2s bound level occurs at a photon energy of 6.6 meV, with a cross section of ˜10-14 cm^2. Simultaneous excitation of both exciton levels, in turn, drives quantum beats which lead to efficient terahertz emission sharply peaked at the difference frequency. Our results demonstrate a new fundamental process of THz quantum optics and highlight analogies and differences between excitonic and atomic systems. [1] R. A. Kaindl et al., Nature 423, 734 (2003). [2] M. Kubouchi et al., Phys. Rev. Lett. 94, 016403 (2005). [3] R. Huber et al., Phys. Rev. Lett., to appear.

Effects of Carrier Confinement and Intervalley Scattering on Photoexcited Electron Plasma in Silicon.

PubMed

Sieradzki, A; Kuznicki, Z T

2013-01-01

The ultrafast reflectivity of silicon, excited and probed with femtosecond laser pulses, is studied for different wavelengths and energy densities. The confinement of carriers in a thin surface layer delimited by a nanoscale Si-layered system buried in a Si heavily-doped wafer reduces the critical density of carriers necessary to create the electron plasma by a factor of ten. We performed two types of reflectivity measurements, using either a single beam or two beams. The plasma strongly depends on the photon energy density because of the intervalley scattering of the electrons revealed by two different mechanisms assisted by the electron-phonon interaction. One mechanism leads to a negative differential reflectivity that can be attributed to an induced absorption in X valleys. The other mechanism occurs, when the carrier population is thermalizing and gives rise to a positive differential reflectivity corresponding to Pauli-blocked intervalley gamma to X scattering. These results are important for improving the efficiency of Si light-to-electricity converters, in which there is a possibility of multiplying carriers by nanostructurization of Si.

Experimental station for ultrafast extreme ultraviolet spectroscopy for non-equilibrium dynamics in warm dense matter

NASA Astrophysics Data System (ADS)

Lee, Jong-won; Geng, Xiaotao; Jung, Jae Hyung; Cho, Min Sang; Yang, Seong Hyeok; Jo, Jawon; Lee, Chang-lyoul; Cho, Byoung Ick; Kim, Dong-Eon

2018-07-01

Recent interest in highly excited matter generated by intense femtosecond laser pulses has led to experimental methods that directly investigate ultrafast non-equilibrium electronic and structural dynamics. We present a tabletop experimental station for the extreme ultraviolet (EUV) spectroscopy used to trace L-edge dynamics in warm dense aluminum with a temporal resolution of a hundred femtoseconds. The system consists of the EUV probe generation part via a high-order harmonic generation process of femtosecond laser pulses with atomic clusters, a beamline with high-throughput optics and a sample-refreshment system of nano-foils utilizing the full repetition rate of the probe, and a flat-field EUV spectrograph. With the accumulation of an order of a hundred shots, a clear observation of the change in the aluminum L-shell absorption was achieved with a temporal resolution of 90 fs in a 600-fs window. The signature of a non-equilibrium electron distribution over a 10-eV range and its evolution to a 1-eV Fermi distribution are observed. This demonstrates the capability of this apparatus to capture the non-equilibrium electron-hole dynamics in highly excited warm dense matter conditions.

NuSTAR REVEALS RELATIVISTIC REFLECTION BUT NO ULTRA-FAST OUTFLOW IN THE QUASAR PG 1211+143

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zoghbi, A.; Miller, J. M.; Walton, D. J.

We report on four epochs of observations of the quasar PG 1211+143 using NuSTAR. The net exposure time is 300 ks. Prior work on this source found suggestive evidence of an ultra-fast outflow (UFO) in the Fe K band with a velocity of approximately 0.1c. The putative flow would carry away a high-mass flux and kinetic power, with broad implications for feedback and black hole--galaxy co-evolution. NuSTAR detects PG 1211+143 out to 30 keV, meaning that the continuum is well-defined both through and above the Fe K band. A characteristic relativistic disk reflection spectrum is clearly revealed via a broadmore » Fe K emission line and Compton back-scattering curvature. The data offer only weak constraints on the spin of the black hole. A careful search for UFOs shows no significant absorption feature above 90% confidence. The limits are particularly tight when relativistic reflection is included. We discuss the statistics and the implications of these results in terms of connections between accretion onto quasars, Seyferts, and stellar-mass black holes, and feedback into their host environments.« less

NuSTAR Reveals Relativistic Reflection But No Ultra-Fast Outflow in the Quasar Pg∼1211+143

NASA Astrophysics Data System (ADS)

Zoghbi, A.; Miller, J. M.; Walton, D. J.; Harrison, F. A.; Fabian, A. C.; Reynolds, C. S.; Boggs, S. E.; Christensen, F. E.; Craig, W.; Hailey, C. J.; Stern, D.; Zhang, W. W.

2015-01-01

We report on four epochs of observations of the quasar PG 1211+143 using NuSTAR. The net exposure time is 300 ks. Prior work on this source found suggestive evidence of an ultra-fast outflow (UFO) in the Fe K band with a velocity of approximately 0.1c. The putative flow would carry away a high-mass flux and kinetic power, with broad implications for feedback and black hole--galaxy co-evolution. NuSTAR detects PG 1211+143 out to 30 keV, meaning that the continuum is well-defined both through and above the Fe K band. A characteristic relativistic disk reflection spectrum is clearly revealed via a broad Fe K emission line and Compton back-scattering curvature. The data offer only weak constraints on the spin of the black hole. A careful search for UFOs shows no significant absorption feature above 90% confidence. The limits are particularly tight when relativistic reflection is included. We discuss the statistics and the implications of these results in terms of connections between accretion onto quasars, Seyferts, and stellar-mass black holes, and feedback into their host environments.

Q-switched all-fiber laser with short pulse duration based on tungsten diselenide

NASA Astrophysics Data System (ADS)

Li, Wenyi; OuYang, Yuyi; Ma, Guoli; Liu, Mengli; Liu, Wenjun

2018-05-01

Fiber lasers are widely used in industrial processing, sensing, medical and communications applications due to their simple structure, good stability and low cost. With the rapid development of fiber lasers and the sustained improvement of industrial laser quality requirements, researchers in ultrafast optics focus on how to get laser pulses with high output power and narrow pulse duration. Q-switched technology is one of the most effective techniques to generate ultrashort pulses. In this paper, a tungsten diselenide saturable absorber with 16.82% modulation depth is prepared by chemical vapor deposition. Experimental results show that when the pump power changes from 115.7 mW to 630 mW, the all-fiber laser can achieve a stable Q-switched pulse output. The repetition rate of the output pulse varies from 80.32 kHz to 204.2 kHz, the pulse duration is 581 ns, the maximum output power is 17.1 mW and the maximum pulse energy is 83.7 nJ. Results in this paper show that tungsten diselenide can be applied to ultrafast optics, which is a kind of saturable absorption material with excellent properties.

Ultrafast Carrier dynamics of InxGa1-xN nanostructures grown directly on Si(111)

NASA Astrophysics Data System (ADS)

Kumar, Praveen; Devi, Pooja; Rodriguez⁠, P. E. D. S.; Kumar, Manish; Shivling, V. D.; Noetzel, Richard; Sharma, Chhavi; Sinha, R. K.; Kumar, Mahesh

2018-05-01

We show a flux dependence changes in structural, optical and electronic properties of InxGa1-xN nanostructures (NSs) namely nanocolumns (NCs), nanoflakes (NFs) and nanowall network (NWN) grown directly on Si(111) surface. Field emission scanning electron microscopy (FESEM) images were recorded to see morphological changes from NFs to NCs and NWNc etc, while high-resolution X-ray diffraction (HRXRD) ω-2θ scans were used to determine In incorporation. The maximum In incorporation was observed to be 20, 33 and 38% for the sharp transition from NFs to NCs and NWNs, respectively. The charge carrier dynamics of these grown NSs were probed using Ultrafast Femtosecond Transient Absorption Spectroscopy (UFTAS) with excitation at 350 nm pump wavelength. The UFTAS studies show the comparative charge carriers dynamics of the NWS, NCs and NFs. The charge carrier studies show a higher lifetime in NWNs as compare to NCs and NFs. Further, to examine electronic structure and level of degeneracy of these NSs, core-level and valence band spectra were analyzed by X-ray photoelectron spectroscopy (XPS), which manifest the upward band bending ranging from 0.2 eV to 0.4 eV.

Ultrafast investigation of photoinduced charge transfer in aminoanthraquinone pharmaceutical product

NASA Astrophysics Data System (ADS)

Zhang, Song; Sun, Simei; Zhou, Miaomiao; Wang, Lian; Zhang, Bing

2017-02-01

We investigated the mechanism of intramolecular charge transfer and the following radiationless dynamics of the excited states of 1-aminoanthraquinone using steady state and time-resolved absorption spectroscopy combined with quantum chemical calculations. Following photoexcitation with 460 nm, conformational relaxation via twisting of the amino group, charge transfer and the intersystem crossing (ISC) processes have been established to be the major relaxation pathways responsible for the ultrafast nonradiative of the excited S1 state. Intramolecular proton transfer, which could be induced by intramolecular hydrogen bonding is inspected and excluded. Time-dependent density functional theory (TDDFT) calculations reveal the change of the dipole moments of the S0 and S1 states along the twisted coordinate of the amino group, indicating the mechanism of twisted intra-molecular charge transfer (TICT). The timescale of TICT is measured to be 5 ps due to the conformational relaxation and a barrier on the S1 potential surface. The ISC from the S1 state to the triplet manifold is a main deactivation pathway with the decay time of 28 ps. Our results observed here have yield a physically intuitive and complete picture of the photoinduced charge transfer and radiationless dynamics in anthraquinone pharmaceutial products.

Watching the dynamics of electrons and atoms at work in solar energy conversion.

PubMed

Canton, S E; Zhang, X; Liu, Y; Zhang, J; Pápai, M; Corani, A; Smeigh, A L; Smolentsev, G; Attenkofer, K; Jennings, G; Kurtz, C A; Li, F; Harlang, T; Vithanage, D; Chabera, P; Bordage, A; Sun, L; Ott, S; Wärnmark, K; Sundström, V

2015-01-01

The photochemical reactions performed by transition metal complexes have been proposed as viable routes towards solar energy conversion and storage into other forms that can be conveniently used in our everyday applications. In order to develop efficient materials, it is necessary to identify, characterize and optimize the elementary steps of the entire process on the atomic scale. To this end, we have studied the photoinduced electronic and structural dynamics in two heterobimetallic ruthenium-cobalt dyads, which belong to the large family of donor-bridge-acceptor systems. Using a combination of ultrafast optical and X-ray absorption spectroscopies, we can clock the light-driven electron transfer processes with element and spin sensitivity. In addition, the changes in local structure around the two metal centers are monitored. These experiments show that the nature of the connecting bridge is decisive for controlling the forward and the backward electron transfer rates, a result supported by quantum chemistry calculations. More generally, this work illustrates how ultrafast optical and X-ray techniques can disentangle the influence of spin, electronic and nuclear factors on the intramolecular electron transfer process. Finally, some implications for further improving the design of bridged sensitizer-catalysts utilizing the presented methodology are outlined.