Quantum simulation of ultrafast dynamics using trapped ultracold atoms.

PubMed

Senaratne, Ruwan; Rajagopal, Shankari V; Shimasaki, Toshihiko; Dotti, Peter E; Fujiwara, Kurt M; Singh, Kevin; Geiger, Zachary A; Weld, David M

2018-05-25

Ultrafast electronic dynamics are typically studied using pulsed lasers. Here we demonstrate a complementary experimental approach: quantum simulation of ultrafast dynamics using trapped ultracold atoms. Counter-intuitively, this technique emulates some of the fastest processes in atomic physics with some of the slowest, leading to a temporal magnification factor of up to 12 orders of magnitude. In these experiments, time-varying forces on neutral atoms in the ground state of a tunable optical trap emulate the electric fields of a pulsed laser acting on bound charged particles. We demonstrate the correspondence with ultrafast science by a sequence of experiments: nonlinear spectroscopy of a many-body bound state, control of the excitation spectrum by potential shaping, observation of sub-cycle unbinding dynamics during strong few-cycle pulses, and direct measurement of carrier-envelope phase dependence of the response to an ultrafast-equivalent pulse. These results establish cold-atom quantum simulation as a complementary tool for studying ultrafast dynamics.

Ultrafast Multi-Level Logic Gates with Spin-Valley Coupled Polarization Anisotropy in Monolayer MoS2

PubMed Central

Wang, Yu-Ting; Luo, Chih-Wei; Yabushita, Atsushi; Wu, Kaung-Hsiung; Kobayashi, Takayoshi; Chen, Chang-Hsiao; Li, Lain-Jong

2015-01-01

The inherent valley-contrasting optical selection rules for interband transitions at the K and K′ valleys in monolayer MoS2 have attracted extensive interest. Carriers in these two valleys can be selectively excited by circularly polarized optical fields. The comprehensive dynamics of spin valley coupled polarization and polarized exciton are completely resolved in this work. Here, we present a systematic study of the ultrafast dynamics of monolayer MoS2 including spin randomization, exciton dissociation, free carrier relaxation, and electron-hole recombination by helicity- and photon energy-resolved transient spectroscopy. The time constants for these processes are 60 fs, 1 ps, 25 ps, and ~300 ps, respectively. The ultrafast dynamics of spin polarization, valley population, and exciton dissociation provides the desired information about the mechanism of radiationless transitions in various applications of 2D transition metal dichalcogenides. For example, spin valley coupled polarization provides a promising way to build optically selective-driven ultrafast valleytronics at room temperature. Therefore, a full understanding of the ultrafast dynamics in MoS2 is expected to provide important fundamental and technological perspectives. PMID:25656222

Self-amplified photo-induced gap quenching in a correlated electron material

DOE PAGES

Mathias, S.; Eich, S.; Urbancic, J.; ...

2016-10-04

Capturing the dynamic electronic band structure of a correlated material presents a powerful capability for uncovering the complex couplings between the electronic and structural degrees of freedom. When combined with ultrafast laser excitation, new phases of matter can result, since far-from-equilibrium excited states are instantaneously populated. Here, we elucidate a general relation between ultrafast non-equilibrium electron dynamics and the size of the characteristic energy gap in a correlated electron material. Here, we show that carrier multiplication via impact ionization can be one of the most important processes in a gapped material, and that the speed of carrier multiplication critically dependsmore » on the size of the energy gap. In the case of the charge-density wave material 1T-TiSe 2, our data indicate that carrier multiplication and gap dynamics mutually amplify each other, which explains—on a microscopic level—the extremely fast response of this material to ultrafast optical excitation.« less

Ultrafast carrier dynamics in LT-GaAs doped with Si delta layers

NASA Astrophysics Data System (ADS)

Khusyainov, D. I.; Dekeyser, C.; Buryakov, A. M.; Mishina, E. D.; Galiev, G. B.; Klimov, E. A.; Pushkarev, S. S.; Klochkov, A. N.

2017-10-01

We characterized the ultrafast properties of LT-GaAs doped with silicon δ-layers and introduced delta-doping (δ-doping) as efficient method for enhancing the properties of GaAs-based structures which can be useful for terahertz (THz) antenna, ultrafast switches and other high frequency applications. Low temperature grown GaAs (LT-GaAs) became one of the most promising materials for ultrafast optical and THz devices due to its short carrier lifetime and high carrier mobility. Low temperature growth leads to a large number of point defects and an excess of arsenic. Annealing of LT-GaAs creates high resistivity through the formation of As-clusters, which appear due to the excess of arsenic. High resistivity is very important for THz antennas so that voltage can be applied without the risk of breakdown. With δ-Si doping, control of As-clusters is possible, since after annealing, clusters align in the plane where the δ-doping occurs. In this paper, we compare the properties of LT-GaAs-based planar structures with and without δ-Si doping and subsequent annealing. We used pump-probe transient reflectivity as a probe for ultrafast carrier dynamics in LT-GaAs. The results of the experiment were interpreted using the Ortiz model and show that the δ-Si doping increases deep donor and acceptor concentrations and decreases the photoinduced carrier lifetime as compared with LT-GaAs with same growth and annealing temperatures, but without doping.

Revealing the ultrafast charge carrier dynamics in organo metal halide perovskite solar cell materials using time resolved THz spectroscopy

NASA Astrophysics Data System (ADS)

Ponseca, C. S., Jr.; Sundström, V.

2016-03-01

Ultrafast charge carrier dynamics in organo metal halide perovskite has been probed using time resolved terahertz (THz) spectroscopy (TRTS). Current literature on its early time characteristics is unanimous: sub-ps charge carrier generation, highly mobile charges and very slow recombination rationalizing the exceptionally high power conversion efficiency for a solution processed solar cell material. Electron injection from MAPbI3 to nanoparticles (NP) of TiO2 is found to be sub-ps while Al2O3 NPs do not alter charge dynamics. Charge transfer to organic electrodes, Spiro-OMeTAD and PCBM, is sub-ps and few hundreds of ps respectively, which is influenced by the alignment of energy bands. It is surmised that minimizing defects/trap states is key in optimizing charge carrier extraction from these materials.

Probing Ultrafast Electron Dynamics at Surfaces Using Soft X-Ray Transient Reflectivity Spectroscopy

NASA Astrophysics Data System (ADS)

Baker, L. Robert; Husek, Jakub; Biswas, Somnath; Cirri, Anthony

The ability to probe electron dynamics with surface sensitivity on the ultrafast time scale is critical for understanding processes such as charge separation, injection, and surface trapping that mediate efficiency in catalytic and energy conversion materials. Toward this goal, we have developed a high harmonic generation (HHG) light source for femtosecond soft x-ray reflectivity. Using this light source we investigated the ultrafast carrier dynamics at the surface of single crystalline α-Fe2O3, polycrystalline α-Fe2O3, and the mixed metal oxide, CuFeO2. We have recently demonstrated that CuFeO2 in particular is a selective catalyst for photo-electrochemical CO2 reduction to acetate; however, the role of electronic structure and charge carrier dynamics in mediating catalytic selectivity has not been well understood. Soft x-ray reflectivity measurements probe the M2,3, edges of the 3d transition metals, which provide oxidation and spin state resolution with element specificity. In addition to chemical state specificity, these measurements are also surface sensitive, and by independently simulating the contributions of the real and imaginary components of the complex refractive index, we can differentiate between surface and sub-surface contributions to the excited state spectrum. Accordingly, this work demonstrates the ability to probe ultrafast carrier dynamics in catalytic materials with element and chemical state specificity and with surface sensitivity.

Ultrafast collinear scattering and carrier multiplication in graphene.

PubMed

Brida, D; Tomadin, A; Manzoni, C; Kim, Y J; Lombardo, A; Milana, S; Nair, R R; Novoselov, K S; Ferrari, A C; Cerullo, G; Polini, M

2013-01-01

Graphene is emerging as a viable alternative to conventional optoelectronic, plasmonic and nanophotonic materials. The interaction of light with charge carriers creates an out-of-equilibrium distribution, which relaxes on an ultrafast timescale to a hot Fermi-Dirac distribution, that subsequently cools emitting phonons. Although the slower relaxation mechanisms have been extensively investigated, the initial stages still pose a challenge. Experimentally, they defy the resolution of most pump-probe setups, due to the extremely fast sub-100 fs carrier dynamics. Theoretically, massless Dirac fermions represent a novel many-body problem, fundamentally different from Schrödinger fermions. Here we combine pump-probe spectroscopy with a microscopic theory to investigate electron-electron interactions during the early stages of relaxation. We identify the mechanisms controlling the ultrafast dynamics, in particular the role of collinear scattering. This gives rise to Auger processes, including charge multiplication, which is key in photovoltage generation and photodetectors.

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 pre-breakdown dynamics in Al₂O₃SiO₂ reflector by femtosecond UV laser spectroscopy.

PubMed

Du, Juan; Li, Zehan; Xue, Bing; Kobayashi, Takayoshi; Han, Dongjia; Zhao, Yuanan; Leng, Yuxin

2015-06-29

Ultrafast carrier dynamics in Al2O3/SiO2 high reflectors has been investigated by UV femtosecond laser. It is identified by laser spectroscopy that, the carrier dynamics contributed from the front few layers of Al2O3 play a dominating role in the initial laser-induced damage of the UV reflector. Time-resolved reflection decrease after the UV excitation is observed, and conduction electrons is found to relaxed to a mid-gap defect state locating about one photon below the conduction band . To interpret the laser induced carrier dynamics further, a theoretical model including electrons relaxation to a mid-gap state is built, and agrees very well with the experimental results.. To the best of our knowledge, this is the first study on the pre-damage dynamics in UV high reflector induced by femtosecond UV laser.

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 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 carrier capture and Auger recombination in single GaN/InGaN multiple quantum well nanowires

DOE PAGES

Boubanga-Tombet, Stephane; Wright, Jeremy B.; Lu, Ping; ...

2016-11-04

Ultrafast optical microscopy is an important tool for examining fundamental phenomena in semiconductor nanowires with high temporal and spatial resolution. In this paper, we used this technique to study carrier dynamics in single GaN/InGaN core–shell nonpolar multiple quantum well nanowires. We find that intraband carrier–carrier scattering is the main channel governing carrier capture, while subsequent carrier relaxation is dominated by three-carrier Auger recombination at higher densities and bimolecular recombination at lower densities. Finally, the Auger constants in these nanowires are approximately 2 orders of magnitude lower than in planar InGaN multiple quantum wells, highlighting their potential for future light-emitting devices.

Ultrafast surface carrier dynamics in the topological insulator Bi₂Te₃.

PubMed

Hajlaoui, M; Papalazarou, E; Mauchain, J; Lantz, G; Moisan, N; Boschetto, D; Jiang, Z; Miotkowski, I; Chen, Y P; Taleb-Ibrahimi, A; Perfetti, L; Marsi, M

2012-07-11

We discuss the ultrafast evolution of the surface electronic structure of the topological insulator Bi(2)Te(3) following a femtosecond laser excitation. Using time and angle-resolved photoelectron spectroscopy, we provide a direct real-time visualization of the transient carrier population of both the surface states and the bulk conduction band. We find that the thermalization of the surface states is initially determined by interband scattering from the bulk conduction band, lasting for about 0.5 ps; subsequently, few picoseconds are necessary for the Dirac cone nonequilibrium electrons to recover a Fermi-Dirac distribution, while their relaxation extends over more than 10 ps. The surface sensitivity of our measurements makes it possible to estimate the range of the bulk-surface interband scattering channel, indicating that the process is effective over a distance of 5 nm or less. This establishes a correlation between the nanoscale thickness of the bulk charge reservoir and the evolution of the ultrafast carrier dynamics in the surface Dirac cone.

New Aspects of Photocurrent Generation at Graphene pn Junctions Revealed by Ultrafast Optical Measurements

NASA Astrophysics Data System (ADS)

Aivazian, Grant; Sun, Dong; Jones, Aaron; Ross, Jason; Yao, Wang; Cobden, David; Xu, Xiaodong

2012-02-01

The remarkable electrical and optical properties of graphene make it a promising material for new optoelectronic applications. However, one important, but so far unexplored, property is the role of hot carriers in charge and energy transport at graphene interfaces. Here we investigate the photocurrent (PC) dynamics at a tunable graphene pn junction using ultrafast scanning PC microscopy. Pump-probe measurements show a temperature dependent relaxation time of photogenerated carriers that increases from 1.5ps at 290K to 4ps at 20K; while the amplitude of the PC is independent of the lattice temperature. These observations imply that it is hot carriers, not phonons, which dominate ultrafast energy transport. Gate dependent measurements show many interesting features such as pump induced saturation, enhancement, and sign reversal of probe generated PC. These observations reveal that the underlying PC mechanism is a combination of the thermoelectric and built-in electric field effects. Our results enhance the understanding of non-equilibrium electron dynamics, electron-electron interactions, and electron-phonon interactions in graphene. They also determine fundamental limits on ultrafast device operation speeds (˜500 GHz) for graphene-based photodetectors.

Mapping carrier diffusion in single silicon core-shell nanowires with ultrafast optical microscopy.

PubMed

Seo, M A; Yoo, J; Dayeh, S A; Picraux, S T; Taylor, A J; Prasankumar, R P

2012-12-12

Recent success in the fabrication of axial and radial core-shell heterostructures, composed of one or more layers with different properties, on semiconductor nanowires (NWs) has enabled greater control of NW-based device operation for various applications. (1-3) However, further progress toward significant performance enhancements in a given application is hindered by the limited knowledge of carrier dynamics in these structures. In particular, the strong influence of interfaces between different layers in NWs on transport makes it especially important to understand carrier dynamics in these quasi-one-dimensional systems. Here, we use ultrafast optical microscopy (4) to directly examine carrier relaxation and diffusion in single silicon core-only and Si/SiO(2) core-shell NWs with high temporal and spatial resolution in a noncontact manner. This enables us to reveal strong coherent phonon oscillations and experimentally map electron and hole diffusion currents in individual semiconductor NWs for the first time.

Photo-conductive detection of continuous THz waves via manipulated ultrafast process in nanostructures

NASA Astrophysics Data System (ADS)

Moon, Kiwon; Lee, Eui Su; Lee, Il-Min; Park, Dong Woo; Park, Kyung Hyun

2018-01-01

Time-domain and frequency-domain terahertz (THz) spectroscopy systems often use materials fabricated with exotic and expensive methods that intentionally introduce defects to meet short carrier lifetime requirements. In this study, we demonstrate the development of a nano-photomixer that meets response speed requirements without using defect-incorporated, low-temperature-grown (LTG) semiconductors. Instead, we utilized a thin InGaAs layer grown on a semi-insulating InP substrate by metal-organic chemical vapor deposition (MOCVD) combined with nano-electrodes to manipulate local ultrafast photo-carrier dynamics via a carefully designed field-enhancement and plasmon effect. The developed nano-structured photomixer can detect continuous-wave THz radiation up to a frequency of 2 THz with a peak carrier collection efficiency of 5%, which is approximately 10 times better than the reference efficiency of 0.4%. The better efficiency results from the high carrier mobility of the MOCVD-grown InGaAs thin layer with the coincidence of near-field and plasmon-field distributions in the nano-structure. Our result not only provides a generally applicable methodology for manipulating ultrafast carrier dynamics by means of nano-photonic techniques to break the trade-off relation between the carrier lifetime and mobility in typical LTG semiconductors but also contributes to mass-producible photo-conductive THz detectors to facilitate the widespread application of THz technology.

Anisotropic attosecond charge carrier dynamics and layer decoupling in quasi-2D layered SnS 2

DOE PAGES

Eads, Calley N.; Bandak, Dmytro; Neupane, Mahesh R.; ...

2017-11-08

Strong quantum confinement effects lead to striking new physics in two-dimensional materials such as graphene or transition metal dichalcogenides. While spectroscopic fingerprints of such quantum confinement have been demonstrated widely, the consequences for carrier dynamics are at present less clear, particularly on ultrafast timescales. This is important for tailoring, probing, and understanding spin and electron dynamics in layered and two-dimensional materials even in cases where the desired bandgap engineering has been achieved. Here in this paper we show by means of core–hole clock spectroscopy that SnS 2 exhibits spindependent attosecond charge delocalization times (τ deloc) for carriers confined within amore » layer, τ deloc < 400 as, whereas interlayer charge delocalization is dynamically quenched in excess of a factor of 10, τ deloc > 2.7 fs. These layer decoupling dynamics are a direct consequence of strongly anisotropic screening established within attoseconds, and demonstrate that important two-dimensional characteristics are also present in bulk crystals of van der Waalslayered materials, at least on ultrafast timescales.« less

Ultrafast dynamics of defect-assisted electron-hole recombination in monolayer MoS2.

PubMed

Wang, Haining; Zhang, Changjian; Rana, Farhan

2015-01-14

In this Letter, we present nondegenerate ultrafast optical pump-probe studies of the carrier recombination dynamics in MoS2 monolayers. By tuning the probe to wavelengths much longer than the exciton line, we make the probe transmission sensitive to the total population of photoexcited electrons and holes. Our measurement reveals two distinct time scales over which the photoexcited electrons and holes recombine; a fast time scale that lasts ∼ 2 ps and a slow time scale that lasts longer than ∼ 100 ps. The temperature and the pump fluence dependence of the observed carrier dynamics are consistent with defect-assisted recombination as being the dominant mechanism for electron-hole recombination in which the electrons and holes are captured by defects via Auger processes. Strong Coulomb interactions in two-dimensional atomic materials, together with strong electron and hole correlations in two-dimensional metal dichalcogenides, make Auger processes particularly effective for carrier capture by defects. We present a model for carrier recombination dynamics that quantitatively explains all features of our data for different temperatures and pump fluences. The theoretical estimates for the rate constants for Auger carrier capture are in good agreement with the experimentally determined values. Our results underscore the important role played by Auger processes in two-dimensional atomic materials.

Effects of surface nanostructuring and impurity doping on ultrafast carrier dynamics of silicon photovoltaic cells: a pump-probe study

NASA Astrophysics Data System (ADS)

Chen, Tianyu; Nam, Yoon-Ho; Wang, Xinke; Han, Peng; Sun, Wenfeng; Feng, Shengfei; Ye, Jiasheng; Song, Jae-Won; Lee, Jung-Ho; Zhang, Chao; Zhang, Yan

2018-01-01

We present femtosecond optical pump-terahertz probe studies on the ultrafast dynamical processes of photo-generated charge carriers in silicon photovoltaic cells with various nanostructured surfaces and doping densities. The pump-probe measurements provide direct insight on the lifetime of photo-generated carriers, frequency-dependent complex dielectric response along with photoconductivity of silicon photovoltaic cells excited by optical pump pulses. A lifetime of photo-generated carriers of tens of nanosecond is identified from the time-dependent pump-induced attenuation of the terahertz transmission. In addition, we find a large value of the imaginary part of the dielectric function and of the real part of the photoconductivity in silicon photovoltaic cells with micron length nanowires. We attribute these findings to the result of defect-enhanced electron-photon interactions. Moreover, doping densities of phosphorous impurities in silicon photovoltaic cells are also quantified using the Drude-Smith model with our measured frequency-dependent complex photoconductivities.

Ultrafast Photo-Carrier Dynamics and Coherent Phonon Excitations in Topological Dirac Semimetal Cd3As2

NASA Astrophysics Data System (ADS)

Sun, Fei; Wu, Qiong; Wu, Yanling; Tian, Yichao; Shi, Youguo; Zhao, Jimin

Three dimensional (3D) topological Dirac semimetal has attracted growing research interest owing to its intriguing quantum properties such as high bulk carrier mobility and quantum spin Hall effects. However, so far, the ultrafast dynamics of a typical 3D topological Dirac semimetal, Cd3As2, as well as its coherent phonon has not been thoroughly investigated. Here we report the ultrafast dynamics of Cd3As2 by using femtosecond pump-probe spectroscopy. Two distinct relaxation processes was observed, with the lifetimes (at 5 K) of 2.4 ps and 18.6 ps, respectively. Variable temperature experiment from 5 K to 295 K also reveals signatures of phase transitions. Furthermore, coherent optical (8.1 meV) and acoustic (0.036 THz) phonon modes were generated and detected, respectively, with signatures of hybrid-excitation of the two modes. The National Basic Research Program of China (2012CB821402), the National Natural Science Foundation of China (11274372), and the External Cooperation Program of the Chinese Academy of Sciences (GJHZ1403).

Impact of doping on the carrier dynamics in graphene

PubMed Central

Kadi, Faris; Winzer, Torben; Knorr, Andreas; Malic, Ermin

2015-01-01

We present a microscopic study on the impact of doping on the carrier dynamics in graphene, in particular focusing on its influence on the technologically relevant carrier multiplication in realistic, doped graphene samples. Treating the time- and momentum-resolved carrier-light, carrier-carrier, and carrier-phonon interactions on the same microscopic footing, the appearance of Auger-induced carrier multiplication up to a Fermi level of 300 meV is revealed. Furthermore, we show that doping favors the so-called hot carrier multiplication occurring within one band. Our results are directly compared to recent time-resolved ARPES measurements and exhibit an excellent agreement on the temporal evolution of the hot carrier multiplication for n- and p-doped graphene. The gained insights shed light on the ultrafast carrier dynamics in realistic, doped graphene samples. PMID:26577536

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 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

Spin-controlled ultrafast vertical-cavity surface-emitting lasers

NASA Astrophysics Data System (ADS)

Höpfner, Henning; Lindemann, Markus; Gerhardt, Nils C.; Hofmann, Martin R.

2014-05-01

Spin-controlled semiconductor lasers are highly attractive spintronic devices providing characteristics superior to their conventional purely charge-based counterparts. In particular, spin-controlled vertical-cavity surface emitting lasers (spin-VCSELs) promise to offer lower thresholds, enhanced emission intensity, spin amplification, full polarization control, chirp control and ultrafast dynamics. Most important, the ability to control and modulate the polarization state of the laser emission with extraordinarily high frequencies is very attractive for many applications like broadband optical communication and ultrafast optical switches. We present a novel concept for ultrafast spin-VCSELs which has the potential to overcome the conventional speed limitation for directly modulated lasers by the relaxation oscillation frequency and to reach modulation frequencies significantly above 100 GHz. The concept is based on the coupled spin-photon dynamics in birefringent micro-cavity lasers. By injecting spin-polarized carriers in the VCSEL, oscillations of the coupled spin-photon system can by induced which lead to oscillations of the polarization state of the laser emission. These oscillations are decoupled from conventional relaxation oscillations of the carrier-photon system and can be much faster than these. Utilizing these polarization oscillations is thus a very promising approach to develop ultrafast spin-VCSELs for high speed optical data communication in the near future. Different aspects of the spin and polarization dynamics, its connection to birefringence and bistability in the cavity, controlled switching of the oscillations, and the limitations of this novel approach will be analysed theoretically and experimentally for spin-polarized VCSELs at room temperature.

Ultrafast carrier dynamics in bilayer graphene studied by broadband infrared pump-probe spectroscopy

NASA Astrophysics Data System (ADS)

Limmer, Thomas; da Como, Enrico; Niggebaum, Alexander; Feldmann, Jochen

2010-03-01

Recently, bilayer graphene gained a large interest because of its electrically tunable gap appearing in the middle infrared part of the electromagnetic spectrum. This feature is expected to open a number of applications of bilayer graphene in optoelectronics. In this communication we report on the first pump-probe experiment on a single bilayer flake with an unprecedented probe photon energy interval (0.25 -- 1.3 eV). Single flakes were prepared by mechanical exfoliation of graphite and transferred to calcium fluoride substrates. When illuminated with 800 nm (1.5 eV) pump pulses the induced change in transmission shows an ultrafast saturation of the interband transitions from 1.3 to 0.5 eV. In this energy range the saturation recovery occurs within 3 ps and is consistent with an ultrafast relaxation of hot carriers. Interestingly, we report on the observation of a resonance at 0.4 eV characterized by a longer dynamics. The results are discussed considering many-body interactions.

Charge separation and carrier dynamics in donor-acceptor heterojunction photovoltaic systems.

PubMed

Teuscher, Joël; Brauer, Jan C; Stepanov, Andrey; Solano, Alicia; Boziki, Ariadni; Chergui, Majed; Wolf, Jean-Pierre; Rothlisberger, Ursula; Banerji, Natalie; Moser, Jacques-E

2017-11-01

Electron transfer and subsequent charge separation across donor-acceptor heterojunctions remain the most important areas of study in the field of third-generation photovoltaics. In this context, it is particularly important to unravel the dynamics of individual ultrafast processes (such as photoinduced electron transfer, carrier trapping and association, and energy transfer and relaxation), which prevail in materials and at their interfaces. In the frame of the National Center of Competence in Research "Molecular Ultrafast Science and Technology," a research instrument of the Swiss National Science Foundation, several groups active in the field of ultrafast science in Switzerland have applied a number of complementary experimental techniques and computational simulation tools to scrutinize these critical photophysical phenomena. Structural, electronic, and transport properties of the materials and the detailed mechanisms of photoinduced charge separation in dye-sensitized solar cells, conjugated polymer- and small molecule-based organic photovoltaics, and high-efficiency lead halide perovskite solar energy converters have been scrutinized. Results yielded more than thirty research articles, an overview of which is provided here.

Determination of hot carrier energy distributions from inversion of ultrafast pump-probe reflectivity measurements.

PubMed

Heilpern, Tal; Manjare, Manoj; Govorov, Alexander O; Wiederrecht, Gary P; Gray, Stephen K; Harutyunyan, Hayk

2018-05-10

Developing a fundamental understanding of ultrafast non-thermal processes in metallic nanosystems will lead to applications in photodetection, photochemistry and photonic circuitry. Typically, non-thermal and thermal carrier populations in plasmonic systems are inferred either by making assumptions about the functional form of the initial energy distribution or using indirect sensors like localized plasmon frequency shifts. Here we directly determine non-thermal and thermal distributions and dynamics in thin films by applying a double inversion procedure to optical pump-probe data that relates the reflectivity changes around Fermi energy to the changes in the dielectric function and in the single-electron energy band occupancies. When applied to normal incidence measurements our method uncovers the ultrafast excitation of a non-Fermi-Dirac distribution and its subsequent thermalization dynamics. Furthermore, when applied to the Kretschmann configuration, we show that the excitation of propagating plasmons leads to a broader energy distribution of electrons due to the enhanced Landau damping.

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.

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.

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

Exciton recombination dynamics in CdSe nanowires: bimolecular to three-carrier Auger kinetics.

PubMed

Robel, István; Bunker, Bruce A; Kamat, Prashant V; Kuno, Masaru

2006-07-01

Ultrafast relaxation dynamics of charge carriers in CdSe quantum wires with diameters between 6 and 8 nm are studied as a function of carrier density. At high electron-hole pair densities above 10(19) cm(-3) the dominant process for carrier cooling is the "bimolecular" Auger recombination of one-dimensional (1D) excitons. However, below this excitation level an unexpected transition from a bimolecular (exciton-exciton) to a three-carrier Auger relaxation mechanism occurs. Thus, depending on excitation intensity, electron-hole pair relaxation dynamics in the nanowires exhibit either 1D or 0D (quantum dot) character. This dual nature of the recovery kinetics defines an optimal intensity for achieving optical gain in solution-grown nanowires given the different carrier-density-dependent scaling of relaxation rates in either regime.

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

Eads, Calley N.; Bandak, Dmytro; Neupane, Mahesh R.

Strong quantum confinement effects lead to striking new physics in two-dimensional materials such as graphene or transition metal dichalcogenides. While spectroscopic fingerprints of such quantum confinement have been demonstrated widely, the consequences for carrier dynamics are at present less clear, particularly on ultrafast timescales. This is important for tailoring, probing, and understanding spin and electron dynamics in layered and two-dimensional materials even in cases where the desired bandgap engineering has been achieved. Here in this paper we show by means of core–hole clock spectroscopy that SnS 2 exhibits spindependent attosecond charge delocalization times (τ deloc) for carriers confined within amore » layer, τ deloc < 400 as, whereas interlayer charge delocalization is dynamically quenched in excess of a factor of 10, τ deloc > 2.7 fs. These layer decoupling dynamics are a direct consequence of strongly anisotropic screening established within attoseconds, and demonstrate that important two-dimensional characteristics are also present in bulk crystals of van der Waalslayered materials, at least on ultrafast timescales.« less

Phonon-Assisted Ultrafast Charge Transfer at van der Waals Heterostructure Interface.

PubMed

Zheng, Qijing; Saidi, Wissam A; Xie, Yu; Lan, Zhenggang; Prezhdo, Oleg V; Petek, Hrvoje; Zhao, Jin

2017-10-11

The van der Waals (vdW) interfaces of two-dimensional (2D) semiconductor are central to new device concepts and emerging technologies in light-electricity transduction where the efficient charge separation is a key factor. Contrary to general expectation, efficient electron-hole separation can occur in vertically stacked transition-metal dichalcogenide heterostructure bilayers through ultrafast charge transfer between the neighboring layers despite their weak vdW bonding. In this report, we show by ab initio nonadiabatic molecular dynamics calculations, that instead of direct tunneling, the ultrafast interlayer hole transfer is strongly promoted by an adiabatic mechanism through phonon excitation occurring on 20 fs, which is in good agreement with the experiment. The atomic level picture of the phonon-assisted ultrafast mechanism revealed in our study is valuable both for the fundamental understanding of ultrafast charge carrier dynamics at vdW heterointerfaces as well as for the design of novel quasi-2D devices for optoelectronic and photovoltaic applications.

Charge separation and carrier dynamics in donor-acceptor heterojunction photovoltaic systems

PubMed Central

Teuscher, Joël; Brauer, Jan C.; Stepanov, Andrey; Solano, Alicia; Boziki, Ariadni; Chergui, Majed; Wolf, Jean-Pierre; Rothlisberger, Ursula; Banerji, Natalie; Moser, Jacques-E.

2017-01-01

Electron transfer and subsequent charge separation across donor-acceptor heterojunctions remain the most important areas of study in the field of third-generation photovoltaics. In this context, it is particularly important to unravel the dynamics of individual ultrafast processes (such as photoinduced electron transfer, carrier trapping and association, and energy transfer and relaxation), which prevail in materials and at their interfaces. In the frame of the National Center of Competence in Research “Molecular Ultrafast Science and Technology,” a research instrument of the Swiss National Science Foundation, several groups active in the field of ultrafast science in Switzerland have applied a number of complementary experimental techniques and computational simulation tools to scrutinize these critical photophysical phenomena. Structural, electronic, and transport properties of the materials and the detailed mechanisms of photoinduced charge separation in dye-sensitized solar cells, conjugated polymer- and small molecule-based organic photovoltaics, and high-efficiency lead halide perovskite solar energy converters have been scrutinized. Results yielded more than thirty research articles, an overview of which is provided here. PMID:29308415

Real-Space Imaging of Carrier Dynamics of Materials Surfaces by Second-Generation Four-Dimensional Scanning Ultrafast Electron Microscopy.

PubMed

Sun, Jingya; Melnikov, Vasily A; Khan, Jafar I; Mohammed, Omar F

2015-10-01

In the fields of photocatalysis and photovoltaics, ultrafast dynamical processes, including carrier trapping and recombination on material surfaces, are among the key factors that determine the overall energy conversion efficiency. A precise knowledge of these dynamical events on the nanometer (nm) and femtosecond (fs) scales was not accessible until recently. The only way to access such fundamental processes fully is to map the surface dynamics selectively in real space and time. In this study, we establish a second generation of four-dimensional scanning ultrafast electron microscopy (4D S-UEM) and demonstrate the ability to record time-resolved images (snapshots) of material surfaces with 650 fs and ∼5 nm temporal and spatial resolutions, respectively. In this method, the surface of a specimen is excited by a clocking optical pulse and imaged using a pulsed primary electron beam as a probe pulse, generating secondary electrons (SEs), which are emitted from the surface of the specimen in a manner that is sensitive to the local electron/hole density. This method provides direct and controllable information regarding surface dynamics. We clearly demonstrate how the surface morphology, grains, defects, and nanostructured features can significantly impact the overall dynamical processes on the surface of photoactive-materials. In addition, the ability to access two regimes of dynamical probing in a single experiment and the energy loss of SEs in semiconductor-nanoscale materials will also be discussed.

Ultrafast carrier dynamics in the large-magnetoresistance material WTe 2

DOE PAGES

Dai, Y. M.; Bowlan, J.; Li, H.; ...

2015-10-07

In this study, ultrafast optical pump-probe spectroscopy is used to track carrier dynamics in the large-magnetoresistance material WTe 2. Our experiments reveal a fast relaxation process occurring on a subpicosecond time scale that is caused by electron-phonon thermalization, allowing us to extract the electron-phonon coupling constant. An additional slower relaxation process, occurring on a time scale of ~5–15 ps, is attributed to phonon-assisted electron-hole recombination. As the temperature decreases from 300 K, the time scale governing this process increases due to the reduction of the phonon population. However, below ~50 K, an unusual decrease of the recombination time sets in,more » most likely due to a change in the electronic structure that has been linked to the large magnetoresistance observed in this material.« less

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

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

Ultrafast carrier dynamics in organic molecular crystals and conjugated polymers

NASA Astrophysics Data System (ADS)

Hegmann, Frank

2005-03-01

Organic semiconductors are being extensively studied by many research groups around the world for applications in electronic and photonic devices. For example, much work has focused on the development of organic thin film transistors based on thermally evaporated pentacene films, where the polycrystalline morphology typically results in a thermally-activated carrier mobility. On the other hand, more intrinsic bandlike transport, where the carrier mobility increases as the temperature decreases, has been observed in many organic single crystals. However, the nature of charge transport in organic molecular crystals is still not understood. Also, despite many advances in organic photonics, the nature of photocarrier generation in organic semiconductors is not completely understood and remains controversial even today. The generation of mobile charge carriers in photoexcited organic materials occurs over femtosecond to picosecond time scales, and so ultrafast pump-probe experiments are essential in order to improve our understanding of fundamental processes in these materials. Recently, time-resolved terahertz pulse spectroscopy has been used to directly probe transient photoconductivity in pentacene and functionalized pentacene thin films and single crystals [1,2], revealing photogeneration of mobile charge carriers over sub-picosecond time scales as well as bandlike carrier transport in both single crystal and thin film samples [1]. This talk will provide an overview of ultrafast carrier dynamics in organic semiconductors, and will emphasize how time-resolved terahertz pulse spectroscopy can be used to help understand the nature of photoexcitations and carrier transport in organic materials. (This work was supported by NSERC, CFI, CIPI, the Killam Trust, and ONR. Collaborators for this work are listed in Ref. 1.) [1] O. Ostroverkhova, D. G. Cooke, S. Shcherbyna, R. F. Egerton, F. A. Hegmann, R. R. Tykwinski, and J. E. Anthony, Phys. Rev. B., in press. [2] V. K. Thorsmølle, R. D. Averitt, X. Chi, D. J. Hilton, D. L. Smith, A. P. Ramirez, and A. J. Taylor, Appl. Phys. Lett. 84, 891 (2004).

Ultrafast Photodetection in the Quantum Wells of Single AlGaAs/GaAs-Based Nanowires.

PubMed

Erhard, N; Zenger, S; Morkötter, S; Rudolph, D; Weiss, M; Krenner, H J; Karl, H; Abstreiter, G; Finley, J J; Koblmüller, G; Holleitner, A W

2015-10-14

We investigate the ultrafast optoelectronic properties of single Al0.3Ga0.7As/GaAs core-shell nanowires. The nanowires contain GaAs-based quantum wells. For a resonant excitation of the quantum wells, we find a picosecond photocurrent which is consistent with an ultrafast lateral expansion of the photogenerated charge carriers. This Dember-effect does not occur for an excitation of the GaAs-based core of the nanowires. Instead, the core exhibits an ultrafast displacement current and a photothermoelectric current at the metal Schottky contacts. Our results uncover the optoelectronic dynamics in semiconductor core-shell nanowires comprising quantum wells, and they demonstrate the possibility to use the low-dimensional quantum well states therein for ultrafast photoswitches and photodetectors.

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.

Ultrafast lattice dynamics in lead selenide quantum dot induced by laser excitation

DOE PAGES

Wang, Xuan; Rahmani, Hamidreza; Zhou, Jun; ...

2016-10-10

We directly monitored the lattice dynamics in PbSe quantum dots induced by laser excitation using ultrafast electron di raction. The energy relaxation between the carriers and the lattice took place within 10 ps, showing no evidence of any signi cant phonon bottleneck e ect. Meanwhile, the lattice dilation exhibited some unusual features that could not be explained by the available mechanisms of photon- induced acoustic vibrations in semiconductors alone. The heat transport between the QDs and the substrate deviates signi cantly from Fourier's Law, which opens questions about the heat transfer under nonequilibrium conditions in nanoscale materials.

Ultrafast lattice dynamics in lead selenide quantum dot induced by laser excitation

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

Wang, Xuan; Rahmani, Hamidreza; Zhou, Jun

We directly monitored the lattice dynamics in PbSe quantum dots induced by laser excitation using ultrafast electron di raction. The energy relaxation between the carriers and the lattice took place within 10 ps, showing no evidence of any signi cant phonon bottleneck e ect. Meanwhile, the lattice dilation exhibited some unusual features that could not be explained by the available mechanisms of photon- induced acoustic vibrations in semiconductors alone. The heat transport between the QDs and the substrate deviates signi cantly from Fourier's Law, which opens questions about the heat transfer under nonequilibrium conditions in nanoscale materials.

Ultrafast photocurrents in monolayer MoS2

NASA Astrophysics Data System (ADS)

Parzinger, Eric; Wurstbauer, Ursula; Holleitner, Alexander W.

Two-dimensional transition metal dichalcogenides such as MoS2 have emerged as interesting materials for optoelectronic devices. In particular, the ultrafast dynamics and lifetimes of photoexcited charge carriers have attracted great interest during the last years. We investigate the photocurrent response of monolayer MoS2 on a picosecond time scale utilizing a recently developed pump-probe spectroscopy technique based on coplanar striplines. We discuss the ultrafast dynamics within MoS2 including photo-thermoelectric currents and the impact of built-in fields due to Schottky barriers as well as the Fermi level pinning at the contact region. We acknowledge support by the ERC via Project 'NanoREAL', the DFG via excellence cluster 'Nanosystems Initiative Munich' (NIM), and through the TUM International Graduate School of Science and Engineering (IGSSE) and BaCaTeC.

Ultrafast Spectroscopy of Mid-Infrared Semiconductors Using the Signal and Idler Beams of a Synchronous Optical Parametric Oscillator

DTIC Science & Technology

2008-03-01

then used to fit theoretical models describing radiative and non-radiative relaxation processes. 3.2 Experimental Setup This thesis uses a mode...Russian Efforts. Master’s thesis, Naval Postgraduate School, 2005. 5. Chirsto, Farid C. “Thermochemistry and Kinetics Models for MagnesiumTe- flon/Viton...Coherent Mira Model 900-F Laser. 7. Cooley, William T. Measurement of Ultrafast Carrier Recombination Dynamics in Mid-Infrared Semiconductor Laser Material

Exciton versus free carrier photogeneration in organometal trihalide perovskites probed by broadband ultrafast polarization memory dynamics.

PubMed

Sheng, ChuanXiang; Zhang, Chuang; Zhai, Yaxin; Mielczarek, Kamil; Wang, Weiwei; Ma, Wanli; Zakhidov, Anvar; Vardeny, Z Valy

2015-03-20

We studied the ultrafast transient response of photoexcitations in two hybrid organic-inorganic perovskite films used for high efficiency photovoltaic cells, namely, CH(3)NH(3)PbI(3) and CH(3)NH(3)PbI(1.1)Br(1.9) using polarized broadband pump-probe spectroscopy in the spectral range of 0.3-2.7 eV with 300 fs time resolution. For CH(3)NH(3)PbI(3) with above-gap excitation we found both photogenerated carriers and excitons, but only carriers are photogenerated with below-gap excitation. In contrast, mainly excitons are photogenerated in CH(3)NH(3)PbI(1.1)Br(1.9). Surprisingly, we also discovered in CH(3)NH(3)PbI(3), but not in CH(3)NH(3)PbI(1.1)Br(1.9), transient photoinduced polarization memory for both excitons and photocarriers, which is also reflected in the steady state photoluminescence. From the polarization memory dynamics we obtained the excitons diffusion constant in CH(3)NH(3)PbI(3), D≈0.01  cm(2) s(-1).

Exciton versus Free Carrier Photogeneration in Organometal Trihalide Perovskites Probed by Broadband Ultrafast Polarization Memory Dynamics

NASA Astrophysics Data System (ADS)

Sheng, ChuanXiang; Zhang, Chuang; Zhai, Yaxin; Mielczarek, Kamil; Wang, Weiwei; Ma, Wanli; Zakhidov, Anvar; Vardeny, Z. Valy

2015-03-01

We studied the ultrafast transient response of photoexcitations in two hybrid organic-inorganic perovskite films used for high efficiency photovoltaic cells, namely, CH3NH3PbI3 and CH3NH3PbI1.1Br1.9 using polarized broadband pump-probe spectroscopy in the spectral range of 0.3-2.7 eV with 300 fs time resolution. For CH3NH3PbI3 with above-gap excitation we found both photogenerated carriers and excitons, but only carriers are photogenerated with below-gap excitation. In contrast, mainly excitons are photogenerated in CH3NH3PbI1.1Br1.9 . Surprisingly, we also discovered in CH3NH3PbI3 , but not in CH3NH3PbI1.1Br1.9 , transient photoinduced polarization memory for both excitons and photocarriers, which is also reflected in the steady state photoluminescence. From the polarization memory dynamics we obtained the excitons diffusion constant in CH3NH3PbI3 , D ≈0.01 cm2 s-1 .

Gain and refractive index dynamics in p-doped InAs quantum dash semiconductor optical amplifiers

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

Komolibus, Katarzyna; Tyndall National Institute, University College Cork, Cork T12 R5CP; Piwonski, Tomasz, E-mail: tomasz.piwonski@tyndall.ie

The ultrafast carrier dynamics in a p-doped dash-in-a-well structure at 1.5 μm is experimentally investigated. An analysis of the timescales related to carrier relaxation and escape processes as well as the “dynamical” linewidth enhancement factor is presented and compared with results obtained from similar un-doped materials. Intentional p-doping of the active region results in an enhancement of the intermediate timescale of the gain dynamics associated with phonon-assisted electron capture and a reduction of the α-factor due to increased differential gain.

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.

Evolution of Excited-State Dynamics in Periodic Au 28, Au 36, Au 44, and Au 52 Nanoclusters

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

Zhou, Meng; Zeng, Chenjie; Sfeir, Matthew Y.

An understanding of the correlation between the atomic structure and optical properties of gold nanoclusters is essential for exploration of their functionalities and applications involving light harvesting and electron transfer. We report the femto-nanosecond excited state dynamics of a periodic series of face-centered cubic (FCC) gold nanoclusters (including Au 28, Au 36, Au 44, and Au 52), which exhibit a set of unique features compared with other similar sized clusters. Molecular-like ultrafast S n → S 1 internal conversions (i.e., radiationless electronic transitions) are observed in the relaxation dynamics of FCC periodic series. Excited-state dynamics with near-HOMO–LUMO gap excitation lacksmore » ultrafast decay component, and only the structural relaxation dominates in the dynamical process, which proves the absence of core–shell relaxation. Interestingly, both the relaxation of the hot carriers and the band-edge carrier recombination become slower as the size increases. The evolution in excited-state properties of this FCC series offers new insight into the structure-dependent properties of metal nanoclusters, which will benefit their optical energy harvesting and photocatalytic applications.« less

Evolution of Excited-State Dynamics in Periodic Au 28, Au 36, Au 44, and Au 52 Nanoclusters

DOE PAGES

Zhou, Meng; Zeng, Chenjie; Sfeir, Matthew Y.; ...

2017-08-10

An understanding of the correlation between the atomic structure and optical properties of gold nanoclusters is essential for exploration of their functionalities and applications involving light harvesting and electron transfer. We report the femto-nanosecond excited state dynamics of a periodic series of face-centered cubic (FCC) gold nanoclusters (including Au 28, Au 36, Au 44, and Au 52), which exhibit a set of unique features compared with other similar sized clusters. Molecular-like ultrafast S n → S 1 internal conversions (i.e., radiationless electronic transitions) are observed in the relaxation dynamics of FCC periodic series. Excited-state dynamics with near-HOMO–LUMO gap excitation lacksmore » ultrafast decay component, and only the structural relaxation dominates in the dynamical process, which proves the absence of core–shell relaxation. Interestingly, both the relaxation of the hot carriers and the band-edge carrier recombination become slower as the size increases. The evolution in excited-state properties of this FCC series offers new insight into the structure-dependent properties of metal nanoclusters, which will benefit their optical energy harvesting and photocatalytic applications.« less

Modeling of silicon in femtosecond laser-induced modification regimes: accounting for ambipolar diffusion

NASA Astrophysics Data System (ADS)

Derrien, Thibault J.-Y.; Bulgakova, Nadezhda M.

2017-05-01

During the last decades, femtosecond laser irradiation of materials has led to the emergence of various applications based on functionalization of surfaces at the nano- and microscale. Via inducing a periodic modification on material surfaces (band gap modification, nanostructure formation, crystallization or amorphization), optical and mechanical properties can be tailored, thus turning femtosecond laser to a key technology for development of nanophotonics, bionanoengineering, and nanomechanics. Although modification of semiconductor surfaces with femtosecond laser pulses has been studied for more than two decades, the dynamics of coupling of intense laser light with excited matter remains incompletely understood. In particular, swift formation of a transient overdense electron-hole plasma dynamically modifies optical properties in the material surface layer and induces large gradients of hot charge carriers, resulting in ultrafast charge-transport phenomena. In this work, the dynamics of ultrafast laser excitation of a semiconductor material is studied theoretically on the example of silicon. A special attention is paid to the electron-hole pair dynamics, taking into account ambipolar diffusion effects. The results are compared with previously developed simulation models, and a discussion of the role of charge-carrier dynamics in localization of material modification is provided.

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

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 fluxional exchange dynamics in electrolyte solvation sheath of lithium ion battery

PubMed Central

Lee, Kyung-Koo; Park, Kwanghee; Lee, Hochan; Noh, Yohan; Kossowska, Dorota; Kwak, Kyungwon; Cho, Minhaeng

2017-01-01

Lithium cation is the charge carrier in lithium-ion battery. Electrolyte solution in lithium-ion battery is usually based on mixed solvents consisting of polar carbonates with different aliphatic chains. Despite various experimental evidences indicating that lithium ion forms a rigid and stable solvation sheath through electrostatic interactions with polar carbonates, both the lithium solvation structure and more importantly fluctuation dynamics and functional role of carbonate solvent molecules have not been fully elucidated yet with femtosecond vibrational spectroscopic methods. Here we investigate the ultrafast carbonate solvent exchange dynamics around lithium ions in electrolyte solutions with coherent two-dimensional infrared spectroscopy and find that the time constants of the formation and dissociation of lithium-ion···carbonate complex in solvation sheaths are on a picosecond timescale. We anticipate that such ultrafast microscopic fluxional processes in lithium-solvent complexes could provide an important clue to understanding macroscopic mobility of lithium cation in lithium-ion battery on a molecular level. PMID:28272396

Ultrafast fluxional exchange dynamics in electrolyte solvation sheath of lithium ion battery

NASA Astrophysics Data System (ADS)

Lee, Kyung-Koo; Park, Kwanghee; Lee, Hochan; Noh, Yohan; Kossowska, Dorota; Kwak, Kyungwon; Cho, Minhaeng

2017-03-01

Lithium cation is the charge carrier in lithium-ion battery. Electrolyte solution in lithium-ion battery is usually based on mixed solvents consisting of polar carbonates with different aliphatic chains. Despite various experimental evidences indicating that lithium ion forms a rigid and stable solvation sheath through electrostatic interactions with polar carbonates, both the lithium solvation structure and more importantly fluctuation dynamics and functional role of carbonate solvent molecules have not been fully elucidated yet with femtosecond vibrational spectroscopic methods. Here we investigate the ultrafast carbonate solvent exchange dynamics around lithium ions in electrolyte solutions with coherent two-dimensional infrared spectroscopy and find that the time constants of the formation and dissociation of lithium-ion...carbonate complex in solvation sheaths are on a picosecond timescale. We anticipate that such ultrafast microscopic fluxional processes in lithium-solvent complexes could provide an important clue to understanding macroscopic mobility of lithium cation in lithium-ion battery on a molecular level.

Ultrafast fluxional exchange dynamics in electrolyte solvation sheath of lithium ion battery.

PubMed

Lee, Kyung-Koo; Park, Kwanghee; Lee, Hochan; Noh, Yohan; Kossowska, Dorota; Kwak, Kyungwon; Cho, Minhaeng

2017-03-08

Lithium cation is the charge carrier in lithium-ion battery. Electrolyte solution in lithium-ion battery is usually based on mixed solvents consisting of polar carbonates with different aliphatic chains. Despite various experimental evidences indicating that lithium ion forms a rigid and stable solvation sheath through electrostatic interactions with polar carbonates, both the lithium solvation structure and more importantly fluctuation dynamics and functional role of carbonate solvent molecules have not been fully elucidated yet with femtosecond vibrational spectroscopic methods. Here we investigate the ultrafast carbonate solvent exchange dynamics around lithium ions in electrolyte solutions with coherent two-dimensional infrared spectroscopy and find that the time constants of the formation and dissociation of lithium-ion···carbonate complex in solvation sheaths are on a picosecond timescale. We anticipate that such ultrafast microscopic fluxional processes in lithium-solvent complexes could provide an important clue to understanding macroscopic mobility of lithium cation in lithium-ion battery on a molecular level.

Ultrafast dynamics during the photoinduced phase transition in VO2

NASA Astrophysics Data System (ADS)

Wegkamp, Daniel; Stähler, Julia

2015-12-01

The phase transition of VO2 from a monoclinic insulator to a rutile metal, which occurs thermally at TC = 340 K, can also be driven by strong photoexcitation. The ultrafast dynamics during this photoinduced phase transition (PIPT) have attracted great scientific attention for decades, as this approach promises to answer the question of whether the insulator-to-metal (IMT) transition is caused by electronic or crystallographic processes through disentanglement of the different contributions in the time domain. We review our recent results achieved by femtosecond time-resolved photoelectron, optical, and coherent phonon spectroscopy and discuss them within the framework of a selection of latest, complementary studies of the ultrafast PIPT in VO2. We show that the population change of electrons and holes caused by photoexcitation launches a highly non-equilibrium plasma phase characterized by enhanced screening due to quasi-free carriers and followed by two branches of non-equilibrium dynamics: (i) an instantaneous (within the time resolution) collapse of the insulating gap that precedes charge carrier relaxation and significant ionic motion and (ii) an instantaneous lattice potential symmetry change that represents the onset of the crystallographic phase transition through ionic motion on longer timescales. We discuss the interconnection between these two non-thermal pathways with particular focus on the meaning of the critical fluence of the PIPT in different types of experiments. Based on this, we conclude that the PIPT threshold identified in optical experiments is most probably determined by the excitation density required to drive the lattice potential change rather than the IMT. These considerations suggest that the IMT can be driven by weaker excitation, predicting a transiently metallic, monoclinic state of VO2 that is not stabilized by the non-thermal structural transition and, thus, decays on ultrafast timescales.

Characterization of Nanostructured Semiconductors by Ultrafast Luminescence Imaging

NASA Astrophysics Data System (ADS)

Blake, Jolie

Single nanostructures are predicted to be the building blocks of next generation devices and have already been incorporated into prototypes for solar cells, biomedical devices and lasers. Their role in such applications requires a fundamental understanding of their opto-electronic properties and in particular the charge carrier dynamics occurring on an ultrafast timescale. Luminescence detection is a common approach used to investigate electronic properties of nanostructures because of the contact-less nature of these methods. They are, however, often not equipped to efficiently measure multiple single nanostructures nor do they have the temporal resolution necessary for observing femtosecond dynamics. This dissertation intends to address this paucity of techniques available for the contact-less measurement of single nanostructures through the development of an ultrafast wide-field Kerr-gated microscope system and measurement technique. The setup, operational in both the steady state and transient mode and capable of microscopic and spectroscopic measurements, was developed to measure the transient luminescence of single semiconductor nanostructures. With sub micron spatial resolution and the potential to achieve a temporal resolution greater than 90 fs, the system was used to probe the charge carrier dynamics at multiple discrete locations on single nanowires exhibiting amplified spontaneous emission. Using a rate model for amplified spontaneous emission, the transient emission data was fitted to extract the values of the competing Shockley-Read-Hall, non-geminate and Auger recombination constants. The capabilities of the setup were first demonstrated in the visible detection range, where single nanowires of the ternary alloy CdS x Se1-x were measured. The temporal emission dynamics at two separate locations were compared and calculation of the Langevin mobility revealed that the large carrier densities generated in the nanowire allows access to non-diffusion controlled recombination. In the second phase of this study the setup was configured to the ultraviolet detection range for measuring the nanowires of conductive metal oxides. ZnO was the metal oxide of focus in this research. Ultrafast measurements were conducted on ZnO nanowires and ASE dynamics from multiple regions along a nanowire were again fitted to the ASE model and the recombination constants extracted. The diminished influence of the Shockley-Read-Hall recombination rate on the measured luminescence suggested that leading quadratic term in the model is a measure of a two-body defect mediated recombination rate, from which a defect density could be calculated. The measured change in defect density along the length of the nanowire correlated with changes in the growth conditions that established a defect gradient. The results show that the Kerr-gated system, as well as being a probe of ultrafast dynamics, is also a new tool for measuring changes in defect density in single nanostructures.

Ultrafast non-radiative dynamics of atomically thin MoSe 2

DOE PAGES

Lin, Ming -Fu; Kochat, Vidya; Krishnamoorthy, Aravind; ...

2017-10-17

Non-radiative energy dissipation in photoexcited materials and resulting atomic dynamics provide a promising pathway to induce structural phase transitions in two-dimensional materials. However, these dynamics have not been explored in detail thus far because of incomplete understanding of interaction between the electronic and atomic degrees of freedom, and a lack of direct experimental methods to quantify real-time atomic motion and lattice temperature. Here, we explore the ultrafast conversion of photoenergy to lattice vibrations in a model bi-layered semiconductor, molybdenum diselenide, MoSe 2. Specifically, we characterize sub-picosecond lattice dynamics initiated by the optical excitation of electronic charge carriers in the highmore » electron-hole plasma density regime. Our results focuses on the first ten picosecond dynamics subsequent to photoexcitation before the onset of heat transfer to the substrate, which occurs on a ~100 picosecond time scale. Photoinduced atomic motion is probed by measuring the time dependent Bragg diffraction of a delayed mega-electronvolt femtosecond electron beam. Transient lattice temperatures are characterized through measurement of Bragg peak intensities and calculation of the Debye-Waller factor (DWF). These measurements show a sub-picosecond decay of Bragg diffraction and a correspondingly rapid rise in lattice temperatures. We estimate a high quantum yield for the conversion of excited charge carrier energy to lattice motion under our experimental conditions, indicative of a strong electron-phonon interaction. First principles nonadiabatic quantum molecular dynamics simulations (NAQMD) on electronically excited MoSe 2 bilayers reproduce the observed picosecond-scale increase in lattice temperature and ultrafast conversion of photoenergy to lattice vibrations. Calculation of excited-state phonon dispersion curves suggests that softened vibrational modes in the excited state are involved in efficient and rapid energy transfer between the electronic system and the lattice.« less

Ultrafast non-radiative dynamics of atomically thin MoSe 2

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

Lin, Ming -Fu; Kochat, Vidya; Krishnamoorthy, Aravind

Non-radiative energy dissipation in photoexcited materials and resulting atomic dynamics provide a promising pathway to induce structural phase transitions in two-dimensional materials. However, these dynamics have not been explored in detail thus far because of incomplete understanding of interaction between the electronic and atomic degrees of freedom, and a lack of direct experimental methods to quantify real-time atomic motion and lattice temperature. Here, we explore the ultrafast conversion of photoenergy to lattice vibrations in a model bi-layered semiconductor, molybdenum diselenide, MoSe 2. Specifically, we characterize sub-picosecond lattice dynamics initiated by the optical excitation of electronic charge carriers in the highmore » electron-hole plasma density regime. Our results focuses on the first ten picosecond dynamics subsequent to photoexcitation before the onset of heat transfer to the substrate, which occurs on a ~100 picosecond time scale. Photoinduced atomic motion is probed by measuring the time dependent Bragg diffraction of a delayed mega-electronvolt femtosecond electron beam. Transient lattice temperatures are characterized through measurement of Bragg peak intensities and calculation of the Debye-Waller factor (DWF). These measurements show a sub-picosecond decay of Bragg diffraction and a correspondingly rapid rise in lattice temperatures. We estimate a high quantum yield for the conversion of excited charge carrier energy to lattice motion under our experimental conditions, indicative of a strong electron-phonon interaction. First principles nonadiabatic quantum molecular dynamics simulations (NAQMD) on electronically excited MoSe 2 bilayers reproduce the observed picosecond-scale increase in lattice temperature and ultrafast conversion of photoenergy to lattice vibrations. Calculation of excited-state phonon dispersion curves suggests that softened vibrational modes in the excited state are involved in efficient and rapid energy transfer between the electronic system and the lattice.« less

Ultrafast momentum imaging of pseudospin-flip excitations in graphene

NASA Astrophysics Data System (ADS)

Aeschlimann, S.; Krause, R.; Chávez-Cervantes, M.; Bromberger, H.; Jago, R.; Malić, E.; Al-Temimy, A.; Coletti, C.; Cavalleri, A.; Gierz, I.

2017-07-01

The pseudospin of Dirac electrons in graphene manifests itself in a peculiar momentum anisotropy for photoexcited electron-hole pairs. These interband excitations are in fact forbidden along the direction of the light polarization and are maximum perpendicular to it. Here, we use time- and angle-resolved photoemission spectroscopy to investigate the resulting unconventional hot carrier dynamics, sampling carrier distributions as a function of energy, and in-plane momentum. We first show that the rapidly-established quasithermal electron distribution initially exhibits an azimuth-dependent temperature, consistent with relaxation through collinear electron-electron scattering. Azimuthal thermalization is found to occur only at longer time delays, at a rate that depends on the substrate and the static doping level. Further, we observe pronounced differences in the electron and hole dynamics in n -doped samples. By simulating the Coulomb- and phonon-mediated carrier dynamics we are able to disentangle the influence of excitation fluence, screening, and doping, and develop a microscopic picture of the carrier dynamics in photoexcited graphene. Our results clarify new aspects of hot carrier dynamics that are unique to Dirac materials, with relevance for photocontrol experiments and optoelectronic device applications.

Optically detecting the edge-state of a three-dimensional topological insulator under ambient conditions by ultrafast infrared photoluminescence spectroscopy

PubMed Central

Maezawa, Shun-ya; Watanabe, Hiroshi; Takeda, Masahiro; Kuroda, Kenta; Someya, Takashi; Matsuda, Iwao; Suemoto, Tohru

2015-01-01

Ultrafast infrared photoluminescence spectroscopy was applied to a three-dimensional topological insulator TlBiSe2 under ambient conditions. The dynamics of the luminescence exhibited bulk-insulating and gapless characteristics bounded by the bulk band gap energy. The existence of the topologically protected surface state and the picosecond-order relaxation time of the surface carriers, which was distinguishable from the bulk response, were observed. Our results provide a practical method applicable to topological insulators under ambient conditions for device applications. PMID:26552784

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.

Space-and-time resolved spectroscopy of single GaN nanowires

DOE PAGES

Upadhya, Prashanth C.; Martinez, Julio A.; Li, Qiming; ...

2015-06-30

Gallium nitridenanowires have garnered much attention in recent years due to their attractive optical and electrical properties. An understanding of carrier transport, relaxation, and recombination in these quasi-one-dimensional nanosystems is therefore important in optimizing them for various applications. We present ultrafast optical microscopic measurements on single GaNnanowires. Furthermore, our experiments, performed while varying the light polarization,excitation fluence, and position, give insight into the mechanisms governing carrier dynamics in these nanosystems.

Ultrafast carrier dynamics in bimetallic nanostructure-enhanced methylammonium lead bromide perovskites.

PubMed

Zarick, Holly F; Boulesbaa, Abdelaziz; Puretzky, Alexander A; Talbert, Eric M; DeBra, Zachary R; Soetan, Naiya; Geohegan, David B; Bardhan, Rizia

2017-01-26

In this work, we examine the impact of hybrid bimetallic Au/Ag core/shell nanostructures on the carrier dynamics of methylammonium lead tribromide (MAPbBr 3 ) mesoporous perovskite solar cells (PSCs). Plasmon-enhanced PSCs incorporated with Au/Ag nanostructures demonstrated improved light harvesting and increased power conversion efficiency by 26% relative to reference devices. Two complementary spectral techniques, transient absorption spectroscopy (TAS) and time-resolved photoluminescence (trPL), were employed to gain a mechanistic understanding of plasmonic enhancement processes. TAS revealed a decrease in the photobleach formation time, which suggests that the nanostructures improve hot carrier thermalization to an equilibrium distribution, relieving hot phonon bottleneck in MAPbBr 3 perovskites. TAS also showed a decrease in carrier decay lifetimes, indicating that nanostructures enhance photoinduced carrier generation and promote efficient electron injection into TiO 2 prior to bulk recombination. Furthermore, nanostructure-incorporated perovskite films demonstrated quenching in steady-state PL and decreases in trPL carrier lifetimes, providing further evidence of improved carrier injection in plasmon-enhanced mesoporous PSCs.

Low-power, ultrafast, and dynamic all-optical tunable plasmon induced transparency in two stub resonators side-coupled with a plasmonic waveguide system

NASA Astrophysics Data System (ADS)

Wang, Boyun; Zeng, Qingdong; Xiao, Shuyuan; Xu, Chen; Xiong, Liangbin; Lv, Hao; Du, Jun; Yu, Huaqing

2017-11-01

We theoretically and numerically investigate a low-power, ultrafast, and dynamic all-optical tunable plasmon induced transparency (PIT) in two stub resonators side-coupled with a metal-dielectric-metal (MDM) plasmonic waveguide system. The optical Kerr effect is enhanced by the local electromagnetic field of surface plasmon polaritons (SPPs) and the plasmonic waveguide based on graphene-Ag composite material structures with large effective Kerr nonlinear coefficient. An ultrafast response time of the order of 1 ps is reached because of ultrafast carrier relaxation dynamics of graphene. With dynamically tuning the propagation phase of the plasmonic waveguide, π-phase shift of the transmission spectrum in the PIT system is achieved under excitation of a pump light with an intensity as low as 5.8 MW cm-2. The group delay is controlled between 0.14 and 0.67 ps. Moreover, the tunable bandwidth of about 42 nm is obtained. For the indirect coupling between two stub cavities or the phase coupling scheme, the phase shift multiplication effect of the PIT effect is found. All observed schemes are analyzed rigorously through finite-difference time-domain simulations and coupled-mode formalism. This work not only paves the way towards the realization of on-chip integrated nanophotonic devices but also opens the possibility of the construction of ultrahigh-speed information processing chips based on plasmonic circuits.

A Superconducting Dual-Channel Photonic Switch.

PubMed

Srivastava, Yogesh Kumar; Manjappa, Manukumara; Cong, Longqing; Krishnamoorthy, Harish N S; Savinov, Vassili; Pitchappa, Prakash; Singh, Ranjan

2018-06-05

The mechanism of Cooper pair formation and its underlying physics has long occupied the investigation into high temperature (high-T c ) cuprate superconductors. One of the ways to unravel this is to observe the ultrafast response present in the charge carrier dynamics of a photoexcited specimen. This results in an interesting approach to exploit the dissipation-less dynamic features of superconductors to be utilized for designing high-performance active subwavelength photonic devices with extremely low-loss operation. Here, dual-channel, ultrafast, all-optical switching and modulation between the resistive and the superconducting quantum mechanical phase is experimentally demonstrated. The ultrafast phase switching is demonstrated via modulation of sharp Fano resonance of a high-T c yttrium barium copper oxide (YBCO) superconducting metamaterial device. Upon photoexcitation by femtosecond light pulses, the ultrasensitive cuprate superconductor undergoes dual dissociation-relaxation dynamics, with restoration of superconductivity within a cycle, and thereby establishes the existence of dual switching windows within a timescale of 80 ps. Pathways are explored to engineer the secondary dissociation channel which provides unprecedented control over the switching speed. Most importantly, the results envision new ways to accomplish low-loss, ultrafast, and ultrasensitive dual-channel switching applications that are inaccessible through conventional metallic and dielectric based metamaterials. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Quantum and semiclassical physics behind ultrafast optical nonlinearity in the midinfrared: the role of ionization dynamics within the field half cycle.

PubMed

Serebryannikov, E E; Zheltikov, A M

2014-07-25

Ultrafast ionization dynamics within the field half cycle is shown to be the key physical factor that controls the properties of optical nonlinearity as a function of the carrier wavelength and intensity of a driving laser field. The Schrödinger-equation analysis of a generic hydrogen quantum system reveals universal tendencies in the wavelength dependence of optical nonlinearity, shedding light on unusual properties of optical nonlinearities in the midinfrared. For high-intensity low-frequency fields, free-state electrons are shown to dominate over bound electrons in the overall nonlinear response of a quantum system. In this regime, semiclassical models are shown to offer useful insights into the physics behind optical nonlinearity.

Charge Generation Dynamics in Efficient All-Polymer Solar Cells: Influence of Polymer Packing and Morphology.

PubMed

Gautam, Bhoj R; Lee, Changyeon; Younts, Robert; Lee, Wonho; Danilov, Evgeny; Kim, Bumjoon J; Gundogdu, Kenan

2015-12-23

All-polymer solar cells exhibit rapid progress in power conversion efficiency (PCE) from 2 to 7.7% over the past few years. While this improvement is primarily attributed to efficient charge transport and balanced mobility between the carriers, not much is known about the charge generation dynamics in these systems. Here we measured exciton relaxation and charge separation dynamics using ultrafast spectroscopy in polymer/polymer blends with different molecular packing and morphology. These measurements indicate that preferential face-on configuration with intermixed nanomorphology increases the charge generation efficiency. In fact, there is a direct quantitative correlation between the free charge population in the ultrafast time scales and the external quantum efficiency, suggesting not only the transport but also charge generation is key for the design of high performance all polymer solar cells.

PREFACE: Ultrafast and nonlinear optics in carbon nanomaterials

NASA Astrophysics Data System (ADS)

Kono, Junichiro

2013-02-01

Carbon-based nanomaterials—single-wall carbon nanotubes (SWCNTs) and graphene, in particular—have emerged in the last decade as novel low-dimensional systems with extraordinary properties. Because they are direct-bandgap systems, SWCNTs are one of the leading candidates to unify electronic and optical functions in nanoscale circuitry; their diameter-dependent bandgaps can be utilized for multi-wavelength devices. Graphene's ultrahigh carrier mobilities are promising for high-frequency electronic devices, while, at the same time, it is predicted to have ideal properties for terahertz generation and detection due to its unique zero-gap, zero-mass band structure. There have been a large number of basic optical studies on these materials, but most of them were performed in the weak-excitation, quasi-equilibrium regime. In order to probe and assess their performance characteristics as optoelectronic materials under device-operating conditions, it is crucial to strongly drive them and examine their optical properties in highly non-equilibrium situations and with ultrashot time resolution. In this section, the reader will find the latest results in this rapidly growing field of research. We have assembled contributions from some of the leading experts in ultrafast and nonlinear optical spectroscopy of carbon-based nanomaterials. Specific topics featured include: thermalization, cooling, and recombination dynamics of photo-generated carriers; stimulated emission, gain, and amplification; ultrafast photoluminescence; coherent phonon dynamics; exciton-phonon and exciton-plasmon interactions; exciton-exciton annihilation and Auger processes; spontaneous and stimulated emission of terahertz radiation; four-wave mixing and harmonic generation; ultrafast photocurrents; the AC Stark and Franz-Keldysh effects; and non-perturbative light-mater coupling. We would like to express our sincere thanks to those who contributed their latest results to this special section, and the Journal of Physics: Condensed Matter staff for their help, patience and professionalism. Since this is a fast-moving field, there is absolutely no way of presenting definitive answers to all open questions, but we hope that this special section will provide an overview of the current state of knowledge regarding this topic. Furthermore, we hope that the exciting science and technology described in this section will attract and inspire other researchers and students working in related fields to enter into the study of ultrafast and nonlinear optical phenomena in carbon-based nanostructures. Ultrafast and nonlinear optics in carbon nanomaterials contents Ultrafast and nonlinear optics in carbon nanomaterialsJunichiro Kono The impact of pump fluence on carrier relaxation dynamics in optically excited grapheneT Winzer and E Malic Time-resolved spectroscopy on epitaxial graphene in the infrared spectral range: relaxation dynamics and saturation behaviorS Winnerl, F Göttfert, M Mittendorff, H Schneider, M Helm, T Winzer, E Malic, A Knorr, M Orlita, M Potemski, M Sprinkle, C Berger and W A de Heer Nonlinear optics of graphene in a strong magnetic fieldXianghan Yao and Alexey Belyanin Theory of coherent phonons in carbon nanotubes and graphene nanoribbonsG D Sanders, A R T Nugraha, K Sato, J-H Kim3, J Kono3, R Saito and C J Stanton Non-perturbative effects of laser illumination on the electrical properties of graphene nanoribbons Hernán L Calvo, Pablo M Perez-Piskunow, Horacio M Pastawski, Stephan Roche and Luis E F Foa Torres Transient absorption microscopy studies of energy relaxation in graphene oxide thin film Sean Murphy and Libai Huang Femtosecond dynamics of exciton localization: self-trapping from the small to the large polaron limit F X Morrissey, J G Mance, A D Van Pelt and S L Dexheimer

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.

Ultrafast exciton dynamics and light-driven H2 evolution in colloidal semiconductor nanorods and Pt-tipped nanorods.

PubMed

Wu, Kaifeng; Zhu, Haiming; Lian, Tianquan

2015-03-17

Colloidal quantum confined one-dimensional (1D) semiconductor nanorods (NRs) and related semiconductor-metal heterostructures are promising new materials for efficient solar-to-fuel conversion because of their unique physical and chemical properties. NRs can simultaneously exhibit quantum confinement effects in the radial direction and bulk like carrier transport in the axial direction. The former implies that concepts well-established in zero-dimensional quantum dots, such as size-tunable energetics and wave function engineering through band alignment in heterostructures, can also be applied to NRs; while the latter endows NRs with fast carrier transport to achieve long distance charge separation. Selective growth of catalytic metallic nanoparticles, such as Pt, at the tips of NRs provides convenient routes to multicomponent heterostructures with photocatalytic capabilities and controllable charge separation distances. The design and optimization of such materials for efficient solar-to-fuel conversion require the understanding of exciton and charge carrier dynamics. In this Account, we summarize our recent studies of ultrafast charge separation and recombination kinetics and their effects on steady-state photocatalytic efficiencies of colloidal CdS and CdSe/CdS NRs and related NR-Pt heterostructures. After a brief introduction of their electronic structure, we discuss exciton dynamics of CdS NRs. By transient absorption and time-resolved photoluminescence decay, it is shown that although the conduction band electrons are long-lived, photogenerated holes in CdS NRs are trapped on an ultrafast time scale (∼0.7 ps), which forms localized excitons due to strong Coulomb interaction in 1D NRs. In quasi-type II CdSe/CdS dot-in-rod NRs, a large valence band offset drives the ultrafast localization of holes to the CdSe core, and the competition between this process and ultrafast hole trapping on a CdS rod leads to three types of exciton species with distinct spatial distributions. The effect of the exciton dynamics on photoreduction reactions is illustrated using methyl viologen (MV(2+)) as a model electron acceptor. The steady-state MV(2+) photoreduction quantum yield of CdSe/CdS dot-in-rod NRs approaches unity under rod excitation, much larger than CdSe QDs and CdSe/CdS core/shell QDs. Detailed time-resolved studies show that in quasi-type II CdSe/CdS NRs and type II ZnSe/CdS NRs strong quantum confinement in the radial direction facilitates fast electron transfer and hole removal, whereas the fast carrier mobility along the axial direction enables long distance charge separation and slow charge recombination, which is essential for efficient MV(2+) photoreduction. The NR/MV(2+) relay system can be coupled to Pt nanoparticles in solution for light-driven H2 generation. Alternatively, Pt-tipped CdS and CdSe/CdS NRs provide fully integrated all inorganic systems for light-driven H2 generation. In CdS-Pt and CdSe/CdS-Pt hetero-NRs, ultrafast hole trapping on the CdS rod surface or in CdSe core enables efficient electron transfer from NRs to Pt tips by suppressing hole and energy transfer. It is shown that the quantum yields of photodriven H2 generation using these heterostructures correlate well with measured hole transfer rates from NRs to sacrificial donors, revealing that hole removal is the key efficiency-limiting step. These findings provide important insights for designing more efficient quantum confined NR and NR-Pt based systems for solar-to-fuel conversion.

Ultrafast electron dynamics reveal the high potential of InSe for hot-carrier optoelectronics

NASA Astrophysics Data System (ADS)

Chen, Zhesheng; Giorgetti, Christine; Sjakste, Jelena; Cabouat, Raphael; Véniard, Valérie; Zhang, Zailan; Taleb-Ibrahimi, Amina; Papalazarou, Evangelos; Marsi, Marino; Shukla, Abhay; Peretti, Jacques; Perfetti, Luca

2018-06-01

We monitor the dynamics of hot carriers in InSe by means of two-photon photoelectron spectroscopy (2PPE). The electrons excited by photons of 3.12 eV experience a manifold relaxation. First, they thermalize to electronic states degenerate with the M ¯ valley. Subsequently, the electronic cooling is dictated by Fröhlich coupling with phonons of small momentum transfer. Ab initio calculations predict cooling rates that are in good agreement with the observed dynamics. We argue that electrons accumulating in states degenerate with the M ¯ valley could travel through a multilayer flake of InSe with a lateral size of 1 μ m . The hot carriers pave a viable route to the realization of below-band-gap photodiodes and Gunn oscillators. Our results indicate that these technologies may find a natural implementation in future devices based on layered chalcogenides.

Ultrafast carrier dynamics and third-order nonlinear optical properties of AgInS2/ZnS nanocrystals

NASA Astrophysics Data System (ADS)

Yu, Kuai; Yang, Yang; Wang, Junzhong; Tang, Xiaosheng; Xu, Qing-Hua; Wang, Guo Ping

2018-06-01

Broad photoluminescence (PL) emission, a large Stokes shift and extremely long-lived radiative lifetimes are the characteristics of ternary I–III–VI semiconductor nanocrystals (NCs), such as CuInS2 and AgInS2. However, the lack of understanding regarding the intriguing PL mechanisms and photo-carrier dynamics limits their further applications. Here, AgInS2 and AgInS2/ZnS NCs were chemically synthesized and their carrier dynamics were studied by time-resolved PL spectroscopy. The results demonstrated that the surface defect state, which contributed dominantly to the non-radiative decay processes, was effectively passivated through ZnS alloying. Femtosecond transient absorption spectroscopy was also used to investigate the carrier dynamics, revealing the electron storage at the surface state and donor state. Furthermore, the two photon absorption properties of AgInS2 and AgInS2/ZnS NCs were measured using an open-aperture Z-scan technique. The improved third-order nonlinear susceptibility {χ }(3) of AgInS2 through ZnS alloying demonstrates potential application in two photon PL biological imaging.

Ultrafast dynamics of quasiparticles and coherent acoustic phonons in slightly underdoped (BaK)Fe2As2

PubMed Central

Lin, Kung-Hsuan; Wang, Kuan-Jen; Chang, Chung-Chieh; Wen, Yu-Chieh; Lv, Bing; Chu, Ching-Wu; Wu, Maw-Kuen

2016-01-01

We have utilized ultrafast optical spectroscopy to study carrier dynamics in slightly underdoped (BaK)Fe2As2 crystals without magnetic transition. The photoelastic signals due to coherent acoustic phonons have been quantitatively investigated. According to our temperature-dependent results, we found that the relaxation component of superconducting quasiparticles persisted from the superconducting state up to at least 70 K in the normal state. Our findings suggest that the pseudogaplike feature in the normal state is possibly the precursor of superconductivity. We also highlight that the pseudogap feature of K-doped BaFe2As2 is different from that of other iron-based superconductors, including Co-doped or P-doped BaFe2As2. PMID:27180873

Ultrafast Graphene Light Emitters.

PubMed

Kim, Young Duck; Gao, Yuanda; Shiue, Ren-Jye; Wang, Lei; Aslan, Ozgur Burak; Bae, Myung-Ho; Kim, Hyungsik; Seo, Dongjea; Choi, Heon-Jin; Kim, Suk Hyun; Nemilentsau, Andrei; Low, Tony; Tan, Cheng; Efetov, Dmitri K; Taniguchi, Takashi; Watanabe, Kenji; Shepard, Kenneth L; Heinz, Tony F; Englund, Dirk; Hone, James

2018-02-14

Ultrafast electrically driven nanoscale light sources are critical components in nanophotonics. Compound semiconductor-based light sources for the nanophotonic platforms have been extensively investigated over the past decades. However, monolithic ultrafast light sources with a small footprint remain a challenge. Here, we demonstrate electrically driven ultrafast graphene light emitters that achieve light pulse generation with up to 10 GHz bandwidth across a broad spectral range from the visible to the near-infrared. The fast response results from ultrafast charge-carrier dynamics in graphene and weak electron-acoustic phonon-mediated coupling between the electronic and lattice degrees of freedom. We also find that encapsulating graphene with hexagonal boron nitride (hBN) layers strongly modifies the emission spectrum by changing the local optical density of states, thus providing up to 460% enhancement compared to the gray-body thermal radiation for a broad peak centered at 720 nm. Furthermore, the hBN encapsulation layers permit stable and bright visible thermal radiation with electronic temperatures up to 2000 K under ambient conditions as well as efficient ultrafast electronic cooling via near-field coupling to hybrid polaritonic modes under electrical excitation. These high-speed graphene light emitters provide a promising path for on-chip light sources for optical communications and other optoelectronic applications.

Ultrafast carrier dynamics in bimetallic nanostructure-enhanced methylammonium lead bromide perovskites

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

Zarick, Holly; Boulesbaa, Abdelaziz; Puretzky, Alexander A

In this paper, we examine the impact of hybrid bimetallic Au/Ag core/shell nanostructures on the carrier dynamics of methylammonium lead tribromide (MAPbBr 3) mesoporous perovskite solar cells (PSCs). Plasmon-enhanced PSCs incorporated with Au/Ag nanostructures demonstrated improved light harvesting and increased power conversion efficiency by 26% relative to reference devices. Two complementary spectral techniques, transient absorption spectroscopy (TAS) and time-resolved photoluminescence (trPL), were employed to gain a mechanistic understanding of plasmonic enhancement processes. TAS revealed a decrease in the photobleach formation time, which suggests that the nanostructures improve hot carrier thermalization to an equilibrium distribution, relieving hot phonon bottleneck in MAPbBr3more » perovskites. TAS also showed a decrease in carrier decay lifetimes, indicating that nanostructures enhance photoinduced carrier generation and promote efficient electron injection into TiO 2 prior to bulk recombination. Furthermore, nanostructure-incorporated perovskite films demonstrated quenching in steady-state PL and decreases in trPL carrier lifetimes, providing further evidence of improved carrier injection in plasmon-enhanced mesoporous PSCs.« less

Ultrafast carrier dynamics in bimetallic nanostructure-enhanced methylammonium lead bromide perovskites

DOE PAGES

Zarick, Holly; Boulesbaa, Abdelaziz; Puretzky, Alexander A; ...

2016-12-14

In this paper, we examine the impact of hybrid bimetallic Au/Ag core/shell nanostructures on the carrier dynamics of methylammonium lead tribromide (MAPbBr 3) mesoporous perovskite solar cells (PSCs). Plasmon-enhanced PSCs incorporated with Au/Ag nanostructures demonstrated improved light harvesting and increased power conversion efficiency by 26% relative to reference devices. Two complementary spectral techniques, transient absorption spectroscopy (TAS) and time-resolved photoluminescence (trPL), were employed to gain a mechanistic understanding of plasmonic enhancement processes. TAS revealed a decrease in the photobleach formation time, which suggests that the nanostructures improve hot carrier thermalization to an equilibrium distribution, relieving hot phonon bottleneck in MAPbBr3more » perovskites. TAS also showed a decrease in carrier decay lifetimes, indicating that nanostructures enhance photoinduced carrier generation and promote efficient electron injection into TiO 2 prior to bulk recombination. Furthermore, nanostructure-incorporated perovskite films demonstrated quenching in steady-state PL and decreases in trPL carrier lifetimes, providing further evidence of improved carrier injection in plasmon-enhanced mesoporous PSCs.« less

Ultrafast dynamics of Al-doped zinc oxide under optical excitation (Presentation Recording)

NASA Astrophysics Data System (ADS)

Kinsey, Nathaniel; DeVault, Clayton T.; Kim, Jongbum; Ferrera, Marcello; Kildishev, Alexander V.; Shalaev, Vladimir M.; Boltasseva, Alexandra

2015-09-01

There is a continual need to explore new and promising dynamic materials to power next-generation switchable devices. In recent years, transparent conducting oxides have been shown to be vital materials for such systems, allowing for both optical and electrical tunability. Using a pump-probe technique, we investigate the optical tunability of CMOS-compatible, highly aluminum doped zinc oxide (AZO) thin films. The sample was pumped at 325 nm and probed with a weak beam at 1.3 μm to determine the timescale and magnitude of the changes by altering the temporal delay between the pulses with a delay line. For an incident fluence of 3.9 mJ/cm2 a change of 40% in reflection and 30% (max 6.3dB/μm modulation depth) in transmission is observed which is fully recovered within 1ps. Using a computational model, the experimental results were fitted for the given fluence allowing the recombination time and induced carrier density to be extracted. For a fluence of 3.9 mJ/cm2 the average excess carrier density within the material is 0.7×10^20cm-3 and the recombination time is 88fs. The ultrafast temporal response is the result of Auger recombination due to the extremely high carrier concentration present in our films, ~10^21 cm-3. By measuring and fitting the results at several incident fluence levels, the recombination time versus carrier density was determined and fitted with an Auger model resulting in an Auger coefficient of C = 1.03×10^20cm6/sec. Consequently, AZO is shown to be a unique, promising, and CMOS-compatible material for high performance dynamic devices in the near future.

Charge carrier transfer in tungsten disulfide—black phosphorus heterostructures

NASA Astrophysics Data System (ADS)

Zhao, Siqi; He, Dawei; Wang, Yongsheng; Zhang, Xinwu; He, Jiaqi

2017-11-01

Photocarrier dynamics in tungsten disulfide—black phosphorus (BP) heterostructures were studied by time-resolved differential reflection measurements. The heterostructures were fabricated by stacking together monolayer WS2 and BP flakes that are both fabricated by mechanical exfoliation. Efficient and ultrafast transfer of photocarriers from WS2 to BP flakes was observed. This confirms the type-I band alignment of WS2/BP heterostructures that was predicted by theory. Accompanied with the photocarrier interlayer transfer process from WS2 to BP flakes, the change of the absorption of WS2 persists for several nanoseconds. These results promote the consciousness about the carrier dynamics of interlayer transfer process in van der Waals heterostructures and its application in optoelectronic devices.

Screening in crystalline liquids protects energetic carriers in hybrid perovskites

NASA Astrophysics Data System (ADS)

Zhu, Haiming; Miyata, Kiyoshi; Fu, Yongping; Wang, Jue; Joshi, Prakriti; Niesner, Daniel; Williams, Kristopher; Jin, Song; Zhu, Xiaoyang

Hybrid lead halide perovskites exhibit carrier properties that resemble those of pristine nonpolar semiconductors despite static and dynamic disorder, but how carriers are protected from efficient scattering with charged defects and optical phonons is unknown. Here, we reveal the carrier protection mechanism by comparing three single-crystal lead bromide perovskites: CH3NH3PbBr3, CH(NH2)2PbBr3, and CsPbBr3. We observed hot fluorescence emission from energetic carriers with 102 picosecond lifetimes in CH3NH3PbBr3 or CH(NH,SUB>2)2PbBr3, but not in CsPbBr3. The hot fluorescence is correlated with liquid-like molecular reorientational motions, suggesting that dynamic screening protects energetic carriers via solvation or large polaron formation on time scales competitive with that of ultrafast cooling. Similar protections likely exist for band-edge carriers. The long-lived energetic carriers may enable hot-carrier solar cells with efficiencies exceeding the Shockley-Queisser limit. This work was supported by U.S. Department of Energy Grant ER46980, National Science Foundation, Grant DMR 1420634 (MRSEC), and Department of Energy Award DE-FG02-09ER46664.

Accelerated carrier recombination by grain boundary/edge defects in MBE grown transition metal dichalcogenides

NASA Astrophysics Data System (ADS)

Chen, Ke; Roy, Anupam; Rai, Amritesh; Movva, Hema C. P.; Meng, Xianghai; He, Feng; Banerjee, Sanjay K.; Wang, Yaguo

2018-05-01

Defect-carrier interaction in transition metal dichalcogenides (TMDs) plays important roles in carrier relaxation dynamics and carrier transport, which determines the performance of electronic devices. With femtosecond laser time-resolved spectroscopy, we investigated the effect of grain boundary/edge defects on the ultrafast dynamics of photoexcited carrier in molecular beam epitaxy (MBE)-grown MoTe2 and MoSe2. We found that, comparing with exfoliated samples, the carrier recombination rate in MBE-grown samples accelerates by about 50 times. We attribute this striking difference to the existence of abundant grain boundary/edge defects in MBE-grown samples, which can serve as effective recombination centers for the photoexcited carriers. We also observed coherent acoustic phonons in both exfoliated and MBE-grown MoTe2, indicating strong electron-phonon coupling in this materials. Our measured sound velocity agrees well with the previously reported result of theoretical calculation. Our findings provide a useful reference for the fundamental parameters: carrier lifetime and sound velocity and reveal the undiscovered carrier recombination effect of grain boundary/edge defects, both of which will facilitate the defect engineering in TMD materials for high speed opto-electronics.

Imaging surface acoustic wave dynamics in semiconducting polymers by scanning ultrafast electron microscopy.

PubMed

Najafi, Ebrahim; Liao, Bolin; Scarborough, Timothy; Zewail, Ahmed

2018-01-01

Understanding the mechanical properties of organic semiconductors is essential to their electronic and photovoltaic applications. Despite a large volume of research directed toward elucidating the chemical, physical and electronic properties of these materials, little attention has been directed toward understanding their thermo-mechanical behavior. Here, we report the ultrafast imaging of surface acoustic waves (SAWs) on the surface of the Poly(3-hexylthiophene-2,5-diyl) (P3HT) thin film at the picosecond and nanosecond timescales. We then use these images to measure the propagation velocity of SAWs, which we then employ to determine the Young's modulus of P3HT. We further validate our experimental observation by performing a semi-empirical transient thermoelastic finite element analysis. Our findings demonstrate the potential of ultrafast electron microscopy to not only probe charge carrier dynamics in materials as previously reported, but also to measure their mechanical properties with great accuracy. This is particularly important when in situ characterization of stiffness for thin devices and nanomaterials is required. Copyright © 2017 Elsevier B.V. All rights reserved.

Development of Scanning Ultrafast Electron Microscope Capability.

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

Collins, Kimberlee Chiyoko; Talin, Albert Alec; Chandler, David W.

Modern semiconductor devices rely on the transport of minority charge carriers. Direct examination of minority carrier lifetimes in real devices with nanometer-scale features requires a measurement method with simultaneously high spatial and temporal resolutions. Achieving nanometer spatial resolutions at sub-nanosecond temporal resolution is possible with pump-probe methods that utilize electrons as probes. Recently, a stroboscopic scanning electron microscope was developed at Caltech, and used to study carrier transport across a Si p-n junction [ 1 , 2 , 3 ] . In this report, we detail our development of a prototype scanning ultrafast electron microscope system at Sandia National Laboratoriesmore » based on the original Caltech design. This effort represents Sandia's first exploration into ultrafast electron microscopy.« less

Ultrafast carrier dynamics and third-order nonlinear optical properties of AgInS2/ZnS nanocrystals.

PubMed

Yu, Kuai; Yang, Yang; Wang, Junzhong; Tang, Xiaosheng; Xu, Qing-Hua; Wang, Guo Ping

2018-06-22

Broad photoluminescence (PL) emission, a large Stokes shift and extremely long-lived radiative lifetimes are the characteristics of ternary I-III-VI semiconductor nanocrystals (NCs), such as CuInS 2 and AgInS 2 . However, the lack of understanding regarding the intriguing PL mechanisms and photo-carrier dynamics limits their further applications. Here, AgInS 2 and AgInS 2 /ZnS NCs were chemically synthesized and their carrier dynamics were studied by time-resolved PL spectroscopy. The results demonstrated that the surface defect state, which contributed dominantly to the non-radiative decay processes, was effectively passivated through ZnS alloying. Femtosecond transient absorption spectroscopy was also used to investigate the carrier dynamics, revealing the electron storage at the surface state and donor state. Furthermore, the two photon absorption properties of AgInS 2 and AgInS 2 /ZnS NCs were measured using an open-aperture Z-scan technique. The improved third-order nonlinear susceptibility [Formula: see text] of AgInS 2 through ZnS alloying demonstrates potential application in two photon PL biological imaging.

Ultrafast gating of a mid-infrared laser pulse by a sub-pC relativistic electron beam

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

Cesar, D. B.; Musumeci, P.; Alesini, D.

In this paper we discuss a relative time-of-arrival measurement scheme between an electron beam and a mid-infrared laser pulse based on the electron-beam controlled transmission in semiconductor materials. This technique can be used as a time-stamping diagnostic in ultrafast electron diffraction or microscopy. In particular, our characterization of Germanium demonstrates that sub-ps time-of-arrival sensitivity could be achieved in a single shot and with very low charge beams (<1 pC). Detailed measurements as a function of the beam charge and the laser wavelength offer insights on the free carrier dynamics in the semiconductor upon excitation by the electron beam.

Ultrafast terahertz snapshots of excitonic Rydberg states and electronic coherence in an organometal halide perovskite

DOE PAGES

Luo, Liang; Men, Long; Liu, Zhaoyu; ...

2017-06-01

How photoexcitations evolve into Coulomb-bound electron and hole pairs, called excitons, and unbound charge carriers is a key cross-cutting issue in photovoltaics and optoelectronics. Until now, the initial quantum dynamics following photoexcitation remains elusive in the hybrid perovskite system. Furthermore we reveal excitonic Rydberg states with distinct formation pathways by observing the multiple resonant, internal quantum transitions using ultrafast terahertz quasi-particle transport. Nonequilibrium emergent states evolve with a complex co-existence of excitons, carriers and phonons, where a delayed buildup of excitons under on- and off-resonant pumping conditions allows us to distinguish between the loss of electronic coherence and hot statemore » cooling processes. The nearly ~1 ps dephasing time, efficient electron scattering with discrete terahertz phonons and intermediate binding energy of ~13.5 meV in perovskites are distinct from conventional photovoltaic semiconductors. In addition to providing implications for coherent energy conversion, these are potentially relevant to the development of light-harvesting and electron-transport devices.« less

Evidence for carrier localization in the pseudogap state of cuprate superconductors from coherent quench experiments

PubMed Central

Madan, I.; Kurosawa, T.; Toda, Y.; Oda, M.; Mertelj, T.; Mihailovic, D.

2015-01-01

A ‘pseudogap' was introduced by Mott to describe a state of matter that has a minimum in the density of states at the Fermi level, deep enough for states to become localized. It can arise either from Coulomb repulsion between electrons, and/or incipient charge or spin order. Here we employ ultrafast spectroscopy to study dynamical properties of the normal to pseudogap state transition in the prototype high-temperature superconductor Bi2Sr2CaCu2O8+δ. We perform a systematic temperature and doping dependence study of the pseudogap photodestruction and recovery in coherent quench experiments, revealing marked absence of critical behaviour of the elementary excitations, which implies an absence of collective electronic ordering beyond a few coherence lengths on short timescales. The data imply ultrafast carrier localization into a textured polaronic state arising from a competing Coulomb interaction and lattice strain, enhanced by a Fermi surface instability. PMID:25891310

Ultrafast terahertz snapshots of excitonic Rydberg states and electronic coherence in an organometal halide perovskite

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

Luo, Liang; Men, Long; Liu, Zhaoyu

How photoexcitations evolve into Coulomb-bound electron and hole pairs, called excitons, and unbound charge carriers is a key cross-cutting issue in photovoltaics and optoelectronics. Until now, the initial quantum dynamics following photoexcitation remains elusive in the hybrid perovskite system. Furthermore we reveal excitonic Rydberg states with distinct formation pathways by observing the multiple resonant, internal quantum transitions using ultrafast terahertz quasi-particle transport. Nonequilibrium emergent states evolve with a complex co-existence of excitons, carriers and phonons, where a delayed buildup of excitons under on- and off-resonant pumping conditions allows us to distinguish between the loss of electronic coherence and hot statemore » cooling processes. The nearly ~1 ps dephasing time, efficient electron scattering with discrete terahertz phonons and intermediate binding energy of ~13.5 meV in perovskites are distinct from conventional photovoltaic semiconductors. In addition to providing implications for coherent energy conversion, these are potentially relevant to the development of light-harvesting and electron-transport devices.« less

Nanowires: Enhanced Optoelectronic Performance of a Passivated Nanowire-Based Device: Key Information from Real-Space Imaging Using 4D Electron Microscopy (Small 17/2016).

PubMed

Khan, Jafar I; Adhikari, Aniruddha; Sun, Jingya; Priante, Davide; Bose, Riya; Shaheen, Basamat S; Ng, Tien Khee; Zhao, Chao; Bakr, Osman M; Ooi, Boon S; Mohammed, Omar F

2016-05-01

Selective mapping of surface charge carrier dynamics of InGaN nanowires before and after surface passivation with octadecylthiol (ODT) is reported by O. F. Mohammed and co-workers on page 2313, using scanning ultrafast electron microscopy. In a typical experiment, the 343 nm output of the laser beam is used to excite the microscope tip to generate pulsed electrons for probing, and the 515 nm output is used as a clocking excitation pulse to initiate dynamics. Time-resolved images demonstrate clearly that carrier recombination is significantly slowed after ODT treatment, which supports the efficient removal of surface trap states. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A novel grating-imaging method to measure carrier diffusion coefficient in graphene

NASA Astrophysics Data System (ADS)

Chen, Ke; Wang, Yaguo; Akinwande, Deji; Bank, Seth; Lin, Jung-Fu

Similar to carrier mobility, carrier diffusion coefficient in graphene determines the response rate of future graphene-based electronics. Here we present a simple, sensitive and non-destructive technique integrated with ultrafast pump-probe spectroscopy to measure carrier diffusion in CVD-grown graphene. In the method, the pump and the probe beams pass through the same area of a photomask with metal strips i.e. a transmission amplitude grating, and get diffracted. The diffracted light is collected by an objective lens and focused onto the sample to generate carrier density grating. Relaxation of this carrier density grating is governed by both carrier recombination and carrier diffusion in the sample. Transient transmission change of the probe beams, which reflects this relaxation process, is recorded. The measured diffusion coefficients of multilayer and monolayer CVD-grown graphene are 2000cm2/s and 10000cm2/s, respectively, comparable with the reported values of epitaxial graphene and reduced graphene. This transmission grating technique can be used to measure carrier dynamics in versatile 2D materials.

Ultrafast transient photocarrier dynamics of the bulk-insulating topological insulator B i1.5S b0.5T e1.7S e1.3

NASA Astrophysics Data System (ADS)

Choi, Young Gwan; Zhung, Chan June; Park, Sun-Hee; Park, Joonbum; Kim, Jun Sung; Kim, Seongheun; Park, Jaehun; Lee, J. S.

2018-02-01

Using optical-pump terahertz-probe spectroscopy, we investigated an ultrafast photocarrier relaxation behavior in a B i1.5S b0.5T e1.7S e1.3 (BSTS) single crystal, which is one of the most bulk-insulating topological insulators. Compared to n -type bulk-metallic B i2S e3 , we found that BSTS endows distinct behaviors in its photocarrier dynamics; the relaxation time turns out to be an order of magnitude longer, and the transient conductance spectrum exhibits a nonlinear increase as a function of the pumping power. Also, we observed an abrupt reduction of the photocarrier scattering rate in several picoseconds after the initial photoexcitation. We discuss these intriguing experimental observations based on a bulk-to-surface carrier injection assisted by the built-in electric field near the surface and electron-phonon scattering.

Hot electron dynamics at semiconductor surfaces: Implications for quantum dot photovoltaics

NASA Astrophysics Data System (ADS)

Tisdale, William A., III

Finding a viable supply of clean, renewable energy is one of the most daunting challenges facing the world today. Solar cells have had limited impact in meeting this challenge because of their high cost and low power conversion efficiencies. Semiconductor nanocrystals, or quantum dots, are promising materials for use in novel solar cells because they can be processed with potentially inexpensive solution-based techniques and because they are predicted to have novel optoelectronic properties that could enable the realization of ultra-efficient solar power converters. However, there is a lack of fundamental understanding regarding the behavior of highly-excited, or "hot," charge carriers near quantum-dot and semiconductor interfaces, which is of paramount importance to the rational design of high-efficiency devices. The elucidation of these ultrafast hot electron dynamics is the central aim of this Dissertation. I present a theoretical framework for treating the electronic interactions between quantum dots and bulk semiconductor surfaces and propose a novel experimental technique, time-resolved surface second harmonic generation (TR-SHG), for probing these interactions. I then describe a series of experimental investigations into hot electron dynamics in specific quantum-dot/semiconductor systems. A two-photon photoelectron spectroscopy (2PPE) study of the technologically-relevant ZnO(1010) surface reveals ultrafast (sub-30fs) cooling of hot electrons in the bulk conduction band, which is due to strong electron-phonon coupling in this highly polar material. The presence of a continuum of defect states near the conduction band edge results in Fermi-level pinning and upward (n-type) band-bending at the (1010) surface and provides an alternate route for electronic relaxation. In monolayer films of colloidal PbSe quantum dots, chemical treatment with either hydrazine or 1,2-ethanedithiol results in strong and tunable electronic coupling between neighboring quantum dots. A TR-SHG study of these electronically-coupled quantum-dot films reveals temperature-activated cooling of hot charge carriers and coherent excitation of a previously-unidentified surface optical phonon. Finally, I report the first experimental observation of ultrafast electron transfer from the higher excited states of a colloidal quantum dot (PbSe) to delocalized conduction band states of a widely-used electron acceptor (TiO2). The electric field resulting from ultrafast (<50fs) separation of charge carriers across the PbSe/TiO2(110) interface excites coherent vibration of the TiO2 surface atoms, whose collective motions can be followed in real time.

Ultrafast Coherent Dynamics of a Photonic Crystal All-Optical Switch.

PubMed

Colman, Pierre; Lunnemann, Per; Yu, Yi; Mørk, Jesper

2016-12-02

We present pump-probe measurements of an all-optical photonic crystal switch based on a nanocavity, resolving fast coherent temporal dynamics. The measurements demonstrate the importance of coherent effects typically neglected when considering nanocavity dynamics. In particular, we report the observation of an idler pulse and more than 10 dB parametric gain. The measurements are in good agreement with a theoretical model that ascribes the observation to oscillations of the free-carrier population in the nanocavity. The effect opens perspectives for the realization of new all-optical photonic crystal switches with unprecedented switching contrast.

Revealing charge carrier dynamics in squaraine:[6, 6]-phenyl-C 71-butyric acid methyl ester based organic solar cells

NASA Astrophysics Data System (ADS)

Rana, Aniket; Sharma, Chhavi; Prabhu, Deepak D.; Kumar, Mahesh; Karuvath, Yoosaf; Das, Suresh; Chand, Suresh; Singh, Rajiv K.

2018-04-01

Ultrafast charge carrier dynamics as well as the generation of polaron pair in squaraine (SQ) and squaraine:[6,6]-phenyl-C 71-butyric acid methyl ester (SQ:PCBM71) have been studied using ultrafast transient absorption spectroscopy (UTAS). The current study reveals that the pure SQ exhibits the creation of singlet and triplet states; however, incorporation of PCBM71 in SQ results in the formation of polaron pairs with ˜550ps lifetime, which in turn leads to the creation of free electrons in the device. We show that the considerable increment in monomolecular and bimolecular recombination in SQ:PCBM71 compared to pure SQ which describes the interfacial compatibility of SQ and PCBMC71 molecules. The present work not only provides the information about the carrier generation in SQ and SQ:PCBM71 but also gives the facts relating to the effect of PCBM71 mixing into the SQ which is very significant because the SQ has donor-acceptor-donor (D-A-D) structure and mixing one more acceptor can introduce more complex recombinations in the blend. These findings have been complimented by the charge transport study in the device using impedance spectroscopy. The various important transport parameters are transit time (τt), diffusion constant (Dn), global mobility (μ) and carrier lifetime (τr). The values of these parameters are 26.38 μs, 4.64x10-6 cm2s-1, 6.12x10-6 cm2V-1s-1 and 399 μs, respectively. To the best of our knowledge such study related to SQ is not present in the literature comprehensively.

Effects of environmental conditions on the ultrafast carrier dynamics in graphene revealed by terahertz spectroscopy

NASA Astrophysics Data System (ADS)

Hafez, H. A.; Chai, X.; Sekine, Y.; Takamura, M.; Oguri, K.; Al-Naib, I.; Dignam, M. M.; Hibino, H.; Ozaki, T.

2017-04-01

A thorough understanding of the stability of graphene under ambient environmental conditions is essential for future graphene-based applications. In this paper, we study the effects of ambient temperature on the properties of monolayer graphene using terahertz time-domain spectroscopy as well as time-resolved terahertz spectroscopy enabled by an optical-pump/terahertz-probe technique. The observations show that graphene is extremely sensitive to the ambient environmental conditions and behaves differently depending on the sample preparation technique and the initial Fermi level. The analysis of the spectroscopic data is supported by van der Pauw and Hall effect measurements of the carrier mobility and carrier density at temperatures comparable to those tested in our THz spectroscopic experiments.

Ultrafast electronic dynamics in unipolar n-doped indium gallium arsenide/gallium arsenide self-assembled quantum dots

NASA Astrophysics Data System (ADS)

Wu, Zong-Kwei J.

2006-12-01

Photodetectors based on intraband infrared absorption in the quantum dots have demonstrated improved performance over its quantum well counterpart by lower dark current, relative temperature insensitivity, and its ability for normal incidence operation. Various scattering processes, including phonon emission/absorption and carrier-carrier scattering, are critical in understanding device operation on the fundamental level. In previous studies, our group has investigated carrier dynamics in both low- and high-density regime. Ultrafast electron-hole scattering and the predicted phonon bottleneck effect in intrinsic quantum dots have been observed. Further examination on electron dynamics in unipolar structures is presented in this thesis. We used n-doped quantum dot in mid-infrared photodetector device structure to study the electron dynamics in unipolar structure. Differential transmission spectroscopy with mid-infrared intraband pump and optical interband probe was implemented to measure the electron dynamics directly without creating extra electron-hole pair, Electron relaxation after excitation was measured under various density and temperature conditions. Rapid capture into quantum dot within ˜ 10 ps was observed due to Auger-type electron-electron scattering. Intradot relaxation from the quantum dot excited state to the ground state was also observed on the time scale of 100 ps. With highly doped electron density in the structure, the inter-sublevel relaxation is dominated by Auger-type electron-electron scattering and the phonon bottleneck effect is circumvented. Nanosecond-scale recovery in larger-sized quantum dots was observed, not intrinsic to electron dynamics but due to band-bending and built-in voltage drift. An ensemble Monte Carlo simulation was also established to model the dynamics in quantum dots and in goad agreement with the experimental results. We presented a comprehensive picture of electron dynamics in the unipolar quantum dot structure. Although the phonon bottleneck is circumvented with high doped electron density, relaxation processes in unipolar quantum dots have been measured with time scales longer than that of bipolar systems. The results explain the operation principles of the quantum dot infrared photodetector on a microscopic level and provide basic understanding for future applications and designs.

Ultrafast Hole Trapping and Relaxation Dynamics in p-Type CuS Nanodisks

DOE PAGES

Ludwig, John; An, Li; Pattengale, Brian; ...

2015-06-22

CuS nanocrystals are potential materials for developing low-cost solar energy conversion devices. Understanding the underlying dynamics of photoinduced carriers in CuS nanocrystals is essential to improve their performance in these devices. In this work, we investigated the photoinduced hole dynamics in CuS nanodisks (NDs) using the combination of transient optical (OTA) and X-ray (XTA) absorption spectroscopy. OTA results show that the broad transient absorption in the visible region is attributed to the photoinduced hot and trapped holes. The hole trapping process occurs on a subpicosecond time scale, followed by carrier recombination (~100 ps). The nature of the hole trapping sites,more » revealed by XTA, is characteristic of S or organic ligands on the surface of CuS NDs. Lastly, these results not only suggest the possibility to control the hole dynamics by tuning the surface chemistry of CuS but also represent the first time observation of hole dynamics in semiconductor nanocrystals using XTA.« less

Optical and Nonlinear Optical Response of Light Sensor Thin Films

PubMed Central

Liu, Huimin; Rua, Armando; Vasquez, Omar; Vikhnin, Valentin S.; Fernandez, Felix E.; Fonseca, Luis F.; Resto, Oscar; Weisz, Svi Z.

2005-01-01

For potential ultrafast optical sensor application, both VO2 thin films and nanocomposite crystal-Si enriched SiO2 thin films grown on fused quartz substrates were successfully prepared using pulsed laser deposition (PLD) and RF co-sputtering techniques. In photoluminescence (PL) measurement c-Si/SiO2 film contains nanoparticles of crystal Si exhibits strong red emission with the band maximum ranging from 580 to 750 nm. With ultrashort pulsed laser excitation all films show extremely intense and ultrafast nonlinear optical (NLO) response. The recorded holography from all these thin films in a degenerate-four-wave-mixing configuration shows extremely large third-order response. For VO2 thin films, an optically induced semiconductor-to-metal phase transition (PT) immediately occurred upon laser excitation. it accompanied. It turns out that the fast excited state dynamics was responsible to the induced PT. For c-Si/SiO2 film, its NLO response comes from the contribution of charge carriers created by laser excitation in conduction band of the c-Si nanoparticles. It was verified by introducing Eu3+ which is often used as a probe sensing the environment variations. It turns out that the entire excited state dynamical process associated with the creation, movement and trapping of the charge carriers has a characteristic 500 ps duration.

Bias-induced modulation of ultrafast carrier dynamics in metallic single-walled carbon nanotubes

NASA Astrophysics Data System (ADS)

Maekawa, Keisuke; Yanagi, Kazuhiro; Minami, Yasuo; Kitajima, Masahiro; Katayama, Ikufumi; Takeda, Jun

2018-02-01

The gate bias dependence of excited-state relaxation dynamics in metallic single-walled carbon nanotubes (MCNTs) was investigated using pump-probe transient absorption spectroscopy coupled with electrochemical doping through an ionic liquid. The transient transmittance decayed exponentially with the pump-probe delay time, whose value could be tuned via the Fermi-level modulation of Dirac electrons under a bias voltage. The obtained relaxation time was the shortest when the Fermi level was at the Dirac point of the MCNTs, and exhibited a U-shaped dependence on the bias voltage. Because optical dipole transitions between the Dirac bands are forbidden in MCNTs, the observed dynamics were attributed to carrier relaxation from the E11 band to the Dirac band. Using a model that considers the suppression of electron-electron scattering (impact ionization) due to Pauli blocking, we could qualitatively explain the obtained bias dependence of the relaxation time.

Role of many-body effects in the coherent dynamics of excitons in low-temperature-grown GaAs

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

Webber, D.; Hacquebard, L.; Hall, K. C.

2015-10-05

Femtosecond four-wave mixing experiments on low-temperature-grown (LT-) GaAs indicate a polarization-dependent nonlinear optical response at the exciton, which we attribute to Coulomb-mediated coupling between excitons and electron-hole pairs simultaneously excited by the broad-bandwidth laser pulses. Strong suppression of the exciton response through screening by carriers injected by a third pump pulse was observed, an effect that is transient due to rapid carrier trapping. Our findings highlight the need to account for the complex interplay of disorder and many-body effects in the design of ultrafast optoelectronic devices using this material.

Dynamical anisotropic response of black phosphorus under magnetic field

NASA Astrophysics Data System (ADS)

Liu, Xuefeng; Lu, Wei; Zhou, Xiaoying; Zhou, Yang; Zhang, Chenglong; Lai, Jiawei; Ge, Shaofeng; Sekhar, M. Chandra; Jia, Shuang; Chang, Kai; Sun, Dong

2018-04-01

Black phosphorus (BP) has emerged as a promising material candidate for next generation electronic and optoelectronic devices due to its high mobility, tunable band gap and highly anisotropic properties. In this work, polarization resolved ultrafast mid-infrared transient reflection spectroscopy measurements are performed to study the dynamical anisotropic optical properties of BP under magnetic fields up to 9 T. The relaxation dynamics of photoexcited carrier is found to be insensitive to the applied magnetic field due to the broadening of the Landau levels and large effective mass of carriers. While the anisotropic optical response of BP decreases with increasing magnetic field, its enhancement due to the excitation of hot carriers is similar to that without magnetic field. These experimental results can be well interpreted by the magneto-optical conductivity of the Landau levels of BP thin film, based on an effective k · p Hamiltonian and linear response theory. These findings suggest attractive possibilities of multi-dimensional control of anisotropic response (AR) of BP with light, electric and magnetic field, which further introduces BP to the fantastic magnetic field sensitive applications.

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.

High-frequency polarization dynamics in spin-lasers: pushing the limits

NASA Astrophysics Data System (ADS)

Gerhardt, Nils C.; Lindemann, Markus; Pusch, Tobias; Michalzik, Rainer; Hofmann, Martin R.

2017-09-01

While the high-frequency performance of conventional lasers is limited by the coupled carrier-photon dynamics, spin-polarized lasers have a high potential to overcome this limitation and to push the direct modulation bandwidth beyond 100 GHz. The key is to utilize the ultrafast polarization dynamics in spin-polarized vertical cavity surface-emitting lasers (spin-VCSELs) which is decoupled from the intensity dynamics and its fundamental limitations. The polarization dynamics in such devices, characterized by the polarization oscillation resonance frequency, is mainly determined by the amount of birefringence in the cavity. Using an approach for manipulating the birefringence via mechanical strain we were able to increase the polarization dynamics to resonance frequencies of more than 40 GHz. Up to now these values are only limited by the setup to induce birefringence and do not reflect any fundamental limitations. Taking our record results for the birefringence-induced mode splitting of more than 250 GHz into account, the concept has the potential to provide polarization modulation in spin-VCSELs with modulation frequencies far beyond 100 GHz. This makes them ideal devices for next-generation fast optical interconnects. In this paper we present experimental results for ultrafast polarization dynamics up to 50 GHz and compare them to numerical simulations.

Transient transition from free carrier metallic state to exciton insulating state in GaAs by ultrafast photoexcitation

NASA Astrophysics Data System (ADS)

Nie, X. C.; Song, Hai-Ying; Zhang, Xiu; Gu, Peng; Liu, Shi-Bing; Li, Fan; Meng, Jian-Qiao; Duan, Yu-Xia; Liu, H. Y.

2018-03-01

We present systematic studies of the transient dynamics of GaAs by ultrafast time-resolved reflectivity. In photoexcited non-equilibrium states, we found a sign reverse in reflectivity change ΔR/R, from positive around room temperature to negative at cryogenic temperatures. The former corresponds to a free carrier metallic state, while the latter is attributed to an exciton insulating state, in which the transient electronic properties is mostly dominated by excitons, resulting in a transient metal–insulator transition (MIT). Two transition temperatures (T 1 and T 2) are well identified by analyzing the intensity change of the transient reflectivity. We found that photoexcited MIT starts emerging at T 1 as high as ∼ 230 K, in terms of a dip feature at 0.4 ps, and becomes stabilized below T 2 that is up to ∼ 180 K, associated with a negative constant after 40 ps. Our results address a phase diagram that provides a framework for the inducing of MIT through temperature and photoexcitation, and may shed light on the understanding of light-semiconductor interaction and exciton physics.

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.

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.

Nonequilibrium phase transitions in cuprates observed by ultrafast electron crystallography.

PubMed

Gedik, Nuh; Yang, Ding-Shyue; Logvenov, Gennady; Bozovic, Ivan; Zewail, Ahmed H

2007-04-20

Nonequilibrium phase transitions, which are defined by the formation of macroscopic transient domains, are optically dark and cannot be observed through conventional temperature- or pressure-change studies. We have directly determined the structural dynamics of such a nonequilibrium phase transition in a cuprate superconductor. Ultrafast electron crystallography with the use of a tilted optical geometry technique afforded the necessary atomic-scale spatial and temporal resolutions. The observed transient behavior displays a notable "structural isosbestic" point and a threshold effect for the dependence of c-axis expansion (Deltac) on fluence (F), with Deltac/F = 0.02 angstrom/(millijoule per square centimeter). This threshold for photon doping occurs at approximately 0.12 photons per copper site, which is unexpectedly close to the density (per site) of chemically doped carriers needed to induce superconductivity.

Ultrafast Band Engineering and Transient Spin Currents in Antiferromagnetic Oxides

NASA Astrophysics Data System (ADS)

Gu, Mingqiang; Rondinelli, James M.

2016-04-01

We report a dynamic structure and band engineering strategy with experimental protocols to induce indirect-to-direct band gap transitions and coherently oscillating pure spin-currents in three-dimensional antiferromagnets (AFM) using selective phononic excitations. In the Mott insulator LaTiO3, we show that a photo-induced nonequilibrium phonon mode amplitude destroys the spin and orbitally degenerate ground state, reduces the band gap by 160 meV and renormalizes the carrier masses. The time scale of this process is a few hundreds of femtoseconds. Then in the hole-doped correlated metallic titanate, we show how pure spin-currents can be achieved to yield spin-polarizations exceeding those observed in classic semiconductors. Last, we demonstrate the generality of the approach by applying it to the non-orbitally degenerate AFM CaMnO3. These results advance our understanding of electron-lattice interactions in structures out-of-equilibrium and establish a rational framework for designing dynamic phases that may be exploited in ultrafast optoelectronic and optospintronic devices.

Ultrafast Band Engineering and Transient Spin Currents in Antiferromagnetic Oxides.

PubMed

Gu, Mingqiang; Rondinelli, James M

2016-04-29

We report a dynamic structure and band engineering strategy with experimental protocols to induce indirect-to-direct band gap transitions and coherently oscillating pure spin-currents in three-dimensional antiferromagnets (AFM) using selective phononic excitations. In the Mott insulator LaTiO3, we show that a photo-induced nonequilibrium phonon mode amplitude destroys the spin and orbitally degenerate ground state, reduces the band gap by 160 meV and renormalizes the carrier masses. The time scale of this process is a few hundreds of femtoseconds. Then in the hole-doped correlated metallic titanate, we show how pure spin-currents can be achieved to yield spin-polarizations exceeding those observed in classic semiconductors. Last, we demonstrate the generality of the approach by applying it to the non-orbitally degenerate AFM CaMnO3. These results advance our understanding of electron-lattice interactions in structures out-of-equilibrium and establish a rational framework for designing dynamic phases that may be exploited in ultrafast optoelectronic and optospintronic devices.

Ultrafast band engineering and transient spin currents in antiferromagnetic oxides

DOE PAGES

Gu, Mingqiang; Rondinelli, James M.

2016-04-29

Here, we report a dynamic structure and band engineering strategy with experimental protocols to induce indirect-to-direct band gap transitions and coherently oscillating pure spin-currents in three-dimensional antiferromagnets (AFM) using selective phononic excitations. In the Mott insulator LaTiO 3, we show that a photo-induced nonequilibrium phonon mode amplitude destroys the spin and orbitally degenerate ground state, reduces the band gap by 160 meV and renormalizes the carrier masses. The time scale of this process is a few hundreds of femtoseconds. Then in the hole-doped correlated metallic titanate, we show how pure spin-currents can be achieved to yield spin-polarizations exceeding those observedmore » in classic semiconductors. Last, we demonstrate the generality of the approach by applying it to the non-orbitally degenerate AFM CaMnO 3. These results advance our understanding of electron-lattice interactions in structures out-of-equilibrium and establish a rational framework for designing dynamic phases that may be exploited in ultrafast optoelectronic and optospintronic devices.« less

Ultrafast Band Engineering and Transient Spin Currents in Antiferromagnetic Oxides

PubMed Central

Gu, Mingqiang; Rondinelli, James M.

2016-01-01

We report a dynamic structure and band engineering strategy with experimental protocols to induce indirect-to-direct band gap transitions and coherently oscillating pure spin-currents in three-dimensional antiferromagnets (AFM) using selective phononic excitations. In the Mott insulator LaTiO3, we show that a photo-induced nonequilibrium phonon mode amplitude destroys the spin and orbitally degenerate ground state, reduces the band gap by 160 meV and renormalizes the carrier masses. The time scale of this process is a few hundreds of femtoseconds. Then in the hole-doped correlated metallic titanate, we show how pure spin-currents can be achieved to yield spin-polarizations exceeding those observed in classic semiconductors. Last, we demonstrate the generality of the approach by applying it to the non-orbitally degenerate AFM CaMnO3. These results advance our understanding of electron-lattice interactions in structures out-of-equilibrium and establish a rational framework for designing dynamic phases that may be exploited in ultrafast optoelectronic and optospintronic devices. PMID:27126354

Multidimensional infrared spectroscopy reveals the vibrational and solvation dynamics of isoniazid

NASA Astrophysics Data System (ADS)

Shaw, Daniel J.; Adamczyk, Katrin; Frederix, Pim W. J. M.; Simpson, Niall; Robb, Kirsty; Greetham, Gregory M.; Towrie, Michael; Parker, Anthony W.; Hoskisson, Paul A.; Hunt, Neil T.

2015-06-01

The results of infrared spectroscopic investigations into the band assignments, vibrational relaxation, and solvation dynamics of the common anti-tuberculosis treatment Isoniazid (INH) are reported. INH is known to inhibit InhA, a 2-trans-enoyl-acyl carrier protein reductase enzyme responsible for the maintenance of cell walls in Mycobacterium tuberculosis but as new drug-resistant strains of the bacterium appear, next-generation therapeutics will be essential to combat the rise of the disease. Small molecules such as INH offer the potential for use as a biomolecular marker through which ultrafast multidimensional spectroscopies can probe drug binding and so inform design strategies but a complete characterization of the spectroscopy and dynamics of INH in solution is required to inform such activity. Infrared absorption spectroscopy, in combination with density functional theory calculations, is used to assign the vibrational modes of INH in the 1400-1700 cm-1 region of the infrared spectrum while ultrafast multidimensional spectroscopy measurements determine the vibrational relaxation dynamics and the effects of solvation via spectral diffusion of the carbonyl stretching vibrational mode. These results are discussed in the context of previous linear spectroscopy studies on solid-phase INH and its usefulness as a biomolecular probe.

Terahertz Emission from Hybrid Perovskites Driven by Ultrafast Charge Separation and Strong Electron-Phonon Coupling.

PubMed

Guzelturk, Burak; Belisle, Rebecca A; Smith, Matthew D; Bruening, Karsten; Prasanna, Rohit; Yuan, Yakun; Gopalan, Venkatraman; Tassone, Christopher J; Karunadasa, Hemamala I; McGehee, Michael D; Lindenberg, Aaron M

2018-03-01

Unusual photophysical properties of organic-inorganic hybrid perovskites have not only enabled exceptional performance in optoelectronic devices, but also led to debates on the nature of charge carriers in these materials. This study makes the first observation of intense terahertz (THz) emission from the hybrid perovskite methylammonium lead iodide (CH 3 NH 3 PbI 3 ) following photoexcitation, enabling an ultrafast probe of charge separation, hot-carrier transport, and carrier-lattice coupling under 1-sun-equivalent illumination conditions. Using this approach, the initial charge separation/transport in the hybrid perovskites is shown to be driven by diffusion and not by surface fields or intrinsic ferroelectricity. Diffusivities of the hot and band-edge carriers along the surface normal direction are calculated by analyzing the emitted THz transients, with direct implications for hot-carrier device applications. Furthermore, photogenerated carriers are found to drive coherent terahertz-frequency lattice distortions, associated with reorganizations of the lead-iodide octahedra as well as coupled vibrations of the organic and inorganic sublattices. This strong and coherent carrier-lattice coupling is resolved on femtosecond timescales and found to be important both for resonant and far-above-gap photoexcitation. This study indicates that ultrafast lattice distortions play a key role in the initial processes associated with charge transport. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Tunable Ultrafast Photon Source and Imaging System for Studying Carrier Dynamics in Graphene Devices

DTIC Science & Technology

2015-07-23

structure systems (for example of graphene and boron nitride ). Figure 2 shows the homebuilt Raman spectroscopy setup in our lab which uses two...pseudo-Dirac points in the band structure. In a run up to these studies, we have also developed a technique to optically image boron nitride flakes with...2) Dheeraj Golla, K. Chattrakun, K. Watanabe, T. Taniguchi, Brian J. LeRoy, Arvinder Sandhu, “Optical thickness determination of hexagonal boron

Ultrafast Silicon Photonics with Visible to Mid-Infrared Pumping of Silicon Nanocrystals.

PubMed

Diroll, Benjamin T; Schramke, Katelyn S; Guo, Peijun; Kortshagen, Uwe R; Schaller, Richard D

2017-10-11

Dynamic optical control of infrared (IR) transparency and refractive index is achieved using boron-doped silicon nanocrystals excited with mid-IR optical pulses. Unlike previous silicon-based optical switches, large changes in transmittance are achieved without a fabricated structure by exploiting strong light coupling of the localized surface plasmon resonance (LSPR) produced from free holes of p-type silicon nanocrystals. The choice of optical excitation wavelength allows for selectivity between hole heating and carrier generation through intraband or interband photoexcitation, respectively. Mid-IR optical pumping heats the free holes of p-Si nanocrystals to effective temperatures greater than 3500 K. Increases of the hole effective mass at high effective hole temperatures lead to a subpicosecond change of the dielectric function, resulting in a redshift of the LSPR, modulating mid-IR transmission by as much as 27%, and increasing the index of refraction by more than 0.1 in the mid-IR. Low hole heat capacity dictates subpicosecond hole cooling, substantially faster than carrier recombination, and negligible heating of the Si lattice, permitting mid-IR optical switching at terahertz repetition frequencies. Further, the energetic distribution of holes at high effective temperatures partially reverses the Burstein-Moss effect, permitting the modulation of transmittance at telecommunications wavelengths. The results presented here show that doped silicon, particularly in micro- or nanostructures, is a promising dynamic metamaterial for ultrafast IR photonics.

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

Ultrafast Silicon Photonics with Visible to Mid-Infrared Pumping of Silicon Nanocrystals

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

Diroll, Benjamin T.; Schramke, Katelyn S.; Guo, Peijun

Dynamic optical control of infrared (IR) transparency and refractive index is achieved using boron-doped silicon nanocrystals excited with mid-IR optical pulses. Also, unlike previous silicon-based optical switches, large changes in transmittance are achieved without a fabricated structure by exploiting strong light coupling of the localized surface plasmon resonance (LSPR) produced from free holes of p-type silicon nanocrystals. The choice of optical excitation wavelength allows selectivity between hole heating and carrier generation through intraband or interband photoexcitation, respectively. Mid-IR optical pumping heats the free holes of p-Si nanocrystals to effective temperatures greater than 3500 K. Increases of the hole effective massmore » at high effective hole temperatures lead to a sub-picosecond change of the dielectric function resulting in a redshift of the LSPR, modulating mid-IR transmission by as much as 27% and increasing the index of refraction by more than 0.1 in the mid-IR. Low hole heat capacity dictates sub-picosecond hole cooling, substantially faster than carrier recombination, and negligible heating of the Si lattice, permitting mid-IR optical switching at terahertz repetition frequencies. Further, the energetic distribution of holes at high effective temperatures partially reverses the Burstein-Moss effect, permitting modulation of transmittance at telecommunications wavelengths. Lastly, the results presented here show that doped silicon, particularly in micro- or nanostructures, is a promising dynamic metamaterial for ultrafast IR photonics.« less

Ultrafast photocarrier dynamics related to defect states of Si1-xGex nanowires measured by optical pump-THz probe spectroscopy.

PubMed

Bae, Jung Min; Lee, Woo-Jung; Jung, Seonghoon; Ma, Jin Won; Jeong, Kwang-Sik; Oh, Seung Hoon; Kim, Seongsin M; Suh, Dongchan; Song, Woobin; Kim, Sunjung; Park, Jaehun; Cho, Mann-Ho

2017-06-14

Slightly tapered Si 1-x Ge x nanowires (NWs) (x = 0.29-0.84) were synthesized via a vapor-liquid-solid procedure using Au as a catalyst. We measured the optically excited carrier dynamics of Si 1-x Ge x NWs as a function of Ge content using optical pump-THz probe spectroscopy. The measured -ΔT/T 0 signals of Si 1-x Ge x NWs were converted into conductivity in the THz region. We developed a fitting formula to apply to indirect semiconductors such as Si 1-x Ge x , which explains the temporal population of photo-excited carriers in the band structure and the relationship between the trapping time and the defect states on an ultrafast time scale. From the fitting results, we extracted intra- and inter-valley transition times and trapping times of electrons and holes of Si 1-x Ge x NWs as a function of Ge content. On the basis of theoretical reports, we suggest a physical model to interpret the trapping times related to the species of interface defect states located at the oxide/NW: substoichiometric oxide states of Si(Ge) 0+,1+,2+ , but not Si(Ge) 3+ , could function as defect states capturing photo-excited electrons or holes and could determine the different trapping times of electrons and holes depending on negatively or neutrally charged states.

Ultrafast Silicon Photonics with Visible to Mid-Infrared Pumping of Silicon Nanocrystals

DOE PAGES

Diroll, Benjamin T.; Schramke, Katelyn S.; Guo, Peijun; ...

2017-09-11

Dynamic optical control of infrared (IR) transparency and refractive index is achieved using boron-doped silicon nanocrystals excited with mid-IR optical pulses. Also, unlike previous silicon-based optical switches, large changes in transmittance are achieved without a fabricated structure by exploiting strong light coupling of the localized surface plasmon resonance (LSPR) produced from free holes of p-type silicon nanocrystals. The choice of optical excitation wavelength allows selectivity between hole heating and carrier generation through intraband or interband photoexcitation, respectively. Mid-IR optical pumping heats the free holes of p-Si nanocrystals to effective temperatures greater than 3500 K. Increases of the hole effective massmore » at high effective hole temperatures lead to a sub-picosecond change of the dielectric function resulting in a redshift of the LSPR, modulating mid-IR transmission by as much as 27% and increasing the index of refraction by more than 0.1 in the mid-IR. Low hole heat capacity dictates sub-picosecond hole cooling, substantially faster than carrier recombination, and negligible heating of the Si lattice, permitting mid-IR optical switching at terahertz repetition frequencies. Further, the energetic distribution of holes at high effective temperatures partially reverses the Burstein-Moss effect, permitting modulation of transmittance at telecommunications wavelengths. Lastly, the results presented here show that doped silicon, particularly in micro- or nanostructures, is a promising dynamic metamaterial for ultrafast IR photonics.« less

Extended hot carrier lifetimes observed in bulk In{sub 0.265±0.02}Ga{sub 0.735}N under high-density photoexcitation

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

Zhang, Yi; Tayebjee, Murad J. Y.; Smyth, Suntrana

2016-03-28

We have investigated the ultrafast carrier dynamics in a 1 μm bulk In{sub 0.265}Ga{sub 0.735}N thin film grown using energetic neutral atom-beam lithography/epitaxy molecular beam epitaxy. Cathodoluminescence and X-ray diffraction experiments are used to observe the existence of indium-rich domains in the sample. These domains give rise to a second carrier population and bi-exponential carrier cooling is observed with characteristic lifetimes of 1.6 and 14 ps at a carrier density of 1.3 × 10{sup 16 }cm{sup −3}. A combination of band-filling, screening, and hot-phonon effects gives rise to a two-fold enhanced mono-exponential cooling rate of 28 ps at a carrier density of 8.4 × 10{sup 18 }cm{sup −3}. Thismore » is the longest carrier thermalization time observed in bulk InGaN alloys to date.« less

Attosecond optical-field-enhanced carrier injection into the GaAs conduction band

NASA Astrophysics Data System (ADS)

Schlaepfer, F.; Lucchini, M.; Sato, S. A.; Volkov, M.; Kasmi, L.; Hartmann, N.; Rubio, A.; Gallmann, L.; Keller, U.

2018-06-01

Resolving the fundamental carrier dynamics induced in solids by strong electric fields is essential for future applications, ranging from nanoscale transistors1,2 to high-speed electro-optical switches3. How fast and at what rate can electrons be injected into the conduction band of a solid? Here, we investigate the sub-femtosecond response of GaAs induced by resonant intense near-infrared laser pulses using attosecond transient absorption spectroscopy. In particular, we unravel the distinct role of intra- versus interband transitions. Surprisingly, we found that despite the resonant driving laser, the optical response during the light-matter interaction is dominated by intraband motion. Furthermore, we observed that the coupling between the two mechanisms results in a significant enhancement of the carrier injection from the valence into the conduction band. This is especially unexpected as the intraband mechanism itself can accelerate carriers only within the same band. This physical phenomenon could be used to control ultrafast carrier excitation and boost injection rates in electronic switches in the petahertz regime.

Evidence for impact ionization in vanadium dioxide

DOE PAGES

Holleman, Joshua; Bishop, Michael M.; Garcia, Carlos; ...

2016-10-17

Pump-probe optical spectroscopy was used to investigate charge carrier multiplication via impact ionization in the M 1 insulating phase of VO 2. By comparing the transient reflectivities of the film when pumped at less than and then more than twice the band-gap energy, we observed an enhancement of the ultrafast response with the higher energy pump color while the film was still transiently in the insulating phase. We additionally identified multiple timescales within the charge dynamics and analyzed how these changed when the pump and probe wavelengths were varied. This experiment provided evidence that impact ionization acts efficiently as amore » carrier multiplication process in this prototypical strongly-correlated insulator.« less

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

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

Negligible Electronic Interaction between Photoexcited Electron–Hole Pairs and Free Electrons in Phosphorus–Boron Co-Doped Silicon Nanocrystals

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

Limpens, Rens; Fujii, Minoru; Neale, Nathan R.

Phosphorus (P) and boron (B) co-doped Si nanocrystals (NCs) have raised interest in the optoelectronic industry due to their electronic tunability, optimal carrier multiplication properties, and straightforward dispersibility in polar solvents. Yet a basic understanding of the interaction of photoexcited electron-hole (e-h) pairs with new physical features that are introduced by the co-doping process (free carriers, defect states, and surface chemistry) is missing. Here, we present the first study of the ultrafast carrier dynamics in SiO2-embedded P-B co-doped Si NC ensembles using induced absorption spectroscopy through a two-step approach. First, the induced absorption data show that the large fraction ofmore » the dopants residing on the NC surface slows down carrier relaxation dynamics within the first 20 ps relative to intrinsic (undoped) Si NCs, which we interpret as enhanced surface passivation. On longer time-scales (picosecond to nanosecond regime), we observe a speeding up of the carrier relaxation dynamics and ascribe it to doping-induced trap states. This argument is deduced from the second part of the study, where we investigate multiexciton interactions. From a stochastic modeling approach we show that localized carriers, which are introduced by the P or B dopants, have minor electronic interactions with the photoexcited e-h pairs. This is understood in light of the strong localization of the introduced carriers on their original P- or B-dopant atoms, due to the strong quantum confinement regime in these relatively small NCs (<6 nm).« less

Negligible Electronic Interaction between Photoexcited Electron-Hole Pairs and Free Electrons in Phosphorus-Boron Co-Doped Silicon Nanocrystals

DOE PAGES

Limpens, Rens; Neale, Nathan R; Fujii, Minoru; ...

2018-03-05

Phosphorus (P) and boron (B) co-doped Si nanocrystals (NCs) have raised interest in the optoelectronic industry due to their electronic tunability, optimal carrier multiplication properties, and straightforward dispersibility in polar solvents. Yet a basic understanding of the interaction of photoexcited electron-hole (e-h) pairs with new physical features that are introduced by the co-doping process (free carriers, defect states, and surface chemistry) is missing. Here, we present the first study of the ultrafast carrier dynamics in SiO2-embedded P-B co-doped Si NC ensembles using induced absorption spectroscopy through a two-step approach. First, the induced absorption data show that the large fraction ofmore » the dopants residing on the NC surface slows down carrier relaxation dynamics within the first 20 ps relative to intrinsic (undoped) Si NCs, which we interpret as enhanced surface passivation. On longer time-scales (picosecond to nanosecond regime), we observe a speeding up of the carrier relaxation dynamics and ascribe it to doping-induced trap states. This argument is deduced from the second part of the study, where we investigate multiexciton interactions. From a stochastic modeling approach we show that localized carriers, which are introduced by the P or B dopants, have minor electronic interactions with the photoexcited e-h pairs. This is understood in light of the strong localization of the introduced carriers on their original P- or B-dopant atoms, due to the strong quantum confinement regime in these relatively small NCs (<6 nm).« less

Negligible Electronic Interaction between Photoexcited Electron–Hole Pairs and Free Electrons in Phosphorus–Boron Co-Doped Silicon Nanocrystals

DOE PAGES

Limpens, Rens; Fujii, Minoru; Neale, Nathan R.; ...

2018-02-28

Phosphorus (P) and boron (B) co-doped Si nanocrystals (NCs) have raised interest in the optoelectronic industry due to their electronic tunability, optimal carrier multiplication properties, and straightforward dispersibility in polar solvents. Yet a basic understanding of the interaction of photoexcited electron-hole (e-h) pairs with new physical features that are introduced by the co-doping process (free carriers, defect states, and surface chemistry) is missing. Here, we present the first study of the ultrafast carrier dynamics in SiO2-embedded P-B co-doped Si NC ensembles using induced absorption spectroscopy through a two-step approach. First, the induced absorption data show that the large fraction ofmore » the dopants residing on the NC surface slows down carrier relaxation dynamics within the first 20 ps relative to intrinsic (undoped) Si NCs, which we interpret as enhanced surface passivation. On longer time-scales (picosecond to nanosecond regime), we observe a speeding up of the carrier relaxation dynamics and ascribe it to doping-induced trap states. This argument is deduced from the second part of the study, where we investigate multiexciton interactions. From a stochastic modeling approach we show that localized carriers, which are introduced by the P or B dopants, have minor electronic interactions with the photoexcited e-h pairs. This is understood in light of the strong localization of the introduced carriers on their original P- or B-dopant atoms, due to the strong quantum confinement regime in these relatively small NCs (<6 nm).« less

Ultrahigh photoconductivity of bandgap-graded CdSxSe1-x nanowires probed by terahertz spectroscopy

NASA Astrophysics Data System (ADS)

Liu, Hongwei; Lu, Junpeng; Yang, Zongyin; Teng, Jinghua; Ke, Lin; Zhang, Xinhai; Tong, Limin; Sow, Chorng Haur

2016-06-01

Superiorly high photoconductivity is desirable in optoelectronic materials and devices for information transmission and processing. Achieving high photoconductivity via bandgap engineering in a bandgap-graded semiconductor nanowire has been proposed as a potential strategy. In this work, we report the ultrahigh photoconductivity of bandgap-graded CdSxSe1-x nanowires and its detailed analysis by means of ultrafast optical-pump terahertz-probe (OPTP) spectroscopy. The recombination rates and carrier mobility are quantitatively obtained via investigation of the transient carrier dynamics in the nanowires. By analysis of the terahertz (THz) spectra, we obtain an insight into the bandgap gradient and band alignment to carrier transport along the nanowires. The demonstration of the ultrahigh photoconductivity makes bandgap-graded CdSxSe1-x nanowires a promising candidate as building blocks for nanoscale electronic and photonic devices.

Rhodium doped InGaAs: A superior ultrafast photoconductor

NASA Astrophysics Data System (ADS)

Kohlhaas, R. B.; Globisch, B.; Nellen, S.; Liebermeister, L.; Schell, M.; Richter, P.; Koch, M.; Semtsiv, M. P.; Masselink, W. T.

2018-03-01

The properties of rhodium (Rh) as a deep-level dopant in InGaAs lattice matched to InP grown by molecular beam epitaxy are investigated. When InGaAs:Rh is used as an ultrafast photoconductor, carrier lifetimes as short as 100 fs for optically excited electrons are measured. Rh doping compensates free carriers so that a near intrinsic carrier concentration can be achieved. At the same time, InGaAs:Rh exhibits a large electron mobility of 1000 cm2/V s. Therefore, this material is a very promising candidate for application as a semi-insulating layer, THz antenna, or semiconductor saturable absorber mirror.

Applications of ultrashort laser pulses in science and technology; Proceedings of the Meeting, The Hague, Netherlands, Mar. 12, 13, 1990

NASA Technical Reports Server (NTRS)

Antonetti, Andre (Editor)

1990-01-01

Topics discussed are on the generation of high-intensity femtosecond lasers, the high-repetition and infrared femtosecond pulses, and physics of semiconductors and applications. Papers are presented on the femtosecond pulse generation at 193 nm; the generation of intense subpicosecond and femtosecond pulses; intense tunable subpicosecond and femtosecond pulses in the visible and infrared, generated by optical parametric oscillators; a high-efficiency high-energy optical amplifier for femtosecond pulses; and the generation of solitons, periodic pulsing, and nonlinearities in GaAs. Other papers are on ultrafast relaxation dynamics of photoexcited carriers in GaAs, high-order optical nonlinear susceptibilities of transparent glasses, subnanosecond risetime high-power pulse generation using photoconductive bulk GaAs devices, femtosecond studies of plasma formation in crystalline and amorphous silicon, and subpicosecond dynamics of hot carrier relaxation in InP and GaAs.

Field propagation-induced directionality of carrier-envelope phase-controlled photoemission from nanospheres

PubMed Central

Süßmann, F.; Seiffert, L.; Zherebtsov, S.; Mondes, V.; Stierle, J.; Arbeiter, M.; Plenge, J.; Rupp, P.; Peltz, C.; Kessel, A.; Trushin, S. A.; Ahn, B.; Kim, D.; Graf, C.; Rühl, E.; Kling, M. F.; Fennel, T.

2015-01-01

Near-fields of non-resonantly laser-excited nanostructures enable strong localization of ultrashort light fields and have opened novel routes to fundamentally modify and control electronic strong-field processes. Harnessing spatiotemporally tunable near-fields for the steering of sub-cycle electron dynamics may enable ultrafast optoelectronic devices and unprecedented control in the generation of attosecond electron and photon pulses. Here we utilize unsupported sub-wavelength dielectric nanospheres to generate near-fields with adjustable structure and study the resulting strong-field dynamics via photoelectron imaging. We demonstrate field propagation-induced tunability of the emission direction of fast recollision electrons up to a regime, where nonlinear charge interaction effects become dominant in the acceleration process. Our analysis supports that the timing of the recollision process remains controllable with attosecond resolution by the carrier-envelope phase, indicating the possibility to expand near-field-mediated control far into the realm of high-field phenomena. PMID:26264422

Electronic cooling via interlayer Coulomb coupling in multilayer epitaxial graphene

PubMed Central

Mihnev, Momchil T.; Tolsma, John R.; Divin, Charles J.; Sun, Dong; Asgari, Reza; Polini, Marco; Berger, Claire; de Heer, Walt A.; MacDonald, Allan H.; Norris, Theodore B.

2015-01-01

In van der Waals bonded or rotationally disordered multilayer stacks of two-dimensional (2D) materials, the electronic states remain tightly confined within individual 2D layers. As a result, electron–phonon interactions occur primarily within layers and interlayer electrical conductivities are low. In addition, strong covalent in-plane intralayer bonding combined with weak van der Waals interlayer bonding results in weak phonon-mediated thermal coupling between the layers. We demonstrate here, however, that Coulomb interactions between electrons in different layers of multilayer epitaxial graphene provide an important mechanism for interlayer thermal transport, even though all electronic states are strongly confined within individual 2D layers. This effect is manifested in the relaxation dynamics of hot carriers in ultrafast time-resolved terahertz spectroscopy. We develop a theory of interlayer Coulomb coupling containing no free parameters that accounts for the experimentally observed trends in hot-carrier dynamics as temperature and the number of layers is varied. PMID:26399955

Field propagation-induced directionality of carrier-envelope phase-controlled photoemission from nanospheres.

PubMed

Süßmann, F; Seiffert, L; Zherebtsov, S; Mondes, V; Stierle, J; Arbeiter, M; Plenge, J; Rupp, P; Peltz, C; Kessel, A; Trushin, S A; Ahn, B; Kim, D; Graf, C; Rühl, E; Kling, M F; Fennel, T

2015-08-12

Near-fields of non-resonantly laser-excited nanostructures enable strong localization of ultrashort light fields and have opened novel routes to fundamentally modify and control electronic strong-field processes. Harnessing spatiotemporally tunable near-fields for the steering of sub-cycle electron dynamics may enable ultrafast optoelectronic devices and unprecedented control in the generation of attosecond electron and photon pulses. Here we utilize unsupported sub-wavelength dielectric nanospheres to generate near-fields with adjustable structure and study the resulting strong-field dynamics via photoelectron imaging. We demonstrate field propagation-induced tunability of the emission direction of fast recollision electrons up to a regime, where nonlinear charge interaction effects become dominant in the acceleration process. Our analysis supports that the timing of the recollision process remains controllable with attosecond resolution by the carrier-envelope phase, indicating the possibility to expand near-field-mediated control far into the realm of high-field phenomena.

Real-Space Mapping of Surface Trap States in CIGSe Nanocrystals Using 4D Electron Microscopy.

PubMed

Bose, Riya; Bera, Ashok; Parida, Manas R; Adhikari, Aniruddha; Shaheen, Basamat S; Alarousu, Erkki; Sun, Jingya; Wu, Tom; Bakr, Osman M; Mohammed, Omar F

2016-07-13

Surface trap states in copper indium gallium selenide semiconductor nanocrystals (NCs), which serve as undesirable channels for nonradiative carrier recombination, remain a great challenge impeding the development of solar and optoelectronics devices based on these NCs. In order to design efficient passivation techniques to minimize these trap states, a precise knowledge about the charge carrier dynamics on the NCs surface is essential. However, selective mapping of surface traps requires capabilities beyond the reach of conventional laser spectroscopy and static electron microscopy; it can only be accessed by using a one-of-a-kind, second-generation four-dimensional scanning ultrafast electron microscope (4D S-UEM) with subpicosecond temporal and nanometer spatial resolutions. Here, we precisely map the collective surface charge carrier dynamics of copper indium gallium selenide NCs as a function of the surface trap states before and after surface passivation in real space and time using S-UEM. The time-resolved snapshots clearly demonstrate that the density of the trap states is significantly reduced after zinc sulfide (ZnS) shelling. Furthermore, the removal of trap states and elongation of carrier lifetime are confirmed by the increased photocurrent of the self-biased photodetector fabricated using the shelled NCs.

Bromine substitution improves excited-state dynamics in mesoporous mixed halide perovskite films.

PubMed

Talbert, Eric M; Zarick, Holly F; Boulesbaa, Abdelaziz; Soetan, Naiya; Puretzky, Alexander A; Geohegan, David B; Bardhan, Rizia

2017-08-24

In this study, ultrafast transient absorption spectroscopy (TAS) is utilized to examine the excited-state dynamics in methylammonium lead iodide/bromide (MAPb(I 1-x Br x ) 3 ) perovskites as a function of bromide content. TAS spectral behavior reveals characteristic lifetimes for thermalization, recombination, and charge carrier injection of MAPb(I 1-x Br x ) 3 from x = 0 to 0.3 infiltrated in mesoporous titania films. Carrier recombination and charge injection lifetimes demonstrated a discernable increase with Br content likely because high carrier populations are supported by the higher density of vacant electronic states in mixed-halide perovskites due to the increased capacity of the conduction band. However, we observe for the first time that carrier thermalization lifetimes significantly decrease with increasing Br. This suggests that the shift in crystal structure from tetragonal towards pseudocubic accelerates carrier cooling, resulting in the relief of the hot phonon bottleneck. Furthermore, the stabilized MAPb(I 1-x Br x ) 3 samples exhibit a lower Burstein-Moss shift of 0.07-0.08 eV compared to pure MAPbI 3 (0.12 eV). Our results provide evidence that Br inclusion contributes to a broadening of the parabolic conduction band and to improvement in electron-phonon coupling and phonon propagation in the lattice.

Bromine substitution improves excited-state dynamics in mesoporous mixed halide perovskite films

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

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

Here in this study, ultrafast transient absorption spectroscopy (TAS) is utilized to examine the excited-state dynamics in methylammonium lead iodide/bromide (MAPb(I 1-xBrx)3) perovskites as a function of bromide content. TAS spectral behavior reveals characteristic lifetimes for thermalization, recombination, and charge carrier injection of MAPb(I 1-xBr x) 3 from x = 0 to 0.3 infiltrated in mesoporous titania films. Carrier recombination and charge injection lifetimes demonstrated a discernable increase with Br content likely because high carrier populations are supported by the higher density of vacant electronic states in mixed-halide perovskites due to the increased capacity of the conduction band. However, wemore » observe for the first time that carrier thermalization lifetimes significantly decrease with increasing Br. This suggests that the shift in crystal structure from tetragonal towards pseudocubic accelerates carrier cooling, resulting in the relief of the hot phonon bottleneck. Furthermore, the stabilized MAPb(I 1-xBrx) 3 samples exhibit a lower Burstein–Moss shift of 0.07–0.08 eV compared to pure MAPbI 3 (0.12 eV). Our results provide evidence that Br inclusion contributes to a broadening of the parabolic conduction band and to improvement in electron–phonon coupling and phonon propagation in the lattice.« less

Bromine substitution improves excited-state dynamics in mesoporous mixed halide perovskite films

DOE PAGES

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

2017-08-02

Here in this study, ultrafast transient absorption spectroscopy (TAS) is utilized to examine the excited-state dynamics in methylammonium lead iodide/bromide (MAPb(I 1-xBrx)3) perovskites as a function of bromide content. TAS spectral behavior reveals characteristic lifetimes for thermalization, recombination, and charge carrier injection of MAPb(I 1-xBr x) 3 from x = 0 to 0.3 infiltrated in mesoporous titania films. Carrier recombination and charge injection lifetimes demonstrated a discernable increase with Br content likely because high carrier populations are supported by the higher density of vacant electronic states in mixed-halide perovskites due to the increased capacity of the conduction band. However, wemore » observe for the first time that carrier thermalization lifetimes significantly decrease with increasing Br. This suggests that the shift in crystal structure from tetragonal towards pseudocubic accelerates carrier cooling, resulting in the relief of the hot phonon bottleneck. Furthermore, the stabilized MAPb(I 1-xBrx) 3 samples exhibit a lower Burstein–Moss shift of 0.07–0.08 eV compared to pure MAPbI 3 (0.12 eV). Our results provide evidence that Br inclusion contributes to a broadening of the parabolic conduction band and to improvement in electron–phonon coupling and phonon propagation in the lattice.« less

Ultrafast dynamics of vibrational symmetry breaking in a charge-ordered nickelate

PubMed Central

Coslovich, Giacomo; Kemper, Alexander F.; Behl, Sascha; Huber, Bernhard; Bechtel, Hans A.; Sasagawa, Takao; Martin, Michael C.; Lanzara, Alessandra; Kaindl, Robert A.

2017-01-01

The ability to probe symmetry-breaking transitions on their natural time scales is one of the key challenges in nonequilibrium physics. Stripe ordering represents an intriguing type of broken symmetry, where complex interactions result in atomic-scale lines of charge and spin density. Although phonon anomalies and periodic distortions attest the importance of electron-phonon coupling in the formation of stripe phases, a direct time-domain view of vibrational symmetry breaking is lacking. We report experiments that track the transient multi-terahertz response of the model stripe compound La1.75Sr0.25NiO4, yielding novel insight into its electronic and structural dynamics following an ultrafast optical quench. We find that although electronic carriers are immediately delocalized, the crystal symmetry remains initially frozen—as witnessed by time-delayed suppression of zone-folded Ni–O bending modes acting as a fingerprint of lattice symmetry. Longitudinal and transverse vibrations react with different speeds, indicating a strong directionality and an important role of polar interactions. The hidden complexity of electronic and structural coupling during stripe melting and formation, captured here within a single terahertz spectrum, opens new paths to understanding symmetry-breaking dynamics in solids. PMID:29202025

Ultrafast energy- and momentum-resolved dynamics of magnetic correlations in the photo-doped Mott insulator Sr2IrO4.

PubMed

Dean, M P M; Cao, Y; Liu, X; Wall, S; Zhu, D; Mankowsky, R; Thampy, V; Chen, X M; Vale, J G; Casa, D; Kim, Jungho; Said, A H; Juhas, P; Alonso-Mori, R; Glownia, J M; Robert, A; Robinson, J; Sikorski, M; Song, S; Kozina, M; Lemke, H; Patthey, L; Owada, S; Katayama, T; Yabashi, M; Tanaka, Yoshikazu; Togashi, T; Liu, J; Rayan Serrao, C; Kim, B J; Huber, L; Chang, C-L; McMorrow, D F; Först, M; Hill, J P

2016-06-01

Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and high-temperature superconductivity. Recently, photo-excitation has been used to induce similarly exotic states transiently. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electron laser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr2IrO4. We find that the non-equilibrium state, 2 ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Néel correlations recover within a few picoseconds, whereas the three-dimensional (3D) long-range magnetic order restores on a fluence-dependent timescale of a few hundred picoseconds. The marked difference in these two timescales implies that the dimensionality of magnetic correlations is vital for our understanding of ultrafast magnetic dynamics.

Ultrafast dynamics of vibrational symmetry breaking in a charge-ordered nickelate

DOE PAGES

Coslovich, Giacomo; Kemper, Alexander F.; Behl, Sascha; ...

2017-11-24

The ability to probe symmetry-breaking transitions on their natural time scales is one of the key challenges in nonequilibrium physics. Stripe ordering represents an intriguing type of broken symmetry, where complex interactions result in atomic-scale lines of charge and spin density. Although phonon anomalies and periodic distortions attest the importance of electron-phonon coupling in the formation of stripe phases, a direct time-domain view of vibrational symmetry breaking is lacking. We report experiments that track the transient multi-terahertz response of the model stripe compound La 1.75Sr 0.25NiO 4, yielding novel insight into its electronic and structural dynamics following an ultrafast opticalmore » quench. We find that although electronic carriers are immediately delocalized, the crystal symmetry remains initially frozen—as witnessed by time-delayed suppression of zone-folded Ni–O bending modes acting as a fingerprint of lattice symmetry. Longitudinal and transverse vibrations react with different speeds, indicating a strong directionality and an important role of polar interactions. As a result, the hidden complexity of electronic and structural coupling during stripe melting and formation, captured here within a single terahertz spectrum, opens new paths to understanding symmetry-breaking dynamics in solids.« less

Ultrafast dynamics of vibrational symmetry breaking in a charge-ordered nickelate

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

Coslovich, Giacomo; Kemper, Alexander F.; Behl, Sascha

The ability to probe symmetry-breaking transitions on their natural time scales is one of the key challenges in nonequilibrium physics. Stripe ordering represents an intriguing type of broken symmetry, where complex interactions result in atomic-scale lines of charge and spin density. Although phonon anomalies and periodic distortions attest the importance of electron-phonon coupling in the formation of stripe phases, a direct time-domain view of vibrational symmetry breaking is lacking. We report experiments that track the transient multi-terahertz response of the model stripe compound La 1.75Sr 0.25NiO 4, yielding novel insight into its electronic and structural dynamics following an ultrafast opticalmore » quench. We find that although electronic carriers are immediately delocalized, the crystal symmetry remains initially frozen—as witnessed by time-delayed suppression of zone-folded Ni–O bending modes acting as a fingerprint of lattice symmetry. Longitudinal and transverse vibrations react with different speeds, indicating a strong directionality and an important role of polar interactions. As a result, the hidden complexity of electronic and structural coupling during stripe melting and formation, captured here within a single terahertz spectrum, opens new paths to understanding symmetry-breaking dynamics in solids.« less

Terahertz science and technology of carbon nanomaterials.

PubMed

Hartmann, R R; Kono, J; Portnoi, M E

2014-08-15

The diverse applications of terahertz (THz) radiation and its importance to fundamental science makes finding ways to generate, manipulate and detect THz radiation one of the key areas of modern applied physics. One approach is to utilize carbon nanomaterials, in particular, single-wall carbon nanotubes and graphene. Their novel optical and electronic properties offer much promise to the field of THz science and technology. This article describes the past, current, and future of THz science and technology of carbon nanotubes and graphene. We will review fundamental studies such as THz dynamic conductivity, THz nonlinearities and ultrafast carrier dynamics as well as THz applications such as THz sources, detectors, modulators, antennas and polarizers.

Femtosecond diffraction dynamics of laser-induced periodic surface structures on fused silica

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

Hoehm, S.; Rosenfeld, A.; Krueger, J.

2013-02-04

The formation of laser-induced periodic surface structures (LIPSS) on fused silica upon irradiation with linearly polarized fs-laser pulses (50 fs pulse duration, 800 nm center wavelength) is studied experimentally using a transillumination femtosecond time-resolved (0.1 ps-1 ns) pump-probe diffraction approach. This allows to reveal the generation dynamics of near-wavelength-sized LIPSS showing a transient diffraction at specific spatial frequencies even before a corresponding permanent surface relief was observed. The results confirm that the ultrafast energy deposition to the materials surface plays a key role and triggers subsequent physical mechanisms such as carrier scattering into self-trapped excitons.

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.

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

Hot carrier and hot phonon coupling during ultrafast relaxation of photoexcited electrons in graphene

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

Iglesias, J. M.; Martín, M. J.; Pascual, E.

2016-01-25

We study, by means of a Monte Carlo simulator, the hot phonon effect on the relaxation dynamics in photoexcited graphene and its quantitative impact as compared with considering an equilibrium phonon distribution. Our multi-particle approach indicates that neglecting the hot phonon effect significantly underestimates the relaxation times in photoexcited graphene. The hot phonon effect is more important for a higher energy of the excitation pulse and photocarrier densities between 1 and 3 × 10{sup 12 }cm{sup −2}. Acoustic intervalley phonons play a non-negligible role, and emitted phonons with wavelengths limited up by a maximum (determined by the carrier concentration) induce a slower carriermore » cooling rate. Intrinsic phonon heating is damped in graphene on a substrate due to the additional cooling pathways, with the hot phonon effect showing a strong inverse dependence with the carrier density.« less

Ultrafast Coulomb-Induced Intervalley Coupling in Atomically Thin WS2.

PubMed

Schmidt, Robert; Berghäuser, Gunnar; Schneider, Robert; Selig, Malte; Tonndorf, Philipp; Malić, Ermin; Knorr, Andreas; Michaelis de Vasconcellos, Steffen; Bratschitsch, Rudolf

2016-05-11

Monolayers of semiconducting transition metal dichalcogenides hold the promise for a new paradigm in electronics by exploiting the valley degree of freedom in addition to charge and spin. For MoS2, WS2, and WSe2, valley polarization can be conveniently initialized and read out by circularly polarized light. However, the underlying microscopic processes governing valley polarization in these atomically thin equivalents of graphene are still not fully understood. Here, we present a joint experiment-theory study on the ultrafast time-resolved intervalley dynamics in monolayer WS2. Based on a microscopic theory, we reveal the many-particle mechanisms behind the observed spectral features. We show that Coulomb-induced intervalley coupling explains the immediate and prominent pump-probe signal in the unpumped valley and the seemingly low valley polarization degrees typically observed in pump-probe measurements compared to photoluminescence studies. The gained insights are also applicable to other light-emitting monolayer transition metal dichalcogenides, such as MoS2 and WSe2, where the Coulomb-induced intervalley coupling also determines the initial carrier dynamics.

Excitons in Single-Walled Carbon Nanotubes and Their Dynamics

NASA Astrophysics Data System (ADS)

Amori, Amanda R.; Hou, Zhentao; Krauss, Todd D.

2018-04-01

Understanding exciton dynamics in single-walled carbon nanotubes (SWCNTs) is essential to unlocking the many potential applications of these materials. This review summarizes recent progress in understanding exciton photophysics and, in particular, exciton dynamics in SWCNTs. We outline the basic physical and electronic properties of SWCNTs, as well as bright and dark transitions within the framework of a strongly bound one-dimensional excitonic model. We discuss the many facets of ultrafast carrier dynamics in SWCNTs, including both single-exciton states (bright and dark) and multiple-exciton states. Photophysical properties that directly relate to excitons and their dynamics, including exciton diffusion lengths, chemical and structural defects, environmental effects, and photoluminescence photon statistics as observed through photon antibunching measurements, are also discussed. Finally, we identify a few key areas for advancing further research in the field of SWCNT excitons and photonics.

Terahertz Emission from Hybrid Perovskites Driven by Ultrafast Charge Separation and Strong Electron-Phonon Coupling

DOE PAGES

Guzelturk, Burak; Belisle, Rebecca A.; Smith, Matthew D.; ...

2018-01-23

Unusual photophysical properties of organic–inorganic hybrid perovskites have not only enabled exceptional performance in optoelectronic devices, but also led to debates on the nature of charge carriers in these materials. This study makes the first observation of intense terahertz (THz) emission from the hybrid perovskite methylammonium lead iodide (CH 3NH 3PbI 3) following photoexcitation, enabling an ultrafast probe of charge separation, hot–carrier transport, and carrier–lattice coupling under 1–sun–equivalent illumination conditions. Using this approach, the initial charge separation/transport in the hybrid perovskites is shown to be driven by diffusion and not by surface fields or intrinsic ferroelectricity. Diffusivities of the hotmore » and band–edge carriers along the surface normal direction are calculated by analyzing the emitted THz transients, with direct implications for hot–carrier device applications. Furthermore, photogenerated carriers are found to drive coherent terahertz–frequency lattice distortions, associated with reorganizations of the lead–iodide octahedra as well as coupled vibrations of the organic and inorganic sublattices. This strong and coherent carrier–lattice coupling is resolved on femtosecond timescales and found to be important both for resonant and far–above–gap photoexcitation. As a result, this study indicates that ultrafast lattice distortions play a key role in the initial processes associated with charge transport.« less

On the photoresponse of several novel functionalized oligoacene and anthradithiophene derivatives

NASA Astrophysics Data System (ADS)

Day, Jonathan

The results of an investigation into carrier dynamics in several novel functionalized and solution-processable pentacene and anthradithiophene derivatives are reported. Measurements were made of real-time photoresponse of polycrystalline thin films of these materials to ultrafast laser pulses, on picosecond to microsecond time-scales, as well as measurements of dark current and current under steady illumination. This data was taken over varied field-strength, light intensity and temperature. The results support a model for carrier generation and transport with the following features. Carrier photo-generation is assisted weakly, if it is assisted at all, thermally or by applied fields. Carriers are initially (picosecond to nanosecond time-scales) in extended states and transport is "bandlike." Carriers then relax into more localized states, transported via thermally assisted hopping (nanosecond to second time-scales). This model was supported by further experiments with the electric behavior of films prepared from a pure anthradithophene derivative, doped with either the buckminsterfullerene C60 or with other molecular dopants. These results also show that samples with traps of known density and depth can be prepared, as a means of manipulating transport dynamics. The electronic and photo-electronic behaviors of films with self-anodized aluminum and of films with gold electrodes were compared, and a model of the particular energy profile and dynamics which exist at the different interfaces between the films and the different contacts was developed. This model views the metal-organic-metal system as an anode-to-anode Schottky strucure, whose I-V relation is shaped both by the nature of the interface dynamics for different metal contacts, and by the different distributions of space-charge in the thin film between different electrodes.

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.

Terahertz radiation from accelerating charge carriers in graphene under ultrafast photoexcitation

NASA Astrophysics Data System (ADS)

Rustagi, Avinash; Stanton, C. J.

2016-11-01

We study the generation of terahertz (THz) radiation from the acceleration of ultrafast photoexcited charge carriers in graphene in the presence of a dc electric field. Our model is based on calculating the transient current density from the time-dependent distribution function which is determined using the Boltzmann transport equation (BTE) within a relaxation time approximation. We include the time-dependent generation of carriers by the pump pulse by solving for the carrier generation rate using the optical Bloch equations in the rotating wave approximation (RWA). The linearly polarized pump pulse generates an anisotropic distribution of photoexcited carriers in the kx-ky plane. The collision integral in the Boltzmann equation includes a term that leads to the thermalization of carriers via carrier-carrier scattering to an effective temperature above the lattice temperature, as well as a cooling term, which leads to energy relaxation via inelastic carrier-phonon scattering. The radiated signal is proportional to the time derivative of the transient current density. In spite of the fact that the magnitude of the velocity is the same for all the carriers in graphene, there is still emitted radiation from the photoexcited charge carriers with frequency components in the THz range due to a change in the direction of velocity of the photoexcited carriers in the external electric field as well as cooling of the photoexcited carriers on a subpicosecond time scale.

Ultrafast and nanoscale diodes

NASA Astrophysics Data System (ADS)

Zhang, Peng; Lau, Y. Y.

2016-10-01

Charge carrier transport across interfaces of dissimilar materials (including vacuum) is the essence of all electronic devices. Ultrafast charge transport across a nanometre length scale is of fundamental importance in the miniaturization of vacuum and plasma electronics. With the combination of recent advances in electronics, photonics and nanotechnology, these miniature devices may integrate with solid-state platforms, achieving superior performance. This paper reviews recent modelling efforts on quantum tunnelling, ultrafast electron emission and transport, and electrical contact resistance. Unsolved problems and challenges in these areas are addressed.

Field propagation-induced directionality of carrier-envelope phase-controlled photoemission from nanospheres

DOE PAGES

SuBmann, F.; Seiffert, L.; Zherebtsov, S.; ...

2015-08-12

Near-fields of non-resonantly laser-excited nanostructures enable strong localization of ultrashort light fields and have opened novel routes to fundamentally modify and control electronic strong-field processes. Harnessing spatiotemporally tunable near-fields for the steering of sub-cycle electron dynamics may enable ultrafast optoelectronic devices and unprecedented control in the generation of attosecond electron and photon pulses. Here we utilize unsupported sub-wavelength dielectric nanospheres to generate near-fields with adjustable structure and study the resulting strong-field dynamics via photoelectron imaging. We demonstrate field propagation-induced tunability of the emission direction of fast recollision electrons up to a regime, where nonlinear charge interaction effects become dominant inmore » the acceleration process. In conclusion, our analysis supports that the timing of the recollision process remains controllable with attosecond resolution by the carrier-envelope phase, indicating the possibility to expand near-field-mediated control far into the realm of high-field phenomena.« less

Field propagation-induced directionality of carrier-envelope phase-controlled photoemission from nanospheres

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

SuBmann, F.; Seiffert, L.; Zherebtsov, S.

Near-fields of non-resonantly laser-excited nanostructures enable strong localization of ultrashort light fields and have opened novel routes to fundamentally modify and control electronic strong-field processes. Harnessing spatiotemporally tunable near-fields for the steering of sub-cycle electron dynamics may enable ultrafast optoelectronic devices and unprecedented control in the generation of attosecond electron and photon pulses. Here we utilize unsupported sub-wavelength dielectric nanospheres to generate near-fields with adjustable structure and study the resulting strong-field dynamics via photoelectron imaging. We demonstrate field propagation-induced tunability of the emission direction of fast recollision electrons up to a regime, where nonlinear charge interaction effects become dominant inmore » the acceleration process. In conclusion, our analysis supports that the timing of the recollision process remains controllable with attosecond resolution by the carrier-envelope phase, indicating the possibility to expand near-field-mediated control far into the realm of high-field phenomena.« less

Phonon properties and slow organic-to-inorganic sub-lattice thermalization in hybrid perovskites

NASA Astrophysics Data System (ADS)

Chan, Maria; Chang, Angela; Xia, Yi; Sadasivam, Sridhar; Guo, Peijun; Kinaci, Alper; Lin, Hao-Wu; Darancet, Pierre; Schaller, Richard

Organic-inorganic hybrid perovskite halide compounds have been investigated extensively for photovoltaics (PVs) and related applications. The thermal transport properties of hybrid perovskites, including phonon-carrier and phonon-phonon interactions, are of significance for their PV and solar thermoelectric applications. The interlocking organic and inorganic sublattices can be thought of as an extreme form of nanostructuring. A result of this nanostructuring is the large gap in phonon frequencies between the organic and inorganic sublattices, which is expected to create bottlenecks in phonon equilibration. In this work, we use a combination of ultrafast spectroscopy including photoluminescence and transient absorption, as well as first principles density functional theory (DFT), ab initio molecular dynamics calculations, phonon lifetimes derived from DFT force constants, and non-equilibrium phonon dynamics accounting for phonon lifetimes, to determine the phonon and charge interaction processes. We find evidence that thermalization of carriers occur at an atypically slow 50-100 ps time scale owing to the complex interplay between electronic and phonon excitations.

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

Doughty, Benjamin; Simpson, Mary Jane; Yang, Bin

Our work aims to simplify multi-dimensional femtosecond transient absorption microscopy (TAM) data into decay associated amplitude maps 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 (CH 3NH 3PbI 3) perovskite thin film allows us to simplify the dataset consisting of a 68 time-resolved images into 4 decay associated amplitude maps. Furthermore, these maps provide 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. Thismore » 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.« less

Ultrafast energy- and momentum-resolved dynamics of magnetic correlations in the photo-doped Mott insulator Sr 2IrO 4

DOE PAGES

Dean, M. P. M.; Cao, Y.; Liu, X.; ...

2016-05-09

Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and high-temperature superconductivity 1, 2, 3, 4. Recently, photo-excitation has been used to induce similarly exotic states transiently 5, 6, 7. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electron laser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr 2IrO 4. We find that the non-equilibrium state, 2more » ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Néel correlations recover within a few picoseconds, whereas the three-dimensional (3D) long-range magnetic order restores on a fluence-dependent timescale of a few hundred picoseconds. In conclusion, the marked difference in these two timescales implies that the dimensionality of magnetic correlations is vital for our understanding of ultrafast magnetic dynamics.« less

Ultrafast Photoinduced Multimode Antiferromagnetic Spin Dynamics in Exchange-Coupled Fe/RFeO3 (R = Er or Dy) Heterostructures.

PubMed

Tang, Jin; Ke, Yajiao; He, Wei; Zhang, Xiangqun; Zhang, Wei; Li, Na; Zhang, Yongsheng; Li, Yan; Cheng, Zhaohua

2018-05-25

Antiferromagnetic spin dynamics is important for both fundamental and applied antiferromagnetic spintronic devices; however, it is rarely explored by external fields because of the strong exchange interaction in antiferromagnetic materials. Here, the photoinduced excitation of ultrafast antiferromagnetic spin dynamics is achieved by capping antiferromagnetic RFeO 3 (R = Er or Dy) with an exchange-coupled ferromagnetic Fe film. Compared with antiferromagnetic spin dynamics of bare RFeO 3 orthoferrite single crystals, which can be triggered effectively by ultrafast laser heating just below the phase transition temperature, the ultrafast photoinduced multimode antiferromagnetic spin dynamic modes, for exchange-coupled Fe/RFeO 3 heterostructures, including quasiferromagnetic resonance, impurity, coherent phonon, and quasiantiferromagnetic modes, are observed in a temperature range of 10-300 K. These experimental results not only offer an effective means to trigger ultrafast antiferromagnetic spin dynamics of rare-earth orthoferrites, but also shed light on the ultrafast manipulation of antiferromagnetic magnetization in Fe/RFeO 3 heterostructures. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

Ultra-Broadband Two-Dimensional Electronic Spectroscopy and Pump-Probe Microscopy of Molecular Systems

NASA Astrophysics Data System (ADS)

Spokoyny, Boris M.

Ultrafast spectroscopy offers an unprecedented view on the dynamic nature of chemical reactions. From charge transfer in semiconductors to folding and isomerization of proteins, these all important processes can now be monitored and in some instances even controlled on real, physical timescales. One of the biggest challenges of ultrafast science is the incredible energetic complexity of most systems. It is not uncommon to encounter macromolecules or materials with absorption spectra spanning significant portions of the visible spectrum. Monitoring a multitude of electronic and vibrational transitions, all dynamically interacting with each other on femtosecond timescales poses a truly daunting experimental task. The first part of this thesis deals with the development of a novel Two-Dimensional Electronic Spectroscopy (2DES) and its associated, advanced detection methodologies. Owing to its ultra-broadband implementation, this technique enables us to monitor femtosecond chemical dynamics that span the energetic landscape of the entire visible spectrum. In order to demonstrate the utility of our method, we apply it to two laser dye molecules, IR-144 and Cresyl Violet. Variation of photophysical properties on a microscopic scale in either man-made or naturally occurring systems can have profound implications on how we understand their macroscopic properties. Recently, inorganic hybrid perovskites have been tapped as the next generation solar energy harvesting materials. Their remarkable properties include low exciton binding energy, low exciton recombination rates and long carrier diffusion lengths. Nevertheless, considerable variability in device properties made with nearly identical preparation methods has puzzled the community. In the second part of this thesis we use non-linear pump probe microscopy to study the heterogeneous nature of femtosecond carrier dynamics in thin film perovskites. We show that the local morphology of the perovskite thin films has a profound influence on the underlying photophysics, opening new avenues for further optimization of device performance.

Acoustic-optical phonon up-conversion and hot-phonon bottleneck in lead-halide perovskites

PubMed Central

Yang, Jianfeng; Wen, Xiaoming; Xia, Hongze; Sheng, Rui; Ma, Qingshan; Kim, Jincheol; Tapping, Patrick; Harada, Takaaki; Kee, Tak W.; Huang, Fuzhi; Cheng, Yi-Bing; Green, Martin; Ho-Baillie, Anita; Huang, Shujuan; Shrestha, Santosh; Patterson, Robert; Conibeer, Gavin

2017-01-01

The hot-phonon bottleneck effect in lead-halide perovskites (APbX3) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA+/MA+/Cs+, X=I−/Br−) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI3. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials. PMID:28106061

Colloidal thallium halide nanocrystals with reasonable luminescence, carrier mobility and diffusion length† †Electronic supplementary information (ESI) available: Synthesis and additional characterization of nanocrystals, characterization of nanocrystal films, temperature-dependent phase transition, coefficient of volume expansion, PL decay dynamics, tabulated best fit parameters, and methodology analysis of ultrafast optical pump THz probe (OPTP) spectroscopy. See DOI: 10.1039/c7sc01219e Click here for additional data file.

PubMed Central

Mir, Wasim J.; Warankar, Avinash; Acharya, Ashutosh; Das, Shyamashis

2017-01-01

Colloidal lead halide based perovskite nanocrystals (NCs) have been recently established as an interesting class of defect-tolerant NCs with potential for superior optoelectronic applications. The electronic band structure of thallium halides (TlX, where X = Br and I) show a strong resemblance to lead halide perovskites, where both Pb2+ and Tl+ exhibit a 6s2 inert pair of electrons and strong spin–orbit coupling. Although the crystal structure of TlX is not perovskite, the similarities of its electronic structure with lead halide perovskites motivated us to prepare colloidal TlX NCs. These TlX NCs exhibit a wide bandgap (>2.5 eV or <500 nm) and the potential to exhibit a reduced density of deep defect states. Optical pump terahertz (THz) probe spectroscopy with excitation fluence in the range of 0.85–5.86 × 1013 photons per cm2 on NC films shows that the TlBr NCs possess high effective carrier mobility (∼220 to 329 cm2 V–1 s–1), long diffusion length (∼0.77 to 0.98 μm), and reasonably high photoluminescence efficiency (∼10%). This combination of properties is remarkable compared to other wide-bandgap (>2.5 eV) semiconductor NCs, which suggests a reduction in the deep-defect states in the TlX NCs. Furthermore, the ultrafast carrier dynamics and temperature-dependent reversible structural phase transition together with its influence on the optical properties of the TlX NCs are studied. PMID:28970882

Ultralow surface recombination velocity in InP nanowires probed by terahertz spectroscopy.

PubMed

Joyce, Hannah J; Wong-Leung, Jennifer; Yong, Chaw-Keong; Docherty, Callum J; Paiman, Suriati; Gao, Qiang; Tan, H Hoe; Jagadish, Chennupati; Lloyd-Hughes, James; Herz, Laura M; Johnston, Michael B

2012-10-10

Using transient terahertz photoconductivity measurements, we have made noncontact, room temperature measurements of the ultrafast charge carrier dynamics in InP nanowires. InP nanowires exhibited a very long photoconductivity lifetime of over 1 ns, and carrier lifetimes were remarkably insensitive to surface states despite the large nanowire surface area-to-volume ratio. An exceptionally low surface recombination velocity (170 cm/s) was recorded at room temperature. These results suggest that InP nanowires are prime candidates for optoelectronic devices, particularly photovoltaic devices, without the need for surface passivation. We found that the carrier mobility is not limited by nanowire diameter but is strongly limited by the presence of planar crystallographic defects such as stacking faults in these predominantly wurtzite nanowires. These findings show the great potential of very narrow InP nanowires for electronic devices but indicate that improvements in the crystallographic uniformity of InP nanowires will be critical for future nanowire device engineering.

Time-domain measurement of terahertz frequency magnetoplasmon resonances in a two-dimensional electron system by the direct injection of picosecond pulsed currents

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

Wu, Jingbo; Mayorov, Alexander S.; Wood, Christopher D.

2016-02-29

We have investigated terahertz (THz) frequency magnetoplasmon resonances in a two-dimensional electron system through the direct injection of picosecond duration current pulses. The evolution of the time-domain signals was measured as a function of magnetic field, and the results were found to be in agreement with calculations using a mode-matching approach for four modes observed in the frequency range above 0.1 THz. This introduces a generic technique suitable for sampling ultrafast carrier dynamics in low-dimensional semiconductor nanostructures at THz frequencies.

The Dawn of Ultrafast Nonlinear Optics in the Terahertz Regime

NASA Astrophysics Data System (ADS)

Blanchard, F.; Razzari, L.; Su, F. H.; Sharma, G.; Morandotti, Roberto; Ozaki, T.; Reid, M.; Hegmann, F. A.

The terahertz (THz) frequency range is a specific region of the electromagnetic spectrum also known as the far-infrared (FIR) region. More precisely, THz waves cover the region from 100 GHz to 20 THz, thus bridging the gap between microwaves and infrared light. Physically, 1 THz is equivalent to a wavelength of 300 μm in vacuum, to 33.3 cm-1 in terms of wave numbers, to a photon energy of 4 meV, or to a temperature of 48 K. THz waves have the ability to penetrate various materials including non-metallic compounds (papers and plastics), organics, gases, and liquids, thus being a powerful tool for spectroscopic sensing [1]. This portion of the electromagnetic spectrum has been accessible for some time by various means including molecular gas lasers, gyrotrons, and free-electron lasers [2]. Due to complexity, cost, and limited frequencies of operation, these sources have traditionally made it difficult to gain full access to the terahertz frequency range. Nevertheless, there were several pioneering works in nonlinear FIR spectroscopy already in the early 1970s, about one decade after the advent of the laser (readers may find a review in [3]). In particular, saturated absorption in the FIR region was first studied in 1970, which led to the optically pumped FIR gas laser [4]. In the 1980s, the first demonstration of THz radiation coherently generated and detected was made. This result coincided with the development of ultrafast lasers and was obtained using a photoconductive antenna emitter [5], where photoexcited carriers induced by an ultrafast laser pulse are accelerated by a biasing electric field. The resulting time varying current J(t) radiates an electromagnetic transient, E ∝ partial J/partial t , whose amplitude and phase depend on various parameters such as carrier mobility, carrier lifetime, bias field, and on the impurity doping concentration [6]. This allowed the birth of coherent time-domain THz spectroscopy (TDTS) [1], which provided unprecedented insights into the nature of molecular vibrations, carrier dynamics in semiconductors, and protein kinetics [7-12]. Even with 30 years of rapid advances in the study of light-matter interactions at THz frequencies, lack of efficient emitters and sensitive detectors in this frequency range has for long time slowed down THz linear and nonlinear spectroscopy.

Ultrafast dynamic computed tomography myelography for the precise identification of high-flow cerebrospinal fluid leaks caused by spiculated spinal osteophytes.

PubMed

Thielen, Kent R; Sillery, John C; Morris, Jonathan M; Hoxworth, Joseph M; Diehn, Felix E; Wald, John T; Rosebrock, Richard E; Yu, Lifeng; Luetmer, Patrick H

2015-03-01

Precise localization and understanding of the origin of spontaneous high-flow spinal CSF leaks is required prior to targeted treatment. This study demonstrates the utility of ultrafast dynamic CT myelography for the precise localization of high-flow CSF leaks caused by spiculated spinal osteophytes. This study reports a series of 14 patients with high-flow CSF leaks caused by spiculated spinal osteophytes who underwent ultrafast dynamic CT myelography between March 2009 and December 2010. There were 10 male and 4 female patients, with an average age of 49 years (range 37-74 years). The value of ultrafast dynamic CT myelography in depicting the CSF leak site was qualitatively assessed. In all 14 patients, ultrafast dynamic CT myelography was technically successful at precisely demonstrating the site of the CSF leak, the causative spiculated osteophyte piercing the dura, and the relationship of the implicated osteophyte to adjacent structures. Leak sites included 3 cervical, 11 thoracic, and 0 lumbar levels, with 86% of the leaks occurring from C-5 to T-7. Information obtained from the ultrafast dynamic CT myelogram was considered useful in all treated CSF leaks. Spinal osteophytes piercing the dura are a more frequent cause of high-flow CSF leaks than previously recognized. Ultrafast dynamic CT myelography adds value beyond standard dynamic myelography or digital subtraction myelography in the diagnosis and anatomical characterization of high-flow spinal CSF leaks caused by these osteophytes. This information allows for appropriate planning for percutaneous or surgical treatment.

Revealing giant internal magnetic fields due to spin fluctuations in magnetically doped colloidal nanocrystals

DOE PAGES

Rice, William D.; Liu, Wenyong; Baker, Thomas A.; ...

2015-11-23

Strong quantum confinement in semiconductors can compress the wavefunctions of band electrons and holes to nanometre-scale volumes, significantly enhancing interactions between themselves and individual dopants. In magnetically doped semiconductors, where paramagnetic dopants (such as Mn 2+, Co 2+ and so on) couple to band carriers via strong sp–d spin exchange, giant magneto-optical effects can therefore be realized in confined geometries using few or even single impurity spins. Importantly, however, thermodynamic spin fluctuations become increasingly relevant in this few-spin limit. In nanoscale volumes, the statistical √N fluctuations of N spins are expected to generate giant effective magnetic fields B eff, whichmore » should dramatically impact carrier spin dynamics, even in the absence of any applied field. In this paper, we directly and unambiguously reveal the large B eff that exist in Mn 2+-doped CdSe colloidal nanocrystals using ultrafast optical spectroscopy. At zero applied magnetic field, extremely rapid (300–600 GHz) spin precession of photoinjected electrons is observed, indicating B eff ~ 15-30 T for electrons. Precession frequencies exceed 2 THz in applied magnetic fields. Finally, these signals arise from electron precession about the random fields due to statistically incomplete cancellation of the embedded Mn 2+ moments, thereby revealing the initial coherent dynamics of magnetic polaron formation, and highlighting the importance of magnetization fluctuations on carrier spin dynamics in nanomaterials.« less

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.

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.

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

Optoelectronic forces with quantum wells for cavity optomechanics in GaAs/AlAs semiconductor microcavities

NASA Astrophysics Data System (ADS)

Villafañe, V.; Sesin, P.; Soubelet, P.; Anguiano, S.; Bruchhausen, A. E.; Rozas, G.; Carbonell, C. Gomez; Lemaître, A.; Fainstein, A.

2018-05-01

Radiation pressure, electrostriction, and photothermal forces have been investigated to evidence backaction, nonlinearities, and quantum phenomena in cavity optomechanics. We show here through a detailed study of the relative intensity of the cavity mechanical modes observed when exciting with pulsed lasers close to the GaAs optical gap that optoelectronic forces involving real carrier excitation and deformation potential interaction are the strongest mechanism of light-to-sound transduction in semiconductor GaAs/AlAs distributed Bragg reflector optomechanical resonators. We demonstrate that the ultrafast spatial redistribution of the photoexcited carriers in microcavities with massive GaAs spacers leads to an enhanced coupling to the fundamental 20-GHz vertically polarized mechanical breathing mode. The carrier diffusion along the growth axis of the device can be enhanced by increasing the laser power, or limited by embedding GaAs quantum wells in the cavity spacer, a strategy used here to prove and engineer the optoelectronic forces in phonon generation with real carriers. The wavelength dependence of the observed phenomena provide further proof of the role of optoelectronic forces. The optical forces associated with the different intervening mechanisms and their relevance for dynamical backaction in optomechanics are evaluated using finite-element methods. The results presented open the path to the study of hitherto seldom investigated dynamical backaction in optomechanical solid-state resonators in the presence of optoelectronic forces.

Tracking the ultrafast motion of a single molecule by femtosecond orbital imaging

NASA Astrophysics Data System (ADS)

Cocker, Tyler L.; Peller, Dominik; Yu, Ping; Repp, Jascha; Huber, Rupert

2016-11-01

Watching a single molecule move on its intrinsic timescale has been one of the central goals of modern nanoscience, and calls for measurements that combine ultrafast temporal resolution with atomic spatial resolution. Steady-state experiments access the requisite spatial scales, as illustrated by direct imaging of individual molecular orbitals using scanning tunnelling microscopy or the acquisition of tip-enhanced Raman and luminescence spectra with sub-molecular resolution. But tracking the intrinsic dynamics of a single molecule directly in the time domain faces the challenge that interactions with the molecule must be confined to a femtosecond time window. For individual nanoparticles, such ultrafast temporal confinement has been demonstrated by combining scanning tunnelling microscopy with so-called lightwave electronics, which uses the oscillating carrier wave of tailored light pulses to directly manipulate electronic motion on timescales faster even than a single cycle of light. Here we build on ultrafast terahertz scanning tunnelling microscopy to access a state-selective tunnelling regime, where the peak of a terahertz electric-field waveform transiently opens an otherwise forbidden tunnelling channel through a single molecular state. It thereby removes a single electron from an individual pentacene molecule’s highest occupied molecular orbital within a time window shorter than one oscillation cycle of the terahertz wave. We exploit this effect to record approximately 100-femtosecond snapshot images of the orbital structure with sub-ångström spatial resolution, and to reveal, through pump/probe measurements, coherent molecular vibrations at terahertz frequencies directly in the time domain. We anticipate that the combination of lightwave electronics and the atomic resolution of our approach will open the door to visualizing ultrafast photochemistry and the operation of molecular electronics on the single-orbital scale.

Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3

PubMed Central

Braun, Lukas; Mussler, Gregor; Hruban, Andrzej; Konczykowski, Marcin; Schumann, Thomas; Wolf, Martin; Münzenberg, Markus; Perfetti, Luca; Kampfrath, Tobias

2016-01-01

Three-dimensional topological insulators are fascinating materials with insulating bulk yet metallic surfaces that host highly mobile charge carriers with locked spin and momentum. Remarkably, surface currents with tunable direction and magnitude can be launched with tailored light beams. To better understand the underlying mechanisms, the current dynamics need to be resolved on the timescale of elementary scattering events (∼10 fs). Here, we excite and measure photocurrents in the model topological insulator Bi2Se3 with a time resolution of 20 fs by sampling the concomitantly emitted broadband terahertz (THz) electromagnetic field from 0.3 to 40 THz. Strikingly, the surface current response is dominated by an ultrafast charge transfer along the Se–Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to be orders of magnitude smaller than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are of direct relevance for broadband optoelectronic devices based on topological-insulator surface currents. PMID:27796297

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.

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.

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.

Charge Carrier Dynamics of Quantum Confined Semiconductor Nanoparticles Analyzed via Transient Absorption Spectroscopy

NASA Astrophysics Data System (ADS)

Thibert, Arthur Joseph, III

Semiconductor nanoparticles are tiny crystalline structures (typically range from 1 - 100 nm) whose shape in many cases can be dictated through tailored chemical synthesis with atomic scale precision. The small size of these nanoparticles often results in quantum confinement (spatial confinement of wave functions), which imparts the ability to manipulate band-gap energies thus allowing them to be optimally engineered for different applications (i.e., photovoltaics, photocatalysis, imaging). However, charge carriers excited within these nanoparticles are often involved in many different processes: trapping, trap migration, Auger recombination, non-radiative relaxation, radiative relaxation, oxidation / reduction, or multiple exciton generation. Broadband ultrafast transient absorption laser spectroscopy is used to spectrally resolve the fate of excited charge carriers in both wavelength and time, providing insight as to what synthetic developments or operating conditions will be necessary to optimize their efficiency for certain applications. This thesis outlines the effort of resolving the dynamics of excited charge carriers for several Cd and Si based nanoparticle systems using this experimental technique. The thesis is organized into five chapters and two appendices as indicated below. Chapter 1 provides a brief introduction to the photophysics of semiconductor nanoparticles. It begins by defining what nanoparticles, semiconductors, charge carriers, and quantum confinement are. From there it details how the study of charge carrier dynamics within nanoparticles can lead to increased efficiency in applications such as photocatalysis. Finally, the experimental methodology associated with ultrafast transient absorption spectroscopy is introduced and its power in mapping charge carrier dynamics is established. Chapter 2 (JPCC, 19647, 2011) introduces the first of the studied samples: water-solubilized 2D CdSe nanoribbons (NRs), which were synthesized in the Osterloh laboratory (UCD). The measured signals were decomposed into the constituent dynamics of three transient populations: hot tightly bound excitons, relaxed tightly bound excitons, and separated trapped carriers (holes and electrons). The influenes of three external factors affecting the observed dynamics were explored: (1) excitation wavelength, (2) excitation fluence, and (3) presence of the hole scavenger HS -. Both higher-energy excitation photons and higher-intensity excitation induce slower relaxation of charge carriers to the band edge due to the need to dissipate excess excitation energy. Nonlinear decay kinetics of the relaxed exciton population is observed and demonstrated to arise from bimolecular trapping of excitons with low-density trap sites located at CdSe NR surface sites instead of the commonly resolved multiparticle Auger recombination mechanism. This is supported by the observed linear excitation-fluence dependence of the trapped-carrier population that is n umerically simulated and found to deviate from the excitation fluence dependence expected of Auger recombination kinetics. Introducing hole scavenging HS- has a negligible effect on the exciton kinetics, including migration and dissociation, and instead passivates surface trap states to induce the rapid elimination of holes after exciton dissociation. This increases the lifetime of the reactive electron population and increases measured photocatalytic H2 generation activity. A broad (200 nm) and persistent (20 ps) stimulated emission observed in the tightly bound excitons suggests their potential use as broadband microlasers. In chapter 3 (JPCL, 2688, 2011), the photocatalytic H2O splitting activities of CdSe and CdSe/CdS core/shell quantum dots, which were also synthesized in the Osterloh laboratory (UCD) are contrasted. CdSe/CdS core/shell quantum dots constructed from 4.0 nm CdSe quantum dots are shown to be strongly active for visible-light-driven photocatalytic H2 evolution in 0.1M Na 2S/Na2SO3 solution with a turnover number of 9.94 after 5 h at 103.9 μmol/h. CdSe quantum dots themselves are only marginally active in 0.1 M Na2S/Na2SO3 solution with a turnover number of 1.10 after 5 h at 11.53 μmol/h, while CdSe quantum dots in pure H2O are found to be completely inactive. Broad-band transient absorption spectroscopy is used to elucidate the mechanisms that facilitate the enhancement in the CdSe core/shell quantum dots, which is attributed to passivation of surface-deep trap states with energies lying below the reduction potential necessary for H2O reduction. Thus, it is shown that surface trapping dynamics and energetics can be manipulated to dictate the photocatalytic activities of novel CdSe quantum dot based photocatalytic materials. Chapter 4 builds upon this work examining the differences in dynamics that occur upon passivation of water soluble CdZnS alloy cores with ZnS shells, which were produced in the Snee laboratory (UI Chicago), via 400 nm pump broadband probe ultrafast transient absorption spectroscopy, and global analysis modeling. We also examine the perturbation invoked on charge carrier dynamics caused by growing Pd nanoparticles on the CdZnS/ZnS shell surface in-situ and note the cyclical charge carrier transfer that takes place. Both the CdZnS core and CdZnS/ZnS core/shell quantum dots exhibit unusually long lived excited states (much > 8 ns) while the CdZnS/ZnS.Pd tandem core/shell quantum dots recover much quicker (~3 ns). Additionally, ultrafast excitation fluence dependencies are used to characterize Auger recombination and the presence of two different trap state populations observable in the visible spectrum. In chapter 5 (JACS, 20664, 2011), we switch from examining direct band-gap chalcogenide based quantum dots to Si quantum dots synthesized in the Kauzlarich laboratory (UCD), which exhibit an indirect band-gap. Here a microwave-assisted reaction to produce hydrogen-terminated silicon quantum dots is discussed. The Si quantum dots were passivated for water solubility via two different methods: hydrosilylation produced 3-aminopropenyl-terminated Si quantum dots, and a modified Stöber process produced silica-encapsulated Si quantum dots. Both methods produce water-soluble quantum dots with maximum emission at 414 nm, and after purification, the quantum dots exhibit intrinsic fluorescence quantum yield efficiencies of 15 and 23%, respectively. Even though the quantum dots have different surfaces, they exhibit nearly identical absorption and fluorescence spectra. Femtosecond transient absorption spectroscopy was used for temporal resolution of the photoexcited carrier dynamics between the quantum dots and ligand. The transient dynamics of the 3-aminopropenyl-terminated Si quantum dots is interpreted as a formation and decay of a charge-transfer excited state between the delocalized π electrons of the carbon linker and the Si core excitons. This charge transfer state is stable for ~4 ns before reverting back to a more stable, long-living species. The silica-encapsulated Si QDs show a simpler spectrum without charge transfer dynamics. Appendix I (Chem. Mat., 1220, 2010), addresses the long-time (μs) transient kinetics associated with TiO2 and layered titanates (TBA2 2Ti4O9), which were synthesized in the Osterloh laboratory (UCD). Transient absorption data reveal that photogenerated electrons become trapped in mid band-gap states, from which they decay exponentially with a time-constant of 43.67 + 0.28 ms in titanates, which is much slower than the 68 + 1 ns observed for TiO2 nanocrystals. The slower kinetics observed for the TBA 2Ti4O9 nanosheets originates either from the presence of deeper trap sites on the sheets vs. the nanoparticles, more trap sites, or from more effective electron-hole separation because of the micrometer dimensions of the 2D lattice. Appendix II, depicts the visible solar spectrum at sea level detailing the percentage of photons and energy that exist within certain wavelength ranges.

Mechanism for Broadband White-Light Emission from Two-Dimensional (110) Hybrid Perovskites.

PubMed

Hu, Te; Smith, Matthew D; Dohner, Emma R; Sher, Meng-Ju; Wu, Xiaoxi; Trinh, M Tuan; Fisher, Alan; Corbett, Jeff; Zhu, X-Y; Karunadasa, Hemamala I; Lindenberg, Aaron M

2016-06-16

The recently discovered phenomenon of broadband white-light emission at room temperature in the (110) two-dimensional organic-inorganic perovskite (N-MEDA)[PbBr4] (N-MEDA = N(1)-methylethane-1,2-diammonium) is promising for applications in solid-state lighting. However, the spectral broadening mechanism and, in particular, the processes and dynamics associated with the emissive species are still unclear. Herein, we apply a suite of ultrafast spectroscopic probes to measure the primary events directly following photoexcitation, which allows us to resolve the evolution of light-induced emissive states associated with white-light emission at femtosecond resolution. Terahertz spectra show fast free carrier trapping and transient absorption spectra show the formation of self-trapped excitons on femtosecond time-scales. Emission-wavelength-dependent dynamics of the self-trapped exciton luminescence are observed, indicative of an energy distribution of photogenerated emissive states in the perovskite. Our results are consistent with photogenerated carriers self-trapped in a deformable lattice due to strong electron-phonon coupling, where permanent lattice defects and correlated self-trapped states lend further inhomogeneity to the excited-state potential energy surface.

Carrier-envelope phase-dependent ionization of Xe in intense, ultrafast (two-cycle) laser fields

NASA Astrophysics Data System (ADS)

Vasa, Parinda; Dharmadhikari, Aditya K.; Mathur, Deepak

2018-01-01

We report an experimental study that shows the dependence of the tunnel ionization of Xe by two-cycle, intense, near infrared light on the carrier-envelope-phase (CEP) of incident laser pulses. At low values of the optical field (E), the ionization yield is found to be maximum for cos-like pulses; the CEP dependence of the ion yield becomes stronger for higher charge states. At higher E-values, the CEP dependence either washes out or flips. A simple phenomenological model is used to confirm that our results fall within the ambit of the current understanding of ionization dynamics in strong, ultrashort optical fields. In the observed tunnel ionization of Xe, CEP effects appear to persist for longer, eight-cycle, pulses. Electron rescattering is observed to play a relatively unimportant role in the observed CEP dependence. These results provide fresh perspectives in the ionization mechanisms of multielectron systems in the few-cycle regime.

Theory and computation of hot carriers generated by surface plasmon polaritons in noble metals

PubMed Central

Bernardi, Marco; Mustafa, Jamal; Neaton, Jeffrey B.; Louie, Steven G.

2015-01-01

Hot carriers (HC) generated by surface plasmon polaritons (SPPs) in noble metals are promising for application in optoelectronics, plasmonics and renewable energy. However, existing models fail to explain key quantitative details of SPP-to-HC conversion experiments. Here we develop a quantum mechanical framework and apply first-principles calculations to study the energy distribution and scattering processes of HCs generated by SPPs in Au and Ag. We find that the relative positions of the s and d bands of noble metals regulate the energy distribution and mean free path of the HCs, and that the electron–phonon interaction controls HC energy loss and transport. Our results prescribe optimal conditions for HC generation and extraction, and invalidate previously employed free-electron-like models. Our work combines density functional theory, GW and electron–phonon calculations to provide microscopic insight into HC generation and ultrafast dynamics in noble metals. PMID:26033445

Nonlinear terahertz devices utilizing semiconducting plasmonic metamaterials

DOE PAGES

Seren, Huseyin R.; Zhang, Jingdi; Keiser, George R.; ...

2016-01-26

The development of responsive metamaterials has enabled the realization of compact tunable photonic devices capable of manipulating the amplitude, polarization, wave vector and frequency of light. Integration of semiconductors into the active regions of metallic resonators is a proven approach for creating nonlinear metamaterials through optoelectronic control of the semiconductor carrier density. Metal-free subwavelength resonant semiconductor structures offer an alternative approach to create dynamic metamaterials. We present InAs plasmonic disk arrays as a viable resonant metamaterial at terahertz frequencies. Importantly, InAs plasmonic disks exhibit a strong nonlinear response arising from electric field-induced intervalley scattering, resulting in a reduced carrier mobilitymore » thereby damping the plasmonic response. here, we demonstrate nonlinear perfect absorbers configured as either optical limiters or saturable absorbers, including flexible nonlinear absorbers achieved by transferring the disks to polyimide films. Nonlinear plasmonic metamaterials show potential for use in ultrafast terahertz (THz) optics and for passive protection of sensitive electromagnetic devices.« less

Nonlinear terahertz devices utilizing semiconducting plasmonic metamaterials

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

Seren, Huseyin R.; Zhang, Jingdi; Keiser, George R.

The development of responsive metamaterials has enabled the realization of compact tunable photonic devices capable of manipulating the amplitude, polarization, wave vector and frequency of light. Integration of semiconductors into the active regions of metallic resonators is a proven approach for creating nonlinear metamaterials through optoelectronic control of the semiconductor carrier density. Metal-free subwavelength resonant semiconductor structures offer an alternative approach to create dynamic metamaterials. We present InAs plasmonic disk arrays as a viable resonant metamaterial at terahertz frequencies. Importantly, InAs plasmonic disks exhibit a strong nonlinear response arising from electric field-induced intervalley scattering, resulting in a reduced carrier mobilitymore » thereby damping the plasmonic response. here, we demonstrate nonlinear perfect absorbers configured as either optical limiters or saturable absorbers, including flexible nonlinear absorbers achieved by transferring the disks to polyimide films. Nonlinear plasmonic metamaterials show potential for use in ultrafast terahertz (THz) optics and for passive protection of sensitive electromagnetic devices.« less

Reduced Carrier Recombination in PbS - CuInS2 Quantum Dot Solar Cells

PubMed Central

Sun, Zhenhua; Sitbon, Gary; Pons, Thomas; Bakulin, Artem A.; Chen, Zhuoying

2015-01-01

Energy loss due to carrier recombination is among the major factors limiting the performance of TiO2/PbS colloidal quantum dot (QD) heterojunction solar cells. In this work, enhanced photocurrent is achieved by incorporating another type of hole-transporting QDs, Zn-doped CuInS2 (Zn-CIS) QDs into the PbS QD matrix. Binary QD solar cells exhibit a reduced charge recombination associated with the spatial charge separation between these two types of QDs. A ~30% increase in short-circuit current density and a ~20% increase in power conversion efficiency are observed in binary QD solar cells compared to cells built from PbS QDs only. In agreement with the charge transfer process identified through ultrafast pump/probe spectroscopy between these two QD components, transient photovoltage characteristics of single-component and binary QDs solar cells reveal longer carrier recombination time constants associated with the incorporation of Zn-CIS QDs. This work presents a straightforward, solution-processed method based on the incorporation of another QDs in the PbS QD matrix to control the carrier dynamics in colloidal QD materials and enhance solar cell performance. PMID:26024021

Steering optical comb frequencies by rotating the polarization state

NASA Astrophysics Data System (ADS)

Zhang, Yanyan; Zhang, Xiaofei; Yan, Lulu; Zhang, Pan; Rao, Bingjie; Han, Wei; Guo, Wenge; Zhang, Shougang; Jiang, Haifeng

2017-12-01

Optical frequency combs, with precise control of repetition rate and carrier-envelope-offset frequency, have revolutionized many fields, such as fine optical spectroscopy, optical frequency standards, ultra-fast science research, ultra-stable microwave generation and precise ranging measurement. However, existing high bandwidth frequency control methods have small dynamic range, requiring complex hybrid control techniques. To overcome this limitation, we develop a new approach, where a home-made intra-cavity electro-optic modulator tunes polarization state of laser signal rather than only optical length of the cavity, to steer frequencies of a nonlinear-polarization-rotation mode-locked laser. By taking advantage of birefringence of the whole cavity, this approach results in not only broadband but also relative large-dynamic frequency control. Experimental results show that frequency control dynamic range increase at least one order in comparison with the traditional intra-cavity electro-optic modulator technique. In additional, this technique exhibits less side-effect than traditional frequency control methods.

Dynamic Optical Tuning of Interlayer Interactions in the Transition Metal Dichalcogenides

DOE PAGES

Mannebach, Ehren M.; Nyby, Clara; Ernst, Friederike; ...

2017-11-09

Modulation of weak interlayer interactions between quasi-two-dimensional atomic planes in the transition metal dichalcogenides (TMDCs) provides avenues for tuning their functional properties. Here we show that above-gap optical excitation in the TMDCs leads to an unexpected large-amplitude, ultrafast compressive force between the two-dimensional layers, as probed by in situ measurements of the atomic layer spacing at femtosecond time resolution. We show that this compressive response arises from a dynamic modulation of the interlayer van der Waals interaction and that this represents the dominant light-induced stress at low excitation densities. A simple analytic model predicts the magnitude and carrier density dependencemore » of the measured strains. Furthermore, this work establishes a new method for dynamic, nonequilibrium tuning of correlation-driven dispersive interactions and of the optomechanical functionality of TMDC quasi-two-dimensional materials.« less

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 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

Ultrafast and nonlinear surface-enhanced Raman spectroscopy.

PubMed

Gruenke, Natalie L; Cardinal, M Fernanda; McAnally, Michael O; Frontiera, Renee R; Schatz, George C; Van Duyne, Richard P

2016-04-21

Ultrafast surface-enhanced Raman spectroscopy (SERS) has the potential to study molecular dynamics near plasmonic surfaces to better understand plasmon-mediated chemical reactions such as plasmonically-enhanced photocatalytic or photovoltaic processes. This review discusses the combination of ultrafast Raman spectroscopic techniques with plasmonic substrates for high temporal resolution, high sensitivity, and high spatial resolution vibrational spectroscopy. First, we introduce background information relevant to ultrafast SERS: the mechanisms of surface enhancement in Raman scattering, the characterization of plasmonic materials with ultrafast techniques, and early complementary techniques to study molecule-plasmon interactions. We then discuss recent advances in surface-enhanced Raman spectroscopies with ultrafast pulses with a focus on the study of molecule-plasmon coupling and molecular dynamics with high sensitivity. We also highlight the challenges faced by this field by the potential damage caused by concentrated, highly energetic pulsed fields in plasmonic hotspots, and finally the potential for future ultrafast SERS studies.

Laser-plasma accelerator-based single-cycle attosecond undulator source

NASA Astrophysics Data System (ADS)

Tibai, Z.; Tóth, Gy.; Nagyváradi, A.; Sharma, A.; Mechler, M. I.; Fülöp, J. A.; Almási, G.; Hebling, J.

2018-06-01

Laser-plasma accelerators (LPAs), producing high-quality electron beams, provide an opportunity to reduce the size of free-electron lasers (FELs) to only a few meters. A complete system is proposed here, which is based on FEL technology and consists of an LPA, two undulators, and other magnetic devices. The system is capable to generate carrier-envelope phase stable attosecond pulses with engineered waveform. Pulses with up to 60 nJ energy and 90-400 attosecond duration in the 30-120 nm wavelength range are predicted by numerical simulation. These pulses can be used to investigate ultrafast field-driven electron dynamics in matter.

Synthesis and Characterization of Manganese Doped Silicon Nanoparticles

PubMed Central

Zhang, Xiaoming; Brynda, Marcin; Britt, R. David; Carroll, Elizabeth; Larsen, Delmar S.; Louie, Angelique Y.; Kauzlarich, Susan M.

2008-01-01

Mn doped Si nanoparticles have been synthesized via a low temperature solution route and characterize by X-ray powder diffraction, TEM, optical and emission spectroscopy and by EPR. The particle diameter was 4 nm and the surface was capped by octyl groups. 5% Mn doping resulted in a green emission with slightly lower quantum yield than undoped Si nanoparticles prepared by the same method. Mn2+ doped into the nanoparticle is confirmed by epr hyperfine and the charge carrier dynamics were probed by ultrafast transient absorption spectroscopy. Both techniques are consistent with Mn2+ on or close to the surface of the nanoparticle. PMID:17691792

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.

Multiple exciton generation in cluster-free alloy Cd(x)Hg(1-x)Te colloidal quantum dots synthesized in water.

PubMed

Kershaw, Stephen V; Kalytchuk, Sergii; Zhovtiuk, Olga; Shen, Qing; Oshima, Takuya; Yindeesuk, Witoon; Toyoda, Taro; Rogach, Andrey L

2014-12-21

A number of different composition CdxHg1-xTe alloy quantum dots have been synthesized using a modified aqueous synthesis and ion exchange method. The benefits of good stoichiometric control and high emission quantum yield were retained whilst also ensuring that the tendency to form gel-like clusters and adsorb excess cations in the stabilizing ligand shells was mitigated using a sequestering method to remove excess ionic material during and after the synthesis. This was highly desirable for ultrafast carrier dynamics measurements, avoiding strong photocharging effects which may mask fundamental carrier signals. Transient grating measurements revealed a composition dependent carrier multiplication process which competes with phonon mediated carrier cooling to deplete the initial hot carrier population. The interplay between these two mechanisms is strongly dependent on the electron effective mass which in these alloys has a marked composition dependence and may be considerably lower than the hole effective mass. For a composition x = 0.52 we measured a maximum carrier multiplication quantum yield of 199 ± 19% with pump photon energy 3 times the bandgap energy, Eg, whilst the threshold energy is calculated to be just 2.15Eg. There is some evidence to suggest an impact ionization process analogous to the inverse Auger S mechanism seen in bulk CdxHg1-xTe.

Ultrafast spectroscopy of exciton and exciton dynamics in mono and few layers of WS2

NASA Astrophysics Data System (ADS)

Khadka, Sudiksha; Aleithan, Shrouq; Livshits, Max; Rack, Jeffrey J.; Kordesch, Martin; Stinaff, Eric

Single layer of Transitional metal dichalcogenides (MX2) are 2D semiconductors that have a direct band gap in visible spectrum and fill the gap in between 2D metallic and insulating materials. They have possible application in optoelectronic devices, photovoltaics and photodetection, molecular sensing, 'valleytronics', and flexible transparent electronics. Tungsten Disulphide (WS2), one of the member of MX2 family, has a direct band gap of 2.2 eV and a large valley splitting of about 0.4 eV. Here, we present a detailed study of exciton states and their decay mechanisms in mono and few layer WS2 using femto-second transient absorption spectroscopy. We report a new peak at 3.01+/-0.1 eV whose origin in k space is believed to be at or around K point and further investigation is under way. The exponential fitting of decay curve of the exciton A reveals three time components as 1.7+/-0.3 ps, 33.5+/-10 ps and 670+/-15 ps, most likely corresponding to carrier-carrier scattering, carrier-phonon scattering, and radiative relaxation respectively.

Ultrafast spectroscopy of exciton and exciton dynamics in mono and few layers of WS2

NASA Astrophysics Data System (ADS)

Khadka, Sudiksha; Aleithan, Shrouq; Livshits, Max; Rack, Jeffrey J.; Kordesch, Martin; Stinaff, Eric

Single layer of Transitional metal dichalcogenides (MX2) are 2D semiconductors that have a direct band gap in visible spectrum and fill the gap in between 2D metallic and insulating materials. They have possible application in optoelectronic devices, photovoltaics and photodetection, molecular sensing, 'valleytronics', and flexible transparent electronics. Tungsten Disulphide (WS2), one of the member of MX2 family, has a direct band gap of 2.2 eV and a large valley splitting of about 0.4 eV. Here, we present a detailed study of exciton states and their decay mechanisms in mono and few layer WS2 using femto-second transient absorption spectroscopy. We report a new peak at 3.01+/-0.1 eV whose origin in k space is believed to be at or around K point and further investigation is underway. The exponential fitting of decay curve of the exciton A reveals three time components as 1.7+/-0.3 ps, 33.5+/-10 ps and 670+/-15 ps, most likely corresponding to carrier-carrier scattering, carrier-phonon scattering, and radiative relaxation respectively.

Extracting the temperature of hot carriers in time- and angle-resolved photoemission.

PubMed

Ulstrup, Søren; Johannsen, Jens Christian; Grioni, Marco; Hofmann, Philip

2014-01-01

The interaction of light with a material's electronic system creates an out-of-equilibrium (non-thermal) distribution of optically excited electrons. Non-equilibrium dynamics relaxes this distribution on an ultrafast timescale to a hot Fermi-Dirac distribution with a well-defined temperature. The advent of time- and angle-resolved photoemission spectroscopy (TR-ARPES) experiments has made it possible to track the decay of the temperature of the excited hot electrons in selected states in the Brillouin zone, and to reveal their cooling in unprecedented detail in a variety of emerging materials. It is, however, not a straightforward task to determine the temperature with high accuracy. This is mainly attributable to an a priori unknown position of the Fermi level and the fact that the shape of the Fermi edge can be severely perturbed when the state in question is crossing the Fermi energy. Here, we introduce a method that circumvents these difficulties and accurately extracts both the temperature and the position of the Fermi level for a hot carrier distribution by tracking the occupation statistics of the carriers measured in a TR-ARPES experiment.

Probing ultrafast proton induced dynamics in transparent dielectrics

NASA Astrophysics Data System (ADS)

Taylor, M.; Coughlan, M.; Nersisyan, G.; Senje, L.; Jung, D.; Currell, F.; Riley, D.; Lewis, C. L. S.; Zepf, M.; Dromey, B.

2018-05-01

A scheme has been developed permitting the spatial and temporal characterisation of ultrafast dynamics induced by laser driven proton bursts in transparent dielectrics. Advantage is taken of the high degree of synchronicity between the proton bursts generated during laser-foil target interactions and the probing laser to provide the basis for streaking of the dynamics. Relaxation times of electrons (<10‑12 s) are measured following swift excitation across the optical band gap for various glass samples. A temporal resolution of <500 fs is achieved demonstrating that these ultrafast dynamics can be characterized on a single-shot basis.

Preface: Special issue featuring papers from the International Conference on Nonequilibrium Carrier Dynamics in Semiconductors

NASA Astrophysics Data System (ADS)

Reggiani, L.; Bordone, P.; Brunetti, R.

2004-02-01

The International Conference on Nonequilibrium Carrier Dynamics in Semiconductors (HCIS-13) celebrates 30 years since it first took place in Modena. Nonequilibrium dynamics of charge carriers, pioneered by the hot-electron concept, is an important issue for understanding electro-optic transport properties in semiconductor materials and structures. In these 30 years several topics have matured, and new ones have emerged thus fertilizing the field with a variety of physical problems and new ideas. The history of the conference is summarized in the opening paper `30 years of HCIS'. The future of the conference seems secure considering the continued lively interest of the participants. The conference addressed eleven major topics which constitute the backbone of the proceedings and are summarized as follows: carrier transport in low dimensional and nanostructure systems, nonequilibrium carriers in superlattices and devices, small devices and related phenomena, carrier dynamics and fluctuations, carrier quantum dynamics, coherent/incoherent carrier dynamics of optical excitations and ultra-fast optical phenomena, nonlinear optical effects, transport in organic matter, semiconductor-based spintronics, coherent dynamics in solid state systems for quantum processing and communication, novel materials and devices. Nanometric space scale and femtosecond time scale represent the ultimate domains of theoretical, experimental and practical interest. Traditional fields such as bulk properties, quantum transport, fluctuations and chaotic phenomena, etc, have received thorough and continuous attention. Emerging fields from previous conferences, such as quantum processing and communication, have been better assessed. New fields, such as spintronics and electron transport in organic matter, have appeared for the first time. One plenary talk, 11 invited talks, 230 submitted abstracts covering all these topics constituted a single-session conference. Following scientific selection through the Advisory and Program Committees and peer review, 162 papers were selected for publication by the Institute of Physics Publishing in this special issue of Semiconductor Science and Technology. The financial support that allowed conference organization and helped researchers with budget difficulties to attend came from the following institutions which are gratefully acknowledged: Office of Naval Research (ONR), Defense Advanced Research Project Agency (DARPA), Office of Naval Research International Field Office (ONRIFO), International Union of Pure and Applied Physics (IUPAP), Italian Ministry of Education University and Research (MIUR), National Institute for the Physics of Matter (INFM), University of Modena and Reggio Emilia, Dipartimento di Ingegneria dell’ Innovazione of the Lecce University. Finally, sincere thanks are addressed to the technical staff who provided assistance during the conference: G Angelone, M Benassi, F Grossi, M Leuzzi, A Magnani, S Montanto, L Zagni and D Zanfi. The staff of the University Press Office together with F Goggi and N Minto are acknowledged for their excellent job in printing the conference documents.

Sub-cycle control of terahertz high-harmonic generation by dynamical Bloch oscillations

NASA Astrophysics Data System (ADS)

Schubert, O.; Hohenleutner, M.; Langer, F.; Urbanek, B.; Lange, C.; Huttner, U.; Golde, D.; Meier, T.; Kira, M.; Koch, S. W.; Huber, R.

2014-02-01

Ultrafast charge transport in strongly biased semiconductors is at the heart of high-speed electronics, electro-optics and fundamental solid-state physics. Intense light pulses in the terahertz spectral range have opened fascinating vistas. Because terahertz photon energies are far below typical electronic interband resonances, a stable electromagnetic waveform may serve as a precisely adjustable bias. Novel quantum phenomena have been anticipated for terahertz amplitudes, reaching atomic field strengths. We exploit controlled (multi-)terahertz waveforms with peak fields of 72 MV cm-1 to drive coherent interband polarization combined with dynamical Bloch oscillations in semiconducting gallium selenide. These dynamics entail the emission of phase-stable high-harmonic transients, covering the entire terahertz-to-visible spectral domain between 0.1 and 675 THz. Quantum interference of different ionization paths of accelerated charge carriers is controlled via the waveform of the driving field and explained by a quantum theory of inter- and intraband dynamics. Our results pave the way towards all-coherent terahertz-rate electronics.

Simplification of femtosecond transient absorption microscopy data from CH 3NH 3PbI 3 perovskite thin films into decay associated amplitude maps

DOE PAGES

Doughty, Benjamin; Simpson, Mary Jane; Yang, Bin; ...

2016-02-16

Our work aims to simplify multi-dimensional femtosecond transient absorption microscopy (TAM) data into decay associated amplitude maps 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 (CH 3NH 3PbI 3) perovskite thin film allows us to simplify the dataset consisting of a 68 time-resolved images into 4 decay associated amplitude maps. Furthermore, these maps provide 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. Thismore » 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.« less

Kai Zhu | NREL

Science.gov Websites

., "Long-range hot-carrier transport in hybrid perovskites visualized by ultrafast microscopy," perovskites for optoelectronic and electronic applications," Chem. Soc. Rev. 45, 655-689 (2016). Yang, M

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

Excitation and doping dependence of hole-spin relaxation in bulk GaAs

NASA Astrophysics Data System (ADS)

Krauss, Michael; Hilton, David; Schneider, Hans Christian

2009-03-01

We present theoretical and experimental results on ultrafast hole-spin dynamics in bulk GaAs. By combining a sufficiently realistic bandstructure at the level of an 8x8 k .p theory and a dynamical treatment of the relevant scattering mechanisms [1], we obtain quantitative agreement between the microscopic theoretical results and differential transmission measurements [2] for different excitation conditions. In particular, we examine the dependence of the hole-spin relaxation time on the optically excited carrier density, lattice temperature, and doping concentration. Although the spin relaxation is rather insensitive to changes in the optically excited density and temperature, strong p-doping causes a significantly faster relaxation. [1] M. Krauss, M. Aeschlimann, and H. C. Schneider, Phys.Rev.Lett. 100, 256601 (2008)[2] D. J. Hilton and C. L. Tang, Phys. Rev. Lett. 89, 146601 (2002)

Nonlinear dynamics investigation in few-cycle laser seeding of quantum cascade lasers: role of permanent dipole moment

NASA Astrophysics Data System (ADS)

Wu, Erheng; Cao, Qing; You, Jun; Liu, Chengpu

2017-06-01

The ultrafast dynamics in the few-cycle laser seeding of quantum cascade laser (QCL) is numerically investigated via the exact solution of the full-wave Maxwell-Bloch equations. It is found that, with or without taking permanent dipole moment (PDM) into account, the QCL emission is quite different: beyond the fundamental frequency band, additional high and low bands occur for that with PDM, which forms an ultra-broad quasi-comb. The origin for this is closely related to the generation of second order harmonic and direct-current components as a result of PDM breaking down the parity symmetry. Moreover, the carrier-envelope-phase (CEP) of laser seed is locked to the QCL output, no matter with or without PDM, and this phase controlled QCL maybe has more wide and convenient applications in related fields.

A split-step method to include electron–electron collisions via Monte Carlo in multiple rate equation simulations

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

Huthmacher, Klaus; Molberg, Andreas K.; Rethfeld, Bärbel

2016-10-01

A split-step numerical method for calculating ultrafast free-electron dynamics in dielectrics is introduced. The two split steps, independently programmed in C++11 and FORTRAN 2003, are interfaced via the presented open source wrapper. The first step solves a deterministic extended multi-rate equation for the ionization, electron–phonon collisions, and single photon absorption by free-carriers. The second step is stochastic and models electron–electron collisions using Monte-Carlo techniques. This combination of deterministic and stochastic approaches is a unique and efficient method of calculating the nonlinear dynamics of 3D materials exposed to high intensity ultrashort pulses. Results from simulations solving the proposed model demonstrate howmore » electron–electron scattering relaxes the non-equilibrium electron distribution on the femtosecond time scale.« less

Reversible Strain-Induced Electron-Hole Recombination in Silicon Nanowires Observed with Femtosecond Pump-Probe Microscopy

DTIC Science & Technology

2014-01-01

devices with indirect-bandgap materials such as silicon . KEYWORDS: Ultrafast imaging , strained nanomaterials, spectroscopy Lattice strain produced by...photogenerated charge cloud as a result of carrier diffusion . Normalized carrier profiles, generated by integrating the images along the direction normal to the...To test this idea, Figure 2. Charge carrier diffusion in a Si NW locally strained by a bending deformation (A) SEM image of a bent Si nanowire ∼100

Ultrafast Spectroscopic Noninvasive Probe of Vertical Carrier Transport in Heterostructure Devices (Second year of three year progress report)

DTIC Science & Technology

2014-10-01

properties in Army-relevant semiconductor materials and optoelectronic ( OE ) devices by developing and applying ultrafast optical spectroscopy techniques...met our Q6 through Q8 goals of incorporating electrical testing capabilities into our system, investigating OE devices under operating conditions...extending the capabilities of our system into the IR range, and investigating new OE devices. We have made significant progress towards our Q5 goal of

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

Dynamic absorption and scattering of water and hydrogel during high-repetition-rate (>100 MHz) burst-mode ultrafast-pulse laser ablation.

PubMed

Qian, Zuoming; Covarrubias, Andrés; Grindal, Alexander W; Akens, Margarete K; Lilge, Lothar; Marjoribanks, Robin S

2016-06-01

High-repetition-rate burst-mode ultrafast-laser ablation and disruption of biological tissues depends on interaction of each pulse with the sample, but under those particular conditions which persist from previous pulses. This work characterizes and compares the dynamics of absorption and scattering of a 133-MHz repetition-rate, burst-mode ultrafast-pulse laser, in agar hydrogel targets and distilled water. The differences in energy partition are quantified, pulse-by-pulse, using a time-resolving integrating-sphere-based device. These measurements reveal that high-repetition-rate burst-mode ultrafast-laser ablation is a highly dynamical process affected by the persistence of ionization, dissipation of plasma plume, neutral material flow, tissue tensile strength, and the hydrodynamic oscillation of cavitation bubbles.

Direct observation of ultrafast many-body electron dynamics in an ultracold Rydberg gas

PubMed Central

Takei, Nobuyuki; Sommer, Christian; Genes, Claudiu; Pupillo, Guido; Goto, Haruka; Koyasu, Kuniaki; Chiba, Hisashi; Weidemüller, Matthias; Ohmori, Kenji

2016-01-01

Many-body correlations govern a variety of important quantum phenomena such as the emergence of superconductivity and magnetism. Understanding quantum many-body systems is thus one of the central goals of modern sciences. Here we demonstrate an experimental approach towards this goal by utilizing an ultracold Rydberg gas generated with a broadband picosecond laser pulse. We follow the ultrafast evolution of its electronic coherence by time-domain Ramsey interferometry with attosecond precision. The observed electronic coherence shows an ultrafast oscillation with a period of 1 femtosecond, whose phase shift on the attosecond timescale is consistent with many-body correlations among Rydberg atoms beyond mean-field approximations. This coherent and ultrafast many-body dynamics is actively controlled by tuning the orbital size and population of the Rydberg state, as well as the mean atomic distance. Our approach will offer a versatile platform to observe and manipulate non-equilibrium dynamics of quantum many-body systems on the ultrafast timescale. PMID:27849054

Ultrafast Nanoimaging of the Photoinduced Phase Transition Dynamics in VO2.

PubMed

Dönges, Sven A; Khatib, Omar; O'Callahan, Brian T; Atkin, Joanna M; Park, Jae Hyung; Cobden, David; Raschke, Markus B

2016-05-11

Many phase transitions in correlated matter exhibit spatial inhomogeneities with expected yet unexplored effects on the associated ultrafast dynamics. Here we demonstrate the combination of ultrafast nondegenerate pump-probe spectroscopy with far from equilibrium excitation, and scattering scanning near-field optical microscopy (s-SNOM) for ultrafast nanoimaging. In a femtosecond near-field near-IR (NIR) pump and mid-IR (MIR) probe study, we investigate the photoinduced insulator-to-metal (IMT) transition in nominally homogeneous VO2 microcrystals. With pump fluences as high as 5 mJ/cm(2), we can reach three distinct excitation regimes. We observe a spatial heterogeneity on ∼50-100 nm length scales in the fluence-dependent IMT dynamics ranging from <100 fs to ∼1 ps. These results suggest a high sensitivity of the IMT with respect to small local variations in strain, doping, or defects that are difficult to discern microscopically. We provide a perspective with the distinct requirements and considerations of ultrafast spatiotemporal nanoimaging of phase transitions in quantum materials.

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.

Monitoring Ultrafast Chemical Dynamics by Time-Domain X-ray Photo- and Auger-Electron Spectroscopy.

PubMed

Gessner, Oliver; Gühr, Markus

2016-01-19

The directed flow of charge and energy is at the heart of all chemical processes. Extraordinary efforts are underway to monitor and understand the concerted motion of electrons and nuclei with ever increasing spatial and temporal sensitivity. The element specificity, chemical sensitivity, and temporal resolution of ultrafast X-ray spectroscopy techniques hold great promise to provide new insight into the fundamental interactions underlying chemical dynamics in systems ranging from isolated molecules to application-like devices. Here, we focus on the potential of ultrafast X-ray spectroscopy techniques based on the detection of photo- and Auger electrons to provide new fundamental insight into photochemical processes of systems with various degrees of complexity. Isolated nucleobases provide an excellent testing ground for our most fundamental understanding of intramolecular coupling between electrons and nuclei beyond the traditionally applied Born-Oppenheimer approximation. Ultrafast electronic relaxation dynamics enabled by the breakdown of this approximation is the major component of the nucleobase photoprotection mechanisms. Transient X-ray induced Auger electron spectroscopy on photoexcited thymine molecules provides atomic-site specific details of the extremely efficient coupling that converts potentially bond changing ultraviolet photon energy into benign heat. In particular, the time-dependent spectral shift of a specific Auger band is sensitive to the length of a single bond within the molecule. The X-ray induced Auger transients show evidence for an electronic transition out of the initially excited state within only ∼200 fs in contrast to theoretically predicted picosecond population trapping behind a reaction barrier. Photoinduced charge transfer dynamics between transition metal complexes and semiconductor nanostructures are of central importance for many emerging energy and climate relevant technologies. Numerous demonstrations of photovoltaic and photocatalytic activity have been performed based on the combination of strong light absorption in dye molecules with charge separation and transport in adjacent semiconductor nanostructures. However, a fundamental understanding of the enabling and limiting dynamics on critical atomic length- and time scales is often still lacking. Femtosecond time-resolved X-ray photoelectron spectroscopy is employed to gain a better understanding of a short-lived intermediate that may be linked to the unexpectedly limited performance of ZnO based dye-sensitized solar cells by delaying the generation of free charge carriers. The transient spectra strongly suggest that photoexcited dye molecules attached to ZnO nanocrystals inject their charges into the substrate within less than 1 ps but the electrons are then temporarily trapped at the surface of the semiconductor in direct vicinity of the injecting molecules. The experiments are extended to monitor the electronic response of the semiconductor substrate to the collective injection from a monolayer of dye molecules and the subsequent electron-ion recombination dynamics. The results indicate some qualitative similarities but quantitative differences between the recombination dynamics at molecule-semiconductor interfaces and previously studied bulk-surface electron-hole recombination dynamics in photoexcited semiconductors.

Ultrafast Spectroscopic Noninvasive Probe of Vertical Carrier Transport in Heterostructure Devices

DTIC Science & Technology

2016-03-01

where barriers, tunneling , scattering, strong polarization-induced fields, or carrier localization due to Type I or Type II quantum-well structures can... tunneling across junctions, scattering at heterointerfaces, and internal fields. For light-emitting devices, poor charge transport across multilayer...localization of holes and rapid electron tunneling .5 However, direct transport properties were Approved for public release; distribution is

Extreme ultraviolet probing of nonequilibrium dynamics in high energy density germanium

NASA Astrophysics Data System (ADS)

Principi, E.; Giangrisostomi, E.; Mincigrucci, R.; Beye, M.; Kurdi, G.; Cucini, R.; Gessini, A.; Bencivenga, F.; Masciovecchio, C.

2018-05-01

Intense femtosecond infrared laser pulses induce a nonequilibrium between thousands of Kelvin hot valence electrons and room-temperature ions in a germanium sample foil. The evolution of this exotic state of matter is monitored with time-resolved extreme ultraviolet absorption spectroscopy across the Ge M2 ,3 edge (≃30 eV ) using the FERMI free-electron laser. We analyze two distinct regimes in the ultrafast dynamics in laser-excited Ge: First, on a subpicosecond time scale, the electron energy distribution thermalizes to an extreme temperature unreachable in equilibrium solid germanium; then, during the following picoseconds, the lattice reacts strongly altering the electronic structure and resulting in melting to a metallic state alongside a breakdown of the local atomic order. Data analysis, based on a hybrid approach including both numerical and analytical calculations, provides an estimation of the electron and ion temperatures, the electron density of states, the carrier-phonon relaxation time, as well as the carrier density and lattice heat capacity under those extreme nonequilibrium conditions. Related structural anomalies, such as the occurrence of a transient low-density liquid phase and the possible drop in lattice heat capacity are discussed.

Ponderomotive electron acceleration in a silicon-based nanoplasmonic waveguide.

PubMed

Sederberg, S; Elezzabi, A Y

2014-10-17

Ponderomotive electron acceleration is demonstrated in a semiconductor-loaded nanoplasmonic waveguide. Photogenerated free carriers are accelerated by the tightly confined nanoplasmonic fields and reach energies exceeding the threshold for impact ionization. Broadband (375 nm ≤ λ ≤ 650  nm) white light emission is observed from the nanoplasmonic waveguides. Exponential growth of visible light emission confirms the exponential growth of the electron population, demonstrating the presence of an optical-field-driven electron avalanche. Electron sweeping dynamics are visualized using pump-probe measurements, and a sweeping time of 1.98 ± 0.40 ps is measured. These findings offer a means to harness the potential of the emerging field of ultrafast nonlinear nanoplasmonics.

Coherent electronic wave packet motion in C(60) controlled by the waveform and polarization of few-cycle laser fields.

PubMed

Li, H; Mignolet, B; Wachter, G; Skruszewicz, S; Zherebtsov, S; Süssmann, F; Kessel, A; Trushin, S A; Kling, Nora G; Kübel, M; Ahn, B; Kim, D; Ben-Itzhak, I; Cocke, C L; Fennel, T; Tiggesbäumker, J; Meiwes-Broer, K-H; Lemell, C; Burgdörfer, J; Levine, R D; Remacle, F; Kling, M F

2015-03-27

Strong laser fields can be used to trigger an ultrafast molecular response that involves electronic excitation and ionization dynamics. Here, we report on the experimental control of the spatial localization of the electronic excitation in the C_{60} fullerene exerted by an intense few-cycle (4 fs) pulse at 720 nm. The control is achieved by tailoring the carrier-envelope phase and the polarization of the laser pulse. We find that the maxima and minima of the photoemission-asymmetry parameter along the laser-polarization axis are synchronized with the localization of the coherent electronic wave packet at around the time of ionization.

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

Ultrafast monoenergetic electron source by optical waveform control of surface plasmons.

PubMed

Dombi, Péter; Rácz, Péter

2008-03-03

We propose coherent control of photoelectron acceleration at metal surfaces mediated by surface plasmon polaritons. A high degree of spectral and spatial control of the emission process can be exercised by amplitude and phase controlling the optical waveform (including the carrier-envelope phase) of the plasmon generating few-cycle laser pulse. Numerical results show that the emitted electron beam is highly directional and monoenergetic suggesting applications in contemporary ultrafast methods where ultrashort, well-behaved electron pulses are required.

Photovoltaic Properties and Ultrafast Plasmon Relaxation Dynamics of Diamond-Like Carbon Nanocomposite Films with Embedded Ag Nanoparticles.

PubMed

Meškinis, Šarūnas; Peckus, Domantas; Vasiliauskas, Andrius; Čiegis, Arvydas; Gudaitis, Rimantas; Tamulevičius, Tomas; Yaremchuk, Iryna; Tamulevičius, Sigitas

2017-12-01

Ultrafast relaxation dynamics of diamond-like carbon (DLC) films with embedded Ag nanoparticles (DLC:Ag) and photovoltaic properties of heterojunctions consisting of DLC:Ag and crystalline silicon (DLC:Ag/Si) were investigated by means of transient absorption (TAS) spectroscopy and photovoltaic measurements. The heterojunctions using both p type and n type silicon were studied. It was found that TAS spectra of DLC:Ag films were dependent on the used excitation wavelength. At wavelengths where Ag nanoparticles absorbed light most intensively, only DLC signal was registered. This result is in good accordance with an increase of the DLC:Ag/Si heterojunction short circuit current and open circuit voltage with the excitation wavelength in the photovoltaic measurements. The dependence of the TAS spectra of DLC:Ag films and photovoltaic properties of DLC:Ag/Si heterostructures on the excitation wavelength was explained as a result of trapping of the photoexcited hot charge carriers in DLC matrix. The negative photovoltaic effect was observed for DLC:Ag/p-Si heterostructures and positive ("conventional") for DLC:Ag/n-Si ones. It was explained by the excitation of hot plasmonic holes in the Ag nanoparticles embedded into DLC matrix. Some decrease of DLC:Ag/Si heterostructures photovoltage as well as photocurrent with DLC:Ag film thickness was observed, indicating role of the interface in the charge transfer process of photocarriers excited in Ag nanoparticles.

Photovoltaic Properties and Ultrafast Plasmon Relaxation Dynamics of Diamond-Like Carbon Nanocomposite Films with Embedded Ag Nanoparticles

NASA Astrophysics Data System (ADS)

Meškinis, Šarūnas; Peckus, Domantas; Vasiliauskas, Andrius; Čiegis, Arvydas; Gudaitis, Rimantas; Tamulevičius, Tomas; Yaremchuk, Iryna; Tamulevičius, Sigitas

2017-04-01

Ultrafast relaxation dynamics of diamond-like carbon (DLC) films with embedded Ag nanoparticles (DLC:Ag) and photovoltaic properties of heterojunctions consisting of DLC:Ag and crystalline silicon (DLC:Ag/Si) were investigated by means of transient absorption (TAS) spectroscopy and photovoltaic measurements. The heterojunctions using both p type and n type silicon were studied. It was found that TAS spectra of DLC:Ag films were dependent on the used excitation wavelength. At wavelengths where Ag nanoparticles absorbed light most intensively, only DLC signal was registered. This result is in good accordance with an increase of the DLC:Ag/Si heterojunction short circuit current and open circuit voltage with the excitation wavelength in the photovoltaic measurements. The dependence of the TAS spectra of DLC:Ag films and photovoltaic properties of DLC:Ag/Si heterostructures on the excitation wavelength was explained as a result of trapping of the photoexcited hot charge carriers in DLC matrix. The negative photovoltaic effect was observed for DLC:Ag/p-Si heterostructures and positive ("conventional") for DLC:Ag/n-Si ones. It was explained by the excitation of hot plasmonic holes in the Ag nanoparticles embedded into DLC matrix. Some decrease of DLC:Ag/Si heterostructures photovoltage as well as photocurrent with DLC:Ag film thickness was observed, indicating role of the interface in the charge transfer process of photocarriers excited in Ag nanoparticles.

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.

Charge Carrier Hopping Dynamics in Homogeneously Broadened PbS Quantum Dot Solids.

PubMed

Gilmore, Rachel H; Lee, Elizabeth M Y; Weidman, Mark C; Willard, Adam P; Tisdale, William A

2017-02-08

Energetic disorder in quantum dot solids adversely impacts charge carrier transport in quantum dot solar cells and electronic devices. Here, we use ultrafast transient absorption spectroscopy to show that homogeneously broadened PbS quantum dot arrays (σ hom 2 :σ inh 2 > 19:1, σ inh /k B T < 0.4) can be realized if quantum dot batches are sufficiently monodisperse (δ ≲ 3.3%). The homogeneous line width is found to be an inverse function of quantum dot size, monotonically increasing from ∼25 meV for the largest quantum dots (5.8 nm diameter/0.92 eV energy) to ∼55 meV for the smallest (4.1 nm/1.3 eV energy). Furthermore, we show that intrinsic charge carrier hopping rates are faster for smaller quantum dots. This finding is the opposite of the mobility trend commonly observed in device measurements but is consistent with theoretical predictions. Fitting our data to a kinetic Monte Carlo model, we extract charge carrier hopping times ranging from 80 ps for the smallest quantum dots to over 1 ns for the largest, with the same ethanethiol ligand treatment. Additionally, we make the surprising observation that, in slightly polydisperse (δ ≲ 4%) quantum dot solids, structural disorder has a greater impact than energetic disorder in inhibiting charge carrier transport. These findings emphasize how small improvements in batch size dispersity can have a dramatic impact on intrinsic charge carrier hopping behavior and will stimulate further improvements in quantum dot device performance.

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.

Spatial localization of excitons and charge carriers in hybrid perovskite thin films

DOE PAGES

Simpson, Mary Jane; Doughty, Benjamin; Yang, Bin; ...

2015-07-21

The fundamental photophysics underlying the remarkably high power conversion efficiency of organic-inorganic hybrid perovskite-based solar cells has been increasingly studied using complementary spectroscopic techniques. The spatially heterogeneous polycrystalline morphology of the photoactive layers owing to the presence of distinct crystalline grains has been generally neglected in optical measurements and therefore the reported results are typically averaged over hundreds or even thousands of such grains. Here, we apply femtosecond transient absorption microscopy to spatially and temporally probe ultrafast electronic excited-state dynamics in pristine methylammonium lead tri-iodide (CH 3NH 3PbI 3) thin films and composite structures. We found that the electronic excited-statemore » relaxation kinetics are extremely sensitive to the sample location probed, which was manifested by position-dependent decay timescales and transient signals. As a result, analysis of transient absorption kinetics acquired at distinct spatial positions enabled us to identify contributions of excitons and free charge carriers.« 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.

Isolated Attosecond Pulse Generation without the Need to Stabilize the Carrier-Envelope Phase of Driving Lasers

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

Gilbertson, Steve; Khan, Sabih D.; Wu Yi

2010-08-27

Single isolated attosecond pulses can be extracted from a pulse train with an ultrafast gate in the generation target. By setting the gate width sufficiently narrow with the generalized double optical gating, we demonstrate that single isolated attosecond pulses can be generated with any arbitrary carrier-envelope phase value of the driving laser. The carrier-envelope phase only affects the photon flux, not the pulse duration or contrast. Our results show that isolated attosecond pulses can be generated using carrier-envelope phase unstabilized 23 fs pulses directly from chirped pulse amplifiers.

Mega-electron-volt ultrafast electron diffraction at SLAC National Accelerator Laboratory

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

Weathersby, S. P.; Brown, G.; Chase, T. F.

Ultrafast electron probes are powerful tools, complementary to x-ray free-electron lasers, used to study structural dynamics in material, chemical, and biological sciences. High brightness, relativistic electron beams with femtosecond pulse duration can resolve details of the dynamic processes on atomic time and length scales. SLAC National Accelerator Laboratory recently launched the Ultrafast Electron Diffraction (UED) and microscopy Initiative aiming at developing the next generation ultrafast electron scattering instruments. As the first stage of the Initiative, a mega-electron-volt (MeV) UED system has been constructed and commissioned to serve ultrafast science experiments and instrumentation development. The system operates at 120-Hz repetition ratemore » with outstanding performance. In this paper, we report on the SLAC MeV UED system and its performance, including the reciprocal space resolution, temporal resolution, and machine stability.« less

Mega-electron-volt ultrafast electron diffraction at SLAC National Accelerator Laboratory.

PubMed

Weathersby, S P; Brown, G; Centurion, M; Chase, T F; Coffee, R; Corbett, J; Eichner, J P; Frisch, J C; Fry, A R; Gühr, M; Hartmann, N; Hast, C; Hettel, R; Jobe, R K; Jongewaard, E N; Lewandowski, J R; Li, R K; Lindenberg, A M; Makasyuk, I; May, J E; McCormick, D; Nguyen, M N; Reid, A H; Shen, X; Sokolowski-Tinten, K; Vecchione, T; Vetter, S L; Wu, J; Yang, J; Dürr, H A; Wang, X J

2015-07-01

Ultrafast electron probes are powerful tools, complementary to x-ray free-electron lasers, used to study structural dynamics in material, chemical, and biological sciences. High brightness, relativistic electron beams with femtosecond pulse duration can resolve details of the dynamic processes on atomic time and length scales. SLAC National Accelerator Laboratory recently launched the Ultrafast Electron Diffraction (UED) and microscopy Initiative aiming at developing the next generation ultrafast electron scattering instruments. As the first stage of the Initiative, a mega-electron-volt (MeV) UED system has been constructed and commissioned to serve ultrafast science experiments and instrumentation development. The system operates at 120-Hz repetition rate with outstanding performance. In this paper, we report on the SLAC MeV UED system and its performance, including the reciprocal space resolution, temporal resolution, and machine stability.

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

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 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.

Model for Ultrafast Carrier Scattering in Semiconductors

DTIC Science & Technology

2012-11-14

energy transfer between semi-classical carrier drift-diffusion under an electric field and quantum kinetics of interband /intersubband transitions...from an electron during each phonon-emission event. The net rate of phonon emission is determined by the Boltzmann scattering equation which depends ...energy-drift term under a strong dc field was demonstrated to reduce the field- dependent drift velocity and mobility. The Doppler shift in the energy

Rippling ultrafast dynamics of suspended 2D monolayers, graphene

PubMed Central

Hu, Jianbo; Vanacore, Giovanni M.; Cepellotti, Andrea; Marzari, Nicola; Zewail, Ahmed H.

2016-01-01

Here, using ultrafast electron crystallography (UEC), we report the observation of rippling dynamics in suspended monolayer graphene, the prototypical and most-studied 2D material. The high scattering cross-section for electron/matter interaction, the atomic-scale spatial resolution, and the ultrafast temporal resolution of UEC represent the key elements that make this technique a unique tool for the dynamic investigation of 2D materials, and nanostructures in general. We find that, at early time after the ultrafast optical excitation, graphene undergoes a lattice expansion on a time scale of 5 ps, which is due to the excitation of short-wavelength in-plane acoustic phonon modes that stretch the graphene plane. On a longer time scale, a slower thermal contraction with a time constant of 50 ps is observed and associated with the excitation of out-of-plane phonon modes, which drive the lattice toward thermal equilibrium with the well-known negative thermal expansion coefficient of graphene. From our results and first-principles lattice dynamics and out-of-equilibrium relaxation calculations, we quantitatively elucidate the deformation dynamics of the graphene unit cell. PMID:27791028

Rippling ultrafast dynamics of suspended 2D monolayers, graphene.

PubMed

Hu, Jianbo; Vanacore, Giovanni M; Cepellotti, Andrea; Marzari, Nicola; Zewail, Ahmed H

2016-10-25

Here, using ultrafast electron crystallography (UEC), we report the observation of rippling dynamics in suspended monolayer graphene, the prototypical and most-studied 2D material. The high scattering cross-section for electron/matter interaction, the atomic-scale spatial resolution, and the ultrafast temporal resolution of UEC represent the key elements that make this technique a unique tool for the dynamic investigation of 2D materials, and nanostructures in general. We find that, at early time after the ultrafast optical excitation, graphene undergoes a lattice expansion on a time scale of 5 ps, which is due to the excitation of short-wavelength in-plane acoustic phonon modes that stretch the graphene plane. On a longer time scale, a slower thermal contraction with a time constant of 50 ps is observed and associated with the excitation of out-of-plane phonon modes, which drive the lattice toward thermal equilibrium with the well-known negative thermal expansion coefficient of graphene. From our results and first-principles lattice dynamics and out-of-equilibrium relaxation calculations, we quantitatively elucidate the deformation dynamics of the graphene unit cell.

Ultrafast Terahertz Nonlinear Optics of Landau Level Transitions in a Monolayer Graphene

NASA Astrophysics Data System (ADS)

Yumoto, Go; Matsunaga, Ryusuke; Hibino, Hiroki; Shimano, Ryo

2018-03-01

We investigated the ultrafast terahertz (THz) nonlinearity in a monolayer graphene under the strong magnetic field using THz pump-THz probe spectroscopy. An ultrafast suppression of the Faraday rotation associated with inter-Landau level (LL) transitions is observed, reflecting the Dirac electron character of nonequidistant LLs with large transition dipole moments. A drastic modulation of electron distribution in LLs is induced by far off-resonant THz pulse excitation in the transparent region. Numerical simulation based on the density matrix formalism without rotating-wave approximation reproduces the experimental results. Our results indicate that the strong light-matter coupling regime is realized in graphene, with the Rabi frequency exceeding the carrier wave frequency and even the relevant energy scale of the inter-LL transition.

Real-time visualization of soliton molecules with evolving behavior in an ultrafast fiber laser

NASA Astrophysics Data System (ADS)

Liu, Meng; Li, Heng; Luo, Ai-Ping; Cui, Hu; Xu, Wen-Cheng; Luo, Zhi-Chao

2018-03-01

Ultrafast fiber lasers have been demonstrated to be great platforms for the investigation of soliton dynamics. The soliton molecules, as one of the most fascinating nonlinear phenomena, have been a hot topic in the field of nonlinear optics in recent years. Herein, we experimentally observed the real-time evolving behavior of soliton molecule in an ultrafast fiber laser by using the dispersive Fourier transformation technology. Several types of evolving soliton molecules were obtained in our experiments, such as soliton molecules with monotonically or chaotically evolving phase, flipping and hopping phase. These results would be helpful to the communities interested in soliton nonlinear dynamics as well as ultrafast laser technologies.

Singlet-to-triplet intermediates and triplet exciton dynamics in pentacene thinfilms

NASA Astrophysics Data System (ADS)

Thorsmolle, Verner; Korber, Michael; Obergfell, Emanuel; Kuhlman, Thomas; Campbell, Ian; Crone, Brian; Taylor, Antoinette; Averitt, Richard; Demsar, Jure

Singlet-to-triplet fission in organic semiconductors is a spin-conserving multiexciton process in which one spin-zero singlet excitation is converted into two spin-one triplet excitations on an ultrafast timescale. Current scientific interest into this carrier multiplication process is largely driven by prospects of enhancing the efficiency in photovoltaic applications by generating two long-lived triplet excitons by one photon. The fission process is known to involve intermediate states, known as correlated triplet pairs, with an overall singlet character, before being interchanged into uncorrelated triplets. Here we use broadband femtosecond real-time spectroscopy to study the excited state dynamics in pentacene thin films, elucidating the fission process and the role of intermediate triplet states. VKT and AJT acknowledge support by the LDRD program at Los Alamos National Laboratory and the Department of Energy, Grant No. DE-FG02-04ER118. MK, MO and JD acknowledge support by the Alexander von Humboldt Foundation.

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

Electrically-driven GHz range ultrafast graphene light emitter (Conference Presentation)

NASA Astrophysics Data System (ADS)

Kim, Youngduck; Gao, Yuanda; Shiue, Ren-Jye; Wang, Lei; Aslan, Ozgur Burak; Kim, Hyungsik; Nemilentsau, Andrei M.; Low, Tony; Taniguchi, Takashi; Watanabe, Kenji; Bae, Myung-Ho; Heinz, Tony F.; Englund, Dirk R.; Hone, James

2017-02-01

Ultrafast electrically driven light emitter is a critical component in the development of the high bandwidth free-space and on-chip optical communications. Traditional semiconductor based light sources for integration to photonic platform have therefore been heavily studied over the past decades. However, there are still challenges such as absence of monolithic on-chip light sources with high bandwidth density, large-scale integration, low-cost, small foot print, and complementary metal-oxide-semiconductor (CMOS) technology compatibility. Here, we demonstrate the first electrically driven ultrafast graphene light emitter that operate up to 10 GHz bandwidth and broadband range (400 1600 nm), which are possible due to the strong coupling of charge carriers in graphene and surface optical phonons in hBN allow the ultrafast energy and heat transfer. In addition, incorporation of atomically thin hexagonal boron nitride (hBN) encapsulation layers enable the stable and practical high performance even under the ambient condition. Therefore, electrically driven ultrafast graphene light emitters paves the way towards the realization of ultrahigh bandwidth density photonic integrated circuits and efficient optical communications networks.

Ultrafast electron dynamics in phenylalanine initiated by attosecond pulses.

PubMed

Calegari, F; Ayuso, D; Trabattoni, A; Belshaw, L; De Camillis, S; Anumula, S; Frassetto, F; Poletto, L; Palacios, A; Decleva, P; Greenwood, J B; Martín, F; Nisoli, M

2014-10-17

In the past decade, attosecond technology has opened up the investigation of ultrafast electronic processes in atoms, simple molecules, and solids. Here, we report the application of isolated attosecond pulses to prompt ionization of the amino acid phenylalanine and the subsequent detection of ultrafast dynamics on a sub-4.5-femtosecond temporal scale, which is shorter than the vibrational response of the molecule. The ability to initiate and observe such electronic dynamics in polyatomic molecules represents a crucial step forward in attosecond science, which is progressively moving toward the investigation of more and more complex systems. Copyright © 2014, American Association for the Advancement of Science.

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

Light-field-driven currents in graphene

NASA Astrophysics Data System (ADS)

Higuchi, Takuya; Heide, Christian; Ullmann, Konrad; Weber, Heiko B.; Hommelhoff, Peter

2017-10-01

The ability to steer electrons using the strong electromagnetic field of light has opened up the possibility of controlling electron dynamics on the sub-femtosecond (less than 10-15 seconds) timescale. In dielectrics and semiconductors, various light-field-driven effects have been explored, including high-harmonic generation, sub-optical-cycle interband population transfer and the non-perturbative change of the transient polarizability. In contrast, much less is known about light-field-driven electron dynamics in narrow-bandgap systems or in conductors, in which screening due to free carriers or light absorption hinders the application of strong optical fields. Graphene is a promising platform with which to achieve light-field-driven control of electrons in a conducting material, because of its broadband and ultrafast optical response, weak screening and high damage threshold. Here we show that a current induced in monolayer graphene by two-cycle laser pulses is sensitive to the electric-field waveform, that is, to the exact shape of the optical carrier field of the pulse, which is controlled by the carrier-envelope phase, with a precision on the attosecond (10-18 seconds) timescale. Such a current, dependent on the carrier-envelope phase, shows a striking reversal of the direction of the current as a function of the driving field amplitude at about two volts per nanometre. This reversal indicates a transition of light-matter interaction from the weak-field (photon-driven) regime to the strong-field (light-field-driven) regime, where the intraband dynamics influence interband transitions. We show that in this strong-field regime the electron dynamics are governed by sub-optical-cycle Landau-Zener-Stückelberg interference, composed of coherent repeated Landau-Zener transitions on the femtosecond timescale. Furthermore, the influence of this sub-optical-cycle interference can be controlled with the laser polarization state. These coherent electron dynamics in graphene take place on a hitherto unexplored timescale, faster than electron-electron scattering (tens of femtoseconds) and electron-phonon scattering (hundreds of femtoseconds). We expect these results to have direct ramifications for band-structure tomography and light-field-driven petahertz electronics.

Light-field-driven currents in graphene.

PubMed

Higuchi, Takuya; Heide, Christian; Ullmann, Konrad; Weber, Heiko B; Hommelhoff, Peter

2017-10-12

The ability to steer electrons using the strong electromagnetic field of light has opened up the possibility of controlling electron dynamics on the sub-femtosecond (less than 10 -15 seconds) timescale. In dielectrics and semiconductors, various light-field-driven effects have been explored, including high-harmonic generation, sub-optical-cycle interband population transfer and the non-perturbative change of the transient polarizability. In contrast, much less is known about light-field-driven electron dynamics in narrow-bandgap systems or in conductors, in which screening due to free carriers or light absorption hinders the application of strong optical fields. Graphene is a promising platform with which to achieve light-field-driven control of electrons in a conducting material, because of its broadband and ultrafast optical response, weak screening and high damage threshold. Here we show that a current induced in monolayer graphene by two-cycle laser pulses is sensitive to the electric-field waveform, that is, to the exact shape of the optical carrier field of the pulse, which is controlled by the carrier-envelope phase, with a precision on the attosecond (10 -18 seconds) timescale. Such a current, dependent on the carrier-envelope phase, shows a striking reversal of the direction of the current as a function of the driving field amplitude at about two volts per nanometre. This reversal indicates a transition of light-matter interaction from the weak-field (photon-driven) regime to the strong-field (light-field-driven) regime, where the intraband dynamics influence interband transitions. We show that in this strong-field regime the electron dynamics are governed by sub-optical-cycle Landau-Zener-Stückelberg interference, composed of coherent repeated Landau-Zener transitions on the femtosecond timescale. Furthermore, the influence of this sub-optical-cycle interference can be controlled with the laser polarization state. These coherent electron dynamics in graphene take place on a hitherto unexplored timescale, faster than electron-electron scattering (tens of femtoseconds) and electron-phonon scattering (hundreds of femtoseconds). We expect these results to have direct ramifications for band-structure tomography and light-field-driven petahertz electronics.

Enhanced hot-carrier cooling and ultrafast spectral diffusion in strongly coupled PbSe quantum-dot solids.

PubMed

Gao, Yunan; Talgorn, Elise; Aerts, Michiel; Trinh, M Tuan; Schins, Juleon M; Houtepen, Arjan J; Siebbeles, Laurens D A

2011-12-14

PbSe quantum-dot solids are of great interest for low cost and efficient photodetectors and solar cells. We have prepared PbSe quantum-dot solids with high charge carrier mobilities using layer-by-layer dip-coating with 1,2-ethanediamine as substitute capping ligands. Here we present a time and energy resolved transient absorption spectroscopy study on the kinetics of photogenerated charge carriers, focusing on 0-5 ps after photoexcitation. We compare the observed carrier kinetics to those for quantum dots in dispersion and show that the intraband carrier cooling is significantly faster in quantum-dot solids. In addition we find that carriers diffuse from higher to lower energy sites in the quantum-dot solid within several picoseconds.

Coherent Femtosecond Spectroscopy and Nonlinear Optical Imaging on the Nanoscale

NASA Astrophysics Data System (ADS)

Kravtsov, Vasily

Optical properties of many materials and macroscopic systems are defined by ultrafast dynamics of electronic, vibrational, and spin excitations localized on the nanoscale. Harnessing these excitations for material engineering, optical computing, and control of chemical reactions has been a long-standing goal in science and technology. However, it is challenging due to the lack of spectroscopic techniques that can resolve processes simultaneously on the nanometer spatial and femtosecond temporal scales. This thesis describes the fundamental principles, implementation, and experimental demonstration of a novel type of ultrafast microscopy based on the concept of adiabatic plasmonic nanofocusing. Simultaneous spatio-temporal resolution on a nanometer-femtosecond scale is achieved by using a near-field nonlinear optical response induced by ultrafast surface plasmon polaritons nanofocused on a metal tip. First, we study the surface plasmon response in metallic structures and evaluate its prospects and limitations for ultrafast near-field microscopy. Through plasmon emission-based spectroscopy, we investigate dephasing times and interplay between radiative and non-radiative decay rates of localized plasmons and their modification due to coupling. We identify a new regime of quantum plasmonic coupling, which limits the achievable spatial resolution to several angstroms but at the same time provides a potential channel for generating ultrafast electron currents at optical frequencies. Next, we study propagation of femtosecond wavepackets of surface plasmon polaritons on a metal tip. In time-domain interferometric measurements we detect group delays that correspond to slowing of the plasmon polaritons down to 20% of the speed of light at the tip apex. This provides direct experimental verification of the plasmonic nanofocusing mechanism and suggests enhanced nonlinear optical interactions at the tip apex. We then measure a plasmon-generated third-order nonlinear optical four-wave mixing response from the tip apex and investigate its microscopic mechanism. Our results reveal a significant contribution to the third order nonlinearity of plasmonic structures due to large near-field gradients associated with nanofocused plasmons. In combination with scanning probe imaging and femtosecond pulse shaping, the nanofocused four-wave mixing response provides a basis for a novel type of ultrafast optical microscopy on the nanoscale. We demonstrate its capabilities by nano-imaging the coherent dynamics of localized plasmonic modes in a rough gold film edge with simultaneous sub-50 nm spatial and sub-5 fs temporal resolution. We capture the coherent decay and extract the dephasing times of individual plasmonic modes. Lastly, we apply our technique to study nanoscale spatial heterogeneity of the nonlinear optical response in novel two-dimensional materials: monolayer and few-layer graphene. An enhanced four-wave mixing signal is revealed on the edges of graphene flakes. We investigate the mechanism of this enhancement by performing nano-imaging on a graphene field-effect transistor with the variable carrier density controlled by electrostatic gating.

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.

Dynamical photo-induced electronic properties of molecular junctions

NASA Astrophysics Data System (ADS)

Beltako, K.; Michelini, F.; Cavassilas, N.; Raymond, L.

2018-03-01

Nanoscale molecular-electronic devices and machines are emerging as promising functional elements, naturally flexible and efficient, for next-generation technologies. A deeper understanding of carrier dynamics in molecular junctions is expected to benefit many fields of nanoelectronics and power devices. We determine time-resolved charge current flowing at the donor-acceptor interface in molecular junctions connected to metallic electrodes by means of quantum transport simulations. The current is induced by the interaction of the donor with a Gaussian-shape femtosecond laser pulse. Effects of the molecular internal coupling, metal-molecule tunneling, and light-donor coupling on photocurrent are discussed. We then define the time-resolved local density of states which is proposed as an efficient tool to describe the absorbing molecule in contact with metallic electrodes. Non-equilibrium reorganization of hybridized molecular orbitals through the light-donor interaction gives rise to two phenomena: the dynamical Rabi shift and the appearance of Floquet-like states. Such insights into the dynamical photoelectronic structure of molecules are of strong interest for ultrafast spectroscopy and open avenues toward the possibility of analyzing and controlling the internal properties of quantum nanodevices with pump-push photocurrent spectroscopy.

Picosecond time-resolved photoluminescence using picosecond excitation correlation spectroscopy

NASA Astrophysics Data System (ADS)

Johnson, M. B.; McGill, T. C.; Hunter, A. T.

1988-03-01

We present a study of the temporal decay of photoluminescence (PL) as detected by picosecond excitation correlation spectroscopy (PECS). We analyze the correlation signal that is obtained from two simple models; one where radiative recombination dominates, the other where trapping processes dominate. It is found that radiative recombination alone does not lead to a correlation signal. Parallel trapping type processes are found to be required to see a signal. To illustrate this technique, we examine the temporal decay of the PL signal for In-alloyed, semi-insulating GaAs substrates. We find that the PL signal indicates a carrier lifetime of roughly 100 ps, for excitation densities of 1×1016-5×1017 cm-3. PECS is shown to be an easy technique to measure the ultrafast temporal behavior of PL processes because it requires no ultrafast photon detection. It is particularly well suited to measuring carrier lifetimes.

SALUTE Grid Application using Message-Oriented Middleware

NASA Astrophysics Data System (ADS)

Atanassov, E.; Dimitrov, D. Sl.; Gurov, T.

2009-10-01

Stochastic ALgorithms for Ultra-fast Transport in sEmiconductors (SALUTE) is a grid application developed for solving various computationally intensive problems which describe ultra-fast carrier transport in semiconductors. SALUTE studies memory and quantum effects during the relaxation process due to electronphonon interaction in one-band semiconductors or quantum wires. Formally, SALUTE integrates a set of novel Monte Carlo, quasi-Monte Carlo and hybrid algorithms for solving various computationally intensive problems which describe the femtosecond relaxation process of optically excited carriers in one-band semiconductors or quantum wires. In this paper we present application-specific job submission and reservation management tool named a Job Track Server (JTS). It is developed using Message-Oriented middleware to implement robust, versatile job submission and tracing mechanism, which can be tailored to application specific failover and quality of service requirements. Experience from using the JTS for submission of SALUTE jobs is presented.

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

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

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

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.

Modeling ultrafast exciton migration within the electron donor domains of bulk heterojunction organic photovoltaics

DOE PAGES

Bednarz, Mateusz; Lapin, Joel; McGillicuddy, Ryan; ...

2017-02-21

Recent experimental studies revealed that charge carriers harvested by bulk heterojunction organic photovoltaics can be collected on ultrafast time scales. To investigate ultrafast exciton mobility, we construct simple, nonatomistic models of a common polymeric electron donor material. We first explore the relationship between the magnitude of energetic noise in the model Hamiltonian and the spatial extent of resulting eigenstates. We then employ a quantum master equation approach to simulate migration of chromophore-localized initial excited states. Excitons initially localized on a single chromophore at the center of the model delocalize down polymer chains and across pi-stacked chromophores through a coherent, wavelikemore » mechanism during the first few tens of femtoseconds. We explore the dependence of this coherent delocalization on coupling strength and on the magnitude of energetic noise. At longer times we observe continued migration toward a uniform population distribution that proceeds through an incoherent, diffusive mechanism. A series of simulations modeling exciton harvesting in domains of varying size demonstrates that smaller domains enhance ultrafast exciton harvesting yield. Finally, our nonatomistic model falls short of quantitative accuracy but demonstrates that excitons are mobile within electron donor domains on ultrafast time scales and that coherent exciton transport can enhance ultrafast exciton harvesting.« less

Modeling ultrafast exciton migration within the electron donor domains of bulk heterojunction organic photovoltaics

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

Bednarz, Mateusz; Lapin, Joel; McGillicuddy, Ryan

Recent experimental studies revealed that charge carriers harvested by bulk heterojunction organic photovoltaics can be collected on ultrafast time scales. To investigate ultrafast exciton mobility, we construct simple, nonatomistic models of a common polymeric electron donor material. We first explore the relationship between the magnitude of energetic noise in the model Hamiltonian and the spatial extent of resulting eigenstates. We then employ a quantum master equation approach to simulate migration of chromophore-localized initial excited states. Excitons initially localized on a single chromophore at the center of the model delocalize down polymer chains and across pi-stacked chromophores through a coherent, wavelikemore » mechanism during the first few tens of femtoseconds. We explore the dependence of this coherent delocalization on coupling strength and on the magnitude of energetic noise. At longer times we observe continued migration toward a uniform population distribution that proceeds through an incoherent, diffusive mechanism. A series of simulations modeling exciton harvesting in domains of varying size demonstrates that smaller domains enhance ultrafast exciton harvesting yield. Finally, our nonatomistic model falls short of quantitative accuracy but demonstrates that excitons are mobile within electron donor domains on ultrafast time scales and that coherent exciton transport can enhance ultrafast exciton harvesting.« less

Polarization-dependent force driving the Eg mode in bismuth under optical excitation: comparison of first-principles theory with ultra-fast x-ray experiments

NASA Astrophysics Data System (ADS)

Fahy, Stephen; Murray, Eamonn

2015-03-01

Using first principles electronic structure methods, we calculate the induced force on the Eg (zone centre transverse optical) phonon mode in bismuth immediately after absorption of a ultrafast pulse of polarized light. To compare the results with recent ultra-fast, time-resolved x-ray diffraction experiments, we include the decay of the force due to carrier scattering, as measured in optical Raman scattering experiments, and simulate the optical absorption process, depth-dependent atomic driving forces, and x-ray diffraction in the experimental geometry. We find excellent agreement between the theoretical predictions and the observed oscillations of the x-ray diffraction signal, indicating that first-principles theory of optical absorption is well suited to the calculation of initial atomic driving forces in photo-excited materials following ultrafast excitation. This work is supported by Science Foundation Ireland (Grant No. 12/IA/1601) and EU Commission under the Marie Curie Incoming International Fellowships (Grant No. PIIF-GA-2012-329695).

Ultrafast fluorescence upconversion technique and its applications to proteins.

PubMed

Chosrowjan, Haik; Taniguchi, Seiji; Tanaka, Fumio

2015-08-01

The basic principles and main characteristics of the ultrafast time-resolved fluorescence upconversion technique (conventional and space-resolved), including requirements for nonlinear crystals, mixing spectral bandwidth, acceptance angle, etc., are presented. Applications to flavoproteins [wild-type (WT) FMN-binding protein and its W32Y, W32A, E13R, E13K, E13Q and E13T mutants] and photoresponsive proteins [WT photoactive yellow protein and its R52Q mutant in solution and as single crystals] are demonstrated. For flavoproteins, investigations elucidating the effects of ionic charges on ultrafast electron transfer (ET) dynamics are summarized. It is shown that replacement of the ionic amino acid Glu13 and the resulting modification of the electrostatic charge distribution in the protein chromphore-binding pocket substantially alters the ultrafast fluorescence quenching dynamics and ET rate in FMN-binding protein. It is concluded that, together with donor-acceptor distances, electrostatic interactions between ionic photoproducts and other ionic groups in the proteins are important factors influencing the ET rates. In WT photoactive yellow protein and the R52Q mutant, ultrafast photoisomerization dynamics of the chromophore (deprotonated trans-p-coumaric acid) in liquid and crystal phases are investigated. It is shown that the primary dynamics in solution and single-crystal phases are quite similar; hence, the photocycle dynamics and structural differences observed at longer time scales arise mostly from the structural restraints imposed by the crystal lattice rigidity versus the flexibility in solution. © 2014 FEBS.

Dynamic trapping of a polarization rotation vector soliton in a fiber laser.

PubMed

Liu, Meng; Luo, Ai-Ping; Luo, Zhi-Chao; Xu, Wen-Cheng

2017-01-15

Ultrafast fiber laser, as a dissipative nonlinear optical system, plays an important role in investigating various nonlinear phenomena and soliton dynamics. Vector features of solitons, including polarization locked and polarization rotation vector solitons (PRVSs), are interesting nonlinear dynamics in ultrafast fiber lasers. Herein, we experimentally reveal the trapping characteristics of PRVSs for the first time, to the best of our best knowledge. We show that, for the conventional soliton trapping in the ultrafast fiber laser, the soliton central wavelengths of the two polarization components are constant at the laser output port. However, it is found that the dynamic trapping can be observed for the PRVS. That is, the peak frequencies along the two orthogonal polarization directions are dynamically alternating, depending on the relative intensities of the two polarization components. The obtained results would further unveil the physical mechanism of PRVSs.

Ultrafast Time-Resolved Photoluminescence Studies of Gallium-Arsenide

NASA Astrophysics Data System (ADS)

Johnson, Matthew Bruce

This thesis concerns the study of ultrafast phenomena in GaAs using time-resolved photoluminescence (PL). The thesis consists of five chapters. Chapter one is an introduction, which discusses the study of ultrafast phenomena in semiconductors. Chapter two is a description of the colliding-pulse mode-locked (CPM) ring dye laser, which is at the heart of the experimental apparatus used in this thesis. Chapter three presents a detailed experimental and theoretical investigation of photoluminescence excitation correlation spectroscopy (PECS), the novel technique which is used to time-resolve ultrafast PL phenomena. Chapters 4 and 5 discuss two applications of the PECS technique. In Chapter 4 the variation of PL intensity in In-alloyed GaAs substrate material is studied, while Chapter 5 discusses the variation of carrier lifetimes in ion-damaged GaAs used in photo-conductive circuit elements (PCEs). PECS is a pulse-probe technique that measures the cross correlation of photo-excited carrier populations. The theoretical model employed in this thesis is based upon the rate equation for a simple three-level system consisting of valence and conduction bands and a single trap level. In the limit of radiative band-to-band dominated recombination, no PECS signal should be observed; while in the capture -dominated recombination limit, the PECS signal from the band-to-band PL measures the cross correlation of the excited electron and hole populations and thus, the electron and hole lifetimes. PECS is experimentally investigated using a case study of PL in semi-insulating (SI) GaAs and In -alloyed GaAs. At 77 K, the PECS signal is characteristic of a capture-dominated system, yielding an electron-hole lifetime of about 200 ps. However, at 5 K the behavior is more complicated and shows saturation effects due to the C acceptor level, which is un-ionized at 5 K. As a first application, PECS is used to investigate the large band-to-band PL contrast observed near dislocations in In-alloyed GaAs. It is found that the PL intensity contrast between bright and dark areas correlates with the ratio of the lifetimes measured using PECS in these areas. Thus, the PL intensity contrast is due to the difference in the carrier lifetimes in the different regions. The carrier lifetimes in the bright and dark regions have different temperature dependences. (Abstract shortened with permission of author.).

Conformational Control of Ultrafast Molecular Rotor Property: Tuning Viscosity Sensing Efficiency by Twist Angle Variation.

PubMed

Ghosh, Rajib; Kushwaha, Archana; Das, Dipanwita

2017-09-21

Fluorescent molecular rotors find widespread application in sensing and imaging of microscopic viscosity in complex chemical and biological media. Development of viscosity-sensitive ultrafast molecular rotor (UMR) relies upon the understanding of the excited-state dynamics and their implications for viscosity-dependent fluorescence signaling. Unraveling the structure-property relationship of UMR behavior is of significance toward development of an ultrasensitive fluorescence microviscosity sensor. Herein we show that the ground-state equilibrium conformation has an important role in the ultrafast twisting dynamics of UMRs and consequent viscosity sensing efficiency. Synthesis, photophysics, and ultrafast spectroscopic experiments in conjunction with quantum chemical calculation of a series of UMRs based on dimethylaniline donor and benzimidazolium acceptor with predefined ground-state torsion angle led us to unravel that the ultrafast torsional dynamics around the bond connecting donor and acceptor groups profoundly influences the molecular rotor efficiency. This is the first experimental demonstration of conformational control of small-molecule-based UMR efficiencies which can have wider implication toward development of fluorescence sensors based on the UMR principle. Conformation-controlled UMR efficiency has been shown to exhibit commensurate fluorescence enhancement upon DNA binding.

The influence of surfaces on the transient terahertz conductivity and electron mobility of GaAs nanowires

NASA Astrophysics Data System (ADS)

Joyce, Hannah J.; Baig, Sarwat A.; Parkinson, Patrick; Davies, Christopher L.; Boland, Jessica L.; Tan, H. Hoe; Jagadish, Chennupati; Herz, Laura M.; Johnston, Michael B.

2017-06-01

Bare unpassivated GaAs nanowires feature relatively high electron mobilities (400-2100 cm2 V-1 s-1) and ultrashort charge carrier lifetimes (1-5 ps) at room temperature. These two properties are highly desirable for high speed optoelectronic devices, including photoreceivers, modulators and switches operating at microwave and terahertz frequencies. When engineering these GaAs nanowire-based devices, it is important to have a quantitative understanding of how the charge carrier mobility and lifetime can be tuned. Here we use optical-pump-terahertz-probe spectroscopy to quantify how mobility and lifetime depend on the nanowire surfaces and on carrier density in unpassivated GaAs nanowires. We also present two alternative frameworks for the analysis of nanowire photoconductivity: one based on plasmon resonance and the other based on Maxwell-Garnett effective medium theory with the nanowires modelled as prolate ellipsoids. We find the electron mobility decreases significantly with decreasing nanowire diameter, as charge carriers experience increased scattering at nanowire surfaces. Reducing the diameter from 50 nm to 30 nm degrades the electron mobility by up to 47%. Photoconductivity dynamics were dominated by trapping at saturable states existing at the nanowire surface, and the trapping rate was highest for the nanowires of narrowest diameter. The maximum surface recombination velocity, which occurs in the limit of all traps being empty, was calculated as 1.3  ×  106 cm s-1. We note that when selecting the optimum nanowire diameter for an ultrafast device, there is a trade-off between achieving a short lifetime and a high carrier mobility. To achieve high speed GaAs nanowire devices featuring the highest charge carrier mobilities and shortest lifetimes, we recommend operating the devices at low charge carrier densities.

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.

Transverse Mode Dynamics and Ultrafast Modulation of Vertical-Cavity Surface-Emitting Lasers

NASA Technical Reports Server (NTRS)

Ning, Cun-Zheng; Biegel, Bryan A. (Technical Monitor)

2002-01-01

We show that multiple transverse mode dynamics of VCSELs (Vertical-Cavity Surface-Emitting Lasers) can be utilized to generate ultrafast intensity modulation at a frequency over 100 GHz, much higher than the relaxation oscillation frequency. Such multimode beating can be greatly enhanced by taking laser output from part of the output facet.

H2: the benchmark molecule for ultrafast science and technologies

NASA Astrophysics Data System (ADS)

Ibrahim, Heide; Lefebvre, Catherine; Bandrauk, André D.; Staudte, André; Légaré, François

2018-02-01

This review article focuses on imaging and controlling ultrafast dynamics of the hydrogen molecule and its cation, initiated by ultrashort laser pulses. We discuss the mechanisms underlying these dynamics and theoretical methods to describe them. A broad variety of defining and influencing theoretical and experimental results is presented. We put special emphasis on the required experimental techniques, many of which have been developed in view of imaging the fastest of all nuclear dynamics.

Structural dynamics of lipid bilayers using ultrafast electron crystallography

NASA Astrophysics Data System (ADS)

Chen, Songye; Seidel, Marco; Zewail, Ahmed

2007-03-01

The structures and dynamics of bilayers of crystalline fatty acids and phospholipids were studied using ultrafast electron crystallography (UEC). The systems investigated are arachidic (eicosanoic) acid and dimyristoyl phosphatidic acid (DMPA), deposited on a substrate by the Langmuir-Blodgett technique. The atomic structures under different preparation conditions were determined. The structural dynamics following a temperature jump induced by femtosecond laser on the substrates were obtained and compared to the equilibrium temperature dependence.

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.

Modeling of Optoelectronic Devices

NASA Technical Reports Server (NTRS)

Li, Jian-Zhong; Woo, Alex C. (Technical Monitor)

2000-01-01

Ultrafast modulation of semiconductor quantum well (QW) laser is of technological importance for information technology. Improvement by order(s) of magnitude in data transfer rate is possible as terahertz (THz) radiation is available for heating the laser at picosecond time scale. Optical gain modulation in the QW is achieved via temperature modulation of electron-hole plasma (EHP). Applications include free-space THz communication, optical switching, and pulse generation. The EHP in the semiconductor QW is described with a two-band model. Semiconductor Bloch equations with many-body effects are used to derive a hydrodynamical model for the active QW region. Because of ultrafast carrier-carrier scatterings in the order of 50 fs, EHP follows quasiequilibrium Fermi-Dirac distributions and THz field interacts incoherently with it. Carrier-longitudinal optical (LO) phonon scatterings and coherent laser-EHP interaction are treated microscopically in our physical model. A set of hydrodynamical equations for plasma density, temperature, and laser envelop amplitude are derived and Runge-Kutta method is adopted for numerical simulation. A typical 8 nm GaAs/Al(0.3)Ga(0.7) As single QW at 300 K is used. Additional information is contained in the original extended abstract.

Impact of charge-transfer excitons in regioregular polythiophene on the charge separation at polythiophene-fullerene heterojunctions

NASA Astrophysics Data System (ADS)

Polkehn, M.; Tamura, H.; Burghardt, I.

2018-01-01

This study addresses the mechanism of ultrafast charge separation in regioregular oligothiophene-fullerene assemblies representative of poly-3-hexylthiophene (P3HT)-[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) heterojunctions, with special emphasis on the inclusion of charge transfer excitons in the oligothiophene phase. The formation of polaronic inter-chain charge separated species in highly ordered oligothiophene has been demonstrated in recent experiments and could have a significant impact on the net charge transfer to the fullerene acceptor. The present approach combines a first-principles parametrized multi-site Hamiltonian, based on time-dependent density functional theory calculations, with accurate quantum dynamics simulations using the multi-layer multi-configuration time-dependent Hartree method. Quantum dynamical studies are carried out for up to 182 electronic states and 112 phonon modes. The present analysis follows up on our previous study of (Huix-Rotllant et al 2015 J. Phys. Chem. Lett. 6 1702) and significantly expands the scope of this analysis by including the dynamical role of charge transfer excitons. Our investigation highlights the pronounced mixing of photogenerated Frenkel excitons with charge transfer excitons in the oligothiophene domain, and the opening of new transfer channels due the creation of such charge-separated species. As a result, it turns out that the interfacial donor/acceptor charge transfer state can be largely circumvented due to the presence of charge transfer excitons. However, the latter states in turn act as a trap, such that the free carrier yield observed on ultrafast time scales is tangibly reduced. The present analysis underscores the complexity of the transfer pathways at P3HT-PCBM type junctions.

Deviation from threshold model in ultrafast laser ablation of graphene at sub-micron scale

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

Gil-Villalba, A.; Xie, C.; Salut, R.

We investigate a method to measure ultrafast laser ablation threshold with respect to spot size. We use structured complex beams to generate a pattern of craters in CVD graphene with a single laser pulse. A direct comparison between beam profile and SEM characterization allows us to determine the dependence of ablation probability on spot-size, for crater diameters ranging between 700 nm and 2.5 μm. We report a drastic decrease of ablation probability when the crater diameter is below 1 μm which we interpret in terms of free-carrier diffusion.

Reversible ultrafast melting in bulk CdSe

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

Wu, Wenzhi; Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712; He, Feng

2016-02-07

In this work, transient reflectivity changes in bulk CdSe have been measured with two-color femtosecond pump-probe spectroscopy under a wide range of pump fluences. Three regions of reflectivity change with pump fluences have been consistently revealed for excited carrier density, coherent phonon amplitude, and lattice temperature. For laser fluences from 13 to 19.3 mJ/cm{sup 2}, ultrafast melting happens in first several picoseconds. This melting process is purely thermal and reversible. A complete phase transformation in bulk CdSe may be reached when the absorbed laser energy is localized long enough, as observed in nanocrystalline CdSe.

Ultrafast all-optical switching and error-free 10 Gbit/s wavelength conversion in hybrid InP-silicon on insulator nanocavities using surface quantum wells

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

Bazin, Alexandre; Monnier, Paul; Beaudoin, Grégoire

Ultrafast switching with low energies is demonstrated using InP photonic crystal nanocavities embedding InGaAs surface quantum wells heterogeneously integrated to a silicon on insulator waveguide circuitry. Thanks to the engineered enhancement of surface non radiative recombination of carriers, switching time is obtained to be as fast as 10 ps. These hybrid nanostructures are shown to be capable of achieving systems level performance by demonstrating error free wavelength conversion at 10 Gbit/s with 6 mW switching powers.

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

Phonon-coupled ultrafast interlayer charge oscillation at van der Waals heterostructure interfaces

NASA Astrophysics Data System (ADS)

Zheng, Qijing; Xie, Yu; Lan, Zhenggang; Prezhdo, Oleg V.; Saidi, Wissam A.; Zhao, Jin

2018-05-01

Van der Waals (vdW) heterostructures of transition-metal dichalcogenide (TMD) semiconductors are central not only for fundamental science, but also for electro- and optical-device technologies where the interfacial charge transfer is a key factor. Ultrafast interfacial charge dynamics has been intensively studied, however, the atomic scale insights into the effects of the electron-phonon (e-p) coupling are still lacking. In this paper, using time dependent ab initio nonadiabatic molecular dynamics, we study the ultrafast interfacial charge transfer dynamics of two different TMD heterostructures MoS2/WS2 and MoSe2/WSe2 , which have similar band structures but different phonon frequencies. We found that MoSe2/WSe2 has softer phonon modes compared to MoS2/WS2 , and thus phonon-coupled charge oscillation can be excited with sufficient phonon excitations at room temperature. In contrast, for MoS2/WS2 , phonon-coupled interlayer charge oscillations are not easily excitable. Our study provides an atomic level understanding on how the phonon excitation and e-p coupling affect the interlayer charge transfer dynamics, which is valuable for both the fundamental understanding of ultrafast dynamics at vdW hetero-interfaces and the design of novel quasi-two-dimensional devices for optoelectronic and photovoltaic applications.

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.

Particle Size/ Grain Size Correlation and Mechanical Properties of Spark Plasma Sintered 8Y-ZrO2, MgAl2O4, and Al2O3 Based Composites

NASA Astrophysics Data System (ADS)

Karandikar, Keyur Kashinath

Solution-processed nanomaterials such as lead sulfide (PbS) colloidal quantum dots (CQDs) combine various manufacturing benefits and facile spectral tunability. However, the low mobility of CQD films limits its power conversion efficiency in photovoltaic cells. Here, I employ a novel femtosecond transient absorption (fs-TA) technique to determine the mobility of PbS CQD films that have undergone state of the art surface treatments. A significant mobility increase from 3 x 10-2 to 5 x 10 -1 cm2 V-1 s-1 was determined for iodide passivated and novel perovskite-shelled PbS CQDs, respectively. I performed, for the first time, temperature-dependent ultrafast carrier dynamics in perovskite-shelled CQDs using fs-TA, and determined an activation energy of 14 meV required for carrier hopping. Complementary studies that used time-of-flight measurements to determine the mobility in solar cell configuration corroborated the fs-TA method. Taken together, these results indicate a promising avenue toward improved CQD solar cells.

Femtosecond dynamics in hydrogen-bonded solvents

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

Castner, E.W. Jr.; Chang, Y.J.

1993-09-01

We present results on the ultrafast dynamics of pure hydrogen-bonding solvents, obtained using femtosecond Fourier-transform optical-heterodyne-detected, Raman-induced Kerr effect spectroscopy. Solvent systems we have studied include the formamides, water, ethylene glycol, and acetic acid. Inertial and diffusive motions are clearly resolved. We comment on the effect that such ultrafast solvent motions have on chemical reactions in solution.

Photoinduced Ultrafast Intramolecular Excited-State Energy Transfer in the Silylene-Bridged Biphenyl and Stilbene (SBS) System: A Nonadiabatic Dynamics Point of View.

PubMed

Wang, Jun; Huang, Jing; Du, Likai; Lan, Zhenggang

2015-07-09

The photoinduced intramolecular excited-state energy-transfer (EET) process in conjugated polymers has received a great deal of research interest because of its important role in the light harvesting and energy transport of organic photovoltaic materials in photoelectric devices. In this work, the silylene-bridged biphenyl and stilbene (SBS) system was chosen as a simplified model system to obtain physical insight into the photoinduced intramolecular energy transfer between the different building units of the SBS copolymer. In the SBS system, the vinylbiphenyl and vinylstilbene moieties serve as the donor (D) unit and the acceptor (A) unit, respectively. The ultrafast excited-state dynamics of the SBS system was investigated from the point of view of nonadiabatic dynamics with the surface-hopping method at the TDDFT level. The first two excited states (S1 and S2) are characterized by local excitations at the acceptor (vinylstilbene) and donor (vinylbiphenyl) units, respectively. Ultrafast S2-S1 decay is responsible for the intramolecular D-A excitonic energy transfer. The geometric distortion of the D moiety play an essential role in this EET process, whereas the A moiety remains unchanged during the nonadiabatic dynamics simulation. The present work provides a direct dynamical approach to understand the ultrafast intramolecular energy-transfer dynamics in SBS copolymers and other similar organic photovoltaic copolymers.

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.

Initial photoinduced dynamics of the photoactive yellow protein.

PubMed

Larsen, Delmar S; van Grondelle, Rienk

2005-05-01

The photoactive yellow protein (PYP) is the photoreceptor protein responsible for initiating the blue-light repellent response of the Halorhodospira halophila bacterium. Optical excitation of the intrinsic chromophore in PYP, p-coumaric acid, leads to the initiation of a photocycle that comprises several distinct intermediates. The dynamical processes responsible for the initiation of the PYP photocycle have been explored with several time-resolved techniques, which include ultrafast electronic and vibrational spectroscopies. Ultrafast electronic spectroscopies, such as pump-visible probe, pump-dump-visible probe, and fluorescence upconversion, are useful in identifying the timescales and connectivity of the transient intermediates, while ultrafast vibrational spectroscopies link these intermediates to dynamic structures. Herein, we present the use of these techniques for exploring the initial dynamics of PYP, and show how these techniques provide the basis for understanding the complex relationship between protein and chromophore, which ultimately results in biological function.

Ultrafast quantum control of ionization dynamics in krypton.

PubMed

Hütten, Konrad; Mittermair, Michael; Stock, Sebastian O; Beerwerth, Randolf; Shirvanyan, Vahe; Riemensberger, Johann; Duensing, Andreas; Heider, Rupert; Wagner, Martin S; Guggenmos, Alexander; Fritzsche, Stephan; Kabachnik, Nikolay M; Kienberger, Reinhard; Bernhardt, Birgitta

2018-02-19

Ultrafast spectroscopy with attosecond resolution has enabled the real time observation of ultrafast electron dynamics in atoms, molecules and solids. These experiments employ attosecond pulses or pulse trains and explore dynamical processes in a pump-probe scheme that is selectively sensitive to electronic state of matter via photoelectron or XUV absorption spectroscopy or that includes changes of the ionic state detected via photo-ion mass spectrometry. Here, we demonstrate how the implementation of combined photo-ion and absorption spectroscopy with attosecond resolution enables tracking the complex multidimensional excitation and decay cascade of an Auger auto-ionization process of a few femtoseconds in highly excited krypton. In tandem with theory, our study reveals the role of intermediate electronic states in the formation of multiply charged ions. Amplitude tuning of a dressing laser field addresses different groups of decay channels and allows exerting temporal and quantitative control over the ionization dynamics in rare gas atoms.

Coherent helix vacancy phonon and its ultrafast dynamics waning in topological Dirac semimetal C d3A s2

NASA Astrophysics Data System (ADS)

Sun, Fei; Wu, Q.; Wu, Y. L.; Zhao, H.; Yi, C. J.; Tian, Y. C.; Liu, H. W.; Shi, Y. G.; Ding, H.; Dai, X.; Richard, P.; Zhao, Jimin

2017-06-01

We report an ultrafast lattice dynamics investigation of the topological Dirac semimetal C d3A s2 . A coherent phonon beating among three evenly spaced A1 g optical phonon modes (of frequencies 1.80, 1.96, and 2.11 THz, respectively) is unambiguously observed. The two side modes originate from the counter helixes composing Cd vacancies. Significantly, such helix vacancy-induced phonon (HVP) modes experience prominent extra waning in their ultrafast dynamics as temperature increases, which is immune to the central mode. Above 200 K, the HVP becomes inactive, which may potentially affect the topological properties. Our results in the lattice degree of freedom suggest the indispensable role of temperature in considering topological properties of such quantum materials.

Carrier multiplication and charge transport in artificial quantum-dot solids probed by ultrafast photocurrent spectroscopy (Conference Presentation)

NASA Astrophysics Data System (ADS)

Klimov, Victor I.

2017-05-01

Understanding and controlling carrier transport and recombination dynamics in colloidal quantum dot films is key to their application in electronic and optoelectronic devices. Towards this end, we have conducted transient photocurrent measurements to monitor transport through quantum confined band edge states in lead selenide quantum dots films as a function of pump fluence, temperature, electrical bias, and surface treatment. Room temperature dynamics reveal two distinct timescales of intra-dot geminate processes followed by non-geminate inter-dot processes. The non-geminate kinetics is well described by the recombination of holes with photoinjected and pre-existing electrons residing in mid-gap states. We find the mobility of the quantum-confined states shows no temperature dependence down to 6 K, indicating a tunneling mechanism of early time photoconductance. We present evidence of the importance of the exciton fine structure in controlling the low temperature photoconductance, whereby the nanoscale enhanced exchange interaction between electrons and holes in quantum dots introduces a barrier to charge separation. Finally, side-by-side comparison of photocurrent transients using excitation with low- and high-photon energies (1.5 vs. 3.0 eV) reveals clear signatures of carrier multiplication (CM), that is, generation of multiple excitons by single photons. Based on photocurrent measurements of quantum dot solids and optical measurements of solution based samples, we conclude that the CM efficiency is unaffected by strong inter-dot coupling. Therefore, the results of previous numerous spectroscopic CM studies conducted on dilute quantum dot suspensions should, in principle, be reproducible in electronically coupled QD films used in devices.

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

Pandit, Bill; Jackson, Nicholas E.; Zheng, Tianyue

Rational design strategies for controlling the energetics of conjugated “donor–acceptor” copolymers are ubiquitous in the literature, as they allow for simple energy-level tuning strategies to be employed for photovoltaic and transistor applications. Utilizing the recently reported PTRn series of conjugated polymers closely related to the widely implemented material PTB7, we investigate the effect of local copolymer block energetics on the generation of transient excitonic and charge carrier species. It is clearly demonstrated that local copolymer block energetics play a much larger role than is apparent from simple energy-level tuning arguments, and drastically affect the ultrafast generation of free-charge carrier andmore » trap state populations. Specifically, we observe an almost complete reversal in the efficient generation of free-charge in PTB7 to the ultrafast creation of a high percentage of trapped pseudo charge-transfer states. The implications of this secondary effect of “donor–acceptor” energy level tuning are discussed, along with strategies for avoiding the generation of trap states in “donor–acceptor” copolymers.« less

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

Deb, Marwan, E-mail: marwan.deb@ipcms.unistra.fr; Vomir, Mircea; Rehspringer, Jean-Luc

Controlling the magnetization dynamics on the femtosecond timescale is of fundamental importance for integrated opto-spintronic devices. For industrial perspectives, it requires to develop simple growth techniques for obtaining large area magneto-optical materials having a high amplitude ultrafast Faraday or Kerr response. Here we report on optical pump probe studies of light induced spin dynamics in high quality bismuth doped iron garnet polycrystalline film prepared by the spin coating method. We demonstrate an ultrafast non-thermal optical control of the spin dynamics using both circularly and linearly polarized pulses.

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

Density matrix-based time-dependent configuration interaction approach to ultrafast spin-flip dynamics

NASA Astrophysics Data System (ADS)

Wang, Huihui; Bokarev, Sergey I.; Aziz, Saadullah G.; Kühn, Oliver

2017-08-01

Recent developments in attosecond spectroscopy yield access to the correlated motion of electrons on their intrinsic timescales. Spin-flip dynamics is usually considered in the context of valence electronic states, where spin-orbit coupling is weak and processes related to the electron spin are usually driven by nuclear motion. However, for core-excited states, where the core-hole has a nonzero angular momentum, spin-orbit coupling is strong enough to drive spin-flips on a much shorter timescale. Using density matrix-based time-dependent restricted active space configuration interaction including spin-orbit coupling, we address an unprecedentedly short spin-crossover for the example of L-edge (2p→3d) excited states of a prototypical Fe(II) complex. This process occurs on a timescale, which is faster than that of Auger decay (∼4 fs) treated here explicitly. Modest variations of carrier frequency and pulse duration can lead to substantial changes in the spin-state yield, suggesting its control by soft X-ray light.

Mode-selective vibrational modulation of charge transport in organic electronic devices

PubMed Central

Bakulin, Artem A.; Lovrincic, Robert; Yu, Xi; Selig, Oleg; Bakker, Huib J.; Rezus, Yves L. A.; Nayak, Pabitra K.; Fonari, Alexandr; Coropceanu, Veaceslav; Brédas, Jean-Luc; Cahen, David

2015-01-01

The soft character of organic materials leads to strong coupling between molecular, nuclear and electronic dynamics. This coupling opens the way to influence charge transport in organic electronic devices by exciting molecular vibrational motions. However, despite encouraging theoretical predictions, experimental realization of such approach has remained elusive. Here we demonstrate experimentally that photoconductivity in a model organic optoelectronic device can be modulated by the selective excitation of molecular vibrations. Using an ultrafast infrared laser source to create a coherent superposition of vibrational motions in a pentacene/C60 photoresistor, we observe that excitation of certain modes in the 1,500–1,700 cm−1 region leads to photocurrent enhancement. Excited vibrations affect predominantly trapped carriers. The effect depends on the nature of the vibration and its mode-specific character can be well described by the vibrational modulation of intermolecular electronic couplings. This presents a new tool for studying electron–phonon coupling and charge dynamics in (bio)molecular materials. PMID:26246039

Mode-selective vibrational modulation of charge transport in organic electronic devices

NASA Astrophysics Data System (ADS)

Bakulin, Artem A.; Lovrincic, Robert; Yu, Xi; Selig, Oleg; Bakker, Huib J.; Rezus, Yves L. A.; Nayak, Pabitra K.; Fonari, Alexandr; Coropceanu, Veaceslav; Brédas, Jean-Luc; Cahen, David

2015-08-01

The soft character of organic materials leads to strong coupling between molecular, nuclear and electronic dynamics. This coupling opens the way to influence charge transport in organic electronic devices by exciting molecular vibrational motions. However, despite encouraging theoretical predictions, experimental realization of such approach has remained elusive. Here we demonstrate experimentally that photoconductivity in a model organic optoelectronic device can be modulated by the selective excitation of molecular vibrations. Using an ultrafast infrared laser source to create a coherent superposition of vibrational motions in a pentacene/C60 photoresistor, we observe that excitation of certain modes in the 1,500-1,700 cm-1 region leads to photocurrent enhancement. Excited vibrations affect predominantly trapped carriers. The effect depends on the nature of the vibration and its mode-specific character can be well described by the vibrational modulation of intermolecular electronic couplings. This presents a new tool for studying electron-phonon coupling and charge dynamics in (bio)molecular materials.

Time-delayed behaviors of transient four-wave mixing signal intensity in inverted semiconductor with carrier-injection pumping

NASA Astrophysics Data System (ADS)

Hu, Zhenhua; Gao, Shen; Xiang, Bowen

2016-01-01

An analytical expression of transient four-wave mixing (TFWM) in inverted semiconductor with carrier-injection pumping was derived from both the density matrix equation and the complex stochastic stationary statistical method of incoherent light. Numerical analysis showed that the TFWM decayed decay is towards the limit of extreme homogeneous and inhomogeneous broadenings in atoms and the decaying time is inversely proportional to half the power of the net carrier densities for a low carrier-density injection and other high carrier-density injection, while it obeys an usual exponential decay with other decaying time that is inversely proportional to half the power of the net carrier density or it obeys an unusual exponential decay with the decaying time that is inversely proportional to a third power of the net carrier density for a moderate carrier-density injection. The results can be applied to studying ultrafast carrier dephasing in the inverted semiconductors such as semiconductor laser amplifier and semiconductor optical amplifier.

Ultrafast Dynamics of Energetic Materials

DTIC Science & Technology

2014-01-23

redistributed in condensed-phase materials. In this subproject we developed a technique termed three-dimensional IR- Raman spectroscopy that allowed us to...Fang, 2011, “The distribution of local enhancement factors in surface enhanced Raman -active substrates and the vibrational dynamics in the liquid phase...3. (invited) “Vibrational energy and molecular thermometers in liquids: Ultrafast IR- Raman spectroscopy”, Brandt C. Pein and Dana D. Dlott, To

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.

Lattice-level measurement of material strength with LCLS during ultrafast dynamic compression

NASA Astrophysics Data System (ADS)

Milathianaki, Despina; Boutet, Sebastien; Ratner, Daniel; White, William; Williams, Garth; Gleason, Arianna; Swift, Damian; Higginbotham, Andrew; Wark, Justin

2013-10-01

An in-depth understanding of the stress-strain behavior of materials during ultrafast dynamic compression requires experiments that offer in-situ observation of the lattice at the pertinent temporal and spatial scales. To date, the lattice response under extreme strain-rate conditions (>108 s-1) has been inferred predominantly from continuum-level measurements and multi-million atom molecular dynamics simulations. Several time-resolved x-ray diffraction experiments have captured important information on plasticity kinetics, while limited to nanosecond timescales due to the lack of high brilliance ultrafast x-ray sources. Here we present experiments at LCLS combining ultrafast laser-shocks and serial femtosecond x-ray diffraction. The high spectral brightness (~1012 photons per pulse, ΔE/E = 0.2%) and subpicosecond temporal resolution (<100 fs pulsewidth) of the LCLS x-ray free electron laser allow investigations that link simulations and experiments at the fundamental temporal and spatial scales for the first time. We present movies of the lattice undergoing rapid shock-compression, composed by a series of single femtosecond x-ray snapshots, demonstrating the transient behavior while successfully decoupling the elastic and plastic response in polycrystalline Cu.

Ultrafast exciton migration in an HJ-aggregate: Potential surfaces and quantum dynamics

NASA Astrophysics Data System (ADS)

Binder, Robert; Polkehn, Matthias; Ma, Tianji; Burghardt, Irene

2017-01-01

Quantum dynamical and electronic structure calculations are combined to investigate the mechanism of exciton migration in an oligothiophene HJ aggregate, i.e., a combination of oligomer chains (J-type aggregates) and stacked aggregates of such chains (H-type aggregates). To this end, a Frenkel exciton model is parametrized by a recently introduced procedure [Binder et al., J. Chem. Phys. 141, 014101 (2014)] which uses oligomer excited-state calculations to perform an exact, point-wise mapping of coupled potential energy surfaces to an effective Frenkel model. Based upon this parametrization, the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method is employed to investigate ultrafast dynamics of exciton transfer in a small, asymmetric HJ aggregate model composed of 30 sites and 30 active modes. For a partially delocalized initial condition, it is shown that a torsional defect confines the trapped initial exciton, and planarization induces an ultrafast resonant transition between an HJ-aggregated segment and a covalently bound "dangling chain" end. This model is a minimal realization of experimentally investigated mixed systems exhibiting ultrafast exciton transfer between aggregated, highly planarized chains and neighboring disordered segments.

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.

Understanding the free energy barrier and multiple timescale dynamics of charge separation in organic photovoltaic cells.

PubMed

Yan, Yaming; Song, Linze; Shi, Qiang

2018-02-28

By employing several lattice model systems, we investigate the free energy barrier and real-time dynamics of charge separation in organic photovoltaic (OPV) cells. It is found that the combined effects of the external electric field, entropy, and charge delocalization reduce the free energy barrier significantly. The dynamic disorder reduces charge carrier delocalization and results in the increased charge separation barrier, while the effect of static disorder is more complicated. Simulation of the real-time dynamics indicates that the free charge generation process involves multiple time scales, including an ultrafast component within hundreds of femtoseconds, an intermediate component related to the relaxation of the hot charge transfer (CT) state, and a slow component on the time scale of tens of picoseconds from the thermally equilibrated CT state. Effects of hot exciton dissociation as well as its dependence on the energy offset between the Frenkel exciton and the CT state are also analyzed. The current results indicate that only a small energy offset between the band gap and the lowest energy CT state is needed to achieve efficient free charge generation in OPV devices, which agrees with recent experimental findings.

Understanding the free energy barrier and multiple timescale dynamics of charge separation in organic photovoltaic cells

NASA Astrophysics Data System (ADS)

Yan, Yaming; Song, Linze; Shi, Qiang

2018-02-01

By employing several lattice model systems, we investigate the free energy barrier and real-time dynamics of charge separation in organic photovoltaic (OPV) cells. It is found that the combined effects of the external electric field, entropy, and charge delocalization reduce the free energy barrier significantly. The dynamic disorder reduces charge carrier delocalization and results in the increased charge separation barrier, while the effect of static disorder is more complicated. Simulation of the real-time dynamics indicates that the free charge generation process involves multiple time scales, including an ultrafast component within hundreds of femtoseconds, an intermediate component related to the relaxation of the hot charge transfer (CT) state, and a slow component on the time scale of tens of picoseconds from the thermally equilibrated CT state. Effects of hot exciton dissociation as well as its dependence on the energy offset between the Frenkel exciton and the CT state are also analyzed. The current results indicate that only a small energy offset between the band gap and the lowest energy CT state is needed to achieve efficient free charge generation in OPV devices, which agrees with recent experimental findings.

Ultrafast studies of coexisting electronic order in cuprate superconductors

NASA Astrophysics Data System (ADS)

Hinton, James; Thewalt, Eric; Alpichshev, Zhanybek; Sternbach, Aaron; McLeod, Alex; Ji, L.; Veit, Mike; Dorrow, Chelsey; Koralek, Jake; Xhao, Xudong; Barisic, Neven; Kemper, Alexander; Gedik, Nuh; Greven, Martin; Basov, Dimitri; Orenstein, Joe

The cuprate family of high temperature superconductors displays a variety of electronic phases which emerge when charge carriers are added to the antiferromagnetic parent compound. These electronic phases are characterized by subtle differences in the low energy electronic excitations. Ultrafast time-resolved reflectivity (TRR) provides an ideal tool for investigating the cuprate phase diagram, as small changes in the electronic structure can produce significant contrast in the non-equilibrium reflectivity. Here we present TRR measurements of cuprate superconductors, focusing on the model single-layer cuprate HgBa2CuO4+δ. We observe a cusp-like feature in the quasiparticle lifetime near the superconducting transition temperature Tc. This feature can be understood using a model of coherently-mixed charge-density wave and superconducting pairing. We propose extending this technique to the nanoscale using ultrafast scattering scanning near-field microscopy (u-SNOM). This will allow us to explore how these electronic phases coexist and compete in real-space.

Coherent fifth-order visible-infrared spectroscopies: ultrafast nonequilibrium vibrational dynamics in solution.

PubMed

Lynch, Michael S; Slenkamp, Karla M; Cheng, Mark; Khalil, Munira

2012-07-05

Obtaining a detailed description of photochemical reactions in solution requires measuring time-evolving structural dynamics of transient chemical species on ultrafast time scales. Time-resolved vibrational spectroscopies are sensitive probes of molecular structure and dynamics in solution. In this work, we develop doubly resonant fifth-order nonlinear visible-infrared spectroscopies to probe nonequilibrium vibrational dynamics among coupled high-frequency vibrations during an ultrafast charge transfer process using a heterodyne detection scheme. The method enables the simultaneous collection of third- and fifth-order signals, which respectively measure vibrational dynamics occurring on electronic ground and excited states on a femtosecond time scale. Our data collection and analysis strategy allows transient dispersed vibrational echo (t-DVE) and dispersed pump-probe (t-DPP) spectra to be extracted as a function of electronic and vibrational population periods with high signal-to-noise ratio (S/N > 25). We discuss how fifth-order experiments can measure (i) time-dependent anharmonic vibrational couplings, (ii) nonequilibrium frequency-frequency correlation functions, (iii) incoherent and coherent vibrational relaxation and transfer dynamics, and (iv) coherent vibrational and electronic (vibronic) coupling as a function of a photochemical reaction.

Early-Time Excited-State Relaxation Dynamics of Iridium Compounds: Distinct Roles of Electron and Hole Transfer.

PubMed

Liu, Xiang-Yang; Zhang, Ya-Hui; Fang, Wei-Hai; Cui, Ganglong

2018-06-28

Excited-state and photophysical properties of Ir-containing complexes have been extensively studied because of their potential applications as organic light-emitting diode emitting materials. However, their early time excited-state relaxation dynamics are less explored computationally. Herein we have employed our recently implemented TDDFT-based generalized surface-hopping dynamics method to simulate excited-state relaxation dynamics of three Ir(III) compounds having distinct ligands. According to our multistate dynamics simulations including five excited singlet states i.e., S n ( n = 1-5) and ten excited triplet states, i.e., T n ( n = 1-10), we have found that the intersystem crossing (ISC) processes from the S n to T n are very efficient and ultrafast in these three Ir(III) compounds. The corresponding ISC rates are estimated to be 65, 81, and 140 fs, which are reasonably close to the experimentally measured ca. 80, 80, and 110 fs. In addition, the internal conversion (IC) processes within respective singlet and triplet manifolds are also ultrafast. These ultrafast IC and ISC processes are caused by large nonadiabatic and spin-orbit couplings, respectively, as well as small energy gaps. Importantly, although these Ir(III) complexes share similar macroscopic phenomena, i.e., ultrafast IC and ISC, their microscopic excited-state relaxation mechanism and dynamics are qualitatively distinct. Specifically, the dynamical behaviors of electron and hole and their roles are variational in modulating the excited-state relaxation dynamics of these Ir(III) compounds. In other words, the electronic properties of the ligands that are coordinated with the central Ir(III) atom play important roles in regulating the microscopic excited-state relaxation dynamics. These gained insights could be useful for rationally designing Ir(III) compounds with excellent photoluminescence.

Model of THz Magnetization Dynamics.

PubMed

Bocklage, Lars

2016-03-09

Magnetization dynamics can be coherently controlled by THz laser excitation, which can be applied in ultrafast magnetization control and switching. Here, transient magnetization dynamics are calculated for excitation with THz magnetic field pulses. We use the ansatz of Smit and Beljers, to formulate dynamic properties of the magnetization via partial derivatives of the samples free energy density, and extend it to solve the Landau-Lifshitz-equation to obtain the THz transients of the magnetization. The model is used to determine the magnetization response to ultrafast multi- and single-cycle THz pulses. Control of the magnetization trajectory by utilizing the THz pulse shape and polarization is demonstrated.

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).

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

Charge carrier dynamics and surface plasmon interaction in gold nanorod-blended organic solar cell

NASA Astrophysics Data System (ADS)

Rana, Aniket; Gupta, Neeraj; Lochan, Abhiram; Sharma, G. D.; Chand, Suresh; Kumar, Mahesh; Singh, Rajiv K.

2016-08-01

The inclusion of plasmonic nanoparticles into organic solar cell enhances the light harvesting properties that lead to higher power conversion efficiency without altering the device configuration. This work defines the consequences of the nanoparticle overloading amount and energy transfer process between gold nanorod and polymer (active matrix) in organic solar cells. We have studied the hole population decay dynamics coupled with gold nanorods loading amount which provides better understanding about device performance limiting factors. The exciton and plasmon together act as an interacting dipole; however, the energy exchange between these two has been elucidated via plasmon resonance energy transfer (PRET) mechanism. Further, the charge species have been identified specifically with respect to their energy levels appearing in ultrafast time domain. The specific interaction of these charge species with respective surface plasmon resonance mode, i.e., exciton to transverse mode of oscillation and polaron pair to longitudinal mode of oscillations, has been explained. Thus, our analysis reveals that PRET enhances the carrier population density in polymer via non-radiative process beyond the concurrence of a particular plasmon resonance oscillation mode and polymer absorption range. These findings give new insight and reveal specifically the factors that enhance and control the performance of gold nanorods blended organic solar cells. This work would lead in the emergence of future plasmon based efficient organic electronic devices.

Charge carrier dynamics and surface plasmon interaction in gold nanorod-blended organic solar cell

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

Rana, Aniket; Lochan, Abhiram; Chand, Suresh

The inclusion of plasmonic nanoparticles into organic solar cell enhances the light harvesting properties that lead to higher power conversion efficiency without altering the device configuration. This work defines the consequences of the nanoparticle overloading amount and energy transfer process between gold nanorod and polymer (active matrix) in organic solar cells. We have studied the hole population decay dynamics coupled with gold nanorods loading amount which provides better understanding about device performance limiting factors. The exciton and plasmon together act as an interacting dipole; however, the energy exchange between these two has been elucidated via plasmon resonance energy transfer (PRET)more » mechanism. Further, the charge species have been identified specifically with respect to their energy levels appearing in ultrafast time domain. The specific interaction of these charge species with respective surface plasmon resonance mode, i.e., exciton to transverse mode of oscillation and polaron pair to longitudinal mode of oscillations, has been explained. Thus, our analysis reveals that PRET enhances the carrier population density in polymer via non-radiative process beyond the concurrence of a particular plasmon resonance oscillation mode and polymer absorption range. These findings give new insight and reveal specifically the factors that enhance and control the performance of gold nanorods blended organic solar cells. This work would lead in the emergence of future plasmon based efficient organic electronic devices.« less

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.

Ultrafast 25-fs relaxation in highly excited states of methyl azide mediated by strong nonadiabatic coupling.

PubMed

Peters, William K; Couch, David E; Mignolet, Benoit; Shi, Xuetao; Nguyen, Quynh L; Fortenberry, Ryan C; Schlegel, H Bernhard; Remacle, Françoise; Kapteyn, Henry C; Murnane, Margaret M; Li, Wen

2017-12-26

Highly excited electronic states are challenging to explore experimentally and theoretically-due to the large density of states and the fact that small structural changes lead to large changes in electronic character with associated strong nonadiabatic dynamics. They can play a key role in astrophysical and ionospheric chemistry, as well as the detonation chemistry of high-energy density materials. Here, we implement ultrafast vacuum-UV (VUV)-driven electron-ion coincidence imaging spectroscopy to directly probe the reaction pathways of highly excited states of energetic molecules-in this case, methyl azide. Our data, combined with advanced theoretical simulations, show that photoexcitation of methyl azide by a 10-fs UV pulse at 8 eV drives fast structural changes and strong nonadiabatic coupling that leads to relaxation to other excited states on a surprisingly fast timescale of 25 fs. This ultrafast relaxation differs from dynamics occurring on lower excited states, where the timescale required for the wavepacket to reach a region of strong nonadiabatic coupling is typically much longer. Moreover, our theoretical calculations show that ultrafast relaxation of the wavepacket to a lower excited state occurs along one of the conical intersection seams before reaching the minimum energy conical intersection. These findings are important for understanding the unique strongly coupled non-Born-Oppenheimer molecular dynamics of VUV-excited energetic molecules. Although such observations have been predicted for many years, this study represents one of the few where such strongly coupled non-Born-Oppenheimer molecular dynamics of VUV-excited energetic molecules have been conclusively observed directly, making it possible to identify the ultrafast reaction pathways.

Tracking the ultrafast motion of a single molecule by femtosecond orbital imaging

PubMed Central

Yu, Ping; Repp, Jascha; Huber, Rupert

2017-01-01

Watching a single molecule move on its intrinsic time scale—one of the central goals of modern nanoscience—calls for measurements that combine ultrafast temporal resolution1–8 with atomic spatial resolution9–30. Steady-state experiments achieve the requisite spatial resolution, as illustrated by direct imaging of individual molecular orbitals using scanning tunnelling microscopy9–11 or the acquisition of tip-enhanced Raman and luminescence spectra with sub-molecular resolution27–29. But tracking the dynamics of a single molecule directly in the time domain faces the challenge that single-molecule excitations need to be confined to an ultrashort time window. A first step towards overcoming this challenge has combined scanning tunnelling microscopy with so-called ‘lightwave electronics”1–8, which uses the oscillating carrier wave of tailored light pulses to directly manipulate electronic motion on time scales faster even than that of a single cycle of light. Here we use such ultrafast terahertz scanning tunnelling microscopy to access a state-selective tunnelling regime, where the peak of a terahertz electric-field waveform transiently opens an otherwise forbidden tunnelling channel through a single molecular state and thereby removes a single electron from an individual pentacene molecule’s highest occupied molecular orbital within a time window shorter than one oscillation cycle of the terahertz wave. We exploit this effect to record ~100 fs snapshot images of the structure of the orbital involved, and to reveal through pump-probe measurements coherent molecular vibrations at terahertz frequencies directly in the time domain and with sub-angstrom spatial resolution. We anticipate that the combination of lightwave electronics1–8 and atomic resolution of our approach will open the door to controlling electronic motion inside individual molecules at optical clock rates. PMID:27830788

Towards ultrafast dynamics with split-pulse X-ray photon correlation spectroscopy at free electron laser sources

DOE PAGES

Roseker, W.; Hruszkewycz, S. O.; Lehmkuhler, F.; ...

2018-04-27

One of the important challenges in condensed matter science is to understand ultrafast, atomic-scale fluctuations that dictate dynamic processes in equilibrium and non-equilibrium materials. Here, we report an important step towards reaching that goal by using a state-of-the-art perfect crystal based split-and-delay system, capable of splitting individual X-ray pulses and introducing femtosecond to nanosecond time delays. We show the results of an ultrafast hard X-ray photon correlation spectroscopy experiment at LCLS where split X-ray pulses were used to measure the dynamics of gold nanoparticles suspended in hexane. We show how reliable speckle contrast values can be extracted even from verymore » low intensity free electron laser (FEL) speckle patterns by applying maximum likelihood fitting, thus demonstrating the potential of a split-and-delay approach for dynamics measurements at FEL sources. This will enable the characterization of equilibrium and, importantly also reversible non-equilibrium processes in atomically disordered materials.« less

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.

Towards ultrafast dynamics with split-pulse X-ray photon correlation spectroscopy at free electron laser sources

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

Roseker, W.; Hruszkewycz, S. O.; Lehmkuhler, F.

One of the important challenges in condensed matter science is to understand ultrafast, atomic-scale fluctuations that dictate dynamic processes in equilibrium and non-equilibrium materials. Here, we report an important step towards reaching that goal by using a state-of-the-art perfect crystal based split-and-delay system, capable of splitting individual X-ray pulses and introducing femtosecond to nanosecond time delays. We show the results of an ultrafast hard X-ray photon correlation spectroscopy experiment at LCLS where split X-ray pulses were used to measure the dynamics of gold nanoparticles suspended in hexane. We show how reliable speckle contrast values can be extracted even from verymore » low intensity free electron laser (FEL) speckle patterns by applying maximum likelihood fitting, thus demonstrating the potential of a split-and-delay approach for dynamics measurements at FEL sources. This will enable the characterization of equilibrium and, importantly also reversible non-equilibrium processes in atomically disordered materials.« less

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.

Capturing Structural Dynamics in Crystalline Silicon Using Chirped Electrons from a Laser Wakefield Accelerator

PubMed Central

He, Z.-H.; Beaurepaire, B.; Nees, J. A.; Gallé, G.; Scott, S. A.; Pérez, J. R. Sánchez; Lagally, M. G.; Krushelnick, K.; Thomas, A. G. R.; Faure, J.

2016-01-01

Recent progress in laser wakefield acceleration has led to the emergence of a new generation of electron and X-ray sources that may have enormous benefits for ultrafast science. These novel sources promise to become indispensable tools for the investigation of structural dynamics on the femtosecond time scale, with spatial resolution on the atomic scale. Here, we demonstrate the use of laser-wakefield-accelerated electron bunches for time-resolved electron diffraction measurements of the structural dynamics of single-crystal silicon nano-membranes pumped by an ultrafast laser pulse. In our proof-of-concept study, we resolve the silicon lattice dynamics on a picosecond time scale by deflecting the momentum-time correlated electrons in the diffraction peaks with a static magnetic field to obtain the time-dependent diffraction efficiency. Further improvements may lead to femtosecond temporal resolution, with negligible pump-probe jitter being possible with future laser-wakefield-accelerator ultrafast-electron-diffraction schemes. PMID:27824086

Capturing Structural Dynamics in Crystalline Silicon Using Chirped Electrons from a Laser Wakefield Accelerator

DOE PAGES

He, Z. -H.; Beaurepaire, B.; Nees, J. A.; ...

2016-11-08

Recent progress in laser wakefield acceleration has led to the emergence of a new generation of electron and X-ray sources that may have enormous benefits for ultrafast science. These novel sources promise to become indispensable tools for the investigation of structural dynamics on the femtosecond time scale, with spatial resolution on the atomic scale. Here in this paper, we demonstrate the use of laser-wakefield-accelerated electron bunches for time-resolved electron diffraction measurements of the structural dynamics of single-crystal silicon nano-membranes pumped by an ultrafast laser pulse. In our proof-of-concept study, we resolve the silicon lattice dynamics on a picosecond time scalemore » by deflecting the momentum-time correlated electrons in the diffraction peaks with a static magnetic field to obtain the time-dependent diffraction efficiency. Further improvements may lead to femtosecond temporal resolution, with negligible pump-probe jitter being possible with future laser-wakefield-accelerator ultrafast-electron-diffraction schemes.« less

Structural dynamics of surfaces by ultrafast electron crystallography: experimental and multiple scattering theory.

PubMed

Schäfer, Sascha; Liang, Wenxi; Zewail, Ahmed H

2011-12-07

Recent studies in ultrafast electron crystallography (UEC) using a reflection diffraction geometry have enabled the investigation of a wide range of phenomena on the femtosecond and picosecond time scales. In all these studies, the analysis of the diffraction patterns and their temporal change after excitation was performed within the kinematical scattering theory. In this contribution, we address the question, to what extent dynamical scattering effects have to be included in order to obtain quantitative information about structural dynamics. We discuss different scattering regimes and provide diffraction maps that describe all essential features of scatterings and observables. The effects are quantified by dynamical scattering simulations and examined by direct comparison to the results of ultrafast electron diffraction experiments on an in situ prepared Ni(100) surface, for which structural dynamics can be well described by a two-temperature model. We also report calculations for graphite surfaces. The theoretical framework provided here allows for further UEC studies of surfaces especially at larger penetration depths and for those of heavy-atom materials. © 2011 American Institute of Physics

Ultrafast dynamics in atomic clusters: Analysis and control

PubMed Central

Bonačić-Koutecký, Vlasta; Mitrić, Roland; Werner, Ute; Wöste, Ludger; Berry, R. Stephen

2006-01-01

We present a study of dynamics and ultrafast observables in the frame of pump–probe negative-to-neutral-to-positive ion (NeNePo) spectroscopy illustrated by the examples of bimetallic trimers Ag2Au−/Ag2Au/Ag2Au+ and silver oxides Ag3O2−/Ag3O2/Ag3O2+ in the context of cluster reactivity. First principle multistate adiabatic dynamics allows us to determine time scales of different ultrafast processes and conditions under which these processes can be experimentally observed. Furthermore, we present a strategy for optimal pump–dump control in complex systems based on the ab initio Wigner distribution approach and apply it to tailor laser fields for selective control of the isomerization process in Na3F2. The shapes of pulses can be assigned to underlying processes, and therefore control can be used as a tool for analysis. PMID:16740664

Ultrafast dynamics in atomic clusters: analysis and control.

PubMed

Bonacić-Koutecký, Vlasta; Mitrić, Roland; Werner, Ute; Wöste, Ludger; Berry, R Stephen

2006-07-11

We present a study of dynamics and ultrafast observables in the frame of pump-probe negative-to-neutral-to-positive ion (NeNePo) spectroscopy illustrated by the examples of bimetallic trimers Ag2Au-/Ag2Au/Ag2Au+ and silver oxides Ag3O2-/Ag3O2/Ag3O2+ in the context of cluster reactivity. First principle multistate adiabatic dynamics allows us to determine time scales of different ultrafast processes and conditions under which these processes can be experimentally observed. Furthermore, we present a strategy for optimal pump-dump control in complex systems based on the ab initio Wigner distribution approach and apply it to tailor laser fields for selective control of the isomerization process in Na3F2. The shapes of pulses can be assigned to underlying processes, and therefore control can be used as a tool for analysis.

Ordered water structure at hydrophobic graphite interfaces observed by 4D, ultrafast electron crystallography

PubMed Central

Yang, Ding-Shyue; Zewail, Ahmed H.

2009-01-01

Interfacial water has unique properties in various functions. Here, using 4-dimensional (4D), ultrafast electron crystallography with atomic-scale spatial and temporal resolution, we report study of structure and dynamics of interfacial water assembly on a hydrophobic surface. Structurally, vertically stacked bilayers on highly oriented pyrolytic graphite surface were determined to be ordered, contrary to the expectation that the strong hydrogen bonding of water on hydrophobic surfaces would dominate with suppressed interfacial order. Because of its terrace morphology, graphite plays the role of a template. The dynamics is also surprising. After the excitation of graphite by an ultrafast infrared pulse, the interfacial ice structure undergoes nonequilibrium “phase transformation” identified in the hydrogen-bond network through the observation of structural isosbestic point. We provide the time scales involved, the nature of ice-graphite structural dynamics, and relevance to properties related to confined water. PMID:19246378

Probing ultrafast spin dynamics through a magnon resonance in the antiferromagnetic multiferroic HoMnO 3

DOE PAGES

Bowlan, P.; Trugman, S. A.; Bowlan, J.; ...

2016-09-26

Here, we demonstrate an approach for directly tracking antiferromagnetic (AFM) spin dynamics by measuring ultrafast changes in a magnon resonance. We also test this idea on the multiferroic HoMnO 3 by optically photoexciting electrons, after which changes in the spin order are probed with a THz pulse tuned to a magnon resonance. This reveals a photoinduced change in the magnon line shape that builds up over 5–12 picoseconds, which we show to be the spin-lattice thermalization time, indicating that electrons heat the spins via phonons. We compare our results to previous studies of spin-lattice thermalization in ferromagnetic manganites, giving insightmore » into fundamental differences between the two systems. Finally, our work sheds light on the microscopic mechanism governing spin-phonon interactions in AFMs and demonstrates a powerful approach for directly monitoring ultrafast spin dynamics.« less

Imaging CF3I conical intersection and photodissociation dynamics by ultrafast electron diffraction

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

Yang, Jie

Conical intersections play a critical role in excited state dynamics of polyatomic molecules, as they govern the reaction pathways of many nonadiabatic processes. However, ultrafast probes have lacked sufficient spatial resolution to image wavepacket trajectories through these intersections directly. Here we present the simultaneous experimental characterization of one-photon and two-photon excitation channels in isolated CF3I molecules using ultrafast gas phase electron diffraction. In the two-photon channel, we have mapped out the real space trajectories of a coherent nuclear wavepacket, which bifurcates onto two potential energy surfaces when passing through a conical intersection. In the one-photon channel, we have resolved excitationmore » of both the umbrella and the breathing vibrational modes in the CF3 fragment in multiple nuclear dimensions. These findings benchmark and validate ab-initio nonadiabatic dynamics calculations.« less

Probing ultrafast spin dynamics through a magnon resonance in the antiferromagnetic multiferroic HoMnO 3

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

Bowlan, P.; Trugman, S. A.; Bowlan, J.

Here, we demonstrate an approach for directly tracking antiferromagnetic (AFM) spin dynamics by measuring ultrafast changes in a magnon resonance. We also test this idea on the multiferroic HoMnO 3 by optically photoexciting electrons, after which changes in the spin order are probed with a THz pulse tuned to a magnon resonance. This reveals a photoinduced change in the magnon line shape that builds up over 5–12 picoseconds, which we show to be the spin-lattice thermalization time, indicating that electrons heat the spins via phonons. We compare our results to previous studies of spin-lattice thermalization in ferromagnetic manganites, giving insightmore » into fundamental differences between the two systems. Finally, our work sheds light on the microscopic mechanism governing spin-phonon interactions in AFMs and demonstrates a powerful approach for directly monitoring ultrafast spin dynamics.« less

Dynamic diffraction effects and coherent breathing oscillations in ultrafast electron diffraction in layered 1T-TaSeTe

PubMed Central

Wei, Linlin; Sun, Shuaishuai; Guo, Cong; Li, Zhongwen; Sun, Kai; Liu, Yu; Lu, Wenjian; Sun, Yuping; Tian, Huanfang; Yang, Huaixin; Li, Jianqi

2017-01-01

Anisotropic lattice movements due to the difference between intralayer and interlayer bonding are observed in the layered transition-metal dichalcogenide 1T-TaSeTe following femtosecond laser pulse excitation. Our ultrafast electron diffraction investigations using 4D-transmission electron microscopy (4D-TEM) clearly reveal that the intensity of Bragg reflection spots often changes remarkably due to the dynamic diffraction effects and anisotropic lattice movement. Importantly, the temporal diffracted intensity from a specific crystallographic plane depends on the deviation parameter s, which is commonly used in the theoretical study of diffraction intensity. Herein, we report on lattice thermalization and structural oscillations in layered 1T-TaSeTe, analyzed by dynamic diffraction theory. Ultrafast alterations of satellite spots arising from the charge density wave in the present system are also briefly discussed. PMID:28470025

The complete optical oscilloscope

NASA Astrophysics Data System (ADS)

Lei, Cheng; Goda, Keisuke

2018-04-01

Observing ultrafast transient dynamics in optics is a challenging task. Two teams in Europe have now independently developed `optical oscilloscopes' that can capture both amplitude and phase information of ultrafast optical signals. Their schemes yield new insights into the nonlinear physics that takes place inside optical fibres.

Ultrafast spintronics roadmap: from femtosecond spin current pulses to terahertz non-uniform spin dynamics via nano-confined spin transfer torques (Conference Presentation)

NASA Astrophysics Data System (ADS)

Melnikov, Alexey; Razdolski, Ilya; Alekhin, Alexandr; Ilin, Nikita; Meyburg, Jan; Diesing, Detlef; Roddatis, Vladimir; Rungger, Ivan; Stamenova, Maria; Sanvito, Stefano; Bovensiepen, Uwe

2016-10-01

Further development of spintronics requires miniaturization and reduction of characteristic timescales of spin dynamics combining the nanometer spatial and femtosecond temporal ranges. These demands shift the focus of interest towards the fundamental open question of the interaction of femtosecond spin current (SC) pulses with a ferromagnet (FM). The spatio-temporal properties of the spin transfer torque (STT) exerted by ultrashort SC pulses on the FM open the time domain for studying STT fingerprint on spatially non-uniform magnetization dynamics. Using the sensitivity of magneto-induced second harmonic generation to SC, we develop technique for SC monitoring. With 20 fs resolution, we demonstrate the generation of 250 fs-long SC pulses in Fe/Au/Fe/MgO(001) structures. Their temporal profile indicates (i) nearly-ballistic hot electron transport in Au and (ii) that the pulse duration is primarily determined by the thermalization time of laser-excited hot carriers in Fe. Together with strongly spin-dependent Fe/Au interface transmission calculated for these carriers, this suggests the non-thermal spin-dependent Seebeck effect dominating the generation of ultrashort SC pulses. The analysis of SC transmission/reflection at the Au/Fe interface shows that hot electron spins orthogonal to the Fe magnetization rotate gaining huge parallel (anti-parallel) projection in transmitted (reflected) SC. This is accompanied by a STT-induced perturbation of the magnetization localized at the interface, which excites the inhomogeneous high-frequency spin dynamics in the FM. Time-resolved magneto-optical studies reveal the excitation of several standing spin wave modes in the Fe film with their spectrum extending up to 0.6 THz and indicating the STT spatial confinement to 2 nm.

Femtosecond optical characterization and applications in cadmium(manganese) telluride diluted magnetic semiconductors

NASA Astrophysics Data System (ADS)

Wang, Daozhi

This thesis is devoted to the optical characterization of Cd(Mn)Te single crystals. I present the studies of free-carrier dynamics and generation and detection of coherent acoustic phonons (CAPS) using time-resolved femtosecond pump-probe spectroscopy. The giant Faraday effect and ultrafast responsivity of Cd(Mn)Te to sub-picosecond electromagnetic transients are also demonstrated and discussed in detail. The first, few-picosecond-long electronic process after the initial optical excitation exhibits very distinct characteristic dependence on the excitation condition, and in case of Cd(Mn)Te, it has been attributed to the collective effects of band filling, band renormalization, and two-photon absorption. A closed-form, analytic expression for the differential reflectivity induced by the CAPs is derived based on the propagating-strain-pulse model and it accounts very well for our experimental observations. The accurate values of the Mn concentration and longitudinal sound velocity nu s in Cd(Mn)Te were obtained by fitting the data of the refractive index dependence on the probe wavelength to the Schubert model. In Cd 0.91Mn0.09Te, nus was found to be 3.6x103 m/s. Our comparison studies from the one-color and two-color experiments reveal that the intrinsic phonon lifetime in Cd(Mn)Te was at least on the order of nanoseconds, and the observed exponential damping of the CAP oscillations was due to the finite absorption depth of the probe light. Optically-induced electronic stress has been demonstrated to be the main generation mechanism of CAPs. We also present the giant Faraday effect in the Cd(Mn)Te and the spectra of the Verdet constant, which is mainly due to the exchange interaction between the Mn ions and band electrons. The spectral characteristics of the Verdet constant in Cd(Mn)Te exhibit very unique features compared to that in pure semiconductors. In our time-resolved sampling experiments at the room temperature, the response of the Cd(Mn)Te, particularly with low Mn concentrations, to the sub-picosecond electromagnetic pulses has been demonstrated for the first time and studied in detail. The physical origin of the ultrafast responsivity is shown to be the electro-optic (Pockels) effect, simultaneously excluding the magneto-optical (Faraday) effect due to the Mn-ion spin dynamics. The discrepancy between the absence of the low-frequency Pockels effect and the ultrafast sampling results, suggests that in Cd(Mn)Te crystals at low frequencies, the electric field component of the external electromagnetic transients is screened by the free carriers (holes). At very high (THz) frequencies, tested by our sampling experiment, Mn spins are too slow to respond and we observe the very large Pockels effect in Cd(Mn)Te crystals.

Ultrafast Bilateral DCE-MRI of the Breast with Conventional Fourier Sampling: Preliminary Evaluation of Semi-quantitative Analysis.

PubMed

Pineda, Federico D; Medved, Milica; Wang, Shiyang; Fan, Xiaobing; Schacht, David V; Sennett, Charlene; Oto, Aytekin; Newstead, Gillian M; Abe, Hiroyuki; Karczmar, Gregory S

2016-09-01

The study aimed to evaluate the feasibility and advantages of a combined high temporal and high spatial resolution protocol for dynamic contrast-enhanced magnetic resonance imaging of the breast. Twenty-three patients with enhancing lesions were imaged at 3T. The acquisition protocol consisted of a series of bilateral, fat-suppressed "ultrafast" acquisitions, with 6.9- to 9.9-second temporal resolution for the first minute following contrast injection, followed by four high spatial resolution acquisitions with 60- to 79.5-second temporal resolution. All images were acquired with standard uniform Fourier sampling. A filtering method was developed to reduce noise and detect significant enhancement in the high temporal resolution images. Time of arrival (TOA) was defined as the time at which each voxel first satisfied all the filter conditions, relative to the time of initial arterial enhancement. Ultrafast images improved visualization of the vasculature feeding and draining lesions. A small percentage of the entire field of view (<6%) enhanced significantly in the 30 seconds following contrast injection. Lesion conspicuity was highest in early ultrafast images, especially in cases with marked parenchymal enhancement. Although the sample size was relatively small, the average TOA for malignant lesions was significantly shorter than the TOA for benign lesions. Significant differences were also measured in other parameters descriptive of early contrast media uptake kinetics (P < 0.05). Ultrafast imaging in the first minute of dynamic contrast-enhanced magnetic resonance imaging of the breast has the potential to add valuable information on early contrast dynamics. Ultrafast imaging could allow radiologists to confidently identify lesions in the presence of marked background parenchymal enhancement. Copyright © 2016 The Association of University Radiologists. Published by Elsevier Inc. All rights reserved.

Electron theory of fast and ultrafast dissipative magnetization dynamics.

PubMed

Fähnle, M; Illg, C

2011-12-14

For metallic magnets we review the experimental and electron-theoretical investigations of fast magnetization dynamics (on a timescale of ns to 100 ps) and of laser-pulse-induced ultrafast dynamics (few hundred fs). It is argued that for both situations the dominant contributions to the dissipative part of the dynamics arise from the excitation of electron-hole pairs and from the subsequent relaxation of these pairs by spin-dependent scattering processes, which transfer angular momentum to the lattice. By effective field theories (generalized breathing and bubbling Fermi-surface models) it is shown that the Gilbert equation of motion, which is often used to describe the fast dissipative magnetization dynamics, must be extended in several aspects. The basic assumptions of the Elliott-Yafet theory, which is often used to describe the ultrafast spin relaxation after laser-pulse irradiation, are discussed very critically. However, it is shown that for Ni this theory probably yields a value for the spin-relaxation time T(1) in good agreement with the experimental value. A relation between the quantity α characterizing the damping of the fast dynamics in simple situations and the time T(1) is derived. © 2011 IOP Publishing Ltd

Ultrafast properties of femtosecond-laser-ablated GaAs and its application to terahertz optoelectronics.

PubMed

Madéo, Julien; Margiolakis, Athanasios; Zhao, Zhen-Yu; Hale, Peter J; Man, Michael K L; Zhao, Quan-Zhong; Peng, Wei; Shi, Wang-Zhou; Dani, Keshav M

2015-07-15

We report on the first terahertz (THz) emitter based on femtosecond-laser-ablated gallium arsenide (GaAs), demonstrating a 65% enhancement in THz emission at high optical power compared to the nonablated device. Counter-intuitively, the ablated device shows significantly lower photocurrent and carrier mobility. We understand this behavior in terms of n-doping, shorter carrier lifetime, and enhanced photoabsorption arising from the ablation process. Our results show that laser ablation allows for efficient and cost-effective optoelectronic THz devices via the manipulation of fundamental properties of materials.

Carrier-envelope-offset phase control of ultrafast optical rectification in resonantly excited semiconductors.

PubMed

Van Vlack, C; Hughes, S

2007-04-20

Ultrashort pulse light-matter interactions in a semiconductor are investigated within the regime of resonant optical rectification. Using pulse envelope areas of around 1.5-3.5 pi, a single-shot dependence on carrier-envelope-offset phase (CEP) is demonstrated for 5 fs pulse durations. A characteristic phase map is predicted for several different frequency regimes using parameters for thin-film GaAs. We subsequently suggest a possible technique to extract the CEP, in both sign and amplitude, using a solid state detector.

Carrier-envelope phase-dependent effect of high-order sideband generation in ultrafast driven optomechanical system.

PubMed

Xiong, Hao; Si, Liu-Gang; Lü, Xin-You; Yang, Xiaoxue; Wu, Ying

2013-02-01

We analyze the features of the output field of a generic optomechanical system that is driven by a control field and a nanosecond driven pulse, and find a robust high-order sideband generation in optomechanical systems. The typical spectral structure, plateau and cutoff, confirms the nonperturbative nature of the effect, which is similar to high-order harmonic generation in atoms or molecules. Based on the phenomenon, we show that the carrier-envelope phase of laser pulses that contain huge numbers of cycles can cause profound effects.

Proposed imaging of the ultrafast electronic motion in samples using x-ray phase contrast.

PubMed

Dixit, Gopal; Slowik, Jan Malte; Santra, Robin

2013-03-29

Tracing the motion of electrons has enormous relevance to understanding ubiquitous phenomena in ultrafast science, such as the dynamical evolution of the electron density during complex chemical and biological processes. Scattering of ultrashort x-ray pulses from an electronic wave packet would appear to be the most obvious approach to image the electronic motion in real time and real space with the notion that such scattering patterns, in the far-field regime, encode the instantaneous electron density of the wave packet. However, recent results by Dixit et al. [Proc. Natl. Acad. Sci. U.S.A. 109, 11636 (2012)] have put this notion into question and have shown that the scattering in the far-field regime probes spatiotemporal density-density correlations. Here, we propose a possible way to image the instantaneous electron density of the wave packet via ultrafast x-ray phase contrast imaging. Moreover, we show that inelastic scattering processes, which plague ultrafast scattering in the far-field regime, do not contribute in ultrafast x-ray phase contrast imaging as a consequence of an interference effect. We illustrate our general findings by means of a wave packet that lies in the time and energy range of the dynamics of valence electrons in complex molecular and biological systems. This present work offers a potential to image not only instantaneous snapshots of nonstationary electron dynamics, but also the laplacian of these snapshots which provide information about the complex bonding and topology of the charge distributions in the systems.

Proposed Imaging of the Ultrafast Electronic Motion in Samples using X-Ray Phase Contrast

NASA Astrophysics Data System (ADS)

Dixit, Gopal; Slowik, Jan Malte; Santra, Robin

2013-03-01

Tracing the motion of electrons has enormous relevance to understanding ubiquitous phenomena in ultrafast science, such as the dynamical evolution of the electron density during complex chemical and biological processes. Scattering of ultrashort x-ray pulses from an electronic wave packet would appear to be the most obvious approach to image the electronic motion in real time and real space with the notion that such scattering patterns, in the far-field regime, encode the instantaneous electron density of the wave packet. However, recent results by Dixit et al. [Proc. Natl. Acad. Sci. U.S.A. 109, 11 636 (2012)] have put this notion into question and have shown that the scattering in the far-field regime probes spatiotemporal density-density correlations. Here, we propose a possible way to image the instantaneous electron density of the wave packet via ultrafast x-ray phase contrast imaging. Moreover, we show that inelastic scattering processes, which plague ultrafast scattering in the far-field regime, do not contribute in ultrafast x-ray phase contrast imaging as a consequence of an interference effect. We illustrate our general findings by means of a wave packet that lies in the time and energy range of the dynamics of valence electrons in complex molecular and biological systems. This present work offers a potential to image not only instantaneous snapshots of nonstationary electron dynamics, but also the Laplacian of these snapshots which provide information about the complex bonding and topology of the charge distributions in the systems.

Single-shot high aspect ratio bulk nanostructuring of fused silica using chirp-controlled ultrafast laser Bessel beams

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

Bhuyan, M. K.; Velpula, P. K.; Colombier, J. P.

2014-01-13

We report single-shot, high aspect ratio nanovoid fabrication in bulk fused silica using zeroth order chirp-controlled ultrafast laser Bessel beams. We identify a unique laser pulse length and energy dependence of the physical characteristics of machined structures over which nanovoids of diameter in the range 200–400 nm and aspect ratios exceeding 1000 can be fabricated. A mechanism based on the axial energy deposition of nonlinear ultrashort Bessel beams and subsequent material densification or rarefaction in fused silica is proposed, intricating the non-diffractive nature with the diffusing character of laser-generated free carriers. Fluid flow through nanochannel is also demonstrated.

Enhanced all-optical modulation in a graphene-coated fibre with low insertion loss

PubMed Central

Zhang, Haojie; Healy, Noel; Shen, Li; Huang, Chung Che; Hewak, Daniel W.; Peacock, Anna C.

2016-01-01

Graphene is a highly versatile two-dimensional material platform that offers exceptional optical and electrical properties. Of these, its dynamic conductivity and low effective carrier mass are of particular interest for optoelectronic applications as they underpin the material’s broadband nonlinear optical absorption and ultra-fast carrier mobility, respectively. In this paper, we utilize these phenomena to demonstrate a high-speed, in-fibre optical modulator developed on a side-polished optical fibre platform. An especially low insertion loss (<1 dB) was achieved by polishing the fibre to a near atomically smooth surface (<1 nm RMS), which minimized scattering and ensured excellent contact between the graphene film and the fibre. In order to enhance the light-matter interaction, the graphene film is coated with a high index polyvinyl butyral layer, which has the added advantage of acting as a barrier to the surrounding environment. Using this innovative approach, we have fabricated a robust and stable all-fibre device with an extinction ratio as high as 9 dB and operation bandwidth of 0.5 THz. These results represent a key step towards the integration of low-dimensional materials within standard telecoms networks. PMID:27001353

Giant ultrafast Kerr effect in superconductors

NASA Astrophysics Data System (ADS)

Robson, Charles W.; Fraser, Kieran A.; Biancalana, Fabio

2017-06-01

We study the ultrafast Kerr effect and high-harmonic generation in superconductors by formulating a model for a time-varying electromagnetic pulse normally incident on a thin-film superconductor. It is found that superconductors exhibit exceptionally large χ(3 ) due to the progressive destruction of Cooper pairs, and display high-harmonic generation at low incident intensities, and the highest nonlinear susceptibility of all known materials in the THz regime. Our theory opens up avenues for accessible analytical and numerical studies of the ultrafast dynamics of superconductors.

Femtosecond timing measurement and control using ultrafast organic thin films

NASA Astrophysics Data System (ADS)

Naruse, Makoto; Mitsu, Hiroyuki; Furuki, Makoto; Iwasa, Izumi; Sato, Yasuhiro; Tatsuura, Satoshi; Tian, Minquan

2003-12-01

We show a femtosecond timing measurement and control technique using a squarylium dye J-aggregate film, which is an organic thin film that acts as an ultrafast two-dimensional optical switch. Optical pulse timing is directly mapped to space-domain position on the film, and the large area and ultrafast response offer a femtosecond-resolved, large dynamic range, real-time, multichannel timing measurement capability. A timing fluctuation (jitter, wander, and skew) reduction architecture is presented and experimentally demonstrated.

Semiconductor Nonlinear Dynamics Study by Broadband Terahertz Spectroscopy

NASA Astrophysics Data System (ADS)

Ho, I.-Chen

Semiconductor nonlinearity in the terahertz (THz) frequency range has been attracting considerable attention due to the recent development of high-power semiconductor-based nanodevices. However, the underlying physics concerning carrier dynamics in the presence of high-field THz transients is still obscure. This thesis introduces an ultrafast, time-resolved THz pump/THz probe approach to the study of semiconductor properties in the nonlinear regime. The carrier dynamics regarding two mechanisms, intervalley scattering and impact ionization, is observed for doped InAs on a sub-picosecond time scale. In addition, polaron modulation driven by intense THz pulses is experimentally and theoretically investigated. The observed polaron dynamics verifies the interaction between energetic electrons and a phonon field. In contrast to previous work which reports optical phonon responses, acoustic phonon modulations are addressed in this study. A further understanding of the intense field interacting with solid materials will accelerate the development of semiconductor devices. This thesis starts with the design and performance of a table-top THz spectrometer which has the advantages of ultra-broad bandwidth (one order higher bandwidth compared to a conventional ZnTe sensor) and high electric field strength (>100 kV/cm). Unlike the conventional THz time-domain spectroscopy, the spectrometer integrates a novel THz air-biased-coherent-detection (THz-ABCD) technique and utilizes selected gases as THz emitters and sensors. In comparison with commonly used electro-optic (EO) crystals or photoconductive (PC) dipole antennas, the gases have the benefits of no phonon absorption as existing in EO crystals and no carrier life time limitation as observed in PC dipole antennas. The newly development THz-ABCD spectrometer with a strong THz field strength capability provides a platform for various research topics especially on the nonlinear carrier dynamics of semiconductors. Two mechanisms, electron intervalley scattering and impact ionization of InAs crystals, are observed under the excitation of intense THz field on a sub-picosecond time scale. These two competing mechanisms are demonstrated by changing the impurity doping type of the semiconductors and varying the strength of the THz field. Another investigation of nonlinear carrier dynamics is the observation of coherent polaron oscillation in n-doped semiconductors excited by intense THz pulses. Through modulations of surface reflection with a THz pump/THz probe technique, this work experimentally verifies the interaction between energetic electrons and a phonon field, which has been theoretically predicted by previous publications, and shows that this interaction applies for the acoustic phonon modes. Usually, two transverse acoustic (2TA) phonon responses are inactive in infrared measurement, while they are detectable in second-order Raman spectroscopy. The study of polaron dynamics, with nonlinear THz spectroscopy (in the far-infrared range), provides a unique method to diagnose the overtones of 2TA phonon responses of semiconductors, and therefore incorporates the abilities of both infrared and Raman spectroscopy. This work presents a new milestone in wave-matter interaction and seeks to benefit the industrial applications in high power, small scale devices.

Magnetic field effects on ultrafast lattice compression dynamics of Si(111) crystal when excited by linearly-polarized femtosecond laser pulses

NASA Astrophysics Data System (ADS)

Hatanaka, Koji; Odaka, Hideho; Ono, Kimitoshi; Fukumura, Hiroshi

2007-03-01

Time-resolved X-ray diffraction measurements of Si (111) single crystal are performed when excited by linearly-polarized femtosecond laser pulses (780 nm, 260 fs, negatively-chirped, 1 kHz) under a magnetic field (0.47 T). Laser fluence on the sample surface is 40 mJ/cm^2, which is enough lower than the ablation threshold at 200 mJ/cm^2. Probing X-ray pulses of iron characteristic X-ray lines at 0.193604 and 0.193998 nm are generated by focusing femtosecond laser pulses onto audio-cassette tapes in air. Linearly-polarized femtosecond laser pulse irradiation onto Si(111) crystal surface induces transient lattice compression in the picosecond time range, which is confirmed by transient angle shift of X-ray diffraction to higher angles. Little difference of compression dynamics is observed when the laser polarization is changed from p to s-pol. without a magnetic field. On the other hand, under a magnetic field, the lattice compression dynamics changes when the laser is p-polarized which is vertical to the magnetic field vector. These results may be assigned to photo-carrier formation and energy-band distortion.

Many-body effects in valleytronics: direct measurement of valley lifetimes in single-layer MoS2.

PubMed

Mai, Cong; Barrette, Andrew; Yu, Yifei; Semenov, Yuriy G; Kim, Ki Wook; Cao, Linyou; Gundogdu, Kenan

2014-01-08

Single layer MoS2 is an ideal material for the emerging field of "valleytronics" in which charge carrier momentum can be finely controlled by optical excitation. This system is also known to exhibit strong many-body interactions as observed by tightly bound excitons and trions. Here we report direct measurements of valley relaxation dynamics in single layer MoS2, by using ultrafast transient absorption spectroscopy. Our results show that strong Coulomb interactions significantly impact valley population dynamics. Initial excitation by circularly polarized light creates electron-hole pairs within the K-valley. These excitons coherently couple to dark intervalley excitonic states, which facilitate fast electron valley depolarization. Hole valley relaxation is delayed up to about 10 ps due to nondegeneracy of the valence band spin states. Intervalley biexciton formation reveals the hole valley relaxation dynamics. We observe that biexcitons form with more than an order of magnitude larger binding energy compared to conventional semiconductors. These measurements provide significant insight into valley specific processes in 2D semiconductors. Hence they could be used to suggest routes to design semiconducting materials that enable control of valley polarization.

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

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

Design, fabrication, and experimental characterization of plasmonic photoconductive terahertz emitters.

PubMed

Berry, Christopher; Hashemi, Mohammad Reza; Unlu, Mehmet; Jarrahi, Mona

2013-07-08

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on photoconduction, which has been one of the most commonly used techniques for terahertz generation (1-8). Terahertz generation in a photoconductive emitter is achieved by pumping an ultrafast photoconductor with a pulsed or heterodyned laser illumination. The induced photocurrent, which follows the envelope of the pump laser, is routed to a terahertz radiating antenna connected to the photoconductor contact electrodes to generate terahertz radiation. Although the quantum efficiency of a photoconductive emitter can theoretically reach 100%, the relatively long transport path lengths of photo-generated carriers to the contact electrodes of conventional photoconductors have severely limited their quantum efficiency. Additionally, the carrier screening effect and thermal breakdown strictly limit the maximum output power of conventional photoconductive terahertz sources. To address the quantum efficiency limitations of conventional photoconductive terahertz emitters, we have developed a new photoconductive emitter concept which incorporates a plasmonic contact electrode configuration to offer high quantum-efficiency and ultrafast operation simultaneously. By using nano-scale plasmonic contact electrodes, we significantly reduce the average photo-generated carrier transport path to photoconductor contact electrodes compared to conventional photoconductors (9). Our method also allows increasing photoconductor active area without a considerable increase in the capacitive loading to the antenna, boosting the maximum terahertz radiation power by preventing the carrier screening effect and thermal breakdown at high optical pump powers. By incorporating plasmonic contact electrodes, we demonstrate enhancing the optical-to-terahertz power conversion efficiency of a conventional photoconductive terahertz emitter by a factor of 50 (10).

Ultrafast electronic dynamics driven by nuclear motion

NASA Astrophysics Data System (ADS)

Vendrell, Oriol

2016-05-01

The transfer of electrical charge on a microscopic scale plays a fundamental role in chemistry, in biology, and in technological applications. In this contribution, we will discuss situations in which nuclear motion plays a central role in driving the electronic dynamics of photo-excited or photo-ionized molecular systems. In particular, we will explore theoretically the ultrafast transfer of a double electron hole between the functional groups of glycine after K-shell ionization and subsequent Auger decay. Although a large energy gap of about 15 eV initially exists between the two electronic states involved and coherent electronic dynamics play no role in the hole transfer, we will illustrate how the double hole can be transferred within 3 to 4 fs between both functional ends of the glycine molecule driven solely by specific nuclear displacements and non-Born-Oppenheimer effects. This finding challenges the common wisdom that nuclear dynamics of the molecular skeleton are unimportant for charge transfer processes at the few-femtosecond time scale and shows that they can even play a prominent role. We thank the Hamburg Centre for Ultrafast Imaging and the Volkswagen Foundation for financial support.

Ultrafast Pulse Generation in an Organic Nanoparticle-Array Laser.

PubMed

Daskalakis, Konstantinos S; Väkeväinen, Aaro I; Martikainen, Jani-Petri; Hakala, Tommi K; Törmä, Päivi

2018-04-11

Nanoscale coherent light sources offer potentially ultrafast modulation speeds, which could be utilized for novel sensors and optical switches. Plasmonic periodic structures combined with organic gain materials have emerged as promising candidates for such nanolasers. Their plasmonic component provides high intensity and ultrafast nanoscale-confined electric fields, while organic gain materials offer fabrication flexibility and a low acquisition cost. Despite reports on lasing in plasmonic arrays, lasing dynamics in these structures have not been experimentally studied yet. Here we demonstrate, for the first time, an organic dye nanoparticle-array laser with more than a 100 GHz modulation bandwidth. We show that the lasing modulation speed can be tuned by the array parameters. Accelerated dynamics is observed for plasmonic lasing modes at the blue side of the dye emission.

Photon gating in four-dimensional ultrafast electron microscopy.

PubMed

Hassan, Mohammed T; Liu, Haihua; Baskin, John Spencer; Zewail, Ahmed H

2015-10-20

Ultrafast electron microscopy (UEM) is a pivotal tool for imaging of nanoscale structural dynamics with subparticle resolution on the time scale of atomic motion. Photon-induced near-field electron microscopy (PINEM), a key UEM technique, involves the detection of electrons that have gained energy from a femtosecond optical pulse via photon-electron coupling on nanostructures. PINEM has been applied in various fields of study, from materials science to biological imaging, exploiting the unique spatial, energy, and temporal characteristics of the PINEM electrons gained by interaction with a "single" light pulse. The further potential of photon-gated PINEM electrons in probing ultrafast dynamics of matter and the optical gating of electrons by invoking a "second" optical pulse has previously been proposed and examined theoretically in our group. Here, we experimentally demonstrate this photon-gating technique, and, through diffraction, visualize the phase transition dynamics in vanadium dioxide nanoparticles. With optical gating of PINEM electrons, imaging temporal resolution was improved by a factor of 3 or better, being limited only by the optical pulse widths. This work enables the combination of the high spatial resolution of electron microscopy and the ultrafast temporal response of the optical pulses, which provides a promising approach to attain the resolution of few femtoseconds and attoseconds in UEM.

Generalized GW+Boltzmann Approach for the Description of Ultrafast Electron Dynamics in Topological Insulators

PubMed Central

Battiato, Marco; Sánchez-Barriga, Jaime

2017-01-01

Quantum-phase transitions between trivial insulators and topological insulators differ from ordinary metal-insulator transitions in that they arise from the inversion of the bulk band structure due to strong spin–orbit coupling. Such topological phase transitions are unique in nature as they lead to the emergence of topological surface states which are characterized by a peculiar spin texture that is believed to play a central role in the generation and manipulation of dissipationless surface spin currents on ultrafast timescales. Here, we provide a generalized GW+Boltzmann approach for the description of ultrafast dynamics in topological insulators driven by electron–electron and electron–phonon scatterings. Taking the prototypical insulator Bi2Te3 as an example, we test the robustness of our approach by comparing the theoretical prediction to results of time- and angle-resolved photoemission experiments. From this comparison, we are able to demonstrate the crucial role of the excited spin texture in the subpicosecond relaxation of transient electrons, as well as to accurately obtain the magnitude and strength of electron–electron and electron–phonon couplings. Our approach could be used as a generalized theory for three-dimensional topological insulators in the bulk-conducting transport regime, paving the way for the realization of a unified theory of ultrafast dynamics in topological materials. PMID:28773171

Generalized GW+Boltzmann Approach for the Description of Ultrafast Electron Dynamics in Topological Insulators.

PubMed

Battiato, Marco; Aguilera, Irene; Sánchez-Barriga, Jaime

2017-07-17

Quantum-phase transitions between trivial insulators and topological insulators differ from ordinary metal-insulator transitions in that they arise from the inversion of the bulk band structure due to strong spin-orbit coupling. Such topological phase transitions are unique in nature as they lead to the emergence of topological surface states which are characterized by a peculiar spin texture that is believed to play a central role in the generation and manipulation of dissipationless surface spin currents on ultrafast timescales. Here, we provide a generalized G W +Boltzmann approach for the description of ultrafast dynamics in topological insulators driven by electron-electron and electron-phonon scatterings. Taking the prototypical insulator Bi 2 Te 3 as an example, we test the robustness of our approach by comparing the theoretical prediction to results of time- and angle-resolved photoemission experiments. From this comparison, we are able to demonstrate the crucial role of the excited spin texture in the subpicosecond relaxation of transient electrons, as well as to accurately obtain the magnitude and strength of electron-electron and electron-phonon couplings. Our approach could be used as a generalized theory for three-dimensional topological insulators in the bulk-conducting transport regime, paving the way for the realization of a unified theory of ultrafast dynamics in topological materials.

Photon gating in four-dimensional ultrafast electron microscopy

PubMed Central

Hassan, Mohammed T.; Liu, Haihua; Baskin, John Spencer; Zewail, Ahmed H.

2015-01-01

Ultrafast electron microscopy (UEM) is a pivotal tool for imaging of nanoscale structural dynamics with subparticle resolution on the time scale of atomic motion. Photon-induced near-field electron microscopy (PINEM), a key UEM technique, involves the detection of electrons that have gained energy from a femtosecond optical pulse via photon–electron coupling on nanostructures. PINEM has been applied in various fields of study, from materials science to biological imaging, exploiting the unique spatial, energy, and temporal characteristics of the PINEM electrons gained by interaction with a “single” light pulse. The further potential of photon-gated PINEM electrons in probing ultrafast dynamics of matter and the optical gating of electrons by invoking a “second” optical pulse has previously been proposed and examined theoretically in our group. Here, we experimentally demonstrate this photon-gating technique, and, through diffraction, visualize the phase transition dynamics in vanadium dioxide nanoparticles. With optical gating of PINEM electrons, imaging temporal resolution was improved by a factor of 3 or better, being limited only by the optical pulse widths. This work enables the combination of the high spatial resolution of electron microscopy and the ultrafast temporal response of the optical pulses, which provides a promising approach to attain the resolution of few femtoseconds and attoseconds in UEM. PMID:26438835

First-Principles Quantum Dynamics of Singlet Fission: Coherent versus Thermally Activated Mechanisms Governed by Molecular π Stacking

NASA Astrophysics Data System (ADS)

Tamura, Hiroyuki; Huix-Rotllant, Miquel; Burghardt, Irene; Olivier, Yoann; Beljonne, David

2015-09-01

Singlet excitons in π -stacked molecular crystals can split into two triplet excitons in a process called singlet fission that opens a route to carrier multiplication in photovoltaics. To resolve controversies about the mechanism of singlet fission, we have developed a first principles nonadiabatic quantum dynamical model that reveals the critical role of molecular stacking symmetry and provides a unified picture of coherent versus thermally activated singlet fission mechanisms in different acenes. The slip-stacked equilibrium packing structure of pentacene derivatives is found to enhance ultrafast singlet fission mediated by a coherent superexchange mechanism via higher-lying charge transfer states. By contrast, the electronic couplings for singlet fission strictly vanish at the C2 h symmetric equilibrium π stacking of rubrene. In this case, singlet fission is driven by excitations of symmetry-breaking intermolecular vibrations, rationalizing the experimentally observed temperature dependence. Design rules for optimal singlet fission materials therefore need to account for the interplay of molecular π -stacking symmetry and phonon-induced coherent or thermally activated mechanisms.

Ultrafast terahertz spectroscopy study of a Kondo insulating thin-film Sm B6 : Evidence for an emergent surface state

NASA Astrophysics Data System (ADS)

Zhang, Jingdi; Yong, Jie; Takeuchi, Ichiro; Greene, Richard L.; Averitt, Richard D.

2018-04-01

We utilize terahertz time domain spectroscopy to investigate thin films of the heavy fermion compound Sm B6 , a prototype Kondo insulator. Temperature-dependent terahertz (THz) conductivity measurements reveal a rapid decrease in the Drude weight and carrier scattering rate at ˜T*=20 K , well below the hybridization gap onset temperature (100 K). Moreover, a low-temperature conductivity plateau (below 20 K) suggests the emergence of a surface state with an effective electron mass of 0.1 me . The conductivity dynamics following optical excitation is also measured and interpreted using Rothwarf-Taylor (R-T) phenomenology, yielding a hybridization gap energy of 17 meV. However, R-T modeling of the conductivity dynamics reveals a deviation from the expected thermally excited quasiparticle density at temperatures below 20 K, indicative of another channel opening up in the low-energy electrodynamics. Taken together, these results are consistent with the onset of a surface state well below the crossover temperature (100 K) after long-range coherence of the f -electron Kondo lattice is established.

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.

Direct characterization of photoinduced lattice dynamics in BaFe 2As 2

DOE PAGES

Gerber, S.; Kim, K. W.; Zhang, Y.; ...

2015-06-08

Ultrafast light pulses can modify electronic properties of quantum materials by perturbing the underlying, intertwined degrees of freedom. In particular, iron-based superconductors exhibit a strong coupling among electronic nematic fluctuations, spins and the lattice, serving as a playground for ultrafast manipulation. Here we use time-resolved X-ray scattering to measure the lattice dynamics of photoexcited BaFe 2As 2. On optical excitation, no signature of an ultrafast change of the crystal symmetry is observed, but the lattice oscillates rapidly in time due to the coherent excitation of an A1g mode that modulates the Fe–As–Fe bond angle. We directly quantify the coherent latticemore » dynamics and show that even a small photoinduced lattice distortion can induce notable changes in the electronic and magnetic properties. Our analysis implies that transient structural modification can be an effective tool for manipulating the electronic properties of multi-orbital systems, where electronic instabilities are sensitive to the orbital character of bands.« less

Direct characterization of photoinduced lattice dynamics in BaFe2As2

PubMed Central

Gerber, S.; Kim, K. W.; Zhang, Y.; Zhu, D.; Plonka, N.; Yi, M.; Dakovski, G. L.; Leuenberger, D.; Kirchmann, P.S.; Moore, R. G.; Chollet, M.; Glownia, J. M.; Feng, Y.; Lee, J.-S.; Mehta, A.; Kemper, A. F.; Wolf, T.; Chuang, Y.-D.; Hussain, Z.; Kao, C.-C.; Moritz, B.; Shen, Z.-X.; Devereaux, T. P.; Lee, W.-S.

2015-01-01

Ultrafast light pulses can modify electronic properties of quantum materials by perturbing the underlying, intertwined degrees of freedom. In particular, iron-based superconductors exhibit a strong coupling among electronic nematic fluctuations, spins and the lattice, serving as a playground for ultrafast manipulation. Here we use time-resolved X-ray scattering to measure the lattice dynamics of photoexcited BaFe2As2. On optical excitation, no signature of an ultrafast change of the crystal symmetry is observed, but the lattice oscillates rapidly in time due to the coherent excitation of an A1g mode that modulates the Fe–As–Fe bond angle. We directly quantify the coherent lattice dynamics and show that even a small photoinduced lattice distortion can induce notable changes in the electronic and magnetic properties. Our analysis implies that transient structural modification can be an effective tool for manipulating the electronic properties of multi-orbital systems, where electronic instabilities are sensitive to the orbital character of bands. PMID:26051704

Ultrafast atomic-scale visualization of acoustic phonons generated by optically excited quantum dots

PubMed Central

Vanacore, Giovanni M.; Hu, Jianbo; Liang, Wenxi; Bietti, Sergio; Sanguinetti, Stefano; Carbone, Fabrizio; Zewail, Ahmed H.

2017-01-01

Understanding the dynamics of atomic vibrations confined in quasi-zero dimensional systems is crucial from both a fundamental point-of-view and a technological perspective. Using ultrafast electron diffraction, we monitored the lattice dynamics of GaAs quantum dots—grown by Droplet Epitaxy on AlGaAs—with sub-picosecond and sub-picometer resolutions. An ultrafast laser pulse nearly resonantly excites a confined exciton, which efficiently couples to high-energy acoustic phonons through the deformation potential mechanism. The transient behavior of the measured diffraction pattern reveals the nonequilibrium phonon dynamics both within the dots and in the region surrounding them. The experimental results are interpreted within the theoretical framework of a non-Markovian decoherence, according to which the optical excitation creates a localized polaron within the dot and a travelling phonon wavepacket that leaves the dot at the speed of sound. These findings indicate that integration of a phononic emitter in opto-electronic devices based on quantum dots for controlled communication processes can be fundamentally feasible. PMID:28852685

Electron and lattice dynamics of transition metal thin films observed by ultrafast electron diffraction and transient optical measurements.

PubMed

Nakamura, A; Shimojima, T; Nakano, M; Iwasa, Y; Ishizaka, K

2016-11-01

We report the ultrafast dynamics of electrons and lattice in transition metal thin films (Au, Cu, and Mo) investigated by a combination of ultrafast electron diffraction (UED) and pump-probe optical methods. For a single-crystalline Au thin film, we observe the suppression of the diffraction intensity occuring in 10 ps, which direcly reflects the lattice thermalization via the electron-phonon interaction. By using the two-temperature model, the electron-phonon coupling constant ( g ) and the electron and lattice temperatures ( T e , T l ) are evaluated from UED, with which we simulate the transient optical transmittance. The simulation well agrees with the experimentally obtained transmittance data, except for the slight deviations at the initial photoexcitation and the relaxed quasi-equilibrium state. We also present the results similarly obtained for polycrystalline Au, Cu, and Mo thin films and demonstrate the electron and lattice dynamics occurring in metals with different electron-phonon coupling strengths.

Organic photovoltaics: elucidating the ultra-fast exciton dissociation mechanism in disordered materials.

PubMed

Heitzer, Henry M; Savoie, Brett M; Marks, Tobin J; Ratner, Mark A

2014-07-14

Organic photovoltaics (OPVs) offer the opportunity for cheap, lightweight and mass-producible devices. However, an incomplete understanding of the charge generation process, in particular the timescale of dynamics and role of exciton diffusion, has slowed further progress in the field. We report a new Kinetic Monte Carlo model for the exciton dissociation mechanism in OPVs that addresses the origin of ultra-fast (<1 ps) dissociation by incorporating exciton delocalization. The model reproduces experimental results, such as the diminished rapid dissociation with increasing domain size, and also lends insight into the interplay between mixed domains, domain geometry, and exciton delocalization. Additionally, the model addresses the recent dispute on the origin of ultra-fast exciton dissociation by comparing the effects of exciton delocalization and impure domains on the photo-dynamics.This model provides insight into exciton dynamics that can advance our understanding of OPV structure-function relationships. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

High-quality infrared imaging with graphene photodetectors at room temperature.

PubMed

Guo, Nan; Hu, Weida; Jiang, Tao; Gong, Fan; Luo, Wenjin; Qiu, Weicheng; Wang, Peng; Liu, Lu; Wu, Shiwei; Liao, Lei; Chen, Xiaoshuang; Lu, Wei

2016-09-21

Graphene, a two-dimensional material, is expected to enable broad-spectrum and high-speed photodetection because of its gapless band structure, ultrafast carrier dynamics and high mobility. We demonstrate a multispectral active infrared imaging by using a graphene photodetector based on hybrid response mechanisms at room temperature. The high-quality images with optical resolutions of 418 nm, 657 nm and 877 nm and close-to-theoretical-limit Michelson contrasts of 0.997, 0.994, and 0.996 have been acquired for 565 nm, 1550 nm, and 1815 nm light imaging measurements by using an unbiased graphene photodetector, respectively. Importantly, by carefully analyzing the results of Raman mapping and numerical simulations for the response process, the formation of hybrid photocurrents in graphene detectors is attributed to the synergistic action of photovoltaic and photo-thermoelectric effects. The initial application to infrared imaging will help promote the development of high performance graphene-based infrared multispectral detectors.

Defect-mediated phonon dynamics in TaS2 and WSe2

PubMed Central

Cremons, Daniel R.; Plemmons, Dayne A.; Flannigan, David J.

2017-01-01

We report correlative crystallographic and morphological studies of defect-dependent phonon dynamics in single flakes of 1T-TaS2 and 2H-WSe2 using selected-area diffraction and bright-field imaging in an ultrafast electron microscope. In both materials, we observe in-plane speed-of-sound acoustic-phonon wave trains, the dynamics of which (i.e., emergence, propagation, and interference) are strongly dependent upon discrete interfacial features (e.g., vacuum/crystal and crystal/crystal interfaces). In TaS2, we observe cross-propagating in-plane acoustic-phonon wave trains of differing frequencies that undergo coherent interference approximately 200 ps after initial emergence from distinct interfacial regions. With ultrafast bright-field imaging, the properties of the interfering wave trains are observed to correspond to the beat frequency of the individual oscillations, while intensity oscillations of Bragg spots generated from selected areas within the region of interest match well with the real-space dynamics. In WSe2, distinct acoustic-phonon dynamics are observed emanating and propagating away from structurally dissimilar morphological discontinuities (vacuum/crystal interface and crystal terrace), and results of ultrafast selected-area diffraction reveal thickness-dependent phonon frequencies. The overall observed dynamics are well-described using finite element analysis and time-dependent linear-elastic continuum mechanics. PMID:28503630

Parametric spectro-temporal analyzer (PASTA) for ultrafast optical performance monitoring

NASA Astrophysics Data System (ADS)

Zhang, Chi; Wong, Kenneth K. Y.

2013-12-01

Ultrafast optical spectrum monitoring is one of the most challenging tasks in observing ultrafast phenomena, such as the spectroscopy, dynamic observation of the laser cavity, and spectral encoded imaging systems. However, conventional method such as optical spectrum analyzer (OSA) spatially disperses the spectrum, but the space-to-time mapping is realized by mechanical rotation of a grating, so are incapable of operating at high speed. Besides the spatial dispersion, temporal dispersion provided by dispersive fiber can also stretches the spectrum in time domain in an ultrafast manner, but is primarily confined in measuring short pulses. In view of these constraints, here we present a real-time spectrum analyzer called parametric spectro-temporal analyzer (PASTA), which is based on the time-lens focusing mechanism. It achieves a 100-MHz frame rate and can measure arbitrary waveforms. For the first time, we observe the dynamic spectrum of an ultrafast swept-source: Fourier domain mode-locked (FDML) laser, and the spectrum evolution of a laser cavity during its stabilizing process. In addition to the basic single-lens structure, the multi-lens configurations (e.g. telescope or wide-angle scope) will provide a versatile operating condition, which can zoom in to achieve 0.05-nm resolution and zoom out to achieve 10-nm observation range, namely 17 times zoom in/out ratio. In view of the goal of achieving spectrum analysis with fine accuracy, PASTA provides a promising path to study the real-time spectrum of some dynamic phenomena and non-repetitive events, with orders of magnitude enhancement in the frame rate over conventional OSAs.

Single-order laser high harmonics in XUV for ultrafast photoelectron spectroscopy of molecular wavepacket dynamics.

PubMed

Fushitani, Mizuho; Hishikawa, Akiyoshi

2016-11-01

We present applications of extreme ultraviolet (XUV) single-order laser harmonics to gas-phase ultrafast photoelectron spectroscopy. Ultrashort XUV pulses at 80 nm are obtained as the 5th order harmonics of the fundamental laser at 400 nm by using Xe or Kr as the nonlinear medium and separated from other harmonic orders by using an indium foil. The single-order laser harmonics is applied for real-time probing of vibrational wavepacket dynamics of I 2 molecules in the bound and dissociating low-lying electronic states and electronic-vibrational wavepacket dynamics of highly excited Rydberg N 2 molecules.

Intrinsic subpicosecond magnetization reversal driven by femtosecond laser pulses in GdFeCo amorphous films

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

Li, Shufa; Gao, Ruixin; Cheng, Chuyuan

2013-12-09

Ultrafast magnetization dynamics in GdFeCo films triggered by femtosecond laser pulses with and without an external field applied is studied experimentally for different excitation fluence. It is found that subpicosecond magnetization reversal occurs simultaneously in the ultrafast dynamics of both saturation and remnant magnetization states and almost identical within 13 ps, whereas relatively slow magnetization reversal across compensation point appears only in the dynamics of saturation magnetization state. It shows the subpicosecond magnetization reversal is external field independent, and originates from intrinsic magnetic evolution in ferrimagnetic system. The intrinsic subpicosecond reversal is qualitatively explained by linear reversal.

Real-Time Observation of Internal Motion within Ultrafast Dissipative Optical Soliton Molecules

NASA Astrophysics Data System (ADS)

Krupa, Katarzyna; Nithyanandan, K.; Andral, Ugo; Tchofo-Dinda, Patrice; Grelu, Philippe

2017-06-01

Real-time access to the internal ultrafast dynamics of complex dissipative optical systems opens new explorations of pulse-pulse interactions and dynamic patterns. We present the first direct experimental evidence of the internal motion of a dissipative optical soliton molecule generated in a passively mode-locked erbium-doped fiber laser. We map the internal motion of a soliton pair molecule by using a dispersive Fourier-transform imaging technique, revealing different categories of internal pulsations, including vibrationlike and phase drifting dynamics. Our experiments agree well with numerical predictions and bring insights to the analogy between self-organized states of lights and states of the matter.

Ballistic and resonant negative photocurrents in semiconducting carbon nanotubes

NASA Astrophysics Data System (ADS)

Karnetzky, Christoph; Sponfeldner, Lukas; Engl, Max; Holleitner, Alexander W.

2017-04-01

Ultrafast photocurrent experiments are performed on semiconducting, single-walled carbon nanotubes under a resonant optical excitation of their subbands. The photogenerated excitons are dissociated at large electric fields and the resulting transport of the charge carriers turns out to be ballistic. Thermionic emission processes to the contacts dominate the photocurrent amplitude. The charge current without laser excitation is well described by a Fowler-Nordheim tunneling. The time-averaged photocurrent changes polarity as soon as sufficient charge carriers are injected from the contacts, which can be explained by an effective population inversion in the optically pumped subbands.

Ballistic and resonant negative photocurrents in single carbon nanotubes

NASA Astrophysics Data System (ADS)

Karnetzky, Christoph; Sponfeldner, Lukas; Engl, Max; Holleitner, Alexander W.

We present ultrafast photocurrent experiments on semiconducting, single-walled carbon nanotubes under a resonant optical excitation of their subbands. We demonstrate that a ballistic transport of the photogenerated charge carriers can be achieved. Moreover, thermionic emission processes to the contacts dominate the photocurrent. In contrast, the charge current without laser excitation is well described by a Fowler-Nordheim tunneling. The time-averaged photocurrent changes polarity as soon as sufficient charge carriers are injected from the contacts, which can be explained by an effective population inversion in the optically pumped subbands. We acknowledge the ERC via the project NanoREAL.

Ultrafast transient grating radiation to optical image converter

DOEpatents

Stewart, Richard E; Vernon, Stephen P; Steel, Paul T; Lowry, Mark E

2014-11-04

A high sensitivity transient grating ultrafast radiation to optical image converter is based on a fixed transmission grating adjacent to a semiconductor substrate. X-rays or optical radiation passing through the fixed transmission grating is thereby modulated and produces a small periodic variation of refractive index or transient grating in the semiconductor through carrier induced refractive index shifts. An optical or infrared probe beam tuned just below the semiconductor band gap is reflected off a high reflectivity mirror on the semiconductor so that it double passes therethrough and interacts with the radiation induced phase grating therein. A small portion of the optical beam is diffracted out of the probe beam by the radiation induced transient grating to become the converted signal that is imaged onto a detector.

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.

Ultrafast spin exchange-coupling torque via photo-excited charge-transfer processes

NASA Astrophysics Data System (ADS)

Ma, X.; Fang, F.; Li, Q.; Zhu, J.; Yang, Y.; Wu, Y. Z.; Zhao, H. B.; Lüpke, G.

2015-10-01

Optical control of spin is of central importance in the research of ultrafast spintronic devices utilizing spin dynamics at short time scales. Recently developed optical approaches such as ultrafast demagnetization, spin-transfer and spin-orbit torques open new pathways to manipulate spin through its interaction with photon, orbit, charge or phonon. However, these processes are limited by either the long thermal recovery time or the low-temperature requirement. Here we experimentally demonstrate ultrafast coherent spin precession via optical charge-transfer processes in the exchange-coupled Fe/CoO system at room temperature. The efficiency of spin precession excitation is significantly higher and the recovery time of the exchange-coupling torque is much shorter than for the demagnetization procedure, which is desirable for fast switching. The exchange coupling is a key issue in spin valves and tunnelling junctions, and hence our findings will help promote the development of exchange-coupled device concepts for ultrafast coherent spin manipulation.

Plasmonic antennas as design elements for coherent ultrafast nanophotonics.

PubMed

Brinks, Daan; Castro-Lopez, Marta; Hildner, Richard; van Hulst, Niek F

2013-11-12

Broadband excitation of plasmons allows control of light-matter interaction with nanometric precision at femtosecond timescales. Research in the field has spiked in the past decade in an effort to turn ultrafast plasmonics into a diagnostic, microscopy, computational, and engineering tool for this novel nanometric-femtosecond regime. Despite great developments, this goal has yet to materialize. Previous work failed to provide the ability to engineer and control the ultrafast response of a plasmonic system at will, needed to fully realize the potential of ultrafast nanophotonics in physical, biological, and chemical applications. Here, we perform systematic measurements of the coherent response of plasmonic nanoantennas at femtosecond timescales and use them as building blocks in ultrafast plasmonic structures. We determine the coherent response of individual nanoantennas to femtosecond excitation. By mixing localized resonances of characterized antennas, we design coupled plasmonic structures to achieve well-defined ultrafast and phase-stable field dynamics in a predetermined nanoscale hotspot. We present two examples of the application of such structures: control of the spectral amplitude and phase of a pulse in the near field, and ultrafast switching of mutually coherent hotspots. This simple, reproducible and scalable approach transforms ultrafast plasmonics into a straightforward tool for use in fields as diverse as room temperature quantum optics, nanoscale solid-state physics, and quantum biology.

High-harmonic spectroscopy of ultrafast many-body dynamics in strongly correlated systems

NASA Astrophysics Data System (ADS)

Silva, R. E. F.; Blinov, Igor V.; Rubtsov, Alexey N.; Smirnova, O.; Ivanov, M.

2018-05-01

We bring together two topics that, until now, have been the focus of intense but non-overlapping research efforts. The first concerns high-harmonic generation in solids, which occurs when an intense light field excites a highly non-equilibrium electronic response in a semiconductor or a dielectric. The second concerns many-body dynamics in strongly correlated systems such as the Mott insulator. We show that high-harmonic generation can be used to time-resolve ultrafast many-body dynamics associated with an optically driven phase transition, with accuracy far exceeding one cycle of the driving light field. Our work paves the way for time-resolving highly non-equilibrium many-body dynamics in strongly correlated systems, with few femtosecond accuracy.

Following strain-induced mosaicity changes of ferroelectric thin films by ultrafast reciprocal space mapping.

PubMed

Schick, D; Bojahr, A; Herzog, M; Gaal, P; Vrejoiu, I; Bargheer, M

2013-03-01

We investigate coherent phonon propagation in a thin film of ferroelectric PbZr(0.2)Ti(0.8)O(3) (PZT) by ultrafast x-ray diffraction experiments, which are analyzed as time-resolved reciprocal space mapping in order to observe the in- and out-of-plane structural dynamics, simultaneously. The mosaic structure of the PZT leads to a coupling of the excited out-of-plane expansion to in-plane lattice dynamics on a picosecond time scale, which is not observed for out-of-plane compression.

Ultrafast Independent N-H and N-C Bond Deformation Investigated with Resonant Inelastic X-Ray Scattering

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

Eckert, Sebastian; Norell, Jesper; Miedema, Piter S.

Here, the femtosecond excited-state dynamics following resonant photoexcitation enable the selective deformation of N-H and N-C chemical bonds in 2-thiopyridone in aqueous solution with optical or X-ray pulses. In combination with multiconfigurational quantum-chemical calculations, the orbital-specific electronic structure and its ultrafast dynamics accessed with resonant inelastic X-ray scattering at the N 1s level using synchrotron radiation and the soft X-ray free-electron laser LCLS provide direct evidence for this controlled photoinduced molecular deformation and its ultrashort timescale.

Ultrafast Independent N-H and N-C Bond Deformation Investigated with Resonant Inelastic X-Ray Scattering

DOE PAGES

Eckert, Sebastian; Norell, Jesper; Miedema, Piter S.; ...

2017-04-04

Here, the femtosecond excited-state dynamics following resonant photoexcitation enable the selective deformation of N-H and N-C chemical bonds in 2-thiopyridone in aqueous solution with optical or X-ray pulses. In combination with multiconfigurational quantum-chemical calculations, the orbital-specific electronic structure and its ultrafast dynamics accessed with resonant inelastic X-ray scattering at the N 1s level using synchrotron radiation and the soft X-ray free-electron laser LCLS provide direct evidence for this controlled photoinduced molecular deformation and its ultrashort timescale.

Colloidal thallium halide nanocrystals with reasonable luminescence, carrier mobility and diffusion length.

PubMed

Mir, Wasim J; Warankar, Avinash; Acharya, Ashutosh; Das, Shyamashis; Mandal, Pankaj; Nag, Angshuman

2017-06-01

Colloidal lead halide based perovskite nanocrystals (NCs) have been recently established as an interesting class of defect-tolerant NCs with potential for superior optoelectronic applications. The electronic band structure of thallium halides (TlX, where X = Br and I) show a strong resemblance to lead halide perovskites, where both Pb 2+ and Tl + exhibit a 6s 2 inert pair of electrons and strong spin-orbit coupling. Although the crystal structure of TlX is not perovskite, the similarities of its electronic structure with lead halide perovskites motivated us to prepare colloidal TlX NCs. These TlX NCs exhibit a wide bandgap (>2.5 eV or <500 nm) and the potential to exhibit a reduced density of deep defect states. Optical pump terahertz (THz) probe spectroscopy with excitation fluence in the range of 0.85-5.86 × 10 13 photons per cm 2 on NC films shows that the TlBr NCs possess high effective carrier mobility (∼220 to 329 cm 2 V -1 s -1 ), long diffusion length (∼0.77 to 0.98 μm), and reasonably high photoluminescence efficiency (∼10%). This combination of properties is remarkable compared to other wide-bandgap (>2.5 eV) semiconductor NCs, which suggests a reduction in the deep-defect states in the TlX NCs. Furthermore, the ultrafast carrier dynamics and temperature-dependent reversible structural phase transition together with its influence on the optical properties of the TlX NCs are studied.

Femtosecond tracking of carrier relaxation in germanium with extreme ultraviolet transient reflectivity

NASA Astrophysics Data System (ADS)

Kaplan, Christopher J.; Kraus, Peter M.; Ross, Andrew D.; Zürch, Michael; Cushing, Scott K.; Jager, Marieke F.; Chang, Hung-Tzu; Gullikson, Eric M.; Neumark, Daniel M.; Leone, Stephen R.

2018-05-01

Extreme ultraviolet (XUV) transient reflectivity around the germanium M4 ,5 edge (3 d core-level to valence transition) at 30 eV is advanced to obtain the transient dielectric function of crystalline germanium [100] on femtosecond to picosecond time scales following photoexcitation by broadband visible-to-infrared (VIS/NIR) pulses. By fitting the transient dielectric function, carrier-phonon induced relaxations are extracted for the excited carrier distribution. The measurements reveal a hot electron relaxation rate of 3.2 ±0.2 ps attributed to the X -L intervalley scattering and a hot hole relaxation rate of 600 ±300 fs ascribed to intravalley scattering within the heavy hole (HH) band, both in good agreement with previous work. An overall energy shift of the XUV dielectric function is assigned to a thermally induced band gap shrinkage by formation of acoustic phonons, which is observed to be on a timescale of 4-5 ps, in agreement with previously measured optical phonon lifetimes. The results reveal that the transient reflectivity signal at an angle of 66∘ with respect to the surface normal is dominated by changes to the real part of the dielectric function, due to the near critical angle of incidence of the experiment (66∘-70∘) for the range of XUV energies used. This work provides a methodology for interpreting XUV transient reflectivity near core-level transitions, and it demonstrates the power of the XUV spectral region for measuring ultrafast excitation dynamics in solids.

Radiomics for ultrafast dynamic contrast-enhanced breast MRI in the diagnosis of breast cancer: a pilot study

NASA Astrophysics Data System (ADS)

Drukker, Karen; Anderson, Rachel; Edwards, Alexandra; Papaioannou, John; Pineda, Fred; Abe, Hiroyuke; Karzcmar, Gregory; Giger, Maryellen L.

2018-02-01

Radiomics for dynamic contrast-enhanced (DCE) breast MRI have shown promise in the diagnosis of breast cancer as applied to conventional DCE-MRI protocols. Here, we investigate the potential of using such radiomic features in the diagnosis of breast cancer applied on ultrafast breast MRI in which images are acquired every few seconds. The dataset consisted of 64 lesions (33 malignant and 31 benign) imaged with both `conventional' and ultrafast DCE-MRI. After automated lesion segmentation in each image sequence, we calculated 38 radiomic features categorized as describing size, shape, margin, enhancement-texture, kinetics, and enhancement variance kinetics. For each feature, we calculated the 95% confidence interval of the area under the ROC curve (AUC) to determine whether the performance of each feature in the task of distinguishing between malignant and benign lesions was better than random guessing. Subsequently, we assessed performance of radiomic signatures in 10-fold cross-validation repeated 10 times using a support vector machine with as input all the features as well as features by category. We found that many of the features remained useful (AUC>0.5) for the ultrafast protocol, with the exception of some features, e.g., those designed for latephase kinetics such as the washout rate. For ultrafast MRI, the radiomics enhancement-texture signature achieved the best performance, which was comparable to that of the kinetics signature for `conventional' DCE-MRI, both achieving AUC values of 0.71. Radiomic developed for `conventional' DCE-MRI shows promise for translation to the ultrafast protocol, where enhancement texture appears to play a dominant role.

The perspectives of femtosecond imaging and spectroscopy of complex materials using electrons

NASA Astrophysics Data System (ADS)

Ruan, Chong-Yu; Duxbury, Phiilp M.; Berz, Martin

2014-09-01

The coexistence of various electronic and structural phases that are close in free-energy is a hallmark in strongly correlated electron systems with emergent properties, such as metal-insulator transition, colossal magnetoresistance, and high-temperature superconductivity. The cooperative phase transitions from one functional state to another can involve entanglements between the electronically and structurally ordered states, hence deciphering the fundamental mechanisms is generally difficult and remains very active in condensed matter physics and functional materials research. We outline the recent ultrafast characterizations of 2D charge-density wave materials, including the nonequilibrium electron dynamics unveiled by ultrafast optical spectroscopy-based techniques sensitive to the electronic order parameter. We also describe the most recent findings from ultrafast electron crystallography, which provide structural aspects to correlate lattice dynamics with electronic evolutions to address the two sides of a coin in the ultrafast switching of a cooperative state. Combining these results brings forth new perspectives and a fuller picture in understanding lightmatter interactions and various switching mechanisms in cooperative systems with many potential applications. We also discuss the prospects of implementing new ultrafast electron imaging as a local probe incorporated with femtosecond select-area diffraction, imaging and spectroscopy to provide a full scope of resolution to tackle the more challenging complex phase transitions on the femtosecond-nanometer scale all at once based on a recent understanding of the spacespace- charge-driven emittance limitation on the ultimate performance of these devices. The projection shows promising parameter space for conducting ultrafast electron micordiffraction at close to single-shot level, which is supported by the latest experimental characterization of such a system.

Ultrafast Nonlinear Microscopy in III-V Semiconductor Nanostructures

DTIC Science & Technology

2016-01-20

SECURITY CLASSIFICATION OF: This project involved the investigation of the photoluminescence properties of individual ZnO nano-rods, characterization ...13. SUPPLEMENTARY NOTES 12. DISTRIBUTION AVAILIBILITY STATEMENT 6. AUTHORS 7. PERFORMING ORGANIZATION NAMES AND ADDRESSES 15. SUBJECT TERMS b...Office P.O. Box 12211 Research Triangle Park, NC 27709-2211 ultrafast imaging, strained nanomaterials , electron-hole plasma dynamics, microscopy

Spectrally- and Time-Resolved Sum Frequency Generation (STiR-SFG): a new tool for ultrafast hydrogen bond dynamics at interfaces.

NASA Astrophysics Data System (ADS)

Benderskii, Alexander; Bordenyuk, Andrey; Weeraman, Champika

2006-03-01

The recently developed spectrally- and time-resolved Sum Frequency Generation (STiR-SFG) is a surface-selective 3-wave mixing (IR+visible) spectroscopic technique capable of measuring ultrafast spectral evolution of vibrational coherences. A detailed description of this measurement will be presented, and a noniterative method or deconvolving the laser pulses will be introduced to obtain the molecular response function. STiR-SFG, combined with the frequency-domain SFG spectroscopy, was applied to study hydrogen bonding dynamics at aqueous interfaces (D2O/CaF2). Spectral dynamics of the OD-stretch on the 50-150 fs time scale provides real-time observation of ultrafast H-bond rearrangement. Tuning the IR wavelength to the blue or red side of the OD-stretch transition, we selectively monitor the dynamics of different sub-ensembles in the distribution of the H-bond structures. The blue-side excitation (weaker H-bonding) shows monotonic red-shift of the OD-frequency. In contrast, the red-side excitation (stronger H-bonding structures) produces a blue-shift and a recursion, which may indicate the presence of an underdamped intermolecular mode of interfacial water. Effect of electrolyte concentration on the H-bond dynamics will be discussed.

Photocarrier generation from interlayer charge-transfer transitions in WS2-graphene heterostructures

PubMed Central

Yuan, Long; Chung, Ting-Fung; Kuc, Agnieszka; Wan, Yan; Xu, Yang; Chen, Yong P.; Heine, Thomas; Huang, Libai

2018-01-01

Efficient interfacial carrier generation in van der Waals heterostructures is critical for their electronic and optoelectronic applications. We demonstrate broadband photocarrier generation in WS2-graphene heterostructures by imaging interlayer coupling–dependent charge generation using ultrafast transient absorption microscopy. Interlayer charge-transfer (CT) transitions and hot carrier injection from graphene allow carrier generation by excitation as low as 0.8 eV below the WS2 bandgap. The experimentally determined interlayer CT transition energies are consistent with those predicted from the first-principles band structure calculation. CT interactions also lead to additional carrier generation in the visible spectral range in the heterostructures compared to that in the single-layer WS2 alone. The lifetime of the charge-separated states is measured to be ~1 ps. These results suggest that interlayer interactions make graphene–two-dimensional semiconductor heterostructures very attractive for photovoltaic and photodetector applications because of the combined benefits of high carrier mobility and enhanced broadband photocarrier generation. PMID:29423439

Modelling ultrafast laser ablation

NASA Astrophysics Data System (ADS)

Rethfeld, Baerbel; Ivanov, Dmitriy S.; E Garcia, Martin; Anisimov, Sergei I.

2017-05-01

This review is devoted to the study of ultrafast laser ablation of solids and liquids. The ablation of condensed matter under exposure to subpicosecond laser pulses has a number of peculiar properties which distinguish this process from ablation induced by nanosecond and longer laser pulses. The process of ultrafast ablation includes light absorption by electrons in the skin layer, energy transfer from the skin layer to target interior by nonlinear electronic heat conduction, relaxation of the electron and ion temperatures, ultrafast melting, hydrodynamic expansion of heated matter accompanied by the formation of metastable states and subsequent formation of breaks in condensed matter. In case of ultrashort laser excitation, these processes are temporally separated and can thus be studied separately. As for energy absorption, we consider peculiarities of the case of metal irradiation in contrast to dielectrics and semiconductors. We discuss the energy dissipation processes of electronic thermal wave and lattice heating. Different types of phase transitions after ultrashort laser pulse irradiation as melting, vaporization or transitions to warm dense matter are discussed. Also nonthermal phase transitions, directly caused by the electronic excitation before considerable lattice heating, are considered. The final material removal occurs from the physical point of view as expansion of heated matter; here we discuss approaches of hydrodynamics, as well as molecular dynamic simulations directly following the atomic movements. Hybrid approaches tracing the dynamics of excited electrons, energy dissipation and structural dynamics in a combined simulation are reviewed as well.

Electric transport of a single-crystal iron chalcogenide FeSe superconductor: Evidence of symmetry-breakdown nematicity and additional ultrafast Dirac cone-like carriers

NASA Astrophysics Data System (ADS)

Huynh, K. K.; Tanabe, Y.; Urata, T.; Oguro, H.; Heguri, S.; Watanabe, K.; Tanigaki, K.

2014-10-01

An SDW antiferromagnetic (SDW-AF) low-temperature phase transition is generally observed and the AF spin fluctuations are considered to play an important role for the superconductivity pairing mechanism in FeAs superconductors. However, a similar magnetic phase transition is not observed in FeSe superconductors, which has caused considerable discussion. We report on the intrinsic electronic states of FeSe as elucidated by electric transport measurements under magnetic fields using a high quality single crystal. A mobility spectrum analysis, an ab initio method that does not make assumptions on the transport parameters in a multicarrier system, provides very important and clear evidence that another hidden order, most likely the symmetry broken from the tetragonal C4 symmetry to the C2 symmetry nematicity associated with the selective d -orbital splitting, exists in the case of superconducting FeSe other than the AF magnetic order spin fluctuations. The intrinsic low-temperature phase in FeSe is in the almost compensated semimetallic states but is additionally accompanied by Dirac cone-like ultrafast electrons ˜104cm2(VS) -1 as minority carriers.

Structure and Dynamics with Ultrafast Electron Microscopes

NASA Astrophysics Data System (ADS)

Siwick, Bradley

In this talk I will describe how combining ultrafast lasers and electron microscopes in novel ways makes it possible to directly `watch' the time-evolving structure of condensed matter, both at the level of atomic-scale structural rearrangements in the unit cell and at the level of a material's nano- microstructure. First, I will briefly describe my group's efforts to develop ultrafast electron diffraction using radio- frequency compressed electron pulses in the 100keV range, a system that rivals the capabilities of xray free electron lasers for diffraction experiments. I will give several examples of the new kinds of information that can be gleaned from such experiments. In vanadium dioxide we have mapped the detailed reorganization of the unit cell during the much debated insulator-metal transition. In particular, we have been able to identify and separate lattice structural changes from valence charge density redistribution in the material on the ultrafast timescale. In doing so we uncovered a previously unreported optically accessible phase/state of vanadium dioxide that has monoclinic crystallography like the insulator, but electronic structure and properties that are more like the rutile metal. We have also combined these dynamic structural measurements with broadband ultrafast spectroscopy to make detailed connections between structure and properties for the photoinduced insulator to metal transition. Second, I will show how dynamic transmission electron microscopy (DTEM) can be used to make direct, real space images of nano-microstructural evolution during laser-induced crystallization of amorphous semiconductors at unprecedented spatio-temporal resolution. This is a remarkably complex process that involves several distinct modes of crystal growth and the development of intricate microstructural patterns on the nanosecond to ten microsecond timescales all of which can be imaged directly with DTEM.

Coherent Amplification of Ultrafast Molecular Dynamics in an Optical Oscillator

NASA Astrophysics Data System (ADS)

Aharonovich, Igal; Pe'er, Avi

2016-02-01

Optical oscillators present a powerful optimization mechanism. The inherent competition for the gain resources between possible modes of oscillation entails the prevalence of the most efficient single mode. We harness this "ultrafast" coherent feedback to optimize an optical field in time, and show that, when an optical oscillator based on a molecular gain medium is synchronously pumped by ultrashort pulses, a temporally coherent multimode field can develop that optimally dumps a general, dynamically evolving vibrational wave packet, into a single vibrational target state. Measuring the emitted field opens a new window to visualization and control of fast molecular dynamics. The realization of such a coherent oscillator with hot alkali dimers appears within experimental reach.

Laser-combined scanning tunnelling microscopy for probing ultrafast transient dynamics.

PubMed

Terada, Yasuhiko; Yoshida, Shoji; Takeuchi, Osamu; Shigekawa, Hidemi

2010-07-07

The development of time-resolved scanning tunnelling microscopy (STM), in particular, attempts to combine STM with ultrafast laser technology, is reviewed with emphasis on observed physical quantities and spatiotemporal resolution. Ultrashort optical pulse technology has allowed us to observe transient phenomena in the femtosecond range, which, however, has the drawback of a relatively low spatial resolution due to the electromagnetic wavelength used. In contrast, STM and its related techniques, although the time resolution is limited by the circuit bandwidth (∼100 kHz), enable us to observe structures at the atomic level in real space. Our purpose has been to combine these two techniques to achieve a new technology that satisfies the requirements for exploring the ultrafast transient dynamics of the local quantum functions in organized small structures, which will advance the pursuit of future nanoscale scientific research in terms of the ultimate temporal and spatial resolutions. © 2010 IOP Publishing Ltd

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.

Imaging Molecular Motion: Femtosecond X-Ray Scattering of an Electrocyclic Chemical Reaction

NASA Astrophysics Data System (ADS)

Minitti, M. P.; Budarz, J. M.; Kirrander, A.; Robinson, J. S.; Ratner, D.; Lane, T. J.; Zhu, D.; Glownia, J. M.; Kozina, M.; Lemke, H. T.; Sikorski, M.; Feng, Y.; Nelson, S.; Saita, K.; Stankus, B.; Northey, T.; Hastings, J. B.; Weber, P. M.

2015-06-01

Structural rearrangements within single molecules occur on ultrafast time scales. Many aspects of molecular dynamics, such as the energy flow through excited states, have been studied using spectroscopic techniques, yet the goal to watch molecules evolve their geometrical structure in real time remains challenging. By mapping nuclear motions using femtosecond x-ray pulses, we have created real-space representations of the evolving dynamics during a well-known chemical reaction and show a series of time-sorted structural snapshots produced by ultrafast time-resolved hard x-ray scattering. A computational analysis optimally matches the series of scattering patterns produced by the x rays to a multitude of potential reaction paths. In so doing, we have made a critical step toward the goal of viewing chemical reactions on femtosecond time scales, opening a new direction in studies of ultrafast chemical reactions in the gas phase.

Imaging Molecular Motion: Femtosecond X-Ray Scattering of an Electrocyclic Chemical Reaction.

PubMed

Minitti, M P; Budarz, J M; Kirrander, A; Robinson, J S; Ratner, D; Lane, T J; Zhu, D; Glownia, J M; Kozina, M; Lemke, H T; Sikorski, M; Feng, Y; Nelson, S; Saita, K; Stankus, B; Northey, T; Hastings, J B; Weber, P M

2015-06-26

Structural rearrangements within single molecules occur on ultrafast time scales. Many aspects of molecular dynamics, such as the energy flow through excited states, have been studied using spectroscopic techniques, yet the goal to watch molecules evolve their geometrical structure in real time remains challenging. By mapping nuclear motions using femtosecond x-ray pulses, we have created real-space representations of the evolving dynamics during a well-known chemical reaction and show a series of time-sorted structural snapshots produced by ultrafast time-resolved hard x-ray scattering. A computational analysis optimally matches the series of scattering patterns produced by the x rays to a multitude of potential reaction paths. In so doing, we have made a critical step toward the goal of viewing chemical reactions on femtosecond time scales, opening a new direction in studies of ultrafast chemical reactions in the gas phase.

Ultrafast dynamics of electrons in ammonia.

PubMed

Vöhringer, Peter

2015-04-01

Solvated electrons were first discovered in solutions of metals in liquid ammonia. The physical and chemical properties of these species have been studied extensively for many decades using an arsenal of electrochemical, spectroscopic, and theoretical techniques. Yet, in contrast to their hydrated counterpart, the ultrafast dynamics of ammoniated electrons remained completely unexplored until quite recently. Femtosecond pump-probe spectroscopy on metal-ammonia solutions and femtosecond multiphoton ionization spectroscopy on the neat ammonia solvent have provided new insights into the optical properties and the reactivities of this fascinating species. This article reviews the nature of the optical transition, which gives the metal-ammonia solutions their characteristic blue appearance, in terms of ultrafast relaxation processes involving bound and continuum excited states. The recombination processes following the injection of an electron via photoionization of the solvent are discussed in the context of the electronic structure of the liquid and the anionic defect associated with the solvated electron.

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 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.

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.

Effect of additional optical pumping injection into the ground-state ensemble on the gain and the phase recovery acceleration of quantum-dot semiconductor optical amplifiers

NASA Astrophysics Data System (ADS)

Kim, Jungho

2014-02-01

The effect of additional optical pumping injection into the ground-state ensemble on the ultrafast gain and the phase recovery dynamics of electrically-driven quantum-dot semiconductor optical amplifiers is numerically investigated by solving 1088 coupled rate equations. The ultrafast gain and the phase recovery responses are calculated with respect to the additional optical pumping power. Increasing the additional optical pumping power can significantly accelerate the ultrafast phase recovery, which cannot be done by increasing the injection current density.

Anti-Stokes resonant x-ray Raman scattering for atom specific and excited state selective dynamics

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

Kunnus, Kristjan; Josefsson, Ida; Rajkovic, Ivan

Here, ultrafast electronic and structural dynamics of matter govern rate and selectivity of chemical reactions, as well as phase transitions and efficient switching in functional materials. Since x-rays determine electronic and structural properties with elemental, chemical, orbital and magnetic selectivity, short pulse x-ray sources have become central enablers of ultrafast science. Despite of these strengths, ultrafast x-rays have been poor at picking up excited state moieties from the unexcited ones. With time-resolved anti-Stokes resonant x-ray Raman scattering (AS-RXRS) performed at the LCLS, and ab initio theory we establish background free excited state selectivity in addition to the elemental, chemical, orbitalmore » and magnetic selectivity of x-rays. This unparalleled selectivity extracts low concentration excited state species along the pathway of photo induced ligand exchange of Fe(CO)5 in ethanol. Conceptually a full theoretical treatment of all accessible insights to excited state dynamics with AS-RXRS with transform-limited x-ray pulses is given—which will be covered experimentally by upcoming transform-limited x-ray sources.« less

Phase diagram of the ultrafast photoinduced insulator-metal transition in vanadium dioxide

NASA Astrophysics Data System (ADS)

Cocker, T. L.; Titova, L. V.; Fourmaux, S.; Holloway, G.; Bandulet, H.-C.; Brassard, D.; Kieffer, J.-C.; El Khakani, M. A.; Hegmann, F. A.

2012-04-01

We use time-resolved terahertz spectroscopy to probe the ultrafast dynamics of the insulator-metal phase transition induced by femtosecond laser pulses in a nanogranular vanadium dioxide (VO2) film. Based on the observed thresholds for characteristic transient terahertz dynamics, a phase diagram of critical pump fluence versus temperature for the insulator-metal phase transition in VO2 is established for the first time over a broad range of temperatures down to 17 K. We find that both Mott and Peierls mechanisms are present in the insulating state and that the photoinduced transition is nonthermal. We propose a critical-threshold model for the ultrafast photoinduced transition based on a critical density of electrons and a critical density of coherently excited phonons necessary for the structural transition to the metallic state. As a result, evidence is found at low temperatures for an intermediate metallic state wherein the Mott state is melted but the Peierls distortion remains intact, consistent with recent theoretical predictions. Finally, the observed terahertz conductivity dynamics above the photoinduced transition threshold reveal nucleation and growth of metallic nanodomains over picosecond time scales.

Anti-Stokes resonant x-ray Raman scattering for atom specific and excited state selective dynamics

DOE PAGES

Kunnus, Kristjan; Josefsson, Ida; Rajkovic, Ivan; ...

2016-10-07

Here, ultrafast electronic and structural dynamics of matter govern rate and selectivity of chemical reactions, as well as phase transitions and efficient switching in functional materials. Since x-rays determine electronic and structural properties with elemental, chemical, orbital and magnetic selectivity, short pulse x-ray sources have become central enablers of ultrafast science. Despite of these strengths, ultrafast x-rays have been poor at picking up excited state moieties from the unexcited ones. With time-resolved anti-Stokes resonant x-ray Raman scattering (AS-RXRS) performed at the LCLS, and ab initio theory we establish background free excited state selectivity in addition to the elemental, chemical, orbitalmore » and magnetic selectivity of x-rays. This unparalleled selectivity extracts low concentration excited state species along the pathway of photo induced ligand exchange of Fe(CO)5 in ethanol. Conceptually a full theoretical treatment of all accessible insights to excited state dynamics with AS-RXRS with transform-limited x-ray pulses is given—which will be covered experimentally by upcoming transform-limited x-ray sources.« less

Ultrafast Charge Transfer in Nickel Phthalocyanine Probed by Femtosecond Raman-Induced Kerr Effect Spectroscopy

PubMed Central

2015-01-01

The recently developed technique of femtosecond stimulated Raman spectroscopy, and its variant, femtosecond Raman-induced Kerr effect spectroscopy (FRIKES), offer access to ultrafast excited-state dynamics via structurally specific vibrational spectra. We have used FRIKES to study the photoexcitation dynamics of nickel(II) phthalocyanine with eight butoxy substituents, NiPc(OBu)8. NiPc(OBu)8 is reported to have a relatively long-lived ligand-to-metal charge-transfer (LMCT) state, an essential characteristic for efficient electron transfer in photocatalysis. Following photoexcitation, vibrational transitions in the FRIKES spectra, assignable to phthalocyanine ring modes, evolve on the femtosecond to picosecond time scales. Correlation of ring core size with the frequency of the ν10 (asymmetric C–N stretching) mode confirms the identity of the LMCT state, which has a ∼500 ps lifetime, as well as that of a precursor d-d excited state. An even earlier (∼0.2 ps) transient is observed and tentatively assigned to a higher-lying Jahn–Teller-active LMCT state. This study illustrates the power of FRIKES spectroscopy in elucidating ultrafast molecular dynamics. PMID:24841906

Uncovering Highly-Excited State Mixing in Acetone Using Ultrafast VUV Pulses and Coincidence Imaging Techniques

DOE PAGES

Couch, David E.; Kapteyn, Henry C.; Murnane, Margaret M.; ...

2017-03-17

Here, understanding the ultrafast dynamics of highly-excited electronic states of small molecules is critical for a better understanding of atmospheric and astrophysical processes, as well as for designing coherent control strategies for manipulating chemical dynamics. In highly excited states, nonadiabatic coupling, electron-electron interactions, and the high density of states govern dynamics. However, these states are computationally and experimentally challenging to access. Fortunately, new sources of ultrafast vacuum ultraviolet pulses, in combination with electron-ion coincidence spectroscopies, provide new tools to unravel the complex electronic landscape. Here we report time-resolved photoelectron-photoion coincidence experiments using 8 eV pump photons to study the highlymore » excited states of acetone. We uncover for the first time direct evidence that the resulting excited state consists of a mixture of both n y → 3p and π → π* character, which decays with a time constant of 330 fs. In the future, this approach can inform models of VUV photochemistry and aid in designing coherent control strategies for manipulating chemical reactions.« less

Uncovering Highly-Excited State Mixing in Acetone Using Ultrafast VUV Pulses and Coincidence Imaging Techniques

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

Couch, David E.; Kapteyn, Henry C.; Murnane, Margaret M.

Here, understanding the ultrafast dynamics of highly-excited electronic states of small molecules is critical for a better understanding of atmospheric and astrophysical processes, as well as for designing coherent control strategies for manipulating chemical dynamics. In highly excited states, nonadiabatic coupling, electron-electron interactions, and the high density of states govern dynamics. However, these states are computationally and experimentally challenging to access. Fortunately, new sources of ultrafast vacuum ultraviolet pulses, in combination with electron-ion coincidence spectroscopies, provide new tools to unravel the complex electronic landscape. Here we report time-resolved photoelectron-photoion coincidence experiments using 8 eV pump photons to study the highlymore » excited states of acetone. We uncover for the first time direct evidence that the resulting excited state consists of a mixture of both n y → 3p and π → π* character, which decays with a time constant of 330 fs. In the future, this approach can inform models of VUV photochemistry and aid in designing coherent control strategies for manipulating chemical reactions.« less

Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO 3

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

Chen, F.; Zhu, Y.; Liu, S.

The dynamical processes associated with electric field manipulation of the polarization in a ferroelectric remain largely unknown but fundamentally determine the speed and functionality of ferroelectric materials and devices. Here we apply subpicosecond duration, single-cycle terahertz pulses as an ultrafast electric field bias to prototypical BaTiO 3 ferroelectric thin films with the atomic-scale response probed by femtosecond x-ray-scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large-amplitude displacements of the titanium atoms along the ferroelectric polarization axis, comparable to that of the built-in displacements associated with the intrinsic polarization and incoherent across unit cells. Thismore » effect is associated with a dynamic rotation of the ferroelectric polarization switching on and then off on picosecond time scales. These transient polarization modulations are followed by long-lived vibrational heating effects driven by resonant excitation of the ferroelectric soft mode, as reflected in changes in the c-axis tetragonality. The ultrafast structural characterization described here enables a direct comparison with first-principles-based molecular-dynamics simulations, with good agreement obtained.« less

Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO 3

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

Chen, F.; Zhu, Y.; Liu, S.

The dynamical processes associated with electric field manipulation of the polarization in a ferroelectric remain largely unknown but fundamentally determine the speed and functionality of ferroelectric materials and devices. Here in this paper we apply subpicosecond duration, single-cycle terahertz pulses as an ultrafast electric field bias to prototypical BaTiO 3 ferroelectric thin films with the atomic-scale response probed by femtosecond x-ray-scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large-amplitude displacements of the titanium atoms along the ferroelectric polarization axis, comparable to that of the built-in displacements associated with the intrinsic polarization and incoherent acrossmore » unit cells. This effect is associated with a dynamic rotation of the ferroelectric polarization switching on and then off on picosecond time scales. These transient polarization modulations are followed by long-lived vibrational heating effects driven by resonant excitation of the ferroelectric soft mode, as reflected in changes in the c-axis tetragonality. The ultrafast structural characterization described here enables a direct comparison with first-principles-based molecular-dynamics simulations, with good agreement obtained.« less

Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO 3

DOE PAGES

Chen, F.; Zhu, Y.; Liu, S.; ...

2016-11-22

The dynamical processes associated with electric field manipulation of the polarization in a ferroelectric remain largely unknown but fundamentally determine the speed and functionality of ferroelectric materials and devices. Here in this paper we apply subpicosecond duration, single-cycle terahertz pulses as an ultrafast electric field bias to prototypical BaTiO 3 ferroelectric thin films with the atomic-scale response probed by femtosecond x-ray-scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large-amplitude displacements of the titanium atoms along the ferroelectric polarization axis, comparable to that of the built-in displacements associated with the intrinsic polarization and incoherent acrossmore » unit cells. This effect is associated with a dynamic rotation of the ferroelectric polarization switching on and then off on picosecond time scales. These transient polarization modulations are followed by long-lived vibrational heating effects driven by resonant excitation of the ferroelectric soft mode, as reflected in changes in the c-axis tetragonality. The ultrafast structural characterization described here enables a direct comparison with first-principles-based molecular-dynamics simulations, with good agreement obtained.« less

Ultrafast photodissociation dynamics of 1,4-diiodobenzene

NASA Astrophysics Data System (ADS)

Stankus, Brian; Zotev, Nikola; Rogers, David M.; Gao, Yan; Odate, Asami; Kirrander, Adam; Weber, Peter M.

2018-05-01

The photodissociation dynamics of 1,4-diiodobenzene is investigated using ultrafast time-resolved photoelectron spectroscopy. Following excitation by laser pulses at 271 nm, the excited-state dynamics is probed by resonance-enhanced multiphoton ionization with 405 nm probe pulses. A progression of Rydberg states, which come into resonance sequentially, provide a fingerprint of the dissociation dynamics of the molecule. The initial excitation decays with a lifetime of 33 ± 4 fs, in good agreement with a previous study. The spectrum is interpreted by reference to ab initio calculations at the CASPT2(18,14) level, including spin-orbit coupling. We propose that both the 5B1 and 6B1 states are excited initially, and based on the calculations, we identify diabatic spin-orbit coupled states corresponding to the main dissociation pathways.

Unravelling Photocarrier Dynamics beyond the Space Charge Region for Photoelectrochemical Water Splitting

DOE PAGES

Zhang, Wenrui; Yan, Danhua; Appavoo, Kannatassen; ...

2017-04-18

Semiconductor photoelectrodes for photoelectrochemical (PEC) water splitting require efficient carrier generation, separation, and transport at and beyond the space charge region (SCR) formed at the aqueous interface. The trade-off between photon collection and minority carrier delivery governs the photoelectrode design and implies maximum water splitting efficiency at an electrode thickness equivalent to the light absorption depth. Here, using planar ZnO thin films as a model system, we identify the photocarriers beyond the SCR as another significant source to substantially enhance the PEC performance. The high-quality ZnO films synthesized by pulsed laser deposition feature very few deep trap states and supportmore » a long photocarrier lifetime. Combined with photoelectrochemical characterization, ultrafast spectroscopy, and numerical calculations, it is revealed that engineering the exciton concentration gradient by film thickness facilitates the inward diffusion of photocarriers from the neighboring illuminated region to the SCR and, therefore, achieves a record high quantum efficiency over 80% at a thickness far beyond its light absorption depth and the SCR width. Furthermore, these results elucidate the important role of the photocarriers beyond SCR for the PEC process and provide new insight into exploring the full potential for efficient photoelectrode materials with large exciton diffusivity.« less

Four-Dimensional Ultrafast Electron Microscopy: Insights into an Emerging Technique.

PubMed

Adhikari, Aniruddha; Eliason, Jeffrey K; Sun, Jingya; Bose, Riya; Flannigan, David J; Mohammed, Omar F

2017-01-11

Four-dimensional ultrafast electron microscopy (4D-UEM) is a novel analytical technique that aims to fulfill the long-held dream of researchers to investigate materials at extremely short spatial and temporal resolutions by integrating the excellent spatial resolution of electron microscopes with the temporal resolution of ultrafast femtosecond laser-based spectroscopy. The ingenious use of pulsed photoelectrons to probe surfaces and volumes of materials enables time-resolved snapshots of the dynamics to be captured in a way hitherto impossible by other conventional techniques. The flexibility of 4D-UEM lies in the fact that it can be used in both the scanning (S-UEM) and transmission (UEM) modes depending upon the type of electron microscope involved. While UEM can be employed to monitor elementary structural changes and phase transitions in samples using real-space mapping, diffraction, electron energy-loss spectroscopy, and tomography, S-UEM is well suited to map ultrafast dynamical events on materials surfaces in space and time. This review provides an overview of the unique features that distinguish these techniques and also illustrates the applications of both S-UEM and UEM to a multitude of problems relevant to materials science and chemistry.

Ultrafast dynamic response of single crystal β-HMX

NASA Astrophysics Data System (ADS)

Zaug, Joseph M.; Armstrong, Michael R.; Crowhurst, Jonathan C.; Radousky, Harry B.; Ferranti, Louis; Swan, Raymond; Gross, Rick; Teslich, Nick E.; Wall, Mark A.; Austin, Ryan A.; Fried, Laurence E.

2017-01-01

We report results from ultrafast compression experiments conducted on β-HMX single crystals. Results consist of nominally 12 picosecond time-resolved wave profile data, (ultrafast time domain interferometry -TDI measurements), that were analyzed to determine high-velocity wave speeds as a function of piston velocity. TDI results are used to validate calculations of anisotropic stress-strain behavior of shocked loaded energetic materials. Our previous results derived using a 350 ps duration compression drive revealed anisotropic elastic wave response in single crystal β-HMX from (110) and (010) impact planes. Here we present results using a 1.05 ns duration compression drive with a 950 ps interferometry window to extend knowledge of the anisotropic dynamic response of β-HMX within eight microns of the initial impact plane. We observe two distinct wave profiles from (010) and three wave profiles from (010) impact planes. The (110) impact plane wave speeds typically exceed (010) impact plane wave speeds at the same piston velocities. The development of multiple hydrodynamic wave profiles begins at 20 GPa for the (110) impact plane and 28 GPa for the (10) impact plane. We compare our ultrafast TDI results with previous gun and plate impact results on β-HMX and PBX9501.

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.

High-power parametric amplification of 11.8-fs laser pulses with carrier-envelope phase control.

PubMed

Zinkstok, R Th; Witte, S; Hogervorst, W; Eikema, K S E

2005-01-01

Phase-stable parametric chirped-pulse amplification of ultrashort pulses from a carrier-envelope phase-stabilized mode-locked Ti:sapphire oscillator (11.0 fs) to 0.25 mJ/pulse at 1 kHz is demonstrated. Compression with a grating compressor and a LCD shaper yields near-Fourier-limited 11.8-fs pulses with an energy of 0.12 mJ. The amplifier is pumped by 532-nm pulses from a synchronized mode-locked laser, Nd:YAG amplifier system. This approach is shown to be promising for the next generation of ultrafast amplifiers aimed at producing terawatt-level phase-controlled few-cycle laser pulses.

Theory of spin and lattice wave dynamics excited by focused laser pulses

NASA Astrophysics Data System (ADS)

Shen, Ka; Bauer, Gerrit E. W.

2018-06-01

We develop a theory of spin wave dynamics excited by ultrafast focused laser pulses in a magnetic film. We take into account both the volume and surface spin wave modes in the presence of applied, dipolar and magnetic anisotropy fields and include the dependence on laser spot exposure size and magnetic damping. We show that the sound waves generated by local heating by an ultrafast focused laser pulse can excite a wide spectrum of spin waves (on top of a dominant magnon–phonon contribution). Good agreement with recent experiments supports the validity of the model.

Recent advances in multidimensional ultrafast spectroscopy

NASA Astrophysics Data System (ADS)

Oliver, Thomas A. A.

2018-01-01

Multidimensional ultrafast spectroscopies are one of the premier tools to investigate condensed phase dynamics of biological, chemical and functional nanomaterial systems. As they reach maturity, the variety of frequency domains that can be explored has vastly increased, with experimental techniques capable of correlating excitation and emission frequencies from the terahertz through to the ultraviolet. Some of the most recent innovations also include extreme cross-peak spectroscopies that directly correlate the dynamics of electronic and vibrational states. This review article summarizes the key technological advances that have permitted these recent advances, and the insights gained from new multidimensional spectroscopic probes.

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.

Time resolved 3D momentum imaging of ultrafast dynamics by coherent VUV-XUV radiation

DOE PAGES

Sturm, F. P.; Wright, T. W.; Ray, D.; ...

2016-06-14

Have we present a new experimental setup for measuring ultrafast nuclear and electron dynamics of molecules after photo-excitation and ionization. We combine a high flux femtosecond vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) source with an internally cold molecular beam and a 3D momentum imaging particle spectrometer to measure electrons and ions in coincidence. We describe a variety of tools developed to perform pump-probe studies in the VUV-XUV spectrum and to modify and characterize the photon beam. First benchmark experiments are presented to demonstrate the capabilities of the system.

Recent advances in multidimensional ultrafast spectroscopy

PubMed Central

2018-01-01

Multidimensional ultrafast spectroscopies are one of the premier tools to investigate condensed phase dynamics of biological, chemical and functional nanomaterial systems. As they reach maturity, the variety of frequency domains that can be explored has vastly increased, with experimental techniques capable of correlating excitation and emission frequencies from the terahertz through to the ultraviolet. Some of the most recent innovations also include extreme cross-peak spectroscopies that directly correlate the dynamics of electronic and vibrational states. This review article summarizes the key technological advances that have permitted these recent advances, and the insights gained from new multidimensional spectroscopic probes. PMID:29410844

Ultrafast hopping dynamics of 5f electrons in the Mott insulator UO₂ studied by femtosecond pump-probe spectroscopy.

PubMed

An, Yong Q; Taylor, Antoinette J; Conradson, Steven D; Trugman, Stuart A; Durakiewicz, Tomasz; Rodriguez, George

2011-05-20

We describe a femtosecond pump-probe study of ultrafast hopping dynamics of 5f electrons in the Mott insulator UO₂ following Mott-gap excitation at temperatures of 5-300 K. Hopping-induced response of the lattice and electrons is probed by transient reflectivity at mid- and above-gap photon energies, respectively. These measurements show an instantaneous hop, subsequent picosecond lattice deformation, followed by acoustic phonon emission and microsecond relaxation. Temperature-dependent studies indicate that the slow relaxation results from Hubbard excitons formed by U³⁺-U⁵⁺ pairs.

Ultrafast spin exchange-coupling torque via photo-excited charge-transfer processes

DOE PAGES

Ma, X.; Fang, F.; Li, Q.; ...

2015-10-28

In this study, optical control of spin is of central importance in the research of ultrafast spintronic devices utilizing spin dynamics at short time scales. Recently developed optical approaches such as ultrafast demagnetization, spin-transfer and spin-orbit torques open new pathways to manipulate spin through its interaction with photon, orbit, charge or phonon. However, these processes are limited by either the long thermal recovery time or the low-temperature requirement. Here we experimentally demonstrate ultrafast coherent spin precession via optical charge-transfer processes in the exchange-coupled Fe/CoO system at room temperature. The efficiency of spin precession excitation is significantly higher and the recoverymore » time of the exchange-coupling torque is much shorter than for the demagnetization procedure, which is desirable for fast switching. The exchange coupling is a key issue in spin valves and tunnelling junctions, and hence our findings will help promote the development of exchange-coupled device concepts for ultrafast coherent spin manipulation.« less

Several new directions for ultrafast fiber lasers [Invited].

PubMed

Fu, Walter; Wright, Logan G; Sidorenko, Pavel; Backus, Sterling; Wise, Frank W

2018-04-16

Ultrafast fiber lasers have the potential to make applications of ultrashort pulses widespread - techniques not only for scientists, but also for doctors, manufacturing engineers, and more. Today, this potential is only realized in refractive surgery and some femtosecond micromachining. The existing market for ultrafast lasers remains dominated by solid-state lasers, primarily Ti:sapphire, due to their superior performance. Recent advances show routes to ultrafast fiber sources that provide performance and capabilities equal to, and in some cases beyond, those of Ti:sapphire, in compact, versatile, low-cost devices. In this paper, we discuss the prospects for future ultrafast fiber lasers built on new kinds of pulse generation that capitalize on nonlinear dynamics. We focus primarily on three promising directions: mode-locked oscillators that use nonlinearity to enhance performance; systems that use nonlinear pulse propagation to achieve ultrashort pulses without a mode-locked oscillator; and multimode fiber lasers that exploit nonlinearities in space and time to obtain unparalleled control over an electric field.

Abbreviated breast magnetic resonance protocol: Value of high-resolution temporal dynamic sequence to improve lesion characterization.

PubMed

Oldrini, Guillaume; Fedida, Benjamin; Poujol, Julie; Felblinger, Jacques; Trop, Isabelle; Henrot, Philippe; Darai, Emile; Thomassin-Naggara, Isabelle

2017-10-01

To evaluate the added value of ULTRAFAST-MR sequence to an abbreviated FAST protocol in comparison with FULL protocol to distinguish benign from malignant lesions in a population of women, regardless of breast MR imaging indication. From March 10th to September 22th, 2014, we retrospectively included a total of 70 consecutive patients with 106 histologically proven lesions (58 malignant and 48 benign) who underwent breast MR imaging for preoperative breast staging (n=38), high-risk screening (n=7), problem solving (n=18), and nipple discharge (n=4) with 12 time resolved imaging of contrast kinetics (TRICKS) acquisitions during contrast inflow interleaved in a regular high-resolution dynamic MRI protocol (FULL protocol). Two readers scored MR exams as either positive or negative and described significant lesions according to Bi-RADS lexicon with a TRICKS images (ULTRAFAST), an abbreviated protocol (FAST) and all images (FULL protocol). Sensitivity, specificity, positive and negative predictive values, and accuracy were calculated for each protocol and compared with McNemar's test. For all readers, the combined FAST-ULTRAFAST protocol significantly improved the reading with a specificity of 83.3% and 70.8% in comparison with FAST protocol or FULL protocol, respectively, without change in sensitivity. By adding ULTRAFAST protocol to FAST protocol, readers 1 and 2 were able to correctly change the diagnosis in 22.9% (11/48) and 10.4% (5/48) of benign lesions, without missing any malignancy, respectively. Both interpretation and image acquisition times for combined FAST-ULTRAFAST protocol and FAST protocol were shorter compared to FULL protocol (p<0.001). Compared to FULL protocol, adding ULTRAFAST to FAST protocol improves specificity, mainly in correctly reclassifying benign masses and reducing interpretation and acquisition time, without decreasing sensitivity. Copyright © 2017 Elsevier B.V. All rights reserved.

Earle K. Plyler Prize Lecture: The Three Pillars of Ultrafast Molecular Science - Time, Phase, Intensity

NASA Astrophysics Data System (ADS)

Stolow, Albert

We discuss the probing and control of molecular wavepacket dynamics in the context of three main `pillars' of light-matter interaction: time, phase, intensity. Time: Using short, coherent laser pulses and perturbative matter-field interactions, we study molecular wavepackets with a focus on the ultrafast non-Born-Oppenheimer dynamics, that is, the coupling of electronic and nuclear motions. Time-Resolved Photoelectron Spectroscopy (TRPES) is a powerful ultrafast probe of these processes in polyatomic molecules because it is sensitive both electronic and vibrational dynamics. Ideally, one would like to observe these ultrafast processes from the molecule's point of view - the Molecular Frame - thereby avoiding loss of information due to orientational averaging. This can be achieved by Time-Resolved Coincidence Imaging Spectroscopy (TRCIS) which images 3D recoil vectors of both photofragments and photoelectrons, in coincidence and as a function of time, permitting direct Molecular Frame imaging of valence electronic dynamics during a molecular dynamics. Phase: Using intermediate strength non-perturbative interactions, we apply the second order (polarizability) Non-Resonant Dynamic Stark Effect (NRDSE) to control molecular dynamics without any net absorption of light. NRDSE is also the interaction underlying molecular alignment and applies to field-free 1D of linear molecules and field-free 3D alignment of general (asymmetric) molecules. Using laser alignment, we can transiently fix a molecule in space, yielding a more general approach to direct Molecular Frame imaging of valence electronic dynamics during a chemical reaction. Intensity: In strong (ionizing) laser fields, a new laser-matter physics emerges for polyatomic systems wherein both the single active electron picture and the adiabatic electron response, both implicit in the standard 3-step models, can fail dramatically. This has important consequences for all attosecond strong field spectroscopies of polyatomic molecules, including high harmonic generation (HHG). We discuss an experimental method, Channel-Resolved Above Threshold Ionization (CRATI), which directly unveils the electronic channels participating in the attosecond molecular strong field ionization response [10]. This work was supported by the National Research Council of Canada and the Natural Sciences & Engineering Research Council.

Conformational Dynamics Guides Coherent Exciton Migration in Conjugated Polymer Materials: First-Principles Quantum Dynamical Study

NASA Astrophysics Data System (ADS)

Binder, Robert; Lauvergnat, David; Burghardt, Irene

2018-06-01

We report on high-dimensional quantum dynamical simulations of photoinduced exciton migration in a single-chain oligothiophene segment, in view of elucidating the controversial nature of the elementary exciton transport steps in semiconducting polymers. A novel first-principles parametrized Frenkel J aggregate Hamiltonian is employed that goes significantly beyond the standard Frenkel-Holstein Hamiltonian. Departing from a nonequilibrium state created by photoexcitation, these simulations provide evidence of an ultrafast two-timescale process at low temperatures, involving exciton-polaron formation within tens of femtoseconds (fs), followed by torsional relaxation on an ˜400 fs timescale. The second step is the driving force for exciton migration, as initial conjugation breaks are removed by dynamical planarization. The quantum coherent nature of the elementary exciton migration step is consistent with experimental observations highlighting the correlated and vibrationally coherent nature of the dynamics on ultrafast timescales.

Conformational Dynamics Guides Coherent Exciton Migration in Conjugated Polymer Materials: First-Principles Quantum Dynamical Study.

PubMed

Binder, Robert; Lauvergnat, David; Burghardt, Irene

2018-06-01

We report on high-dimensional quantum dynamical simulations of photoinduced exciton migration in a single-chain oligothiophene segment, in view of elucidating the controversial nature of the elementary exciton transport steps in semiconducting polymers. A novel first-principles parametrized Frenkel J aggregate Hamiltonian is employed that goes significantly beyond the standard Frenkel-Holstein Hamiltonian. Departing from a nonequilibrium state created by photoexcitation, these simulations provide evidence of an ultrafast two-timescale process at low temperatures, involving exciton-polaron formation within tens of femtoseconds (fs), followed by torsional relaxation on an ∼400  fs timescale. The second step is the driving force for exciton migration, as initial conjugation breaks are removed by dynamical planarization. The quantum coherent nature of the elementary exciton migration step is consistent with experimental observations highlighting the correlated and vibrationally coherent nature of the dynamics on ultrafast timescales.

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.

Light-matter Interactions in Semiconductors and Metals: From Nitride Optoelectronics to Quantum Plasmonics

NASA Astrophysics Data System (ADS)

Narang, Prineha

This thesis puts forth a theory-directed approach coupled with spectroscopy aimed at the discovery and understanding of light-matter interactions in semiconductors and metals. The first part of the thesis presents the discovery and development of Zn-IV nitride materials. The commercial prominence in the optoelectronics industry of tunable semiconductor alloy materials based on nitride semiconductor devices, specifically InGaN, motivates the search for earth-abundant alternatives for use in efficient, high-quality optoelectronic devices. II-IV-N2 compounds, which are closely related to the wurtzite-structured III-N semiconductors, have similar electronic and optical properties to InGaN namely direct band gaps, high quantum efficiencies and large optical absorption coefficients. The choice of different group II and group IV elements provides chemical diversity that can be exploited to tune the structural and electronic properties through the series of alloys. The first theoretical and experimental investigation of the ZnSnxGe1--xN2 series as a replacement for III-nitrides is discussed here. The second half of the thesis shows ab-initio calculations for surface plasmons and plasmonic hot carrier dynamics. Surface plasmons, electromagnetic modes confined to the surface of a conductor-dielectric interface, have sparked renewed interest because of their quantum nature and their broad range of applications. The decay of surface plasmons is usually a detriment in the field of plasmonics, but the possibility to capture the energy normally lost to heat would open new opportunities in photon sensors, energy conversion devices and switching. A theoretical understanding of plasmon-driven hot carrier generation and relaxation dynamics in the ultrafast regime is presented here. Additionally calculations for plasmon-mediated upconversion as well as an energy-dependent transport model for these non-equilibrium carriers are shown. Finally, this thesis gives an outlook on the potential of non-equilibrium phenomena in metals and semiconductors for future light-based technologies.

Attosecond control of electrons emitted from a nanoscale metal tip.

PubMed

Krüger, Michael; Schenk, Markus; Hommelhoff, Peter

2011-07-06

Attosecond science is based on steering electrons with the electric field of well controlled femtosecond laser pulses. It has led to the generation of extreme-ultraviolet pulses with a duration of less than 100 attoseconds (ref. 3; 1 as = 10(-18) s), to the measurement of intramolecular dynamics (by diffraction of an electron taken from the molecule under scrutiny) and to ultrafast electron holography. All these effects have been observed with atoms or molecules in the gas phase. Electrons liberated from solids by few-cycle laser pulses are also predicted to show a strong light-phase sensitivity, but only very small effects have been observed. Here we report that the spectra of electrons undergoing photoemission from a nanometre-scale tungsten tip show a dependence on the carrier-envelope phase of the laser, with a current modulation of up to 100 per cent. Depending on the carrier-envelope phase, electrons are emitted either from a single sub-500-attosecond interval of the 6-femtosecond laser pulse, or from two such intervals; the latter case leads to spectral interference. We also show that coherent elastic re-scattering of liberated electrons takes place at the metal surface. Owing to field enhancement at the tip, a simple laser oscillator reaches the peak electric field strengths required for attosecond experiments at 100-megahertz repetition rates, rendering complex amplified laser systems dispensable. Practically, this work represents a simple, extremely sensitive carrier-envelope phase sensor, which could be shrunk in volume to about one cubic centimetre. Our results indicate that the attosecond techniques developed with (and for) atoms and molecules can also be used with solids. In particular, we foresee subfemtosecond, subnanometre probing of collective electron dynamics (such as plasmon polaritons) in solid-state systems ranging in scale from mesoscopic solids to clusters and to single protruding atoms. ©2011 Macmillan Publishers Limited. All rights reserved

Strong-Field Control of Laser Filamentation Mechanisms

NASA Astrophysics Data System (ADS)

Levis, Robert; Romanov, Dmitri; Filin, Aleskey; Compton, Ryan

2008-05-01

The propagation of short strong-file laser pulses in gas and solution phases often result in formation of filaments. This phenomenon involves many nonlinear processes including Kerr lensing, group velocity dispersion, multi-photon ionization, plasma defocusing, intensity clamping, and self-steepening. Of these, formation and dynamics of pencil-shape plasma areas plays a crucial role. The fundamental understanding of these laser-induced plasmas requires additional effort, because the process is highly nonlinear and complex. We studied the ultrafast laser-generated plasma dynamics both experimentally and theoretically. Ultrafast plasma dynamics was probed using Coherent Anti-Stokes Raman Scattering. The measurements were made in a room temperature gas maintained at 1 atm in a flowing cell. The time dependent scattering was measured by delaying the CARS probe with respect to the intense laser excitation pulse. A general trend is observed between the spacing of the ground state and the first allowed excited state with the rise time for the noble gas series and the molecular gases. This trend is consistent with our theoretical model, which considers the ultrafast dynamics of the strong field generated plasma as a three-step process; (i) strong-field ionization followed by the electron gaining considerable kinetic energy during the pulse; (ii) immediate post-pulse dynamics: fast thermalization, impact-ionization-driven electron multiplication and cooling; (iii) ensuing relaxation: evolution to electron-ion equilibrium and eventual recombination.

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.

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.

Ultrafast All-Optical Switching of Germanium-Based Flexible Metaphotonic Devices.

PubMed

Lim, Wen Xiang; Manjappa, Manukumara; Srivastava, Yogesh Kumar; Cong, Longqing; Kumar, Abhishek; MacDonald, Kevin F; Singh, Ranjan

2018-03-01

Incorporating semiconductors as active media into metamaterials offers opportunities for a wide range of dynamically switchable/tunable, technologically relevant optical functionalities enabled by strong, resonant light-matter interactions within the semiconductor. Here, a germanium-thin-film-based flexible metaphotonic device for ultrafast optical switching of terahertz radiation is experimentally demonstrated. A resonant transmission modulation depth of 90% is achieved, with an ultrafast full recovery time of 17 ps. An observed sub-picosecond decay constant of 670 fs is attributed to the presence of trap-assisted recombination sites in the thermally evaporated germanium film. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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.

Strong negative terahertz photoconductivity in photoexcited graphene

NASA Astrophysics Data System (ADS)

Fu, Maixia; Wang, Xinke; Ye, Jiasheng; Feng, Shengfei; Sun, Wenfeng; Han, Peng; Zhang, Yan

2018-01-01

Terahertz (THz) response of a chemical vapor deposited graphene on a quartz substrate has been investigated by using an ultrafast optical-pump THz-probe spectroscopy. Without photoexcitation, the frequency-dependence optical conductivity shows a strong carrier response owing to the intrinsically doped graphene. Upon photoexcitation, an enhancement in THz transmission is observed and the transmission increases nonlinearly with the increase of pump power, which is rooted in a reduction of intrinsic conductivity arising from the strong enhancement of carrier scattering rather than THz emission occurrence. The modulation depth of 18.8% was experimentally achieved, which is more than four times greater than that of the previous reported. The photoinduced response here highlights the variety of response possible in graphene depending on the sample quality, carrier mobility and doping level. The graphene provides promising applications in high-performance THz modulators and THz photoelectric devices.

Spin-orbit torque-driven skyrmion dynamics revealed by time-resolved X-ray microscopy

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

Woo, Seonghoon; Song, Kyung Mee; Han, Hee-Sung

Magnetic skyrmions are topologically protected spin textures with attractive properties suitable for high-density and low-power spintronic device applications. Much effort has been dedicated to understanding the dynamical behaviours of the magnetic skyrmions. However, experimental observation of the ultrafast dynamics of this chiral magnetic texture in real space, which is the hallmark of its quasiparticle nature, has so far remained elusive. Here, we report nanosecond-dynamics of a 100nm-diameter magnetic skyrmion during a current pulse application, using a time-resolved pump-probe soft X-ray imaging technique. We demonstrate that distinct dynamic excitation states of magnetic skyrmions, triggered by current-induced spin-orbit torques, can be reliablymore » tuned by changing the magnitude of spin-orbit torques. Our findings show that the dynamics of magnetic skyrmions can be controlled by the spin-orbit torque on the nanosecond time scale, which points to exciting opportunities for ultrafast and novel skyrmionic appl ications in the future.« less

Spin-orbit torque-driven skyrmion dynamics revealed by time-resolved X-ray microscopy

DOE PAGES

Woo, Seonghoon; Song, Kyung Mee; Han, Hee-Sung; ...

2017-05-24

Magnetic skyrmions are topologically protected spin textures with attractive properties suitable for high-density and low-power spintronic device applications. Much effort has been dedicated to understanding the dynamical behaviours of the magnetic skyrmions. However, experimental observation of the ultrafast dynamics of this chiral magnetic texture in real space, which is the hallmark of its quasiparticle nature, has so far remained elusive. Here, we report nanosecond-dynamics of a 100nm-diameter magnetic skyrmion during a current pulse application, using a time-resolved pump-probe soft X-ray imaging technique. We demonstrate that distinct dynamic excitation states of magnetic skyrmions, triggered by current-induced spin-orbit torques, can be reliablymore » tuned by changing the magnitude of spin-orbit torques. Our findings show that the dynamics of magnetic skyrmions can be controlled by the spin-orbit torque on the nanosecond time scale, which points to exciting opportunities for ultrafast and novel skyrmionic appl ications in the future.« less

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.

EDITORIAL: Ultrafast magnetization processes

NASA Astrophysics Data System (ADS)

Hillebrands, Burkard

2008-09-01

This Cluster Issue of Journal of Physics D: Applied Physics is devoted to ultrafast magnetization processes. It reports on the scientific yield of the Priority Programme 1133 'Ultrafast Magnetization Processes' which was funded by the Deutsche Forschungsgemeinschaft in the period 2002-2008 in three successive two-year funding periods, supporting research of 17-18 groups in Germany. Now, at the end of this Priority Programme, the members feel that the achievements made in the course of the programme merit communication to the international scientific community in a concerted way. Therefore, each of the projects of the last funding period presents a key result in a published contribution to this Cluster Issue. The purpose of the funding by a Priority Programme is to advance knowledge in an emerging field of research through collaborative networked support over several locations. Priority Programmes are characterized by their enhanced quality of research through the use of new methods and forms of collaboration in emerging fields, by added value through interdisciplinary cooperation, and by networking. The aim of the Priority Programme 1133 'Ultrafast Magnetization Processes' may be well characterized by the call for projects in June 2001 after the programme was approved by the Deutsche Forschungsgemeinschaft: 'The aim of the priority programme is the achievement of a basic understanding of the temporal evolution of fast magnetization processes in magnetically ordered films, multilayers and micro-structured systems. The challenge lies in the advancement of the field of ultrafast magnetization processes into the regime of a few femtoseconds to nanoseconds, a topic not yet well explored. A general aim is to understand the fundamental mechanisms needed for applications in ultrafast magneto-electronic devices. The fundamental topic to be addressed is the response of the magnetization of small structures upon the application of pulsed magnetic fields, laser pulses or injected spin-polarized electron pulses on short time scales, ranging from a small disturbance of the system up to the reversal of the magnetization direction.' Now, seven years later, the subject of ultrafast magnetization processes has grown into a mainstream research direction in modern magnetism. The major international conferences on magnetism, such as the Annual Conference on Magnetism and Magnetic Materials (MMM), the INTERMAG, the International Conference of Magnetism, as well as many regional conferences, schedule dedicated sessions to ultrafast magnetization processes, very often several of them. The large share in research in this field from German scientists has been made possible by this Priority Programme. Since its beginning, new developments have been picked up by the Priority Programme 1133 and addressed by projects. Spin torque phenomena in spin dynamics, although foreseen at the time of establishing the Priority Programme, have been taken up. The field of dissipation has been addressed and extended by several groups, with contributions both from theoretical and experimental groups. A first set of contributions addresses ultrafast dynamics and materials. T Roth et al [article 164001] in this issue] study the dynamics of coercivity in ultrafast pump-probe experiments on the femtosecond time scale. They show that an all optical pump-probe technique is, in general, not suitable for gaining access to the time-dependent behaviour of the coercivity, since the switching in a fixed external field is an irreversible process. They comment on the possible mechanisms leading to the observed reduction of the coercivity with increasing pump power and propose a potential solution to clarify the origin of such a behaviour. B Heitkamp et al [164002] discuss the femtosecond spin dynamics of ferromagnetic CoPt thin films and nanodots, which they probe using spin-polarized photoemission electron microscopy. They show by photoelectron spin analysis, that enhanced optical near fields can be used to induce a local demagnetization of the sample following femtosecond laser excitation. A B Schmidt et al [164003] report a new access to the surface electronic structure of fcc Co films combining spin-resolved one- and two-photon photoemission. The knowledge of surface states is important for interpreting time-resolved measurements of ultrafast magnetization dynamics in this material. An extension of ultrafast dynamics has been made by several groups. A Melnikov et al [164004] report on the ultrafast dynamics at lanthanide surfaces such as Gd(0001) and Tb(0001) using time-resolved second-harmonic generation and photoelectron spectroscopy. These surfaces exhibit a rich dynamics including a collective response of the crystal lattice and the magnetization. Effects of phonon-magnon scattering are discussed. M Fiebig et al [164005] report on experiments of ultrafast magnetization dynamics in antiferromagnetic compounds, and show that the magnetization dynamics in these systems differs noticeably from that of ferromagnetic compounds. They use optical second-harmonic generation and linear reflection to monitor the evolution of the antiferromagnetic order parameter subsequent to an intense optical excitation. In a theory paper, the local light-induced spin manipulation in two-magnetic-centre metallic chains is studied by T Hardenstein et al [164006] using highly correlational ab initio calculations. They show that, as an example of local spin manipulation, the spin on the iron side of a Co-Na-Fe cluster can be switched. S Halm et al [164007] present evidence to manipulate spin states in a diluted magnetic semiconductor on a submicrometer length scale via the magnetic fringe fields of micro-structured magnets. By optically switching the magnetization of the ferromagnet, the magnetization in the semiconductor is manipulated and the limits of a dynamical interaction between the spin states in the ferromagnet and the magnetic semiconductor are discussed. A second set of contributions addresses the field of spin waves and dynamic spin torque phenomena. C W Sandweg et al [164008] discuss the modification of the thermal spin wave spectrum by a domain wall in a narrow stripe and report the observation of a localized mode near the domain wall using the new technique of Brillouin light scattering microscopy. Time-resolved measurements are often made using a stroboscopic approach, thus missing non-periodic responses. P Möhrke et al [164009] report single-shot Kerr magnetometer measurements to observe the real time-domain wall motion in permalloy nanowires. The dynamics in magnetic disks is studied by I Neudecker et al [164010] using in-plane magnetic microwave fields for excitation. The effect of current-induced magnetization dynamics in single and double layer magnetic nanopillars is reported by N Müsgens et al [164011]. A spin-polarized charge current can modify the damping properties of spin waves in magnetic nanostructures. This is reported by V E Demidov et al [164012] using space-resolved Brillouin light scattering. They also present results regarding nonlinear spin-wave propagation and mode coupling in magnetic stripes and squares. D V Berkov and N L Gorn [164013] report on their results of nonlinear magnetization dynamics in nanodevices induced by a spin-polarized current using micromagnetic simulation. A third set of contributions focuses on dissipation phenomena ranging from a phenomenological description to the investigation of the microscopic origin(s). In a theory paper, M Fähnle et al [164014] revisit the Gilbert equation and discuss anisotropic and non-local damping of the magnetization dynamics. They derive their results by a combination of the breathing Fermi surface model with a variant of the ab initio density functional electron theory given by the magnetic force theorem. On the experimental side, S Serrano-Guisan et al [164015] address Gilbert damping in Ni81Fe19 thin films and microstructures using anisotropic magnetoresistance and pulsed inductive microwave magnetometry to measure the time-resolved precessional magnetization dynamics. The intrinsic and non-local Gilbert damping in polycrystalline Ni films is also addressed by J Walowski et al [164016] using femtosecond laser pulses. Several spin-wave modes are observed and their dissipation is studied. Non-local damping by spin currents emitted into a non-magnetic metallic layer of either vanadium, palladium or dysprosium is studied. Dissipation in small magnetic Ni81Fe19 rings is studied using Brillouin light scattering microscopy by H Schultheiss et al [164017]. They investigate the spatial profiles and the decay constants of spin-wave quasi-eigenmodes. We hope that this cluster of papers will help to stimulate and advance a better understanding of this very interesting field of ultrafast magnetization processes.

Desolvation of polymers by ultrafast heating: Influence of hydrophilicity

NASA Astrophysics Data System (ADS)

Sun, Si Neng; Urbassek, Herbert M.

2010-10-01

Using molecular-dynamics simulation, we investigate the consequences of ultrafast laser-induced heating of a small water droplet containing a solvated polymer. Two polymers are studied: polyethylene as an example of a hydrophobic, and polyketone as an example of a hydrophilic polymer. In both cases, when the droplet is heated below the critical temperature of water, strong water evaporation is started, but the polymer remains in contact with a central water cluster. However, upon heating beyond the critical temperature, the hydrophilic polyethylene becomes completely desolvated, while polyketone still remains solvated. We analyze this behavior in terms of the intermolecular interactions and of the expansion dynamics of the heated droplet.

Ultrafast dynamics of localized magnetic moments in the unconventional Mott insulator Sr 2IrO 4

DOE PAGES

Krupin, O.; Dakovski, G. L.; Kim, B. J.; ...

2016-06-16

Here, we report a time-resolved study of the ultrafast dynamics of the magnetic moments formed by themore » $${{J}_{\\text{eff}}}=1/2$$ states in Sr 2IrO 4 by directly probing the localized iridium 5d magnetic state through resonant x-ray diffraction. Using optical pump–hard x-ray probe measurements, two relaxation time scales were determined: a fast fluence-independent relaxation is found to take place on a time scale of 1.5 ps, followed by a slower relaxation on a time scale of 500 ps–1.5 ns.« less

Ultrashort electron pulses as a four-dimensional diagnosis of plasma dynamics.

PubMed

Zhu, P F; Zhang, Z C; Chen, L; Li, R Z; Li, J J; Wang, X; Cao, J M; Sheng, Z M; Zhang, J

2010-10-01

We report an ultrafast electron imaging system for real-time examination of ultrafast plasma dynamics in four dimensions. It consists of a femtosecond pulsed electron gun and a two-dimensional single electron detector. The device has an unprecedented capability of acquiring a high-quality shadowgraph image with a single ultrashort electron pulse, thus permitting the measurement of irreversible processes using a single-shot scheme. In a prototype experiment of laser-induced plasma of a metal target under moderate pump intensity, we demonstrated its unique capability of acquiring high-quality shadowgraph images on a micron scale with a-few-picosecond time resolution.

Ultrafast core-loss spectroscopy in four-dimensional electron microscopy

PubMed Central

van der Veen, Renske M.; Penfold, Thomas J.; Zewail, Ahmed H.

2015-01-01

We demonstrate ultrafast core-electron energy-loss spectroscopy in four-dimensional electron microscopy as an element-specific probe of nanoscale dynamics. We apply it to the study of photoexcited graphite with femtosecond and nanosecond resolutions. The transient core-loss spectra, in combination with ab initio molecular dynamics simulations, reveal the elongation of the carbon-carbon bonds, even though the overall behavior is a contraction of the crystal lattice. A prompt energy-gap shrinkage is observed on the picosecond time scale, which is caused by local bond length elongation and the direct renormalization of band energies due to temperature-dependent electron–phonon interactions. PMID:26798793

Resolving Nonadiabatic Dynamics of Hydrated Electrons Using Ultrafast Photoemission Anisotropy.

PubMed

Karashima, Shutaro; Yamamoto, Yo-Ichi; Suzuki, Toshinori

2016-04-01

We have studied ultrafast nonadiabatic dynamics of excess electrons trapped in the band gap of liquid water using time- and angle-resolved photoemission spectroscopy. Anisotropic photoemission from the first excited state was discovered, which enabled unambiguous identification of nonadiabatic transition to the ground state in 60 fs in H_{2}O and 100 fs in D_{2}O. The photoelectron kinetic energy distribution exhibited a rapid spectral shift in ca. 20 fs, which is ascribed to the librational response of a hydration shell to electronic excitation. Photoemission anisotropy indicates that the electron orbital in the excited state is depolarized in less than 40 fs.

Carrier multiplication detected through transient photocurrent in device-grade films of lead selenide quantum dots

DOE PAGES

Gao, Jianbo; Fidler, Andrew F.; Klimov, Victor I.

2015-09-08

In carrier multiplication, the absorption of a single photon results in two or more electron–hole pairs. Quantum dots are promising materials for implementing carrier multiplication principles in real-life technologies. So far, however, most of research in this area has focused on optical studies of solution samples with yet to be proven relevance to practical devices. We report ultra-fast electro-optical studies of device-grade films of electronically coupled quantum dots that allow us to observe multiplication directly in the photocurrent. Our studies help rationalize previous results from both optical spectroscopy and steady-state photocurrent measurements and also provide new insights into effects ofmore » electric field and ligand treatments on multiexciton yields. Importantly, we demonstrate that using appropriate chemical treatments of the films, extra charges produced by carrier multiplication can be extracted from the quantum dots before they are lost to Auger recombination and hence can contribute to photocurrent of practical devices.« less

Carrier multiplication detected through transient photocurrent in device-grade films of lead selenide quantum dots

PubMed Central

Gao, Jianbo; Fidler, Andrew F.; Klimov, Victor I.

2015-01-01

In carrier multiplication, the absorption of a single photon results in two or more electron–hole pairs. Quantum dots are promising materials for implementing carrier multiplication principles in real-life technologies. So far, however, most of research in this area has focused on optical studies of solution samples with yet to be proven relevance to practical devices. Here we report ultrafast electro-optical studies of device-grade films of electronically coupled quantum dots that allow us to observe multiplication directly in the photocurrent. Our studies help rationalize previous results from both optical spectroscopy and steady-state photocurrent measurements and also provide new insights into effects of electric field and ligand treatments on multiexciton yields. Importantly, we demonstrate that using appropriate chemical treatments of the films, extra charges produced by carrier multiplication can be extracted from the quantum dots before they are lost to Auger recombination and hence can contribute to photocurrent of practical devices. PMID:26345390

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.

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.

Effect of wetting-layer density of states on the gain and phase recovery dynamics of quantum-dot semiconductor optical amplifiers

NASA Astrophysics Data System (ADS)

Kim, Jungho; Yu, Bong-Ahn

2015-03-01

We numerically investigate the effect of the wetting-layer (WL) density of states on the gain and phase recovery dynamics of quantum-dot semiconductor optical amplifiers in both electrical and optical pumping schemes by solving 1088 coupled rate equations. The temporal variations of the ultrafast gain and phase recovery responses at the ground state (GS) are calculated as a function of the WL density of states. The ultrafast gain recovery responses do not significantly depend on the WL density of states in the electrical pumping scheme and the three optical pumping schemes such as the optical pumping to the WL, the optical pumping to the excited state ensemble, and the optical pumping to the GS ensemble. The ultrafast phase recovery responses are also not significantly affected by the WL density of states except the optical pumping to the WL, where the phase recovery component caused by the WL becomes slowed down as the WL density of states increases.

Ultrafast X-ray Auger probing of photoexcited molecular dynamics

DOE PAGES

McFarland, B. K.; Farrell, J. P.; Miyabe, S.; ...

2014-06-23

Here, molecules can efficiently and selectively convert light energy into other degrees of freedom. Disentangling the underlying ultrafast motion of electrons and nuclei of the photoexcited molecule presents a challenge to current spectroscopic approaches. Here we explore the photoexcited dynamics of molecules by an interaction with an ultrafast X-ray pulse creating a highly localized core hole that decays via Auger emission. We discover that the Auger spectrum as a function of photoexcitation—X-ray-probe delay contains valuable information about the nuclear and electronic degrees of freedom from an element-specific point of view. For the nucleobase thymine, the oxygen Auger spectrum shifts towardsmore » high kinetic energies, resulting from a particular C–O bond stretch in the ππ* photoexcited state. A subsequent shift of the Auger spectrum towards lower kinetic energies displays the electronic relaxation of the initial photoexcited state within 200 fs. Ab-initio simulations reinforce our interpretation and indicate an electronic decay to the nπ* state.« less

Materials Properties and Solvated Electron Dynamics of Isolated Nanoparticles and Nanodroplets Probed with Ultrafast Extreme Ultraviolet Beams.

PubMed

Ellis, Jennifer L; Hickstein, Daniel D; Xiong, Wei; Dollar, Franklin; Palm, Brett B; Keister, K Ellen; Dorney, Kevin M; Ding, Chengyuan; Fan, Tingting; Wilker, Molly B; Schnitzenbaumer, Kyle J; Dukovic, Gordana; Jimenez, Jose L; Kapteyn, Henry C; Murnane, Margaret M

2016-02-18

We present ultrafast photoemission measurements of isolated nanoparticles in vacuum using extreme ultraviolet (EUV) light produced through high harmonic generation. Surface-selective static EUV photoemission measurements were performed on nanoparticles with a wide array of compositions, ranging from ionic crystals to nanodroplets of organic material. We find that the total photoelectron yield varies greatly with nanoparticle composition and provides insight into material properties such as the electron mean free path and effective mass. Additionally, we conduct time-resolved photoelectron yield measurements of isolated oleylamine nanodroplets, observing that EUV photons can create solvated electrons in liquid nanodroplets. Using photoemission from a time-delayed 790 nm pulse, we observe that a solvated electron is produced in an excited state and subsequently relaxes to its ground state with a lifetime of 151 ± 31 fs. This work demonstrates that femotosecond EUV photoemission is a versatile surface-sensitive probe of the properties and ultrafast dynamics of isolated nanoparticles.

Alternative Electron-Transfer Channels Ensure Ultrafast Deactivation of Light-Induced Excited States in Riboflavin Binding Protein.

PubMed

Zanetti-Polzi, Laura; Aschi, Massimiliano; Amadei, Andrea; Daidone, Isabella

2017-07-20

Flavoproteins, containing flavin chromophores, are enzymes capable of transferring electrons at very high speeds. The ultrafast photoinduced electron-transfer (ET) kinetics of riboflavin binding protein to the excited riboflavin was studied by femtosecond spectroscopy and found to occur within a few hundred femtoseconds [ Zhong and Zewail, Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 11867-11872 ]. This ultrafast kinetics was attributed to the presence of two aromatic rings that could transfer the electron to riboflavin: the side chains of tryptophan 156 and tyrosine 75. However, the underlying ET mechanism remained unclear. Here, using a hybrid quantum mechanical-molecular dynamics approach, we perform ET dynamics simulations taking into account the motion of the protein and the solvent upon ET. This approach reveals that ET occurs via a major reaction channel involving tyrosine 75 (83%) and a minor one involving tryptophan 156 (17%). We also show that the protein environment is designed to ensure the fast quenching of the riboflavin excited state.

4D imaging of transient structures and morphologies in ultrafast electron microscopy.

PubMed

Barwick, Brett; Park, Hyun Soon; Kwon, Oh-Hoon; Baskin, J Spencer; Zewail, Ahmed H

2008-11-21

With advances in spatial resolution reaching the atomic scale, two-dimensional (2D) and 3D imaging in electron microscopy has become an essential methodology in various fields of study. Here, we report 4D imaging, with in situ spatiotemporal resolutions, in ultrafast electron microscopy (UEM). The ability to capture selected-area-image dynamics with pixel resolution and to control the time separation between pulses for temporal cooling of the specimen made possible studies of fleeting structures and morphologies. We demonstrate the potential for applications with two examples, gold and graphite. For gold, after thermally induced stress, we determined the atomic structural expansion, the nonthermal lattice temperature, and the ultrafast transients of warping/bulging. In contrast, in graphite, striking coherent transients of the structure were observed in both image and diffraction, directly measuring, on the nanoscale, the longitudinal resonance period governed by Young's elastic modulus. The success of these studies demonstrates the promise of UEM in real-space imaging of dynamics.

Localized Excited Charge Carriers Generate Ultrafast Inhomogeneous Strain in the Multiferroic BiFeO3

DTIC Science & Technology

2014-03-03

Crane, Y.-H. Chu, M. B. Holcomb, M. Gajek , M. Huijben, C.-H. Yang, N. Balke, and R. Ramesh, J. Phys. Condens. Matter 20, 434220 (2008). [4] N. A. Hill...L. W. Martin, Y. H. Chu, M. Gajek , R. Ramesh, R. C. Rai, X. Xu, and J. L. Musfeldt, Appl. Phys. Lett. 92, 091905 (2008). [27] F. Zamponi, Z. Ansari, C

Ultrafast High Harmonic, Soft X-Ray Probing of Molecular Dynamics

DTIC Science & Technology

2013-04-30

590 L/s scroll pump and a titanium sublimation pump . A TOF-PES has been designed and constructed to analyze the energy of the photoelectrons...are studied using the quasi-continuous vacuum ultraviolet light of the Advanced Light Source at Lawrence Berkeley National Laboratory. The molecular...34), the method of high order harmonic generation of ultrashort vacuum ultraviolet pulses was used to investigate molecular photodissociation, ultrafast

Ultrafast Magnetization Manipulation Using Single Femtosecond Light and Hot-Electron Pulses.

PubMed

Xu, Yong; Deb, Marwan; Malinowski, Grégory; Hehn, Michel; Zhao, Weisheng; Mangin, Stéphane

2017-11-01

Current-induced magnetization manipulation is a key issue for spintronic applications. This manipulation must be fast, deterministic, and nondestructive in order to function in device applications. Therefore, single- electronic-pulse-driven deterministic switching of the magnetization on the picosecond timescale represents a major step toward future developments of ultrafast spintronic systems. Here, the ultrafast magnetization dynamics in engineered Gd x [FeCo] 1- x -based structures are studied to compare the effect of femtosecond laser and hot-electron pulses. It is demonstrated that a single femtosecond hot-electron pulse causes deterministic magnetization reversal in either Gd-rich and FeCo-rich alloys similarly to a femtosecond laser pulse. In addition, it is shown that the limiting factor of such manipulation for perpendicular magnetized films arises from the formation of a multidomain state due to dipolar interactions. By performing time-resolved measurements under various magnetic fields, it is demonstrated that the same magnetization dynamics are observed for both light and hot-electron excitation, and that the full magnetization reversal takes place within 40 ps. The efficiency of the ultrafast current-induced magnetization manipulation is enhanced due to the ballistic transport of hot electrons before reaching the GdFeCo magnetic layer. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Polar rotor scattering as atomic-level origin of low mobility and thermal conductivity of perovskite CH3NH3PbI3

PubMed Central

Li, Bing; Kawakita, Yukinobu; Liu, Yucheng; Wang, Mingchao; Matsuura, Masato; Shibata, Kaoru; Ohira-Kawamura, Seiko; Yamada, Takeshi; Lin, Shangchao; Nakajima, Kenji; Liu, Shengzhong (Frank)

2017-01-01

Perovskite CH3NH3PbI3 exhibits outstanding photovoltaic performances, but the understanding of the atomic motions remains inadequate even though they take a fundamental role in transport properties. Here, we present a complete atomic dynamic picture consisting of molecular jumping rotational modes and phonons, which is established by carrying out high-resolution time-of-flight quasi-elastic and inelastic neutron scattering measurements in a wide energy window ranging from 0.0036 to 54 meV on a large single crystal sample, respectively. The ultrafast orientational disorder of molecular dipoles, activated at ∼165 K, acts as an additional scattering source for optical phonons as well as for charge carriers. It is revealed that acoustic phonons dominate the thermal transport, rather than optical phonons due to sub-picosecond lifetimes. These microscopic insights provide a solid standing point, on which perovskite solar cells can be understood more accurately and their performances are perhaps further optimized. PMID:28665407

Coherent ultrafast spin-dynamics probed in three dimensional topological insulators

PubMed Central

Boschini, F.; Mansurova, M.; Mussler, G.; Kampmeier, J.; Grützmacher, D.; Braun, L.; Katmis, F.; Moodera, J. S.; Dallera, C.; Carpene, E.; Franz, C.; Czerner, M.; Heiliger, C.; Kampfrath, T.; Münzenberg, M.

2015-01-01

Topological insulators are candidates to open up a novel route in spin based electronics. Different to traditional ferromagnetic materials, where the carrier spin-polarization and magnetization are based on the exchange interaction, the spin properties in topological insulators are based on the coupling of spin- and orbit interaction connected to its momentum. Specific ways to control the spin-polarization with light have been demonstrated: the energy momentum landscape of the Dirac cone provides spin-momentum locking of the charge current and its spin. We investigate a spin-related signal present only during the laser excitation studying real and imaginary part of the complex Kerr angle by disentangling spin and lattice contributions. This coherent signal is only present at the time of the pump-pulses’ light field and can be described in terms of a Raman coherence time. The Raman transition involves states at the bottom edge of the conduction band. We demonstrate a coherent femtosecond control of spin-polarization for electronic states at around the Dirac cone. PMID:26510509

Carrier-envelope offset frequency stabilization of an ultrafast semiconductor laser

NASA Astrophysics Data System (ADS)

Jornod, Nayara; Gürel, Kutan; Wittwer, Valentin J.; Brochard, Pierre; Hakobyan, Sargis; Schilt, Stéphane; Waldburger, Dominik; Keller, Ursula; Südmeyer, Thomas

2018-02-01

We present the self-referenced stabilization of the carrier-envelope offset (CEO) frequency of a semiconductor disk laser. The laser is a SESAM-modelocked VECSEL emitting at a wavelength of 1034 nm with a repetition frequency of 1.8 GHz. The 270-fs pulses are amplified to 3 W and compressed to 120 fs for the generation of a coherent octavespanning supercontinuum spectrum. A quasi-common-path f-to-2f interferometer enables the detection of the CEO beat with a signal-to-noise ratio of 30 dB sufficient for its frequency stabilization. The CEO frequency is phase-locked to an external reference with a feedback signal applied to the pump current.

Effect of 1.5 MeV electron irradiation on β-Ga2O3 carrier lifetime and diffusion length

NASA Astrophysics Data System (ADS)

Lee, Jonathan; Flitsiyan, Elena; Chernyak, Leonid; Yang, Jiancheng; Ren, Fan; Pearton, Stephen J.; Meyler, Boris; Salzman, Y. Joseph

2018-02-01

The influence of 1.5 MeV electron irradiation on minority transport properties of Si doped β-Ga2O3 vertical Schottky rectifiers was observed for fluences up to 1.43 × 1016 cm-2. The Electron Beam-Induced Current technique was used to determine the minority hole diffusion length as a function of temperature for each irradiation dose. This revealed activation energies related to shallow donors at 40.9 meV and radiation-induced defects with energies at 18.1 and 13.6 meV. Time-resolved cathodoluminescence measurements showed an ultrafast 210 ps decay lifetime and reduction in carrier lifetime with increased irradiation.

Single-shot ultrafast tomographic imaging by spectral multiplexing

NASA Astrophysics Data System (ADS)

Matlis, N. H.; Axley, A.; Leemans, W. P.

2012-10-01

Computed tomography has profoundly impacted science, medicine and technology by using projection measurements scanned over multiple angles to permit cross-sectional imaging of an object. The application of computed tomography to moving or dynamically varying objects, however, has been limited by the temporal resolution of the technique, which is set by the time required to complete the scan. For objects that vary on ultrafast timescales, traditional scanning methods are not an option. Here we present a non-scanning method capable of resolving structure on femtosecond timescales by using spectral multiplexing of a single laser beam to perform tomographic imaging over a continuous range of angles simultaneously. We use this technique to demonstrate the first single-shot ultrafast computed tomography reconstructions and obtain previously inaccessible structure and position information for laser-induced plasma filaments. This development enables real-time tomographic imaging for ultrafast science, and offers a potential solution to the challenging problem of imaging through scattering surfaces.

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.

Scanning ultrafast electron microscopy.

PubMed

Yang, Ding-Shyue; Mohammed, Omar F; Zewail, Ahmed H

2010-08-24

Progress has been made in the development of four-dimensional ultrafast electron microscopy, which enables space-time imaging of structural dynamics in the condensed phase. In ultrafast electron microscopy, the electrons are accelerated, typically to 200 keV, and the microscope operates in the transmission mode. Here, we report the development of scanning ultrafast electron microscopy using a field-emission-source configuration. Scanning of pulses is made in the single-electron mode, for which the pulse contains at most one or a few electrons, thus achieving imaging without the space-charge effect between electrons, and still in ten(s) of seconds. For imaging, the secondary electrons from surface structures are detected, as demonstrated here for material surfaces and biological specimens. By recording backscattered electrons, diffraction patterns from single crystals were also obtained. Scanning pulsed-electron microscopy with the acquired spatiotemporal resolutions, and its efficient heat-dissipation feature, is now poised to provide in situ 4D imaging and with environmental capability.

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.

The Evolution of Surface Symmetry in Femtosecond Laser-Induced Transient States of Matter

NASA Astrophysics Data System (ADS)

Garnett, Joy Carleen

Gallium arsenide and other III-V materials are well known for their excellent optical and electronic properties and have led to the development of high-performance photovoltaic cells1,2, photoelectrochemical water splitting3,4, and light emitting diodes (LEDs)5. Several combinations of III-V semiconductors are now being considered as potentially attractive alternatives to silicon for these applications. However, further development requires fundamental understanding of processes that govern light-matter interactions. Specifically, surface strain and ultrafast dynamics are of great interest to the optoelectronic industry. Strained semiconductor surfaces dominate the design of optoelectronics and III-V semiconductor-based LEDs. Currently, the structures of strained surfaces are well characterized with x-ray diffraction (XRD)6 and electron crystallography7-9. However, optically-induced electronic behavior at these interfaces are not fully understood. This has the been one of the stimulants for the research in this dissertation. To further explore opticallyinduced electronic behavior at strained interfaces, I have asked the following questions: 1. How does static optoelectronic behavior change as a function of strain? 2. How does surface symmetry and electronic motion change with respect to strain? 3. How do atomic bonds change as a function of strain? Another main research goal of this work is to understand ultrafast subpicosecond processes after pulsed laser excitation. The knowledge of ultrafast processes dominates the design of devices in industries that require high temporal and spectral resolution. Ultrafast atomic motion has been the major focus of subpicosecond structural dynamics. Currently, these dynamics upon photoexcitation are well characterized with experimental methods such as ultrafast x-ray diffraction (U-XRD), ultrafast electron diffraction (UED), and ultrafast electron crystallography (UEC). However, ultrafast atomic motion does not occur alone. The bonds connecting these moving atoms are also affected during this process. The correlation between structural and electronic dynamics is not well understood. To further explore correlated structural and electronic behavior upon ultrafast laser excitation, I have asked the following questions: 1. How does subpicosecond optoelectronic behavior change as a function of time after femtosecond pulse photoexcitation? 2. How does subpicosecond surface symmetry and electronic motion change with respect to time after femtosecond pulse photoexcitation? 3. How do atomic bonds change as a function of time after femtosecond pulse photoexcitation? To address these questions, I used experimental methods sensitive to both atomic motions and electronic responses: polarization-resolved second harmonic generation (PRSHG) and timeresolved, polarization-resolved second harmonic generation (TRPRSHG). The dissertation covers application of these techniques to III-V semiconductors: gallium arsenide (GaAs), gallium antimonide (GaSb), and aluminum gallium arsenide (AlGaAs). This dissertation is organized as follows. Chapter 2 presents the background of electronic band structures, ultrafast relaxation processes, and the origin of nonlinear optics from the perspectives of classical and quantum mechanics. It thus provides a framework for the static and transient nonlinear optical processes observed in III-V semiconductors under ultrafast pulse excitation. Next, Chapter 3 motivates the use of the experimental and analytical methods as applied to the experimental and theoretical studies outlined in Chapters 4 and 5. Chapter 4 is devoted to the understanding of polarization-resolved second-order nonlinear optical responses of various strained III-V semiconductor heterostructures resulting from defect-conducive growth conditions. Simplified phenomenological expressions for the polarization-resolved second harmonic generation (PRSHG) are first derived using tensor analysis. Afterwards, these expressions are used to fit experimental data. The developed formalism is tested under different conditions to gauge the fit robustness and sensitivity to mechanical and electronic changes in strained IIIV semiconductors. Along that same vein, Chapter 5 extends this analytical fit to describe ultrafast PRSHG responses of GaAs (100) as a function of transient changes in the interatomic potential within the first picosecond after photoexcitation. Finally, the dissertation concludes with Chapter 6 addressing possible directions for future work. The chapter begins with a description of studies to further test the sensitivity and robustness of the PRSHG phenomenological fit and how it can be used to characterize more classes of materials.

Electron and hole dynamics in the electronic and structural phase transitions of VO2

NASA Astrophysics Data System (ADS)

Haglund, Richard

2015-03-01

The ultrafast, optically induced insulator-to-metal transition (IMT) and the associated structural phase transition (SPT) in vanadium dioxide (VO2) have been studied for over a decade. However, only recently have effects due to the combined presence of electron-hole pairs and injected electrons been observed. Here we compare and contrast IMT dynamics when both hot electrons and optically excited electron-hole pairs are involved, in (1) thin films of VO2 overlaid by a thin gold foil, in which hot electrons are generated by 1.5 eV photons absorbed in the foil and accelerated through the VO2 by an applied electric field; (2) VO2 nanoparticles covered with a sparse mesh of gold nanoparticles averaging 20-30 nm in diameter in which hot electrons are generated by resonant excitation and decay of the localized surface plasmon; and (3) bare VO2 thin films excited by intense near-single-cycle THz pulses. In the first case, the IMT is driven by excitation of the bulk gold plasmon, and the SPT appears on a few-picosecond time scale. In the second case, density-functional calculations indicate that above a critical carrier density, the addition of a single electron to a 27-unit supercell drives the catastrophic collapse of the coherent phonon associated with, and leading to, the SPT. In the third case, sub-bandgap-energy photons (approximately 0.1 eV) initiate the IMT, but exhibit the same sub-100 femtosecond switching time and coherent phonon dynamics as observed when the IMT is initiated by 1.5 eV photons. This suggests that the underlying mechanism must be quite different, possibly THz-field induced interband tunneling of spatially separated electron-hole pairs. The implications of these findings for ultrafast switching in opto-electronic devices - such as hybrid VO2 silicon ring resonators - are briefly considered. Support from the National Science Foundation (DMR-1207407), the Office of Science, U.S. Department of Energy (DE-FG02-01ER45916) and the Defense Threat-Reduction Agency (HDTRA1-10-1-0047) for these studies is gratefully acknowledged.

Circularly polarized attosecond pulse generation and applications to ultrafast magnetism

NASA Astrophysics Data System (ADS)

Bandrauk, André D.; Guo, Jing; Yuan, Kai-Jun

2017-12-01

Attosecond science is a growing new field of research and potential applications which relies on the development of attosecond light sources. Achievements in the generation and application of attosecond pulses enable to investigate electron dynamics in the nonlinear nonperturbative regime of laser-matter interactions on the electron’s natural time scale, the attosecond. In this review, we describe the generation of circularly polarized attosecond pulses and their applications to induce attosecond magnetic fields, new tools for ultrafast magnetism. Simulations are performed on aligned one-electron molecular ions by using nonperturbative nonlinear solutions of the time-dependent Schrödinger equation. We discuss how bichromatic circularly polarized laser pulses with co-rotating or counter-rotating components induce electron-parent ion recollisions, thus producing circularly polarized high-order harmonic generation, the source of circularly polarized attosecond pulses. Ultrafast quantum electron currents created by the generated attosecond pulses give rise to attosecond magnetic field pulses. The results provide a guiding principle for producing circularly polarized attosecond pulses and ultrafast magnetic fields in complex molecular systems for future research in ultrafast magneto-optics.

Fusion of Ultraviolet-Visible and Infrared Transient Absorption Spectroscopy Data to Model Ultrafast Photoisomerization.

PubMed

Debus, Bruno; Orio, Maylis; Rehault, Julien; Burdzinski, Gotard; Ruckebusch, Cyril; Sliwa, Michel

2017-08-03

Ultrafast photoisomerization reactions generally start at a higher excited state with excess of internal vibrational energy and occur via conical intersections. This leads to ultrafast dynamics which are difficult to investigate with a single transient absorption spectroscopy technique, be it in the ultraviolet-visible (UV-vis) or infrared (IR) domain. On one hand, the information available in the UV-vis domain is limited as only slight spectral changes are observed for different isomers. On the other hand, the interpretation of vibrational spectra is strongly hindered by intramolecular relaxation and vibrational cooling. These limitations can be circumvented by fusing UV-vis and IR transient absorption spectroscopy data in a multiset multivariate curve resolution analysis. We apply this approach to describe the spectrodynamics of the ultrafast cis-trans photoisomerization around the C-N double bond observed for aromatic Schiff bases. Twisted intermediate states could be elucidated, and isomerization was shown to occur through a continuous complete rotation. More broadly, data fusion can be used to rationalize a vast range of ultrafast photoisomerization processes of interest in photochemistry.

Carbon Nanotubes as an Ultrafast Emitter with a Narrow Energy Spread at Optical Frequency.

PubMed

Li, Chi; Zhou, Xu; Zhai, Feng; Li, Zhenjun; Yao, Fengrui; Qiao, Ruixi; Chen, Ke; Cole, Matthew Thomas; Yu, Dapeng; Sun, Zhipei; Liu, Kaihui; Dai, Qing

2017-08-01

Ultrafast electron pulses, combined with laser-pump and electron-probe technologies, allow ultrafast dynamics to be characterized in materials. However, the pursuit of simultaneous ultimate spatial and temporal resolution of microscopy and spectroscopy is largely subdued by the low monochromaticity of the electron pulses and their poor phase synchronization to the optical excitation pulses. Field-driven photoemission from metal tips provides high light-phase synchronization, but suffers large electron energy spreads (3-100 eV) as driven by a long wavelength laser (>800 nm). Here, ultrafast electron emission from carbon nanotubes (≈1 nm radius) excited by a 410 nm femtosecond laser is realized in the field-driven regime. In addition, the emitted electrons have great monochromaticity with energy spread as low as 0.25 eV. This great performance benefits from the extraordinarily high field enhancement and great stability of carbon nanotubes, superior to metal tips. The new nanotube-based ultrafast electron source opens exciting prospects for extending current characterization to sub-femtosecond temporal resolution as well as sub-nanometer spatial resolution. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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

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.

Ultrafast relaxation dynamics in BiFeO 3/YBa 2Cu 3O 7 bilayers

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

Springer, D.; Nair, Saritha K.; He, Mi

The temperature dependence of the relaxation dynamics in the bilayer thin film heterostructure composed of multiferroic BiFeO 3 (BFO) and superconducting YBa 2Cu 3O 7 (YBCO) grown on (001) SrTiO 3 substrate is studied by time-resolved pump-probe technique, and compared with that of pure YBCO thin film grown under the same growth conditions. The superconductivity of YBCO is found to be retained in the heterostructure. We observe a speeding up of the YBCO recombination dynamics in the superconducting state of the heterostructure, and attribute it to the presence of weak ferromagnetism at the BFO/YBCOinterface as observed inmagnetization data. An extensionmore » of the Rothwarf-Taylor model is used to fit the ultrafast dynamics of BFO/YBCO, that models an increased quasiparticle occupation of the ferromagnetic interfacial layer in the superconducting state of YBCO.« less

Ultrafast relaxation dynamics in BiFeO 3/YBa 2Cu 3O 7 bilayers

DOE PAGES

Springer, D.; Nair, Saritha K.; He, Mi; ...

2016-02-12

The temperature dependence of the relaxation dynamics in the bilayer thin film heterostructure composed of multiferroic BiFeO 3 (BFO) and superconducting YBa 2Cu 3O 7 (YBCO) grown on (001) SrTiO 3 substrate is studied by time-resolved pump-probe technique, and compared with that of pure YBCO thin film grown under the same growth conditions. The superconductivity of YBCO is found to be retained in the heterostructure. We observe a speeding up of the YBCO recombination dynamics in the superconducting state of the heterostructure, and attribute it to the presence of weak ferromagnetism at the BFO/YBCOinterface as observed inmagnetization data. An extensionmore » of the Rothwarf-Taylor model is used to fit the ultrafast dynamics of BFO/YBCO, that models an increased quasiparticle occupation of the ferromagnetic interfacial layer in the superconducting state of YBCO.« less

Tracking of the nuclear wavepacket motion in cyanine photoisomerization by ultrafast pump-dump-probe spectroscopy.

PubMed

Wei, Zhengrong; Nakamura, Takumi; Takeuchi, Satoshi; Tahara, Tahei

2011-06-01

Understanding ultrafast reactions, which proceed on a time scale of nuclear motions, requires a quantitative characterization of the structural dynamics. To track such structural changes with time, we studied a nuclear wavepacket motion in photoisomerization of a prototype cyanine dye, 1,1'-diethyl-4,4'-cyanine, by ultrafast pump-dump-probe measurements in solution. The temporal evolution of wavepacket motion was examined by monitoring the efficiency of stimulated emission dumping, which was obtained from the recovery of a ground-state bleaching signal. The dump efficiency versus pump-dump delay exhibited a finite rise time, and it became longer (97 fs → 330 fs → 390 fs) as the dump pulse was tuned to longer wavelengths (690 nm → 950 nm → 1200 nm). This result demonstrates a continuous migration of the leading edge of the wavepacket on the excited-state potential from the Franck-Condon region toward the potential minimum. A slowly decaying feature of the dump efficiency indicated a considerable broadening of the wavepacket over a wide range of the potential, which results in the spread of a population distribution on the flat S(1) potential energy surface. The rapid migration as well as broadening of the wavepacket manifests a continuous nature of the structural dynamics and provides an intuitive visualization of this ultrafast reaction. We also discussed experimental strategies to evaluate reliable dump efficiencies separately from other ultrafast processes and showed a high capability and possibility of the pump-dump-probe method for spectroscopic investigation of unexplored potential regions such as conical intersections. © 2011 American Chemical Society

Diffraction contrast as a sensitive indicator of femtosecond sub-nanoscale motion in ultrafast transmission electron microscopy

NASA Astrophysics Data System (ADS)

Cremons, Daniel R.; Schliep, Karl B.; Flannigan, David J.

2013-09-01

With ultrafast transmission electron microscopy (UTEM), access can be gained to the spatiotemporal scales required to directly visualize rapid, non-equilibrium structural dynamics of materials. This is achieved by operating a transmission electron microscope (TEM) in a stroboscopic pump-probe fashion by photoelectrically generating coherent, well-timed electron packets in the gun region of the TEM. These probe photoelectrons are accelerated down the TEM column where they travel through the specimen before reaching a standard, commercially-available CCD detector. A second laser pulse is used to excite (pump) the specimen in situ. Structural changes are visualized by varying the arrival time of the pump laser pulse relative to the probe electron packet at the specimen. Here, we discuss how ultrafast nanoscale motions of crystalline materials can be visualized and precisely quantified using diffraction contrast in UTEM. Because diffraction contrast sensitively depends upon both crystal lattice orientation as well as incoming electron wavevector, minor spatial/directional variations in either will produce dynamic and often complex patterns in real-space images. This is because sections of the crystalline material that satisfy the Laue conditions may be heterogeneously distributed such that electron scattering vectors vary over nanoscale regions. Thus, minor changes in either crystal grain orientation, as occurs during specimen tilting, warping, or anisotropic expansion, or in the electron wavevector result in dramatic changes in the observed diffraction contrast. In this way, dynamic contrast patterns observed in UTEM images can be used as sensitive indicators of ultrafast specimen motion. Further, these motions can be spatiotemporally mapped such that direction and amplitude can be determined.

Probing ultra-fast processes with high dynamic range at 4th-generation light sources: Arrival time and intensity binning at unprecedented repetition rates.

PubMed

Kovalev, S; Green, B; Golz, T; Maehrlein, S; Stojanovic, N; Fisher, A S; Kampfrath, T; Gensch, M

2017-03-01

Understanding dynamics on ultrafast timescales enables unique and new insights into important processes in the materials and life sciences. In this respect, the fundamental pump-probe approach based on ultra-short photon pulses aims at the creation of stroboscopic movies. Performing such experiments at one of the many recently established accelerator-based 4th-generation light sources such as free-electron lasers or superradiant THz sources allows an enormous widening of the accessible parameter space for the excitation and/or probing light pulses. Compared to table-top devices, critical issues of this type of experiment are fluctuations of the timing between the accelerator and external laser systems and intensity instabilities of the accelerator-based photon sources. Existing solutions have so far been only demonstrated at low repetition rates and/or achieved a limited dynamic range in comparison to table-top experiments, while the 4th generation of accelerator-based light sources is based on superconducting radio-frequency technology, which enables operation at MHz or even GHz repetition rates. In this article, we present the successful demonstration of ultra-fast accelerator-laser pump-probe experiments performed at an unprecedentedly high repetition rate in the few-hundred-kHz regime and with a currently achievable optimal time resolution of 13 fs (rms). Our scheme, based on the pulse-resolved detection of multiple beam parameters relevant for the experiment, allows us to achieve an excellent sensitivity in real-world ultra-fast experiments, as demonstrated for the example of THz-field-driven coherent spin precession.

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.