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Sample records for optically driven excitons

  1. Excitons and optical spectra of phosphorene nanoribbons

    NASA Astrophysics Data System (ADS)

    Nourbakhsh, Zahra; Asgari, Reza

    2016-07-01

    On the basis of many-body ab initio calculations, using the single-shot G0W0 method and Bethe-Salpeter equation, we study phosphorene nanoribbons (PNRs) in the two typical zigzag and armchair directions. The electronic structure, optical absorption, electron-hole (exciton) binding energy, exciton exchange splitting, and exciton wave functions are calculated for different sizes of PNRs. The typically strong splitting between singlet and triplet excitonic states make PNRs favorable systems for optoelectronic applications. Quantum confinement occurs in both kinds of PNRs, and it is stronger in the zPNRs, which behave like quasi-zero-dimensional systems. Scaling laws are investigated for the size-dependent behaviors of PNRs. The first bright excitonic state in PNRs is explored in detail.

  2. Optical nutation in the exciton range of spectrum

    SciTech Connect

    Khadzhi, P. I.; Vasiliev, V. V.

    2013-08-15

    Optical nutation in the exciton range of spectrum is studied in the mean field approximation taking into account exciton-photon and elastic exciton-exciton interactions. It is shown that the features of nutation development are determined by the initial exciton and photon densities, the resonance detuning, the nonlinearity parameter, and the initial phase difference. For nonzero initial exciton and photon concentrations, three regimes of temporal evolution of excitons and photons exist: periodic conversion of excitons to photons and vice versa, aperiodic conversion of photons to excitons, and the rest regime. In the rest regime, the initial exciton and photon densities are nonzero and do not change with time. The oscillation amplitudes and periods of particle densities determined by the system parameters are found. The exciton self-trapping and photon trapping appearing in the system at threshold values of the nonlinearity parameter were predicted. As this parameter increases, the oscillation amplitudes of the exciton and photon densities sharply change at the critical value of the nonlinearity parameter. These two phenomena are shown to be caused by the elastic exciton-exciton interaction, resulting in the dynamic concentration shift of the exciton level.

  3. Excitons and Optical Properties of {alpha} -Quartz

    SciTech Connect

    Chang, Eric K.; Rohlfing, Michael; Louie, Steven G. [Department of Physics, University of California at Berkeley, Berkeley, California 94720

    2000-09-18

    We present an ab initio study of the optical properties of {alpha} -quartz. The absorption spectrum is calculated by solving the Bethe-Salpeter equation for the interacting electron-hole system and found to be in excellent agreement with the measured spectrum up to 10 eV above the absorption threshold. We find that excitonic effects are crucial in understanding the sharp features in the absorption spectrum in this energy range. They are also crucial in the ab initio computation of the static dielectric constant, significantly enhancing its value. (c) 2000 The American Physical Society.

  4. Electro-optical properties of Rydberg excitons

    NASA Astrophysics Data System (ADS)

    Zielińska-Raczyńska, Sylwia; Ziemkiewicz, David; Czajkowski, Gerard

    2016-07-01

    We show how to compute the electro-optical functions (absorption, reflection, and transmission) when Rydberg exciton-polaritons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field. With the use of the real density matrix approach, numerical calculations applied for the Cu2O crystal are performed. We also examine in detail and explain the dependence of the resonance displacement on the state number and applied electric field strength. We report a fairly good agreement with recently published experimental data.

  5. Exciton Absorption in Semiconductor Quantum Wells Driven by a Strong Intersubband Pump Field

    NASA Technical Reports Server (NTRS)

    Liu, Ansheng; Ning, Cun-Zheng

    1999-01-01

    Optical interband excitonic absorption of semiconductor quantum wells (QW's) driven by a coherent pump field is investigated based on semiconductor Bloch equations. The pump field has a photon energy close to the intersubband spacing between the first two conduction subbands in the QW's. An external weak optical field probes the interband transition. The excitonic effects and pump-induced population redistribution within the conduction subbands in the QW system are included. When the density of the electron-hole pairs in the QW structure is low, the pump field induces an Autler-Townes splitting of the exciton absorption spectrum. The split size and the peak positions of the absorption doublet depend not only on the pump frequency and intensity but also on the carrier density. As the density of the electron-hole pairs is increased, the split contrast (the ratio between the maximum and minimum values) is decreased because the exciton effect is suppressed at higher densities due to the many-body screening.

  6. A measurable force driven by an excitonic condensate

    SciTech Connect

    Hakioğlu, T.; Özgün, Ege; Günay, Mehmet

    2014-04-21

    Free energy signatures related to the measurement of an emergent force (≈10{sup −9}N) due to the exciton condensate (EC) in Double Quantum Wells are predicted and experiments are proposed to measure the effects. The EC-force is attractive and reminiscent of the Casimir force between two perfect metallic plates, but also distinctively different from it by its driving mechanism and dependence on the parameters of the condensate. The proposed experiments are based on a recent experimental work on a driven micromechanical oscillator. Conclusive observations of EC in recent experiments also provide a strong promise for the observation of the EC-force.

  7. Realization of an all optical exciton-polariton router

    SciTech Connect

    Marsault, Félix; Nguyen, Hai Son; Tanese, Dimitrii; Lemaître, Aristide; Galopin, Elisabeth; Sagnes, Isabelle; Amo, Alberto

    2015-11-16

    We report on the experimental realization of an all optical router for exciton-polaritons. This device is based on the design proposed by Flayac and Savenko [Appl. Phys. Lett. 103, 201105 (2013)], in which a zero-dimensional island is connected through tunnel barriers to two periodically modulated wires of different periods. Selective transmission of polaritons injected in the island, into either of the two wires, is achieved by tuning the energy of the island state across the band structure of the modulated wires. We demonstrate routing of ps polariton pulses using an optical control beam which controls the energy of the island quantum states, thanks to polariton-exciton interactions.

  8. Optical dynamics of exciton and polaron formation in molecular aggregates

    NASA Astrophysics Data System (ADS)

    de Boer, Steven; Wiersma, Douwe A.

    1989-03-01

    Results of femtosecond accumulated photon echo, picosecond pump-probe and fluorescence lifetime measurements are reported on aggregates of the dyes pseudoisocyanine (PIC) and substituted thiapyrylium (TPY), embedded in a polycarbonate matrix. It is concluded that in the PIC aggregate, delocalized excitations (excitons) are formed, which are weakly coupled to the aggregate's nuclear frame. In the TPY aggregate, excitons are also initially formed, but through strong local electron-phonon coupling these excitons are not stable and decay into polarons, which become trapped. It is suggested that the nature of the excitations in aggregates crucially depends on the change of electron density distribution upon optical excitation. When this change is large, as revealed by a large change of dipole moment, polarons will be formed. In the other limit of a small change of dipole moment on optical excitation, excitons with an enhanced radiative lifetime are formed, which coherently propagate over that part of the aggregate where the molecules are electronically strongly coupled. The relevance of these findings towards energy transport in photo-biological systems is also discussed.

  9. Interaction of excitons with optical phonons in layer crystals

    NASA Astrophysics Data System (ADS)

    Nitsovich, Bohdan M.; Zenkova, C. Y.; Kramar, N. K.

    2002-02-01

    The investigation is concerned with layer crystals of the GaSe, InSe, GaTe, MoS2-type and other inorganic semiconductors, whose phonon spectrum has a great number of peculiarities, among them the availability of low-energy optical phonons. In this case the dispersion of these phonons can be essential and vary in character. The mass operator of the exciton-phonon system and the light absorption coefficient for different dispersion laws of optical phonons have been calculated. The influence of the sign of the phonon 'effective mass' on the exciton absorption band of layer crystals, which causes the opposite in sign dynamics of the absorption maximum shift, and the change of the absorption curve asymmetry have been determined.

  10. Electrically driven optical antennas

    NASA Astrophysics Data System (ADS)

    Kern, Johannes; Kullock, René; Prangsma, Jord; Emmerling, Monika; Kamp, Martin; Hecht, Bert

    2015-09-01

    Unlike radiowave antennas, so far optical nanoantennas cannot be fed by electrical generators. Instead, they are driven by light or indirectly via excited discrete states in active materials in their vicinity. Here we demonstrate the direct electrical driving of an in-plane optical antenna by the broadband quantum-shot noise of electrons tunnelling across its feed gap. The spectrum of the emitted photons is determined by the antenna geometry and can be tuned via the applied voltage. Moreover, the direction and polarization of the light emission are controlled by the antenna resonance, which also improves the external quantum efficiency by up to two orders of magnitude. The one-material planar design offers facile integration of electrical and optical circuits and thus represents a new paradigm for interfacing electrons and photons at the nanometre scale, for example for on-chip wireless communication and highly configurable electrically driven subwavelength photon sources.

  11. Nonlinear Nano-Optics: Probing One Exciton at a Time

    NASA Astrophysics Data System (ADS)

    Bonadeo, Nicolas H.

    1998-03-01

    Optical studies in single quantum dots (QD's) have recently been possible with the use of high spatial resolution techniques (K. Brunner,et al., Phys. Rev. Lett. 69, 3216 (1992).)^,(H. F. Hess,et al., Science 264, 1740 (1994).)^,(D. Gammon, et al., Phys. Rev. Lett. 76, 3005 (1996).). The various approaches remove the spectral blurring caused by inhomogeneous broadening in ensemble measurements revealing extremely sharp resonances that result from the complete energy quantization of the zero-dimensional exciton. Previous experiments in single QD's have been based uniquely in photoluminescence (PL) detection and were mainly performed in the frequency domain. In this work, we present data from two different experimental approaches that go beyond these limitations and open up a new direction of research for direct measurements of exciton dynamics, coherent transients and optical nonlinearities in QD's. The first set of experiments combines the elegance and power of CW coherent nonlinear optical spectroscopy with the breakthrough of single QD probing producing the first nonlinear measurement in a single QD. The nonlinear measurements allow us to identify an incoherent and coherent contribution to the resonant electronic response, extract the excitation decoherence time and energy relaxation rate, as well as demonstrate a behavior similar to two beam coupling. In the second set of experiments, using two phase-locked lasers pulses, we show the possibility to control the population(A. P. Heberly, J.J. Baumberg, and Kohler, Phys. Rev. Lett. 74, 3596 (1995))of a single QD in times shorter than the excitonic lifetime and taking thus, coherent-control to the ultimate quantum limit of a single exciton per control box. In addition, we performed a series of transient experiments that includes the first direct measurement of the decoherence time in single QD. The measurements are performed at T=6K in a narrow (42 Åsingle MBE grown GaAs quantum well with 250 ÅAl _0.3Ga_0.7As

  12. Excitonic effects and the optical absorption spectrum ofhydrogenated Si clusters

    SciTech Connect

    Rohlfing, Michael; Louie, Steven G.

    1997-10-19

    We calculate the optical absorption spectrum of hydrogen-terminated silicon clusters by solving the Bethe-Salpeter equation for the two-particle Green's function using an ab initio approach. The one-particle Green's function and the electron-hole interaction kernel are calculated within the GW approximation for the electron self-energy operator. Very large exciton binding energies are observed. Our results for the one-particle properties and the optical absorption spectra of the clusters are in very good agreement with available experimental data.

  13. Exciton-Polariton Fano Resonance Driven by Second Harmonic Generation

    NASA Astrophysics Data System (ADS)

    Wang, Yafeng; Liao, Liming; Hu, Tao; Luo, Song; Wu, Lin; Wang, Jun; Zhang, Zhe; Xie, Wei; Sun, Liaoxin; Kavokin, A. V.; Shen, Xuechu; Chen, Zhanghai

    2017-02-01

    Angle-resolved second harmonic generation (SHG) spectra of ZnO microwires show characteristic Fano resonances in the spectral vicinity of exciton-polariton modes. We observe a resonant peak followed by a strong dip in SHG originating from the constructive and destructive interference of the nonresonant SHG and the resonant contribution of the polariton mode. It is demonstrated that the Fano line shape, and thus the Fano asymmetry parameter q , can be tuned by the phase shift of the two channels. We develop a model to calculate the phase-dependent q as a function of the radial angle in the microwire and achieve a good agreement with the experimental results. The deduced phase-to-q relation unveils the crucial information about the dynamics of the system and offers a tool for control on the line shape of the SHG spectra in the vicinity of exciton-polariton modes.

  14. Exciton-Polariton Fano Resonance Driven by Second Harmonic Generation.

    PubMed

    Wang, Yafeng; Liao, Liming; Hu, Tao; Luo, Song; Wu, Lin; Wang, Jun; Zhang, Zhe; Xie, Wei; Sun, Liaoxin; Kavokin, A V; Shen, Xuechu; Chen, Zhanghai

    2017-02-10

    Angle-resolved second harmonic generation (SHG) spectra of ZnO microwires show characteristic Fano resonances in the spectral vicinity of exciton-polariton modes. We observe a resonant peak followed by a strong dip in SHG originating from the constructive and destructive interference of the nonresonant SHG and the resonant contribution of the polariton mode. It is demonstrated that the Fano line shape, and thus the Fano asymmetry parameter q, can be tuned by the phase shift of the two channels. We develop a model to calculate the phase-dependent q as a function of the radial angle in the microwire and achieve a good agreement with the experimental results. The deduced phase-to-q relation unveils the crucial information about the dynamics of the system and offers a tool for control on the line shape of the SHG spectra in the vicinity of exciton-polariton modes.

  15. Tunable optical delay via carrier induced exciton dephasing in semiconductor quantum wells.

    PubMed

    Sarkar, Susanta; Guo, Yan; Wang, Hailin

    2006-04-03

    We report the experimental realization of a tunable optical delay by exploiting unique incoherent nonlinear optical processes in semiconductors. The tunable optical delay takes advantage of the strong Coulomb interactions between excitons and free carriers and uses optical injection of free carriers to broaden and bleach an exciton absorption resonance. Fractional delay exceeding 200% has been obtained for an 8 ps optical pulse propagating near the heavy-hole excitonic transition in a GaAs quantum well structure. Tunable optical delay based on optical injection of free carriers avoids strong absorption of the pump beam and is also robust against variations in the frequency of the pump beam.

  16. All-optical depletion of dark excitons from a semiconductor quantum dot

    SciTech Connect

    Schmidgall, E. R.; Schwartz, I.; Cogan, D.; Gershoni, D.; Gantz, L.; Heindel, T.; Reitzenstein, S.

    2015-05-11

    Semiconductor quantum dots are considered to be the leading venue for fabricating on-demand sources of single photons. However, the generation of long-lived dark excitons imposes significant limits on the efficiency of these sources. We demonstrate a technique that optically pumps the dark exciton population and converts it to a bright exciton population, using intermediate excited biexciton states. We show experimentally that our method considerably reduces the dark exciton population while doubling the triggered bright exciton emission, approaching thereby near-unit fidelity of quantum dot depletion.

  17. Optical bistability and nonlinearity of coherently coupled exciton-plasmon systems.

    PubMed

    Li, Jian-Bo; Kim, Nam-Chol; Cheng, Mu-Tian; Zhou, Li; Hao, Zhong-Hua; Wang, Qu-Quan

    2012-01-16

    We theoretically investigated optical third-order nonlinearity of a coherently coupled exciton-plasmon hybrid system under a strong control field with a weak probe field. The analytic formulas of exciton population and effective third-order optical susceptibility of the hybrid of a metal nanoparticle (MNP) and a semiconductor quantum dot (SQD) were deduced. The bistable exciton population and the induced bistable nonlinear absorption and refraction response were revealed. The bistability region can be tuned by adjusting the size of metal nanoparticle, interparticle distance and intensity of control field. Our results have perspective applications in optical information processing based on resonant coupling of exciton-plasmon.

  18. Excitons and optical properties of alpha-quartz

    SciTech Connect

    Chang, Eric K.; Rohlfing, Michael; Louie, Steven G.

    2000-04-01

    We present an ab initio study of the optical properties of alpa-quartz. The absorption spectrum is calculated by solving the Bethe-Salpeter equation for the interacting electron-hole system and found to be in excellent agreement with the measured spectrum up to 10 eV above the absorption threshold. We find that excitonic effects are crucial in understanding the sharp features in the absorption spectrum in this energy range. The are also crucial in the ab initio computation of the static dielectric constant, significantly enhancing its value.

  19. Excitons and optical properties of alpha-quartz

    PubMed

    Chang; Rohlfing; Louie

    2000-09-18

    We present an ab initio study of the optical properties of alpha-quartz. The absorption spectrum is calculated by solving the Bethe-Salpeter equation for the interacting electron-hole system and found to be in excellent agreement with the measured spectrum up to 10 eV above the absorption threshold. We find that excitonic effects are crucial in understanding the sharp features in the absorption spectrum in this energy range. They are also crucial in the ab initio computation of the static dielectric constant, significantly enhancing its value.

  20. PREFACE: International Conference on Optics of Excitons in Confined Systems

    NASA Astrophysics Data System (ADS)

    Viña, Luis; Tejedor, Carlos; Calleja, José M.

    2010-01-01

    The OECS11 (International Conference on Optics of Excitons in Confined Systems) was the eleventh of a very successful series of conferences that started in 1987 in Rome (Italy). Afterwards the conference was held at Naxos (Sicily, Italy, 1991), Montpellier (France, 1993), Cortona (Italy, 1995), Göttingen (Germany, 1997), Ascona (Switzerland, 1999), Montpellier (France, 2001), Lecce (Italy, 2003), Southampton (UK, 2005) and Patti (Sicily, Italy, 2007). It is addressed to scientists who lead fundamental and applied research on the optical properties of excitons in novel condensed-matter nanostructures. The 2009 meeting (7-11 September 2009) has brought together a large representation of the world leading actors in this domain, with the aim of stimulating the exchange of ideas, promoting international collaborations, and coordinating research on the newest exciton-related issues such as quantum information science and exciton quantum-collective phenomena. The meeting has included invited lectures, contributed oral presentations and posters, covering the following general topics: low-dimensional heterostructures: quantum wells, quantum wires and quantum dots polaritons quantum optics with excitons and polaritons many-body effects under coherent and incoherent excitation coherent optical spectroscopy quantum coherence and quantum-phase manipulation Bose-Einstein condensation and other collective phenomena excitons in novel materials The OECS 11 was held at the campus of the Universidad Autónoma de Madrid in Cantoblanco. The scientific program was composed of more than 200 contributions divided into 16 invited talks, 44 oral contributions and 3 poster sessions with a total of 150 presentations. The scientific level of the presentations was guaranteed by a selection process where each contribution was rated by three members of the Program Committee. The Conference has gathered 238 participants from 21 different countries, with the following distribution: Germany (43

  1. Excitons and Optical Spectrum of the Si(111)- (2{times}1) Surface

    SciTech Connect

    Rohlfing, M.; Louie, S.G.

    1999-07-01

    We investigate excitons at the Si(111)-( 2{times}1) surface and their optical spectrum from first principles. This is done by solving the Bethe-Salpeter equation for the two-particle Green`s function, including the electron-hole interaction. The optical spectrum of the surface is dominated by a surface exciton formed from the {pi} -bonded surface states. The excitonic binding energy is more than 1thinspthinsporder of magnitude larger than in bulk Si. The two-particle wave function of the exciton state is strongly localized at the surface and exhibits distinct anisotropy due to the surface reconstruction. {copyright} {ital 1999} {ital The American Physical Society }

  2. Degradation of Hole Transport Materials via Exciton-Driven Cyclization.

    PubMed

    Bell, Bruce M; Clark, Michael B; Devore, David D; De Vries, Timothy S; Froese, Robert D; Gray, Kaitlyn C; Jackson, David H K; Kuech, T F; Na, Hong-Yeop; Kearns, Kenneth L; Lee, Kyung-Joo; Mukhopadhyay, Sukrit; Rachford, Aaron A; Spencer, Liam P; Woodward, W H Hunter

    2017-04-06

    Organic light-emitting diode (OLED) displays have been an active and intense area of research for well over a decade and have now reached commercial success for displays from cell phones to large format televisions. A more thorough understanding of the many different potential degradation modes which cause OLED device failure will be necessary to develop the next generation of OLED materials, improve device lifetime, and to ultimately improve the cost vs performance ratio. Each of the different organic layers in an OLED device can be susceptible to unique decomposition pathways, however stability toward excitons is critical for emissive layer (EML) materials as well as any layer near the recombination zone. This study will specifically focus on degradation modes within the hole transport layer (HTL) with the goal being to identify the general decomposition paths occurring in an operating device and use this information to design new derivatives which can block these pathways. Through post-mortem analyses of several aged OLED devices, an apparently common intramolecular cyclization pathway has been identified that was not previously reported for arylamine-containing HTL materials and that operates parallel to but faster than the previously described fragmentation pathways.

  3. Exciton formation assisted by longitudinal optical phonons in monolayer transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Thilagam, A.

    2016-09-01

    We examine a mechanism by which excitons are generated via the longitudinal optical (LO) phonon-assisted scattering process after optical excitation of monolayer transition metal dichalcogenides. The exciton formation time is computed as a function of the exciton center-of-mass wavevector, electron and hole temperatures, and carrier densities for known values of the Fröhlich coupling constant, LO phonon energy, lattice temperature, and the exciton binding energy in layered structures. For the monolayer MoS2, we obtain ultrafast exciton formation times on the sub-picosecond time scale at charge densities of 5 × 1011 cm-2 and carrier temperatures less than 300 K, in good agreement with recent experimental findings ( ≈0.3 ps). While excitons are dominantly created at zero center-of-mass wavevectors at low charge carrier temperatures ( ≈30 K), the exciton formation time is most rapid at non-zero wavevectors at higher temperatures ( ≥120 K) of charge carriers. The results show the inverse square-law dependence of the exciton formation times on the carrier density, consistent with a square-law dependence of photoluminescence on the excitation density. Our results show that excitons are formed more rapidly in exemplary monolayer selenide-based dichalcogenides (MoSe2 and WSe2) than sulphide-based dichalcogenides (MoS2 and WS2).

  4. Optical lattices of excitons in InGaN/GaN quantum well systems

    SciTech Connect

    Chaldyshev, V. V. Bolshakov, A. S. Zavarin, E. E.; Sakharov, A. V.; Lundin, V. V.; Tsatsulnikov, A. F.; Yagovkina, M. A.

    2015-01-15

    Optical lattices of excitons in periodic systems of InGaN quantum wells with GaN barriers are designed, implemented, and investigated. Due to the collective interaction of quasi-two-dimensional excitons with light and a fairly high binding energy of excitons in GaN, optical Bragg reflection at room temperature is significantly enhanced. To increase the resonance optical response of the system, new structures with two quantum wells in a periodic supercell are designed and implemented. Resonance reflection of 40% at room temperatures for structures with 60 periods is demonstrated.

  5. Optical properties of MgZnO alloys: Excitons and exciton-phonon complexes

    SciTech Connect

    Neumann, M. D.; Cobet, C.; Esser, N.; Laumer, B.; Wassner, T. A.; Eickhoff, M.; Feneberg, M.; Goldhahn, R.

    2011-07-01

    The characteristics of the excitonic absorption and emission around the fundamental bandgap of wurtzite Mg{sub x}Zn{sub 1-x}O grown on c-plane sapphire substrates by plasma assisted molecular beam epitaxy with Mg contents between x = 0 and x = 0.23 are studied using spectroscopic ellipsometry and photoluminescence (PL) measurements. The ellipsometric data were analyzed using a multilayer model yielding the dielectric function (DF). The imaginary part of the DF for the alloys exhibits a pronounced feature which is attributed to exciton-phonon coupling (EPC) similar to the previously reported results for ZnO. Thus, in order to determine reliable transition energies, the spectral dependence is analyzed by a model which includes free excitonic lines, the exciton continuum, and the enhanced absorption due to EPC. A line shape analysis of the temperature-dependent PL spectra yielded in particular the emission-related free excitonic transition energies, which are compared to the results from the DF line-shape analysis. The PL linewidth is discussed within the framework of an alloy disorder model.

  6. Anomalous Light Cones and Valley Optical Selection Rules of Interlayer Excitons in Twisted Heterobilayers

    NASA Astrophysics Data System (ADS)

    Yu, Hongyi; Wang, Yong; Tong, Qingjun; Xu, Xiaodong; Yao, Wang

    2015-10-01

    We show that, because of the inevitable twist and lattice mismatch in heterobilayers of transition metal dichalcogenides, interlayer excitons have sixfold degenerate light cones anomalously located at finite velocities on the parabolic energy dispersion. The photon emissions at each light cone are elliptically polarized, with the major axis locked to the direction of exciton velocity, and helicity specified by the valley indices of the electron and the hole. These finite-velocity light cones allow unprecedented possibilities for optically injecting valley polarization and valley current, and the observation of both direct and inverse valley Hall effects, by exciting interlayer excitons. Our findings suggest potential excitonic circuits with valley functionalities, and unique opportunities to study exciton dynamics and condensation phenomena in semiconducting 2D heterostructures.

  7. Driven optical matter (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Figliozzi, Patrick; Sule, Nishant; Yan, Zijie; Vaikuntanathan, Suriyanarayanan; Rice, Stuart A.; Scherer, Norbert F.

    2016-09-01

    Optical trapping has enabled studying a wide variety of questions and systems in chemistry, biology, physics, and materials science. For example, optical trapping has been used to understand hydrodynamic interactions in dilute and dense colloidal fluids and discover connections to granular materials. In this presentation we show that shaped optical fields and gradients can be used to study the electrodynamic interactions amongst nanoparticles (NPs) and drive them into new ordered states. We demonstrate the formation and use of NP-based optical matter to study a range of nonequilibrium phenomena in solution; field-driven barrier crossing phenomena and noise-driven ordering. Optical matter, a material that forms only in the presence of an optical field, involves NP interactions by optical scattering and interference. Metal NPs can be formed into regular arrangements in minimally shaped fields; e.g., in focused Gaussian beams, line traps, and optical ring traps. Inter-particle interactions and motions are also affected when the optical matter is driven. Particles recirculate in an optical ring vortex trap allowing long term measurements to examine rare events. In particular, particles can hop between optical binding sites, move past electrodynamic obstacles or pass each other while moving around the ring. The polarization state of the optical beam can be used to produce periodic variations of the NP electrodynamic interactions. As particles circulate this "noise" causes NP clusters to be less stable as if the temperature of the system is increased. Conversely, we observe noise-driven ordering in dense systems. We will explain these phenomena using simulations and theory.

  8. Selectively tunable optical Stark effect of anisotropic excitons in atomically thin ReS2

    PubMed Central

    Sim, Sangwan; Lee, Doeon; Noh, Minji; Cha, Soonyoung; Soh, Chan Ho; Sung, Ji Ho; Jo, Moon-Ho; Choi, Hyunyong

    2016-01-01

    The optical Stark effect is a coherent light–matter interaction describing the modification of quantum states by non-resonant light illumination in atoms, solids and nanostructures. Researchers have strived to utilize this effect to control exciton states, aiming to realize ultra-high-speed optical switches and modulators. However, most studies have focused on the optical Stark effect of only the lowest exciton state due to lack of energy selectivity, resulting in low degree-of-freedom devices. Here, by applying a linearly polarized laser pulse to few-layer ReS2, where reduced symmetry leads to strong in-plane anisotropy of excitons, we control the optical Stark shift of two energetically separated exciton states. Especially, we selectively tune the Stark effect of an individual state with varying light polarization. This is possible because each state has a completely distinct dependence on light polarization due to different excitonic transition dipole moments. Our finding provides a methodology for energy-selective control of exciton states. PMID:27857053

  9. Selectively tunable optical Stark effect of anisotropic excitons in atomically thin ReS2

    NASA Astrophysics Data System (ADS)

    Sim, Sangwan; Lee, Doeon; Noh, Minji; Cha, Soonyoung; Soh, Chan Ho; Sung, Ji Ho; Jo, Moon-Ho; Choi, Hyunyong

    2016-11-01

    The optical Stark effect is a coherent light-matter interaction describing the modification of quantum states by non-resonant light illumination in atoms, solids and nanostructures. Researchers have strived to utilize this effect to control exciton states, aiming to realize ultra-high-speed optical switches and modulators. However, most studies have focused on the optical Stark effect of only the lowest exciton state due to lack of energy selectivity, resulting in low degree-of-freedom devices. Here, by applying a linearly polarized laser pulse to few-layer ReS2, where reduced symmetry leads to strong in-plane anisotropy of excitons, we control the optical Stark shift of two energetically separated exciton states. Especially, we selectively tune the Stark effect of an individual state with varying light polarization. This is possible because each state has a completely distinct dependence on light polarization due to different excitonic transition dipole moments. Our finding provides a methodology for energy-selective control of exciton states.

  10. Exciton effects on the nonlinear optical properties of semiparabolic quantum dot under electric field

    NASA Astrophysics Data System (ADS)

    Bejan, D.

    2017-02-01

    The effects of exciton and electric field on the nonlinear optical properties, such as refraction index change, optical absorption coefficient and optical rectification of semiparabolic one-dimensional quantum dot, were theoretically investigated. The energy eigenvalues and eigenfunctions are calculated numerically within the effective mass approximation for a typical GaAs/ Al0.3Ga0.7 As quantum dot, for the cases where there is an exciton or a single electron/hole in the structure. Optical properties are obtained using the compact density matrix approach and steady state solutions. Our results show that: i) if the increasing electric field is oriented along the growth direction, the refractive index change structure and the resonance peaks of the absorption coefficient and optical rectification present a blue shift and are weakened for exciton and electron systems but have a red shift and are strengthened for the hole system; ii) when the field, oriented against the growth direction, augments, the above optical parameters present a red shift and are increased for exciton and electron systems but have a blue shift and are lowered for the hole system; iii) the exciton presence in the structure enhances the amplitude of the resonant peaks of all optical parameters even at zero electric field.

  11. Electrically driven optical metamaterials

    PubMed Central

    Le-Van, Quynh; Le Roux, Xavier; Aassime, Abdelhanin; Degiron, Aloyse

    2016-01-01

    The advent of metamaterials more than 15 years ago has offered extraordinary new ways of manipulating electromagnetic waves. Yet, progress in this field has been unequal across the electromagnetic spectrum, especially when it comes to finding applications for such artificial media. Optical metamaterials, in particular, are less compatible with active functionalities than their counterparts developed at lower frequencies. One crucial roadblock in the path to devices is the fact that active optical metamaterials are so far controlled by light rather than electricity, preventing them from being integrated in larger electronic systems. Here we introduce electroluminescent metamaterials based on metal nano-inclusions hybridized with colloidal quantum dots. We show that each of these miniature blocks can be individually tuned to exhibit independent optoelectronic properties (both in terms of electrical characteristics, polarization, colour and brightness), illustrate their capabilities by weaving complex light-emitting surfaces and finally discuss their potential for displays and sensors. PMID:27328976

  12. Electrically driven optical metamaterials

    NASA Astrophysics Data System (ADS)

    Le-van, Quynh; Le Roux, Xavier; Aassime, Abdelhanin; Degiron, Aloyse

    2016-06-01

    The advent of metamaterials more than 15 years ago has offered extraordinary new ways of manipulating electromagnetic waves. Yet, progress in this field has been unequal across the electromagnetic spectrum, especially when it comes to finding applications for such artificial media. Optical metamaterials, in particular, are less compatible with active functionalities than their counterparts developed at lower frequencies. One crucial roadblock in the path to devices is the fact that active optical metamaterials are so far controlled by light rather than electricity, preventing them from being integrated in larger electronic systems. Here we introduce electroluminescent metamaterials based on metal nano-inclusions hybridized with colloidal quantum dots. We show that each of these miniature blocks can be individually tuned to exhibit independent optoelectronic properties (both in terms of electrical characteristics, polarization, colour and brightness), illustrate their capabilities by weaving complex light-emitting surfaces and finally discuss their potential for displays and sensors.

  13. Optical functionality of plasmon-exciton nanomaterials in the strong coupling regime

    NASA Astrophysics Data System (ADS)

    Sukharev, Maxim

    Understanding optical plasmon-exciton interaction in hybrid plasmonic nanostructures is important for tuning the optical response, e.g. for applications in nonlinear optics, organic solar cells, or organic light-emitting diodes. In developing such nanostructures, the strong coupling phenomena play crucial role allowing to efficiently transfer energy between plasmons and molecular excitons on a femtosecond time scale. In this talk I will discuss modeling aspects of various optical phenomena at plasmonic interfaces using Maxwell-Bloch equations in three dimensions. Various plasmonic systems including periodic V-grooves, bowtie antennas, nanowires, periodic hole arrays, and others will be considered. In particular, I will demonstrate that one can design hybrid nanomaterials with highly pronounced Fano resonances using femtosecond lasers. I will show that it is possible to use ultra-short laser pulses to materials with desired properties and functionality. Electromagnetic energy transport in systems composed of closely spaced nanowires in a presence of molecular excitons will also be discussed.

  14. Nonlinear optical spectra having characteristics of Fano interferences in coherently coupled lowest exciton biexciton states in semiconductor quantum dots

    SciTech Connect

    Gotoh, Hideki Sanada, Haruki; Yamaguchi, Hiroshi; Sogawa, Tetsuomi

    2014-10-15

    Optical nonlinear effects are examined using a two-color micro-photoluminescence (micro-PL) method in a coherently coupled exciton-biexciton system in a single quantum dot (QD). PL and photoluminescence excitation spectroscopy (PLE) are employed to measure the absorption spectra of the exciton and biexciton states. PLE for Stokes and anti-Stokes PL enables us to clarify the nonlinear optical absorption properties in the lowest exciton and biexciton states. The nonlinear absorption spectra for excitons exhibit asymmetric shapes with peak and dip structures, and provide a distinct contrast to the symmetric dip structures of conventional nonlinear spectra. Theoretical analyses with a density matrix method indicate that the nonlinear spectra are caused not by a simple coherent interaction between the exciton and biexciton states but by coupling effects among exciton, biexciton and continuum states. These results indicate that Fano quantum interference effects appear in exciton-biexciton systems at QDs and offer important insights into their physics.

  15. Optical transitions from the lowest to higher exciton and biexciton Rydberg states in CuCl

    NASA Astrophysics Data System (ADS)

    Miyajima, K.; Sakaniwa, K.; Sugawara, M.

    2016-11-01

    We measured the optical transitions due to the internal energy levels of an exciton and biexciton in a CuCl single crystal using pump-probe spectroscopy. The transient absorption bands due to the transitions from the 1 s to 2 p and from the 1 s to 3 p exciton states were observed at 3 K, which is consistent with their reported energies. Simultaneously, the transient absorption peak due to the biexciton was observed, which corresponds to a transition from the lowest state (composed of two 1 s excitons) to higher states (composed of 1 s and 2 p excitons). The value of the observed biexciton peak energy was reasonable considering the hydrogen molecule orbitals and the electron-to-hole effective mass ratio. In addition, the transient absorption peaks were broadened at 77 K, because of the increase in the homogeneous width of the 2 p exciton state. The transient absorption spectrum was almost completely determined by this width. Our findings are of importance with regard to the optical phenomena in the infrared region related to the quantum coherence of excitons and biexcitons in semiconductors.

  16. Selective optical pumping of charged excitons in unintentionally doped InAs quantum dots.

    PubMed

    Muñoz-Matutano, Guillermo; Alén, Benito; Martínez-Pastor, Juan; Seravalli, Lucca; Frigeri, Paola; Franchi, Secondo

    2008-04-09

    We have investigated the selective optical pumping of charged excitonic species in a sample containing quantum dots of different sizes and low areal density by photoluminescence and excitation of the photoluminescence microspectroscopy. We study the selective optical excitation of negatively charged excitons as an alternative to commonly used electrical methods. We demonstrate that under resonant excitation in impurity related bands, the selective pumping efficiency can be as high as 85% in small quantum dots having one electron shell and emitting at around 930 nm, and around 65% in big quantum dots having four electron shells and emitting at 1160 nm.

  17. Magneto-optical properties of Rydberg excitons: Center-of-mass quantization approach

    NASA Astrophysics Data System (ADS)

    Zielińska-Raczyńska, Sylwia; Ziemkiewicz, David; Czajkowski, Gerard

    2017-02-01

    We show how to compute the magneto-optical functions (absorption, reflection, and transmission) when Rydberg exciton polaritons appear, including the effect of the coherence between the electron-hole pair and the electromagnetic field, and the polaritonic effect. Using the real density-matrix approach the analytical expressions for magneto-optical functions are obtained and numerical calculations for Cu2O crystal are performed. The influence of the strength of applied external magnetic field on the resonance displacement of excitonic spectra is discussed. We report a good agreement with recently published experimental data.

  18. In-situ optical transmission electron microscope study of exciton phonon replicas in ZnO nanowires by cathodoluminescence

    SciTech Connect

    Yang, Shize; Tian, Xuezeng; Wang, Lifen; Wei, Jiake; Qi, Kuo; Li, Xiaomin; Xu, Zhi E-mail: xdbai@iphy.ac.cn Wang, Wenlong; Zhao, Jimin; Bai, Xuedong E-mail: xdbai@iphy.ac.cn; Wang, Enge E-mail: xdbai@iphy.ac.cn

    2014-08-18

    The cathodoluminescence spectrum of single zinc oxide (ZnO) nanowires is measured by in-situ optical Transmission Electron Microscope. The coupling between exciton and longitudinal optical phonon is studied. The band edge emission varies for different excitation spots. This effect is attributed to the exciton propagation along the c axis of the nanowire. Contrary to free exciton emission, the phonon replicas are well confined in ZnO nanowire. They travel along the c axis and emit at the end surface. Bending strain increases the relative intensity of second order phonon replicas when excitons travel along the c-axis.

  19. Optically decomposed near-band-edge structure and excitonic transitions in Ga2S3

    PubMed Central

    Ho, Ching-Hwa; Chen, Hsin-Hung

    2014-01-01

    The band-edge structure and band gap are key parameters for a functional chalcogenide semiconductor to its applications in optoelectronics, nanoelectronics, and photonics devices. Here, we firstly demonstrate the complete study of experimental band-edge structure and excitonic transitions of monoclinic digallium trisulfide (Ga2S3) using photoluminescence (PL), thermoreflectance (TR), and optical absorption measurements at low and room temperatures. According to the experimental results of optical measurements, three band-edge transitions of EA = 3.052 eV, EB = 3.240 eV, and EC = 3.328 eV are respectively determined and they are proven to construct the main band-edge structure of Ga2S3. Distinctly optical-anisotropic behaviors by orientation- and polarization-dependent TR measurements are, respectively, relevant to distinguish the origins of the EA, EB, and EC transitions. The results indicated that the three band-edge transitions are coming from different origins. Low-temperature PL results show defect emissions, bound-exciton and free-exciton luminescences in the radiation spectra of Ga2S3. The below-band-edge transitions are respectively characterized. On the basis of experimental analyses, the optical property of near-band-edge structure and excitonic transitions in the monoclinic Ga2S3 crystal is revealed. PMID:25142550

  20. Nature of the narrow optical band in H*-aggregates: Dozy-chaos–exciton coupling

    SciTech Connect

    Egorov, Vladimir V.

    2014-07-15

    Dozy chaos emerges as a combined effect of the collective chaotic motion of electrons and nuclei, and their chaotic electromagnetic interactions in the transient state of molecules experiencing quantum transitions. Following earlier discussions of the well-known Brönsted relations for proton-transfer reactions; the temperature-dependent electron transfer in Langmuir–Blodgett films; the shape of the optical bands of polymethine dye monomers, their dimers, and J-aggregates, this paper reports one more application of the dozy-chaos theory of molecular quantum transitions. The qualitative and quantitative explanations for shape of a narrow and blue-shifted optical absorption band in H{sup *}-aggregates is given on the basis of the dozy-chaos theory by taking into account the dozy-chaos–exciton coupling effect. It is emphasized that in the H{sup *}-aggregate chromophore (dimer of cyclic bis-thiacarbocyanines) there is a competition between two Frenkel exciton transitions through the chaotic reorganization motion of nuclear environment. As a result, the highly organized quantum transition to the upper exciton state becomes an exciton-induced source of dozy chaos for the low organized transition to the lower exciton state. This manifests itself in appearing the narrow peak and broad wing in the optical spectrum pattern of H{sup *}-aggregates. A similar enhancement in the H{sup *}-effect caused by the strengthening of the exciton coupling in H{sup *}-dimers, which could be achieved by synthesizing tertiary and quarternary thiacarbocyanine monomers, is predicted.

  1. Screening effect on the exciton mediated nonlinear optical susceptibility of semiconductor quantum dots.

    PubMed

    Bautista, Jessica E Q; Lyra, Marcelo L; Lima, R P A

    2014-11-17

    We study the exciton contribution to the third-order optical susceptibility of one-dimensional semiconductor quantum dots and show that the screening of the electron-hole interaction has a strong influence on the nonlinear optical properties in the weak confinement regime. Based on a density matrix formulation, we estimate the spectrum of the third-order optical susceptibility and its contribution to the refraction index and absorption coefficient. In particular, we show that the multipeaked spectrum of the nonlinear susceptibility, which results from the hydrogenoid character of the exciton eigenstates for a purely Coulombian electron-hole coupling, is reverted towards a single peaked structure as the interaction becomes strongly screened, thus leading to a substantial enhancement of the nonlinear optical properties of semiconductor quantum dots.

  2. Photocurrent spectroscopy of exciton and free particle optical transitions in suspended carbon nanotube pn-junctions

    SciTech Connect

    Chang, Shun-Wen; Theiss, Jesse; Hazra, Jubin; Aykol, Mehmet; Kapadia, Rehan; Cronin, Stephen B.

    2015-08-03

    We study photocurrent generation in individual, suspended carbon nanotube pn-junction diodes formed by electrostatic doping using two gate electrodes. Photocurrent spectra collected under various electrostatic doping concentrations reveal distinctive behaviors for free particle optical transitions and excitonic transitions. In particular, the photocurrent generated by excitonic transitions exhibits a strong gate doping dependence, while that of the free particle transitions is gate independent. Here, the built-in potential of the pn-junction is required to separate the strongly bound electron-hole pairs of the excitons, while free particle excitations do not require this field-assisted charge separation. We observe a sharp, well defined E{sub 11} free particle interband transition in contrast with previous photocurrent studies. Several steps are taken to ensure that the active charge separating region of these pn-junctions is suspended off the substrate in a suspended region that is substantially longer than the exciton diffusion length and, therefore, the photocurrent does not originate from a Schottky junction. We present a detailed model of the built-in fields in these pn-junctions, which, together with phonon-assistant exciton dissociation, predicts photocurrents on the same order of those observed experimentally.

  3. Breaking inversion symmetry induces excitonic peak in optical absorption of topological semimetal

    NASA Astrophysics Data System (ADS)

    Dadsetani, Mehrdad; Ebrahimian, Ali

    2017-01-01

    In this work we present ab initio study on linear optical properties of Dirac and Weyl semimetals and tried to find the consequences of inversion symmetry breaking in the optical properties of topological semimetal. The real and imaginary part of dielectric function in addition to energy loss spectra of topological semimetal with and without inversion symmetry have been calculated within Random phase approximation (RPA) then the electron-hole interaction is included by solving the Bethe-Salpeter Equation (BSE) for the electron-hole Green's function. We find that the lack of inversion symmetry and spin-orbit interaction increases the density of states at Fermi level, giving rise to excitonic peak in optical absorption of topological semimetal. It is remarkable that the excitonic effects in high energy range of the spectrum are stronger than in the lower one. To explore the breaking of inversion symmetry related optical properties, we have investigated the optical properties of Dirac semimetals Na3Bi and BaPt and compared them to corresponding ones in Weyl semimetals NbP and Na3Bi0.75Sb0.25. Our calculations show that NbP, which lacks inversion symmetry, has high energy exciton at 10 and 10.8 eV. In contrast with Na3Bi, electron-hole interactions give rise to several weak peaks at different energy in the optical absorption of Na3Bi0.75Sb0.25 while its red shift is less pronounced.

  4. Large polarization-dependent exciton optical Stark effect in lead iodide perovskites

    NASA Astrophysics Data System (ADS)

    Yang, Ye; Yang, Mengjin; Zhu, Kai; Johnson, Justin C.; Berry, Joseph J.; van de Lagemaat, Jao; Beard, Matthew C.

    2016-08-01

    A strong interaction of a semiconductor with a below-bandgap laser pulse causes a blue-shift of the bandgap transition energy, known as the optical Stark effect. The energy shift persists only during the pulse duration with an instantaneous response time. The optical Stark effect has practical relevance for applications, including quantum information processing and communication, and passively mode-locked femtosecond lasers. Here we demonstrate that solution-processable lead-halide perovskites exhibit a large optical Stark effect that is easily resolved at room temperature resulting from the sharp excitonic feature near the bandedge. We also demonstrate that a polarized pump pulse selectively shifts one spin state producing a spin splitting of the degenerate excitonic states. Such selective spin manipulation is an important prerequisite for spintronic applications. Our result implies that such hybrid semiconductors may have great potential for optoelectronic applications beyond photovoltaics.

  5. Large polarization-dependent exciton optical Stark effect in lead iodide perovskites

    PubMed Central

    Yang, Ye; Yang, Mengjin; Zhu, Kai; Johnson, Justin C.; Berry, Joseph J.; van de Lagemaat, Jao; Beard, Matthew C.

    2016-01-01

    A strong interaction of a semiconductor with a below-bandgap laser pulse causes a blue-shift of the bandgap transition energy, known as the optical Stark effect. The energy shift persists only during the pulse duration with an instantaneous response time. The optical Stark effect has practical relevance for applications, including quantum information processing and communication, and passively mode-locked femtosecond lasers. Here we demonstrate that solution-processable lead-halide perovskites exhibit a large optical Stark effect that is easily resolved at room temperature resulting from the sharp excitonic feature near the bandedge. We also demonstrate that a polarized pump pulse selectively shifts one spin state producing a spin splitting of the degenerate excitonic states. Such selective spin manipulation is an important prerequisite for spintronic applications. Our result implies that such hybrid semiconductors may have great potential for optoelectronic applications beyond photovoltaics. PMID:27577007

  6. Quasiparticle energies, excitons, and optical spectra of few-layer black phosphorus

    NASA Astrophysics Data System (ADS)

    Tran, Vy; Fei, Ruixiang; Yang, Li

    2015-12-01

    We report first-principles GW-Bethe-Salpeter-equation (BSE) studies of excited-state properties of few-layer black phosphorus (BP) (phosphorene). With improved GW computational methods, we obtained converged quasiparticle band gaps and optical absorption spectra by the single-shot (G0W0) procedure. Moreover, we reveal fine structures of anisotropic excitons, including the series of one-dimensional like wave functions, spin singlet-triplet splitting, and electron-hole binding energy spectra by solving BSE. An effective-mass model is employed to describe these electron-hole pairs, shedding light on estimating the exciton binding energy of anisotropic two-dimensional semiconductors without expensive ab initio simulations. Finally, the anisotropic optical response of BP is explained by using optical selection rules based on the projected single-particle density of states at band edges.

  7. Large polarization-dependent exciton optical Stark effect in lead iodide perovskites

    DOE PAGES

    Yang, Ye; Yang, Mengjin; Zhu, Kai; ...

    2016-08-31

    A strong interaction of a semiconductor with a below-bandgap laser pulse causes a blue-shift of the bandgap transition energy, known as the optical Stark effect. The energy shift persists only during the pulse duration with an instantaneous response time. The optical Stark effect has practical relevance for applications, including quantum information processing and communication, and passively mode-locked femtosecond lasers. Here we demonstrate that solution-processable lead-halide perovskites exhibit a large optical Stark effect that is easily resolved at room temperature resulting from the sharp excitonic feature near the bandedge. We also demonstrate that a polarized pump pulse selectively shifts one spinmore » state producing a spin splitting of the degenerate excitonic states. Such selective spin manipulation is an important prerequisite for spintronic applications. Lastly, our result implies that such hybrid semiconductors may have great potential for optoelectronic applications beyond photovoltaics.« less

  8. Large polarization-dependent exciton optical Stark effect in lead iodide perovskites

    SciTech Connect

    Yang, Ye; Yang, Mengjin; Zhu, Kai; Johnson, Justin C.; Berry, Joseph J.; van de Lagemaat, Jao; Beard, Matthew C.

    2016-08-31

    A strong interaction of a semiconductor with a below-bandgap laser pulse causes a blue-shift of the bandgap transition energy, known as the optical Stark effect. The energy shift persists only during the pulse duration with an instantaneous response time. The optical Stark effect has practical relevance for applications, including quantum information processing and communication, and passively mode-locked femtosecond lasers. Here we demonstrate that solution-processable lead-halide perovskites exhibit a large optical Stark effect that is easily resolved at room temperature resulting from the sharp excitonic feature near the bandedge. We also demonstrate that a polarized pump pulse selectively shifts one spin state producing a spin splitting of the degenerate excitonic states. Such selective spin manipulation is an important prerequisite for spintronic applications. Lastly, our result implies that such hybrid semiconductors may have great potential for optoelectronic applications beyond photovoltaics.

  9. Excitons in a mirror: Formation of “optical bilayers” using MoS{sub 2} monolayers on gold substrates

    SciTech Connect

    Mertens, Jan; Baumberg, Jeremy J.; Shi, Yumeng; Yang, Hui Ying; Molina-Sánchez, Alejandro; Wirtz, Ludger

    2014-05-12

    We report coupling of excitons in monolayers of molybdenum disulphide to their mirror image in an underlying gold substrate. Excitons at the direct band gap are little affected by the substrate whereas strongly bound C-excitons associated with a van-Hove singularity change drastically. On quartz substrates only one C-exciton is visible (in the blue) but on gold substrates a strong red-shifted extra resonance in the green is seen. Exciton coupling to its image leads to formation of a “mirror biexciton” with enhanced binding energy. Estimates of this energy shift in an emitter-gold system match experiments well. The absorption spectrum of MoS{sub 2} on gold thus resembles a bilayer of MoS{sub 2} which has been created by optical coupling. Additional top-mirrors produce an “optical bulk.”.

  10. Exciton scattering approach for optical spectra calculations in branched conjugated macromolecules

    NASA Astrophysics Data System (ADS)

    Li, Hao; Wu, Chao; Malinin, Sergey V.; Tretiak, Sergei; Chernyak, Vladimir Y.

    2016-12-01

    The exciton scattering (ES) technique is a multiscale approach based on the concept of a particle in a box and developed for efficient calculations of excited-state electronic structure and optical spectra in low-dimensional conjugated macromolecules. Within the ES method, electronic excitations in molecular structure are attributed to standing waves representing quantum quasi-particles (excitons), which reside on the graph whose edges and nodes stand for the molecular linear segments and vertices, respectively. Exciton propagation on the linear segments is characterized by the exciton dispersion, whereas exciton scattering at the branching centers is determined by the energy-dependent scattering matrices. Using these ES energetic parameters, the excitation energies are then found by solving a set of generalized "particle in a box" problems on the graph that represents the molecule. Similarly, unique energy-dependent ES dipolar parameters permit calculations of the corresponding oscillator strengths, thus, completing optical spectra modeling. Both the energetic and dipolar parameters can be extracted from quantum-chemical computations in small molecular fragments and tabulated in the ES library for further applications. Subsequently, spectroscopic modeling for any macrostructure within a considered molecular family could be performed with negligible numerical effort. We demonstrate the ES method application to molecular families of branched conjugated phenylacetylenes and ladder poly-para-phenylenes, as well as structures with electron donor and acceptor chemical substituents. Time-dependent density functional theory (TD-DFT) is used as a reference model for electronic structure. The ES calculations accurately reproduce the optical spectra compared to the reference quantum chemistry results, and make possible to predict spectra of complex macromolecules, where conventional electronic structure calculations are unfeasible.

  11. Optical spectroscopy and imaging of the higher energy excitons and bandgap of monolayer MoS2

    NASA Astrophysics Data System (ADS)

    Borys, Nicholas; Bao, Wei; Barnard, Edward; Ko, Changhyun; Tongay, Sefaatin; Wu, Junqiao; Yang, Li; Schuck, P. James

    Monolayer MoS2 (ML-MoS2) exhibits a rich manifold of excitons that dictate optoelectronic performance and functionality. Disentangling these states, which include the quasi-particle bandgap, is critical for developing 2D optoelectronic devices that operate beyond the optical bandgap. Whereas photoluminescence (PL) spectroscopy only probes the lowest-energy radiative state and absorption spectroscopy fails to discriminate energetically degenerate states, photoluminescence excitation (PLE) spectroscopy selectively probes only the excited states that thermalize to the emissive ground state exciton. Using PLE spectroscopy of ML-MoS2, we identify the Rydberg series of the exciton A and exciton B states as well as signatures of the quasi-particle bandgap and coupling between the indirect C exciton and the lowest-energy A exciton, which have eluded previous PLE studies. The assignment of these states is confirmed with density functional theory. Mapping the PLE spectrum reveals spatial variations of the higher-energy exciton manifold and quasi-particle bandgap which mirror the heterogeneity in the PL but also indicate variations in local exciton thermalization processes and chemical potentials.

  12. Excitonic Effects and the Optical Absorption Spectrum of Hydrogenated Si Clusters

    SciTech Connect

    Rohlfing, M.; Louie, S.G. |

    1998-04-01

    We calculate the optical absorption spectrum of hydrogen-terminated silicon clusters by solving the Bethe-Salpeter equation for the two-particle Green{close_quote}s function using an {ital ab initio} approach. The one-particle Green{close_quote}s function and the electron-hole interaction kernel are calculated within the GW approximation for the electron self-energy operator. Very large exciton binding energies are observed. Our results for the one-particle properties and the optical absorption spectra of the clusters are in very good agreement with available experimental data. {copyright} {ital 1998} {ital The American Physical Society}

  13. Excitonic effects from geometric order and disorder explain broadband optical absorption in eumelanin.

    PubMed

    Chen, Chun-Teh; Chuang, Chern; Cao, Jianshu; Ball, Vincent; Ruch, David; Buehler, Markus J

    2014-05-22

    Eumelanin is a ubiquitous biological pigment, and the origin of its broadband absorption spectrum has long been a topic of scientific debate. Here, we report a first-principles computational investigation to explain its broadband absorption feature. These computations are complemented by experimental results showing a broadening of the absorption spectra of dopamine solutions upon their oxidation. We consider a variety of eumelanin molecular structures supported by experiments or theoretical studies, and calculate the absorption spectra with proper account of the excitonic couplings based on the Frenkel exciton model. The interplay of geometric order and disorder of eumelanin aggregate structures broadens the absorption spectrum and gives rise to a relative enhancement of absorption intensity at the higher-energy end, proportional to the cube of absorption energy. These findings show that the geometric disorder model is as able as the chemical disorder model, and complements this model, to describe the optical properties of eumelanin.

  14. Skyrmion formation and optical spin-Hall effect in an expanding coherent cloud of indirect excitons.

    PubMed

    Vishnevsky, D V; Flayac, H; Nalitov, A V; Solnyshkov, D D; Gippius, N A; Malpuech, G

    2013-06-14

    We provide a theoretical description of the polarization pattern and phase singularities experimentally evidenced recently in a condensate of indirect excitons [H. High et al., Nature 483, 584 (2012)]. We show that the averaging of the electron and hole orbital motion leads to a comparable spin-orbit interaction for both types of carriers. We demonstrate that the interplay between a radial coherent flux of bright indirect excitons and the Dresselhaus spin-orbit interaction results in the formation of spin domains and of topological defects similar to Skyrmions. We reproduce qualitatively all the features of the experimental data and obtain a polarization pattern as in the optical spin-Hall effect despite the different symmetry of the spin-orbit interactions.

  15. Optical spectrum of bottom-up graphene nanoribbons: towards efficient atom-thick excitonic solar cells

    PubMed Central

    Villegas, Cesar E. P.; Mendonça, P. B.; Rocha, A. R.

    2014-01-01

    Recently, atomically well-defined cove-shaped graphene nanoribbons have been obtained using bottom-up synthesis. These nanoribbons have an optical gap in the visible range of the spectrum which make them candidates for donor materials in photovoltaic devices. From the atomistic point of view, their electronic and optical properties are not clearly understood. Therefore, in this work we carry out ab-initio density functional theory calculations combine with many-body perturbation formalism to study their electronic and optical properties. Through the comparison with experimental measurements, we show that an accurate description of the nanoribbon's optical properties requires the inclusion of electron-hole correlation effects. The energy, binding energy and the corresponding excitonic transitions involved are analyzed. We found that in contrast to zigzag graphene nanoribbons, the excitonic peaks in the absorption spectrum are a consequence of a group of transitions involving the first and second conduction and valence bands. Finally, we estimate some relevant optical properties that strengthen the potential of these nanoribbons for acting as a donor materials in photovoltaic. PMID:25301001

  16. Optical spectrum of bottom-up graphene nanoribbons: towards efficient atom-thick excitonic solar cells.

    PubMed

    Villegas, Cesar E P; Mendonça, P B; Rocha, A R

    2014-10-10

    Recently, atomically well-defined cove-shaped graphene nanoribbons have been obtained using bottom-up synthesis. These nanoribbons have an optical gap in the visible range of the spectrum which make them candidates for donor materials in photovoltaic devices. From the atomistic point of view, their electronic and optical properties are not clearly understood. Therefore, in this work we carry out ab-initio density functional theory calculations combine with many-body perturbation formalism to study their electronic and optical properties. Through the comparison with experimental measurements, we show that an accurate description of the nanoribbon's optical properties requires the inclusion of electron-hole correlation effects. The energy, binding energy and the corresponding excitonic transitions involved are analyzed. We found that in contrast to zigzag graphene nanoribbons, the excitonic peaks in the absorption spectrum are a consequence of a group of transitions involving the first and second conduction and valence bands. Finally, we estimate some relevant optical properties that strengthen the potential of these nanoribbons for acting as a donor materials in photovoltaic.

  17. Electro optical tuning of Tamm-plasmon exciton-polaritons

    NASA Astrophysics Data System (ADS)

    Gessler, J.; Baumann, V.; Emmerling, M.; Amthor, M.; Winkler, K.; Höfling, S.; Schneider, C.; Kamp, M.

    2014-11-01

    We report on electro optical tuning of the emission from GaAs quantum wells resonantly coupled to a Tamm-plasmon mode in a hybrid metal/dielectric structure. The structures were studied via momentum resolved photoluminescence and photoreflectance spectroscopy, and the surface metal layer was used as a top gate, which allowed for a precise tuning of the quantum well emission via the quantum confined Stark effect. By tuning the resonance, we were able to observe the characteristic anticrossing behavior of a polaritonic emission in the strong light-matter coupling regime, yielding a Rabi splitting of (9.2 ± 0.2) meV.

  18. Magneto-optical properties and recombination dynamics of isoelectronic bound excitons in ZnO

    SciTech Connect

    Chen, S. L.; Chen, W. M.; Buyanova, I. A.

    2014-02-21

    Magneto-optical and time-resolved photoluminescence (PL) spectroscopies are employed to evaluate electronic structure of a bound exciton (BX) responsible for the 3.364 eV line (labeled as I{sub 1}{sup *}) in bulk ZnO. From time-resolved PL spectroscopy, I{sub 1}{sup *} is concluded to originate from the exciton ground state. Based on performed magneto-PL studies, the g-factors of the involved electron and hole are determined as being g{sub e} = 1.98 and g{sub h}{sup ∥}(g{sub h}{sup ⊥}) = 1.2(1.62), respectively. These values are nearly identical to the reported g-factors for the I{sup *} line in ZnO (Phys. Rev. B 86, 235205 (2012)), which proves that I{sub 1}{sup *} should have a similar origin as I{sup *} and should arise from an exciton bound to an isoelectronic center with a hole-attractive potential.

  19. Optical absorption and refraction index change of a confined exciton in a spherical quantum dot nanostructure

    NASA Astrophysics Data System (ADS)

    Mathan Kumar, K.; John Peter, A.; Lee, C. W.

    2011-12-01

    Electronic energies of an exciton confined in a strained Zn1- x Cd x Se/ZnSe quantum dot have been computed as a function of dot radius with various Cd content. Calculations have been performed using Bessel function as an orthonormal basis for different confinement potentials of barrier height considering the internal electric field induced by the spontaneous and piezoelectric polarizations. The optical absorption coefficients and the refractive index changes between the ground state ( L = 0) and the first excited state ( L = 1) are investigated. It is found that the optical properties in the strained ZnCdSe/ZnSe quantum dot are strongly affected by the confinement potentials and the dot radii. The intensity of the total absorption spectra increases for the transition between higher levels. The obtained optical nonlinearity brings out the fact that it should be considered in calculating the optical properties in low dimensional semiconductors especially in quantum dots.

  20. Ab initio study of the optical properties of crystalline phenanthrene, including the excitonic effects

    NASA Astrophysics Data System (ADS)

    Dadsetani, Mehrdad; Nejatipour, Hajar; Ebrahimian, Ali

    2015-05-01

    Using the ab initio methods for solving the Bethe-Salpeter equation on the basis of the FPLAPW method, optical properties of crystalline phenanthrene were calculated, in a comparison to its isomer, anthracene. It was found that despite the similarity of the structural, electronic, and the overall optical properties in a 40 eV energy range, phenanthrene and anthracene show significant differences in their optical spectra in the energy range below band gaps. Phenanthrene has two spin singlet excitonic features whereas anthracene shows one. The singlet and the lowest triplet binding energies of phenanthrene were found to be larger than anthracene. In this study, in addition, a comparison has been made between the optical spectra in RPA and the existing experimental data.

  1. Transformation Optics Approach to Plasmon-Exciton Strong Coupling in Nanocavities

    NASA Astrophysics Data System (ADS)

    Li, Rui-Qi; Hernángomez-Pérez, D.; García-Vidal, F. J.; Fernández-Domínguez, A. I.

    2016-09-01

    We investigate the conditions yielding plasmon-exciton strong coupling at the single emitter level in the gap between two metal nanoparticles. Inspired by transformation optics ideas, a quasianalytical approach is developed that makes possible a thorough exploration of this hybrid system incorporating the full richness of its plasmonic spectrum. This allows us to reveal that by placing the emitter away from the cavity center, its coupling to multipolar dark modes of both even and odd parity increases remarkably. This way, reversible dynamics in the population of the quantum emitter takes place in feasible implementations of this archetypal nanocavity.

  2. Lowest optical excitations in molecular crystals: bound excitons versus free electron-hole pairs in anthracene.

    PubMed

    Hummer, Kerstin; Puschnig, Peter; Ambrosch-Draxl, Claudia

    2004-04-09

    By solving the Bethe-Salpeter equation for the electron-hole Green function for crystalline anthracene we find the lowest absorption peak generated by strongly bound excitons or by a free electron-hole pair, depending on the polarization direction being parallel to the short or the long molecular axis, respectively. Both excitations are shifted to lower energies by pressure. The physical difference of these excitations is apparent from the electron-hole wave functions. Our findings are a major contribution to solve the long-standing puzzle about the nature of the lowest optical excitations in organic materials.

  3. Dynamic quasi-energy-band modulation and exciton effects in biased superlattices driven by a two-color far-infrared field: Disappearance of dynamic localization

    NASA Astrophysics Data System (ADS)

    Yashima, Kenta; Hino, Ken-Ichi; Toshima, Nobuyuki

    2003-12-01

    A theoretical study of the optical and electronic properties of semiconductor superlattices in ac-dc fields, termed the dynamic Wannier-Stark ladder (DWSL), is done. The biased superlattices are driven by two far-infrared fields with different frequencies and relative phase of δ. Here, the frequency of the first laser is equal to the Bloch frequency ωB of the system under study, while that of the second laser is equal to 2ωB. Quasienergies of the DWSL are calculated based on the Floquet theorem, and the associated linear photoabsorption spectra are evaluated. For δ=0, a gourd-shaped quasi-energy structure characteristic of both dynamic localization (DL) and delocalization (DDL), similar to the usual DWSL driven by a single laser, appears. By changing the ratio of the two laser strengths, however, the width of the quasi-energy band and the locations of both DL and DDL vary noticeably. As for δ≠0, on the other hand, band collapse and the associated DL do not necessarily follow. In fact, DL vanishes and the quasi-energy degeneracy is lifted in a certain range of δ. Just DDL remains over the entire range of the laser strength, eventually resulting in a plateaulike band structure in the linear absorption spectra. The basic physics underlying this phenomenon, which can be readily interpreted in terms of a closed analytical expression, is that all quasi-energies for given crystal momenta are out of phase with each other as a function of laser strength without converging to a single point of energy. This is a feature of this DWSL which sharply distinguishes it from a conventional DWSL generated using a single laser to drive it. Furthermore, an exciton effect is incorporated with the above noninteracting problem, so that exciton dressed states are formed. It is found that this effect gives rise to more involved quasi-energy structures and a more pronounced release of the energy degeneracy of DL, leading again to the formation of a band structure in the absorption

  4. Optical selection rules for excitonic Rydberg series in the massive Dirac cones of hexagonal two-dimensional materials

    NASA Astrophysics Data System (ADS)

    Gong, Pu; Yu, Hongyi; Wang, Yong; Yao, Wang

    2017-03-01

    We investigate the optical transition selection rules for excitonic Rydberg series formed in massive Dirac cones. The entanglement of the exciton envelop function with the pseudospin texture leads to anomalous selection rules for one-photon generation of excitons, where d orbitals can be excited with the opposite helicity selection rule from the s orbitals in a given valley. The trigonal warping effects in realistic hexagonal lattices further renders more excited states bright, where p orbitals can also be accessed by one-photon excitation with the opposite valley selection rules to the s orbitals. The one-photon generation of exciton in the various states and the intraexcitonic transition between these states are both dictated by the discrete in-plane rotational symmetry of the lattices, and our results show that in hexagonal 2D materials the symmetry allowed transitions are enabled when trigonal warping effects are included in the massive Dirac fermion model. In monolayer transition metal dichalcogenides where excitons can be generated by visible light and intraexcitonic transitions can be induced by infrared light, we give the strength of these optical transitions, estimated using modified hydrogenlike envelope functions combined with the optical transition matrix elements between the Bloch states calculated at various k points.

  5. Electronic and optical properties of single excitons and biexcitons in type-II quantum dot nanocrystals

    NASA Astrophysics Data System (ADS)

    Koc, Fatih; Sahin, Mehmet

    2014-05-01

    In this study, a detailed investigation of the electronic and optical properties (i.e., binding energies, absorption wavelength, overlap of the electron-hole wave functions, recombination oscillator strength, etc.) of an exciton and a biexciton in CdTe/CdSe core/shell type-II quantum dot heterostructures has been carried out in the frame of the single band effective mass approximation. In order to determine the electronic properties, we have self-consistently solved the Poisson-Schrödinger equations in the Hartree approximation. We have considered all probable Coulomb interaction effects on both energy levels and also on the corresponding wave functions for both single exciton and biexciton. In addition, we have taken into account the quantum mechanical exchange-correlation effects in the local density approximation between same kinds of particles for biexciton. Also, we have examined the effect of the ligands and dielectric mismatch on the electronic and optical properties. We have used a different approximation proposed by Sahin and Koc [Appl. Phys. Lett. 102, 183103 (2013)] for the recombination oscillator strength of the biexciton for bound and unbound cases. The results obtained have been presented comparatively as a function of the shell thicknesses and probable physical reasons in behind of the results have been discussed in a detail.

  6. Electronic and optical properties of single excitons and biexcitons in type-II quantum dot nanocrystals

    SciTech Connect

    Koc, Fatih; Sahin, Mehmet E-mail: mehsahin@gmail.com

    2014-05-21

    In this study, a detailed investigation of the electronic and optical properties (i.e., binding energies, absorption wavelength, overlap of the electron-hole wave functions, recombination oscillator strength, etc.) of an exciton and a biexciton in CdTe/CdSe core/shell type-II quantum dot heterostructures has been carried out in the frame of the single band effective mass approximation. In order to determine the electronic properties, we have self-consistently solved the Poisson-Schrödinger equations in the Hartree approximation. We have considered all probable Coulomb interaction effects on both energy levels and also on the corresponding wave functions for both single exciton and biexciton. In addition, we have taken into account the quantum mechanical exchange-correlation effects in the local density approximation between same kinds of particles for biexciton. Also, we have examined the effect of the ligands and dielectric mismatch on the electronic and optical properties. We have used a different approximation proposed by Sahin and Koc [Appl. Phys. Lett. 102, 183103 (2013)] for the recombination oscillator strength of the biexciton for bound and unbound cases. The results obtained have been presented comparatively as a function of the shell thicknesses and probable physical reasons in behind of the results have been discussed in a detail.

  7. Exciton scattering approach for branched conjugated molecules and complexes. IV. Transition dipoles and optical spectra

    NASA Astrophysics Data System (ADS)

    Li, Hao; Malinin, Sergey V.; Tretiak, Sergei; Chernyak, Vladimir Y.

    2010-03-01

    The electronic excitation energies and transition dipole moments are the essential ingredients to compute an optical spectrum of any molecular system. Here we extend the exciton scattering (ES) approach, originally developed for computing excitation energies in branched conjugated molecules, to the calculation of the transition dipole moments. The ES parameters that characterize contributions of molecular building blocks to the total transition dipole can be extracted from the quantum-chemical calculations of the excited states in simple molecular fragments. Using these extracted parameters, one can then effortlessly calculate the oscillator strengths and optical spectra of various large molecular structures. We illustrate application of this extended ES approach using an example of phenylacetylene-based molecules. Absorption spectra predicted by the ES approach show close agreement with the results of the reference quantum-chemical calculations.

  8. Signatures of Quantum Coherences in Rydberg Excitons

    NASA Astrophysics Data System (ADS)

    Grünwald, P.; Aßmann, M.; Heckötter, J.; Fröhlich, D.; Bayer, M.; Stolz, H.; Scheel, S.

    2016-09-01

    Coherent optical control of individual particles has been demonstrated both for atoms and semiconductor quantum dots. Here we demonstrate the emergence of quantum coherent effects in semiconductor Rydberg excitons in bulk Cu2O . Because of the spectral proximity between two adjacent Rydberg exciton states, a single-frequency laser may pump both resonances with little dissipation from the detuning. As a consequence, additional resonances appear in the absorption spectrum that correspond to dressed states consisting of two Rydberg exciton levels coupled to the excitonic vacuum, forming a V -type three-level system, but driven only by one laser light source. We show that the level of pure dephasing in this system is extremely low. These observations are a crucial step towards coherently controlled quantum technologies in a bulk semiconductor.

  9. Simultaneous monitoring of singlet and triplet exciton variations in solid organic semiconductors driven by an external static magnetic field

    SciTech Connect

    Ding, Baofu Alameh, Kamal

    2014-07-07

    The research field of organic spintronics has remarkably and rapidly become a promising research area for delivering a range of high-performance devices, such as magnetic-field sensors, spin valves, and magnetically modulated organic light emitting devices (OLEDs). Plenty of microscopic physical and chemical models based on exciton or charge interactions have been proposed to explain organic magneto-optoelectronic phenomena. However, the simultaneous observation of singlet- and triplet-exciton variations in an external magnetic field is still unfeasible, preventing a thorough theoretical description of the spin dynamics in organic semiconductors. Here, we show that we can simultaneously observe variations of singlet excitons and triplet excitons in an external magnetic field, by designing an OLED structure employing a singlet-exciton filtering and detection layer in conjunction with a separate triplet-exciton detection layer. This OLED structure enables the observation of a Lorentzian and a non-Lorentzian line-shape magnetoresponse for singlet excitons and triplet excitons, respectively.

  10. A detailed investigation of electronic and optical properties of the exciton, the biexciton and charged excitons in a multi-shell quantum dot nanocrystal

    NASA Astrophysics Data System (ADS)

    Aktürk, Abdurrahman; Sahin, Mehmet; Koc, Fatih; Erdinc, Ahmet

    2014-07-01

    In the present study, the electronic and optical properties of the exciton (X), the biexciton (XX) and charged excitons (X- and X+) in a multi-shell quantum dot nanocrystal have been systematically explored in detail. The electronic properties have been determined in the framework of the single-band effective mass approximation. For this purpose, the Poisson-Schrödinger equations have been solved self-consistently in the Hartree approximation. In the electronic structure calculations for XX, X- and X+, the quantum mechanical exchange-correlation potentials between particles of the same type have been taken into account in the local density approximation. Some optical parameters, such as the overlap integrals, recombination oscillator strengths, radiative lifetimes, etc, have been determined by using the single-particle energy levels and wavefunctions obtained. A different approximation, reported in Sahin and Koc 2013 Appl. Phys. Lett. 102 183103, has been used in the recombination oscillator strength calculations. The results have been presented comparatively as a function of the shell thicknesses, and the well widths and probable physical reasons underlying them have been discussed in detail.

  11. Control of exciton fluxes in an excitonic integrated circuit.

    PubMed

    High, Alex A; Novitskaya, Ekaterina E; Butov, Leonid V; Hanson, Micah; Gossard, Arthur C

    2008-07-11

    Efficient signal communication uses photons. Signal processing, however, uses an optically inactive medium, electrons. Therefore, an interconnection between electronic signal processing and optical communication is required at the integrated circuit level. We demonstrated control of exciton fluxes in an excitonic integrated circuit. The circuit consists of three exciton optoelectronic transistors and performs operations with exciton fluxes, such as directional switching and merging. Photons transform into excitons at the circuit input, and the excitons transform into photons at the circuit output. The exciton flux from the input to the output is controlled by a pattern of the electrode voltages. The direct coupling of photons, used in communication, to excitons, used as the device-operation medium, may lead to the development of efficient exciton-based optoelectronic devices.

  12. Optically trapped and driven paddle-wheel

    NASA Astrophysics Data System (ADS)

    Asavei, Theodor; Nieminen, Timo A.; Loke, Vincent L. Y.; Stilgoe, Alexander B.; Bowman, Richard; Preece, Daryl; Padgett, Miles J.; Heckenberg, Norman R.; Rubinsztein-Dunlop, Halina

    2013-06-01

    We demonstrate the control and rotation of an optically trapped object, an optical paddle-wheel, with the rotation direction normal to the beam axis. This is in contrast to the usual situation where the rotation is about the beam axis. The paddle-wheel can be optically driven and moved to any position in the field of view of the microscope, which can be of interest for various biological applications where controlled application of a fluid flow is needed in a particular location and in a specific direction. This is of particular interest in signal transduction studies in cells, especially when a cell is flat and spread out on a surface.

  13. Interplay of Cu and oxygen vacancy in optical transitions and screening of excitons in ZnO:Cu films

    SciTech Connect

    Darma, Yudi; Rusydi, Andrivo; Seng Herng, Tun; Marlina, Resti; Fauziah, Resti; Ding, Jun

    2014-02-24

    We study room temperature optics and electronic structures of ZnO:Cu films as a function of Cu concentration using a combination of spectroscopic ellipsometry, photoluminescence, and ultraviolet-visible absorption spectroscopy. Mid-gap optical states, interband transitions, and excitons are observed and distinguishable. We argue that the mid-gap states are originated from interactions of Cu and oxygen vacancy (Vo). They are located below conduction band (Zn4s) and above valence band (O2p) promoting strong green emission and narrowing optical band gap. Excitonic states are screened and its intensities decrease upon Cu doping. Our results show the importance of Cu and Vo driving the electronic structures and optical transitions in ZnO:Cu films.

  14. Interaction Driven Subgap Spin Exciton in the Kondo Insulator SmB6

    DOE PAGES

    Fuhrman, W. T.; Leiner, Jonathan C.; Nikolić, P.; ...

    2015-01-21

    In this paper, using inelastic neutron scattering, we map a 14 meV coherent resonant mode in the topological Kondo insulator SmB6 and describe its relation to the low energy insulating band structure. The resonant intensity is confined to the X and R high symmetry points, repeating outside the first Brillouin zone and dispersing less than 2 meV, with a 5d-like magnetic form factor. We present a slave-boson treatment of the Anderson Hamiltonian with a third neighbor dominated hybridized band structure. This approach produces a spin exciton below the charge gap with features that are consistent with the observed neutron scattering.more » Finally, we find that maxima in the wave vector dependence of the inelastic neutron scattering indicate band inversion.« less

  15. Interaction Driven Subgap Spin Exciton in the Kondo Insulator SmB6

    SciTech Connect

    Fuhrman, W. T.; Leiner, Jonathan C.; Nikolić, P.; Granroth, Garrett E.; Stone, Matthew B.; Lumsden, Mark D.; DeBeer-Schmitt, Lisa M.; Alekseev, Pavel A.; Mignot, Jean-Michel; Koohpayeh, S. M.; Cottingham, P.; Phelan, William Adam; Schoop, L.; McQueen, T. M.; Broholm, C.

    2015-01-21

    In this paper, using inelastic neutron scattering, we map a 14 meV coherent resonant mode in the topological Kondo insulator SmB6 and describe its relation to the low energy insulating band structure. The resonant intensity is confined to the X and R high symmetry points, repeating outside the first Brillouin zone and dispersing less than 2 meV, with a 5d-like magnetic form factor. We present a slave-boson treatment of the Anderson Hamiltonian with a third neighbor dominated hybridized band structure. This approach produces a spin exciton below the charge gap with features that are consistent with the observed neutron scattering. Finally, we find that maxima in the wave vector dependence of the inelastic neutron scattering indicate band inversion.

  16. Excitons and the lifetime of organic semiconductor devices

    PubMed Central

    Forrest, Stephen R.

    2015-01-01

    While excitons are responsible for the many beneficial optical properties of organic semiconductors, their non-radiative recombination within the material can result in material degradation due to the dumping of energy onto localized molecular bonds. This presents a challenge in developing strategies to exploit the benefits of excitons without negatively impacting the device operational stability. Here, we will briefly review the fundamental mechanisms leading to excitonic energy-driven device ageing in two example devices: blue emitting electrophosphorescent organic light emitting devices (PHOLEDs) and organic photovoltaic (OPV) cells. We describe strategies used to minimize or even eliminate this fundamental device degradation pathway. PMID:25987572

  17. Excitons and the lifetime of organic semiconductor devices.

    PubMed

    Forrest, Stephen R

    2015-06-28

    While excitons are responsible for the many beneficial optical properties of organic semiconductors, their non-radiative recombination within the material can result in material degradation due to the dumping of energy onto localized molecular bonds. This presents a challenge in developing strategies to exploit the benefits of excitons without negatively impacting the device operational stability. Here, we will briefly review the fundamental mechanisms leading to excitonic energy-driven device ageing in two example devices: blue emitting electrophosphorescent organic light emitting devices (PHOLEDs) and organic photovoltaic (OPV) cells. We describe strategies used to minimize or even eliminate this fundamental device degradation pathway.

  18. Optical Control of Semiconductor Quantum Dot Spin Qubits with Microcavity Exciton-Polaritons

    NASA Astrophysics Data System (ADS)

    Puri, Shruti; McMahon, Peter L.; Yamamoto, Yoshihisa

    2015-03-01

    Topological surface codes demand the least stringent threshold conditions and are most promising for implementing large quantum algorithms. Based on the resource requirements to reach fault tolerance, we develop a hardware platform for large scale quantum computation with semiconductor quantum dot (QD) electron spin qubits. The current proposals for implementation of two-qubit gates and quantum non demolition (QND) readout in a QuDOS (Quantum Dots with Optically Controlled Spins) architecture suffer from large error rates. In our scheme, the optical manipulation of the QD spin qubits is carried out using their Coulomb exchange interaction with optically excited, spin-polarized, laterally confined quantum well (LcQW) exciton-polaritons. The small mass of polaritons protects them from interaction with their solid-state environment (phonons) and enables strong coupling between spin qubits separated by a few microns. Furthermore, the excitation manifold of the QD is well separated from that of the LcQW polaritons, preventing a spin-flip event during readout. We will outline schemes for implementing fast, high-fidelity, single qubit gate, two-qubit geometric phase gate and single-shot QND measurement and analyze important decoherence mechanisms. The work being presented was carried out at Stanford University. Currently the author is at University of Sherbrooke, Canada.

  19. Entanglement of two optically driven quantum dots mediated by phonons in nanomechanical resonator

    NASA Astrophysics Data System (ADS)

    He, Yong; Jiang, Meiping

    2017-01-01

    The exciton-phonon coupling between an optically driven quantum dot (QD) and a mechanical resonator can be described by Jaynes-Cummings model under a certain condition, revealing phonon absorption and emission. When two optically driven QDs share a common phonon mode, it shows the phonon-mediated coupling between the QDs. Based on the effective master equation for the reduced density matrix of the two QDs, the temporal evolution of each state and the concurrence (quantum entanglement) between them are studied. The results suggest that the stationary concurrence depends strongly on the resonator temperature. The non-negligible entanglement in the hybrid system is advantaged to develop solid-state quantum information processing.

  20. CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES: Study of Squeezed Excitons in Polar Semiconductors

    NASA Astrophysics Data System (ADS)

    Yin, Miao; Cheng, Ze; Wu, Zi-Xia; Ping, Yun-Xia

    2009-03-01

    Some properties of excitons in polar semiconductors are studied theoretically by means of squeezed state variational approach. This method makes it possible to consider bilinear terms of the phonon operators as well as linear terms arising from the Lee-Low-Pines (LLP)-like transformation. The exciton ground state energy and binding energy are calculated numerically. It is shown that the squeezing effect is significant in the case of strong exciton-phonon coupling region.

  1. Hydrodynamic synchronisation of optically driven rotors

    NASA Astrophysics Data System (ADS)

    Debono, Luke J.; Box, Stuart; Phillips, David B.; Simpson, Stephen H.; Hanna, Simon

    2015-08-01

    Hydrodynamic coupling is thought to play a role in the coordinated beating of cilia and flagella, and may inform the future design of artificial swimmers and pumps. In this study, optical tweezers are used to investigate the hydrodynamic coupling between a pair of driven oscillators. The theoretical model of Lenz and Ryskin [P. Lenz and A. Ryskin, Phys. Biol. 3, 285{294 (2006)] is experimentally recreated, in which each oscillator consists of a sphere driven in a circular trajectory. The optical trap position is maintained ahead of the sphere to provide a tangential driving force. The trap is also moved radially to harmonically constrain the sphere to the circular trajectory. Analytically, it has been shown that two oscillators of this type are able to synchronise or phase-lock under certain conditions. We explore the interplay between synchronisation mechanisms and find good agreement between experiment, theory and Brownian dynamics simulations.

  2. Quantum confinement in semiconductor nanofilms: Optical spectra and multiple exciton generation

    NASA Astrophysics Data System (ADS)

    Khmelinskii, Igor; Makarov, Vladimir I.

    2016-04-01

    We report optical absorption and photoluminescence (PL) spectra of Si and SnO2 nanocrystalline films in the UV-vis-NIR range, featuring discrete bands resulting from transverse quantum confinement, observed in the optical spectra of nanofilms for the first time ever. The film thickness ranged from 3.9 to 12.2 nm, depending on the material. The results are interpreted within the particle-in-a-box model, with infinite walls. The calculated values of the effective electron mass are independent on the film thickness and equal to 0.17mo (Si) and 0.21mo (SnO2), with mo the mass of the free electron. The second calculated model parameter, the quantum number n of the HOMO (valence band), was also thickness-independent: 8.00 (Si) and 7.00 (SnO2). The transitions observed in absorption all start at the level n and correspond to Δn = 1, 2, 3, …. The photoluminescence bands exhibit large Stokes shifts, shifting to higher energies with increased excitation energy. In effect, nanolayers of Si, an indirect-gap semiconductor, behave as a direct-gap semiconductor, as regards the transverse-quantized level system. A prototype Si-SnO2 nanofilm photovoltaic cell demonstrated photoelectron quantum yields achieving 2.5, showing clear evidence of multiple exciton generation, for the first time ever in a working nanofilm device.

  3. Tailoring MoS2 Exciton-Plasmon Interaction by Optical Spin-Orbit Coupling.

    PubMed

    Li, Ziwei; Li, Yu; Han, Tianyang; Wang, Xingli; Yu, Ying; Tay, Bengkang; Liu, Zheng; Fang, Zheyu

    2017-02-28

    Molybdenum disulfide (MoS2) monolayer as one of the atomic thickness two-dimensional materials has remarkable electronic and optical properties, which is an ideal candidate for a wide range of optoelectronic applications. However, the atomic monolayer thickness poses a significant challenge in MoS2 photoluminescence emission due to weak light-matter interaction. Here, we investigate the MoS2 exciton-plasmon interaction with spin-orbit coupling of light. The plasmonic spiral rings with subwavelength dimensions are designed and fabricated on hybrid substrates. MoS2 photoluminescence enhancement can be actively controlled by changing the incident optical spin states, laser powers, and the nanospiral geometries, which is arising from the change of field enhancement at near-field region. Planar light-emitting devices based on spin-orbit coupling (SOC) effect were further realized and flexibly controlled by changing the polarization of light. The SOC effect is discussed by the accumulation of geometric and dynamic phases, which can be demonstrated and elaborated by the Majorana sphere model. Our results provide a way to manipulate MoS2 light-matter interaction actively and can be further applied in the spin-dependent light-emitting devices at the nanoscale.

  4. Neutral and charged exciton fine structure in single lead halide perovskite nanocrystals revealed by magneto-optical spectroscopy.

    PubMed

    Fu, Ming; Tamarat, Philippe; Huang, He; Even, Jacky; Rogach, Andrey L; Lounis, Brahim

    2017-02-27

    Revealing the crystal structure of lead halide perovskite nanocrystals is essential for the optimization of stability of these emerging materials in applications such as solar cells, photodetectors and light emitting devices. We use magneto-photoluminescence spectroscopy of individual perovskite CsPbBr3 nanocrystals as a unique tool to determine their crystal structure, which imprints distinct signatures in the excitonic sublevels of charge complexes at low temperatures. At zero magnetic field, the identification of two classes of photoluminescence spectra, displaying either two or three sublevels in their exciton fine structure, shows evidence for the existence of two crystalline structures, namely tetragonal D4h and orthorhombic D2h phases. Magnetic field shifts, splitting and coupling of the sublevels provide a determination of the diamagnetic coefficient and valuable information on the exciton g-factor and its anisotropic character. Moreover, this spectroscopic study reveals the optical properties of charged excitons and allows the extraction of the electron and hole g-factors for perovskite systems.

  5. Influence of the sign of the coupling on the temperature dependence of optical properties of one-dimensional exciton models

    NASA Astrophysics Data System (ADS)

    Cruzeiro, L.

    2008-10-01

    A new physical cause for a temperature-dependent double peak in exciton systems is put forward within a thermal equilibrium approach for the calculation of optical properties of exciton systems. Indeed, it is found that one-dimensional exciton systems with only one molecule per unit cell can have an absorption spectrum characterized by a double peak provided that the coupling between excitations in different molecules is positive. The two peaks, whose relative intensities vary with temperature, are located around the exciton band edges, being separated by an energy of approximately 4V, where V is the average coupling between nearest neighbours. For small amounts of diagonal and off-diagonal disorder, the contributions from the intermediate states in the band are also visible as intermediate structure between the two peaks, this being enhanced for systems with periodic boundary conditions. At a qualitative level, these results correlate well with experimental observations in the molecular aggregates of the thiacarbocyanine dye THIATS and in the organic crystals of acetanilide and N-methylacetamide.

  6. Optical Characterization of Strong UV Luminescence Emitted from the Excitonic Edge of Nickel Oxide Nanotowers

    PubMed Central

    Ho, Ching-Hwa; Kuo, Yi-Ming; Chan, Ching-Hsiang; Ma, Yuan-Ron

    2015-01-01

    NiO had been claimed to have the potential for application in transparent conducting oxide, electrochromic device for light control, and nonvolatile memory device. However, the detailed study of excitonic transition and light-emission property of NiO has rarely been explored to date. In this work, we demonstrate strong exciton-complex emission of high-quality NiO nanotowers grown by hot-filament metal-oxide vapor deposition with photoluminescence as an evaluation tool. Fine and clear emission features coming from the excitonic edge of the NiO are obviously observed in the photoluminescence spectra. A main excitonic emission of ~3.25 eV at 300 K can be decomposed into free exciton, bound excitons, and donor-acceptor-pair irradiations at lowered temperatures down to 10 K. The band-edge excitonic structure for the NiO nanocrystals has been evaluated and analyzed by transmission and thermoreflectacne measurements herein. All the experimental results demonstrate the cubic NiO thin-film nanotower is an applicable direct-band-gap material appropriate for UV luminescence and transparent-conducting-oxide applications. PMID:26506907

  7. Optical properties of halide and oxide compounds including the excitonic effects

    NASA Astrophysics Data System (ADS)

    Shwetha, G.; Kanchana, V.

    2014-04-01

    We have studied the optical properties of alkali halide and alkaline-earth oxide compounds including the excitonic effects by using the newly developed bootstrap kernel approximation for the exchange-correlation kernel of the Time-Dependent Density Functional Theory (TD-DFT) implemented in Full-Potential Linearized Augmented Plane Wave (FP-LAPW) method in the elk code. The bootstrap calculations are computationally less expensive and give results the same quality as the Bethe-Salpeter equation. We found improved results when compared to normal Density Functional Theory calculations, and observed results are comparable with the experiments. The lower energy peak of imaginary part of dielectric spectra shifts to lower energy regions as we move from MgO to BaO indicating the decrease in the band gap of these compounds from MgO to BaO. In all the studied compounds, the lower energy peak of the imaginary part of dielectric function is due to the transition from halogen p or oxide p states to metal derived s/d states.

  8. Optical control of exciton states and enhanced valley Zeeman splitting in WS2(Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Kioseoglou, George; Currie, Marc; Hanbicki, Aubrey T.; Paradisanos, Ioannis; Stratakis, Emmanuel; Fotakis, Costas; Jonker, Berrend T.

    2016-10-01

    Monolayer transition metal dichalcogenides, MX2 (M = Mo, W and X = S, Se), are direct-gap semiconductors with many interesting properties capable of producing an all-surface material applicable to sensing, single-atom storage and other quantum-based technologies. Here we report on the optical control of single layers of MX2 such that the photoluminescence (PL) is solely from the trion state. After trion isolation, changes in the Raman spectra were observed: there is a decrease in the intensity of the out of plane mode and an enhancement of the 2LA mode. The effect is reversible, and our results suggest that the changes of the strength of a particular excitonic state are due to surface interactions with ambient environment. In addition, spatial non-uniformity is probed by studying variations of strain and the PL emission as a function of position on our sample. The boundaries of mechanically exfoliated MX2 as well as boundaries intentionally created via fs laser ablation were investigated. The edges exhibit significant Raman shifts as well as remarkably enhanced PL emission compared to their respective central area. Finally, we probe the degree of circular polarization of the emitted PL as a function of the photo-excitation energy and temperature to elucidate spin-dependent inter- and intra-valley relaxation mechanisms. This work was supported by the FP7-REGPOT-2012-2013-1, under grant agreement 316165.

  9. Adaptive optics assisted reconfigurable liquid-driven optical switch

    NASA Astrophysics Data System (ADS)

    Fuh, Yiin-Kuen; Huang, Wei-Chi

    2013-07-01

    This study demonstrates a mechanical-based, liquid-driven optical switch integrated with adaptive optics and a reconfigurable black liquid (dye-doped liquid). The device aperture can be continuously tuned between 0.6 and 6.9 mm, precisely achieved by a syringe pump for volume control. Adaptive optics (AO) capability and possible enhancement of the lost power intensity of the ink-polluted glass plate have also been experimentally investigated. While measuring power intensity with/without AO indicates only a marginal difference of ˜1%, a significant difference of 3 s in the response characteristic of "switching on" time can be observed. An extremely high contrast ratio of ˜105 for a red-colored light is achieved.

  10. Optical Absorption Spectra and Excitons of Dye-Substrate Interfaces: Catechol on TiO2(110).

    PubMed

    Mowbray, Duncan John; Migani, Annapaola

    2016-06-14

    Optimizing the photovoltaic efficiency of dye-sensitized solar cells (DSSC) based on staggered gap heterojunctions requires a detailed understanding of sub-band gap transitions in the visible from the dye directly to the substrate's conduction band (CB) (type-II DSSCs). Here, we calculate the optical absorption spectra and spatial distribution of bright excitons in the visible region for a prototypical DSSC, catechol on rutile TiO2(110), as a function of coverage and deprotonation of the OH anchoring groups. This is accomplished by solving the Bethe-Salpeter equation (BSE) based on hybrid range-separated exchange and correlation functional (HSE06) density functional theory (DFT) calculations. Such a treatment is necessary to accurately describe the interfacial level alignment and the weakly bound charge transfer transitions that are the dominant absorption mechanism in type-II DSSCs. Our HSE06 BSE spectra agree semiquantitatively with spectra measured for catechol on anatase TiO2 nanoparticles. Our results suggest deprotonation of catechol's OH anchoring groups, while being nearly isoenergetic at high coverages, shifts the onset of the absorption spectra to lower energies, with a concomitant increase in photovoltaic efficiency. Further, the most relevant bright excitons in the visible region are rather intense charge transfer transitions with the electron and hole spatially separated in both the [110] and [001] directions. Such detailed information on the absorption spectra and excitons is only accessible via periodic models of the combined dye-substrate interface.

  11. Strong excitonic effects in the optical properties of graphitic carbon nitride g-C3N4 from first principles

    NASA Astrophysics Data System (ADS)

    Wei, Wei; Jacob, Timo

    2013-02-01

    Graphitic carbon nitride (g-C3N4) has recently triggered extensive investigations due to its potential applications, such as in direct photochemical water splitting, CO2 activation, and transition-metal-free spintronics. However, electronic, and particularly the optical properties of g-C3N4 still have not been well established. Based on one of the state-of-the-art approaches—many-body Green's function theory (i.e., GW + BSE)—absorption of ultraviolet light by g-C3N4 is found to be determined by strong excitonic effects with a significantly large binding energy assigned to the bound excitons. Dark states have also been found in g-C3N4, which can affect the photoluminescence yield of g-C3N4. We find that the band gap of g-C3N4 probably can be tuned by adjusting the condensation (dimensionality) to initiate excitonic absorption in the visible light region, which might help improve the solar energy conversion efficiency.

  12. Laser with optically driven Q-switch

    NASA Technical Reports Server (NTRS)

    Hemmati, Hamid (Inventor)

    1993-01-01

    An optically driven interactive Q-switch, i.e., a Q-switch that responds to a short pulse of light, for example, from external light-emitting diodes (LEDs) or diode lasers, is provided for producing an output laser pulse from electronic energy stored in a laser medium. Q-switching is thus achieved on demand by electrically pulsing the light source to produce a pulse of light directed onto a Q-switch medium in the laser cavity. Electronic control of the light pulse from the external source will thus provide not only efficient Q-switching frequency but also independent control of output laser pulse width with a fast rise time for each output laser pulse.

  13. Laser with optically driven Q-switch

    NASA Technical Reports Server (NTRS)

    Hemmati, Hamid (Inventor)

    1995-01-01

    An optically driven interactive Q-switch, i.e., a Q-switch that responds to a short pulse of light, for example, from external light-emitting diodes (LED's) or diode lasers, is provided for producing an output laser pulse from electronic energy stored in a laser medium. Q-switching is thus achieved on demand by electrically pulsing the light source to produce a pulse of light directed onto a Q-switch medium in the laser cavity. Electronic control of the light pulse from the external source will thus provide not only efficient Q-switching frequency but also independent control of output laser pulse width with a fast rise time for each output laser pulse.

  14. Exciton storage in type-II quantum dots using the optical Aharonov-Bohm effect

    SciTech Connect

    Climente, Juan I.; Planelles, Josep

    2014-05-12

    We investigate the bright-to-dark exciton conversion efficiency in type-II quantum dots subject to a perpendicular magnetic field. To this end, we take the exciton storage protocol recently proposed by Simonin and co-workers [Phys. Rev. B 89, 075304 (2014)] and simulate its coherent dynamics. We confirm the storage is efficient in perfectly circular structures subject to weak external electric fields, where adiabatic evolution is dominant. In practice, however, the efficiency rapidly degrades with symmetry lowering. Besides, the use of excited states is likely unfeasible owing to the fast decay rates. We then propose an adaptation of the protocol which does not suffer from these limitations.

  15. Vibrational Optical Activity of BODIPY Dimers: The Role of Magnetic-Electric Coupling in Vibrational Excitons.

    PubMed

    Abbate, Sergio; Bruhn, Torsten; Pescitelli, Gennaro; Longhi, Giovanna

    2017-01-12

    The vibrational exciton (VE) interpretation of intense bisignated couplets in vibrational circular dichroism (VCD) spectra of a pair of atropisomeric BODIPY (boron dipyrrin) dimers is discussed. The role of intrinsic magnetic moments is crucial to reproduce the different behaviors of quasi-isomeric BODIPY dimers with different aryl junction.

  16. Exciton-related nonlinear optical properties in cylindrical quantum dots with asymmetric axial potential: combined effects of hydrostatic pressure, intense laser field, and applied electric field

    PubMed Central

    2012-01-01

    The exciton binding energy of an asymmetrical GaAs-Ga1−xAlxAs cylindrical quantum dot is studied with the use of the effective mass approximation and a variational calculation procedure. The influence on this quantity of the application of a direct-current electric field along the growth direction of the cylinder, together with that of an intense laser field, is particularly considered. The resulting states are used to calculate the exciton-related nonlinear optical absorption and optical rectification, whose corresponding resonant peaks are reported as functions of the external probes, the quantum dot dimensions, and the aluminum molar fraction in the potential barrier regions. PMID:22971418

  17. Si dielectric function in a local basis representation: Optical properties, local field effects, excitons, and stopping power

    NASA Astrophysics Data System (ADS)

    Gómez, M.; González, P.; Ortega, J.; Flores, F.

    2014-11-01

    An atomiclike basis representation is used to analyze the dielectric function ɛ (q ⃗+G ⃗,q ⃗+G⃗';ω ) of Si. First, we show that a s p3d5 local basis set yields good results for the electronic band structure of this crystal and, then, we analyze the Si optical properties including local field and excitonic effects. In our formulation, we follow Hanke and Sham [W. Hanke and L. J. Sham, Phys. Rev. B 12, 4501 (1975), 10.1103/PhysRevB.12.4501; Phys. Rev. B 21, 4656 (1980), 10.1103/PhysRevB.21.4656], and introduce excitonic effects using a many-body formulation that incorporates a static screened electron-hole interaction. Dynamical effects in this interaction are also analyzed and shown to introduce non-negligible corrections in the optical spectrum. Our results are found in reasonable agreement with the experimental evidence and with other theoretical results calculated with the computationally more demanding plane-wave representation. Finally, calculations for the stopping power of Si are also presented.

  18. Repulsively bound exciton-biexciton states in high-spin fermions in optical lattices

    SciTech Connect

    Argueelles, A.; Santos, L.

    2011-03-15

    We show that the interplay between spin-changing collisions and quadratic Zeeman coupling provides a mechanism for the formation of repulsively bound composites in high-spin fermions, which we illustrate by considering spin flips in an initially polarized hard-core one-dimensional Mott insulator of spin-3/2 fermions. We show that after the flips the dynamics is characterized by the creation of two types of exciton-biexciton composites. We analyze the conditions for the existence of these bound states and discuss their intriguing properties. In particular we show that the effective mass and stability of the composites depends nontrivially on spin-changing collisions, on the quadratic Zeeman effect, and on the initial exciton localization. Finally, we show that the composites may remain stable against inelastic collisions, opening the possibility of interesting quantum composite phases.

  19. Exciton dynamics in conjugated polymer photovoltaics: Steady-state and time-resolved optical spectroscopy

    NASA Astrophysics Data System (ADS)

    Chasteen, Stephanie V.

    The performance of organic photovoltaics is severely limited by poor exciton dissociation and charge transport due in part to high rates of exciton recombination and low charge mobilities in polymers. This challenge can be partially overcome through the use of blended and layered heterojunctions. Such morphologies offer multiple exciton dissociation sites and separate charge pathways, thus limiting exciton recombination, and allowing for thicker, more absorbing, polymer films. I have performed photovoltaic device characterization and time-resolved and steady-state photoluminescence on a variety of donor-acceptor heterojunction. I have used these methods to understand excited state dynamics and how they affect device performance. As hole-transporters I use a derivative of poly-phenylene-vinylene (M3EH-PPV) and poly-3-hexylthiophene (P3HT). As electron-transporters I use the metal oxide titanium dioxide (TiO2), the electron-transporter CN-PPV, and a fullerene derivative (PCBM). These materials are layered and blended together to form donor-acceptor heterojunctions. All heterojunctions result in enhanced device performance, and 1:4 M3EH-PPV:PCBM resulted in the highest efficiencies. M3EH-PPV emission is characterized by single-chain excitations, and the decay is dominated by short components of 0.20 and 0.45 ns. CN-ether-PPV is dominated by interchain excited state species---ie., excimers---with a decay time of 14.0 ns. The broken conjugation imposed by the ether group affect the excited state, resulting in an excited state species that is particularly vulnerable to quenching. This has important ramifications for material design. Hole-transporting polymers blended and layered with CN-ether-PPV have high currents (Jsc up to 3.3 mA/cm2) and good quenching relative to CN-ether-PPV (˜90%) due to charge separation and generation, respectively. Hole-transporters blended with PCBM result in efficient devices (Jsc up to 14 mA/cm2) due to rapid charge transfer and the

  20. Optically Induced Nuclear Spin Polarization in the Quantum Hall Regime: The Effect of Electron Spin Polarization through Exciton and Trion Excitations.

    PubMed

    Akiba, K; Kanasugi, S; Yuge, T; Nagase, K; Hirayama, Y

    2015-07-10

    We study nuclear spin polarization in the quantum Hall regime through the optically pumped electron spin polarization in the lowest Landau level. The nuclear spin polarization is measured as a nuclear magnetic field B(N) by means of the sensitive resistive detection. We find the dependence of B(N) on the filling factor nonmonotonic. The comprehensive measurements of B(N) with the help of the circularly polarized photoluminescence measurements indicate the participation of the photoexcited complexes, i.e., the exciton and trion (charged exciton), in nuclear spin polarization. On the basis of a novel estimation method of the equilibrium electron spin polarization, we analyze the experimental data and conclude that the filling factor dependence of B(N) is understood by the effect of electron spin polarization through excitons and trions.

  1. Correlated structural-optical study of single nanocrystals in a gap-bar antenna: effects of plasmonics on excitonic recombination pathways.

    PubMed

    Wang, Feng; Karan, Niladri S; Nguyen, Hue Minh; Ghosh, Yagnaseni; Sheehan, Chris J; Hollingsworth, Jennifer A; Htoon, Han

    2015-06-07

    We performed time-correlated single-photon counting experiments on individual silica coated CdSe/CdS core/thick-shell nanocrystal quantum dots (a.k.a., giant NQDs [g-NQDs]), placed on the plasmonic gap-bar antennas. Optical properties were directly correlated with the scanning electron microscopy (SEM) images of g-NQD-plasmonic antenna coupled structures. The structures, in which the g-NQDs are located in the gap of the antenna, afford a coupling with up to 9.6 fold enhancement of radiative recombination rates. These coupled g-NQDs are also characterized by a strong enhancement of bi-exciton emission efficiency that increases with their radiative enhancement factor. By analysing these findings with a simple model, we show that the plasmonic field of the antenna does not alter the Auger recombination processes of the bi-exciton states. As a result, enhancements of the single and bi-exciton radiative recombination rates lead directly to bi-exciton emission enhancement. These findings suggest that a plasmonic field can be utilized effectively in achieving a strong bi-exciton emission that is needed for photon pair generation and plasmon-assisted lasing.

  2. Optical driven electromechanical transistor based on tunneling effect.

    PubMed

    Jin, Leisheng; Li, Lijie

    2015-04-15

    A new electromechanical transistor based on an optical driven vibrational ring structure has been postulated. In the device, optical power excites the ring structure to vibrate, which acts as the shuttle transporting electrons from one electrode to the other forming the transistor. The electrical current of the transistor is adjusted by the optical power. Coupled opto-electro-mechanical simulation has been performed. It is shown from the dynamic analysis that the stable working range of the transistor is much wider than that of the optical wave inside the cavity, i.e., the optical resonance enters nonperiodic states while the mechanical vibration of the ring is still periodic.

  3. Optical probes of excitonic phases in quantum Hall bilayers at νT=1*

    NASA Astrophysics Data System (ADS)

    Pellegrini, Vittorio

    2009-03-01

    In this talk we discuss our recent inelastic light scattering results that shed light on the interplay between incompressible and compressible quantum phases of electron bilayers at total filling factor νT = 1. In the regime of finite values of tunneling gaps, we observe a quantum phase transformation between composite fermion (CF) metal and incompressible excitonic states as the tunneling gap is reduced. We show that the transition becomes discontinuous (first-order) by impacts of different terms of the electron-electron interactions that prevail on weak residual disorder [1]. The evidence is based on precise determinations of the excitonic order parameter and of measurements of CF spin excitations by resonant inelastic light scattering close to the phase boundary [2,3]. While there is marked softening of low-lying excitations, our experiments underpin the roles of competing order parameters linked to quasi-particle correlations in removing the divergence of quantum fluctuations [4]. In the regime of vanishingly small tunneling gaps we show that the abrupt disappearing of CF spin excitations below the spin-wave indicates the emergence of the inter-layer correlated quantum Hall state in the vicinity of νT = 1 and when the temperature is lowered below a critical value [5]. Finally, the evolution of the spin-wave mode as a function of the Zeeman energy suggests the occurrence of a spin transition [5]. * Work done in collaboration with: B. Karmakar, A. Pinczuk, L.N. Pfeiffer, K.W. West.[4pt] [1] B. Karmakar, submitted; [2] S. Luin, et al. Phys. Rev. Lett. 94, 146804 (2005); [3] B. Karmakar et al. Solid State Communications 143, 499 (2007); [4] J. Schliemann, S. M. Girvin and A. H. MacDonald, Phys. Rev. Lett. 86, 1849 (2001); [5] B. Karmakar et al. unpublished.

  4. Effect of Lattice Screening on Excitonic and Optical Properties in CH3NH3PbI3 Solar Cell Materials

    NASA Astrophysics Data System (ADS)

    Leveillee, Joshua; Schleife, Andre; Andre Schleife Research Group Team

    Hybrid Organo-Metallic Perovskites have made great strides towards becoming a next generation solar cell material. Though high performing experimental devices have been constructed from these perovskites, the fundamental optical and electronic physics of these systems remains an active area of research. A large lattice dielectric constant in the Methylammonium(CH3NH3)-Lead(Pb)-Iodide(I3) system potentially contributes to the screening of the electron-hole interaction. The strongly increased dielectric screening due to lattice contributions has been suggested to reduce the exciton binding energy and strongly effects the optical band gap. In this study, we seek to understand, from first principles, the interplay between lattice screening and exciton binding energy. We use density functional theory for ground state calculations and the Bethe-Salpeter equation for the optical polarization function, from which we calculate optical spectra and excitonic properties. We will discuss differences between lattice and electronic screening and the effect on the optical properties of multiple CH3NH3PbI3 phases. Supported by NSF Grand Number: CBET-1437230.

  5. Terahertz optics: Terahertz-driven harmonics

    NASA Astrophysics Data System (ADS)

    Kim, K. Y.; You, Y. S.

    2014-02-01

    Researchers have demonstrated high-harmonic generation using strong terahertz pulses in a bulk solid without damaging it. The mechanism underpinning such an extreme nonlinearity also generates coherent electromagnetic radiation covering the terahertz, infrared and optical regions.

  6. Laser-driven polyplanar optic display

    SciTech Connect

    Veligdan, J.T.; Biscardi, C.; Brewster, C.; DeSanto, L.; Beiser, L.

    1998-01-01

    The Polyplanar Optical Display (POD) is a unique display screen which can be used with any projection source. This display screen is 2 inches thick and has a matte-black face which allows for high contrast images. The prototype being developed is a form, fit and functional replacement display for the B-52 aircraft which uses a monochrome ten-inch display. The new display uses a 200 milliwatt green solid-state laser (532 nm) as its optical source. In order to produce real-time video, the laser light is being modulated by a Digital Light Processing (DLP) chip manufactured by Texas Instruments, Inc. A variable astigmatic focusing system is used to produce a stigmatic image on the viewing face of the POD. In addition to the optical design, the authors discuss the DLP chip, the optomechanical design and viewing angle characteristics.

  7. Emulation of lossless exciton-polariton condensates by dual-core optical waveguides: stability, collective modes, and dark solitons.

    PubMed

    Salasnich, Luca; Malomed, Boris A; Toigo, Flavio

    2014-10-01

    We propose a possibility to simulate the exciton-polariton (EP) system in the lossless limit, which is not currently available in semiconductor microcavities, by means of a simple optical dual-core waveguide, with one core carrying the nonlinearity and operating close to the zero-group-velocity-dispersion point, and the other core being linear and dispersive. Both two-dimensional (2D) and one-dimensional (1D) EP systems may be emulated by means of this optical setting. In the framework of this system, we find that, while the uniform state corresponding to the lower branch of the nonlinear dispersion relation is stable against small perturbations, the upper branch is always subject to the modulational instability. The stability and instability are verified by direct simulations too. We analyze collective excitations on top of the stable lower-branch state, which include a Bogoliubov-like gapless mode and a gapped one. Analytical results are obtained for the corresponding sound velocity and energy gap. The effect of a uniform phase gradient (superflow) on the stability is considered too, with a conclusion that the lower-branch state becomes unstable above a critical wave number of the flux. Finally, we demonstrate that the stable 1D state may carry robust dark solitons.

  8. Relationship between molecular stacking and optical properties of 9,10-bis((4-N,N-dialkylamino)styryl) anthracene crystals: the cooperation of excitonic and dipolar coupling.

    PubMed

    Li, Feng; Gao, Na; Xu, Hai; Liu, Wei; Shang, Hui; Yang, Wenjun; Zhang, Ming

    2014-08-04

    Five 9,10-bis((4-N,N-dialkylamino)styryl) anthracene derivatives (DSA-C1-DSA-C7) with different length alkyl chains were synthesized. They showed the same color in dilute solutions but different colors in crystals. The absorption, photoluminescence, and fluorescence decay indicate that there exist both excitonic and dipolar coupling in crystals of DSA-C1-DSA-C7. X-ray crystallographic analysis revealed that all the crystals belong to the triclinic space group P1 with one molecule per unit cell and that the molecules in every crystal have the identical orientation. This offers ideal samples to investigate the impact of the molecular stacking on the optical properties of the crystals. For the first time, the cooperation of excitonic and dipolar coupling has been comprehensively studied, and the contribution to the spectral shift from the excitonic and dipolar couplings quantitatively obtained. The experiments of amplified spontaneous emission (ASE) together with measurements of the quantum efficiency further confirmed this interpretation. The results suggest that the excitonic and dipolar couplings between the adjacent molecules are both important and jointly induce the spectral shifts of the crystals.

  9. Laser induced magneto-Raman optical gain of an exciton and a biexciton in a CdTe/ZnTe quantum dot

    NASA Astrophysics Data System (ADS)

    Sujanah, P.; John Peter, A.; Lee, Chang Woo

    2016-06-01

    Magnetic field and laser field amplitude dependent electronic and optical properties of exciton and biexciton in a CdTe/ZnTe quantum dot nanostructure are brought out taking into account the spatial confinement effect. Binding energies of exciton and biexciton as functions of laser field amplitude and magnetic field strength are computed in a CdTe/ZnTe quantum dot for the constant dot radius 30 Å. Oscillator strength, resonant absorption coefficients and resonant optical Raman intensity of the exciton and biexciton as a function of laser field amplitude are obtained in the presence of magnetic field strength in a CdTe/ZnTe quantum dot. The laser field induced magneto-Raman gain is studied for a constant dot radii. The Coulomb interaction energy which is involved in Hartree potential is obtained numerically. The result shows that the applications of magnetic field strength and the laser field amplitude alter the electronic and optical properties considerably in the CdTe/ZnTe quantum dot.

  10. Acoustically induced stark effect for excitons in intrinsic semiconductors.

    PubMed

    Ivanov, A L; Littlewood, P B

    2001-09-24

    A Stark effect for excitons parametrically driven by coherent acoustic phonons is proposed. Our scheme refers to a low-temperature intrinsic semiconductor or semiconductor nanostructure pumped by an acoustic wave (frequency band nu(ac) approximately equal to 1-40 GHz and intensity range I(ac) approximately equal to 10(-2)-10(2) W/cm(2)) and probed by low-intensity light. Tunable optical band gaps, which strongly change the spectral shape of the exciton line, are induced in the polariton spectrum by acoustic pumping. We develop an exactly solvable model of the acoustic Stark effect and apply our results to GaAs driven by bulk or surface acoustic waves.

  11. Coherent Exciton Dynamics in Atomically Thin Semiconductors

    NASA Astrophysics Data System (ADS)

    Li, Xiaoqin (Elaine)

    The near band-edge optical response of an emerging class of semiconductors, known as the transitional metal dichalcogenides (TMDs), is dominated by tightly-bound excitons and charged excitons (i.e. trions). A fundamental property of these quasiparticles (excitons and trions) is quantum decoherence time, which reflects irreversible quantum dissipation arising from system (excitons and trions) and bath (vacuum and other quasiparticles) interactions and determines the timescale during which excitons can be coherently manipulated. Dephasing time is also equivalent to the intrinsic homogeneous linewidth of exciton resonance. In addition, excitons in TMDs are localized at the corners of the Brillouin zone and provide a convenient way to optical manipulate the valley degree of freedom, which may act as a useful information carrier analogous to electronic charge or spin. Direct measurement of valley coherence time is challenging because it corresponds to a non-radiative coherence between two degenerate states. Using ultrafast multi-dimensional optical spectroscopy, we investigate the intrinsic homogeneous linewidth of excitons, exciton valley coherence as well as coupling between excitons and trions. Our studies reveal coherent electronic dynamics on the order of ~100 fs in monolayer TMDs. We gratefully acknowledge financial support from NSF, AFOSR, and the Welch Foundation.

  12. The nonlinear optical properties of a magneto-exciton in a strained Ga0.2In0.8As/GaAs quantum dot

    NASA Astrophysics Data System (ADS)

    Senthil Kumar, N R..; Peter, A. John; Kyoo, Yoo Chang

    2013-10-01

    The magnetic field-dependent heavy hole excitonic states in a strained Ga0.2In0.8As/GaAs quantum dot are investigated by taking into account the anisotropy, non-parabolicity of the conduction band, and the geometrical confinement. The strained quantum dot is considered as a parabolic dot of InAs embedded in a GaAs barrier material. The dependence of the effective excitonic g-factor as a function of dot radius and the magnetic field strength is numerically measured. The interband optical transition energy as a function of geometrical confinement is computed in the presence of a magnetic field. The magnetic field-dependent oscillator strength of interband transition under the geometrical confinement is studied. The exchange enhancements as a function of dot radius are observed for various magnetic field strengths in a strained Ga0.2In0.8As/GaAs quantum dot. Heavy hole excitonic absorption spectra, the changes in refractive index, and the third-order susceptibility of third-order harmonic generation are investigated in the Ga0.2In0.8As/GaAs quantum dot. The result shows that the effect of magnetic field strength is more strongly dependent on the nonlinear optical property in a low-dimensional semiconductor system.

  13. Contribution of the transition moments, form of the absorption band, exciton, and the correlation effects in the linear and nonlinear optical properties of conjugated polymers

    NASA Astrophysics Data System (ADS)

    Díaz-Ponce, Javier Alejandro

    2017-04-01

    This work compares the linear and nonlinear optical properties of polyenes and polyenynes. The simulation was made for finite and infinite conjugation of conjugated polymers, such as polyacetylene, β-carotene, bis (p-toluene sulfonate) (PTS) polyenyne, and a short conjugated polyenyne poly-2,6-decadyin-1,6-ylene azelate (PHDAz). The resonance energy and degree of conjugation are determined by fitting the linear absorption spectra. These parameters are then used for calculating the two photon and third-order nonlinear optical properties. The contribution of the transition moment, the form of the absorption band, the exciton, and phonons in the optical properties are determined. The difference of the experimental values is assigned to correlation effects. We found that the exciton, the correlation effects, and the conduction band are more important in the optical properties of polyenynes than of polyenes. In this way, the dependence of the optical properties of polyenynes on the conduction band makes it possible to increase their nonlinear optical properties by interaction with photons (θ ≈ 0). The dependence of the optical properties on the conduction band also produces that the finiteness of the conjugation strongly decreases the optical properties of polyenynes in relation to polyenes with similar conjugation. On the other hand, the phonons are more important in the optical properties of polyenes than of polyenynes. Consequently, the band is indirect for the studied polyenes and direct for the polyenynes. Therefore, the nonlinear optical properties in the resonance frequency of polyenyne PTS are higher than those for polyacetylene. We also found that asymmetric oscillations of χ(3) in the Brillouin zone increases the χ(3) final value in polyenynes. In addition, we found a change of sign of the wave function coefficients by the Peierls distortion of polyenes to become polyenynes, but this change of sign does not affect the optical properties. As a corollary

  14. Electronic band gaps and exciton binding energies in monolayer M oxW1 -xS2 transition metal dichalcogenide alloys probed by scanning tunneling and optical spectroscopy

    NASA Astrophysics Data System (ADS)

    Rigosi, Albert F.; Hill, Heather M.; Rim, Kwang Taeg; Flynn, George W.; Heinz, Tony F.

    2016-08-01

    Using scanning tunneling spectroscopy (STS) and optical reflectance contrast measurements, we examine band-gap properties of single layers of transition metal dichalcogenide (TMDC) alloys: Mo S2 , M o0.5W0.5S2 , M o0.25W0.75S2 , M o0.1W0.9S2 , and W S2 . The quasiparticle band gap, spin-orbit separation of the excitonic transitions at the K /K' point in the Brillouin zone, and binding energies of the A exciton are extracted from STS and optical data. The exciton binding energies change roughly linearly with tungsten concentration. For our samples on an insulating substrate, we report quasiparticle band gaps from 2.17 ± 0.04 eV (Mo S2) to 2.38 ± 0.06 eV (W S2) , with A exciton binding energies ranging from 310 to 420 meV.

  15. Tangled nonlinear driven chain reactions of all optical singularities

    NASA Astrophysics Data System (ADS)

    Vasil'ev, V. I.; Soskin, M. S.

    2012-03-01

    Dynamics of polarization optical singularities chain reactions in generic elliptically polarized speckle fields created in photorefractive crystal LiNbO3 was investigated in details Induced speckle field develops in the tens of minutes scale due to photorefractive 'optical damage effect' induced by incident beam of He-Ne laser. It was shown that polarization singularities develop through topological chain reactions of developing speckle fields driven by photorefractive nonlinearities induced by incident laser beam. All optical singularities (C points, optical vortices, optical diabolos,) are defined by instantaneous topological structure of the output wavefront and are tangled by singular optics lows. Therefore, they have develop in tangled way by six topological chain reactions driven by nonlinear processes in used nonlinear medium (photorefractive LiNbO3:Fe in our case): C-points and optical diabolos for right (left) polarized components domains with orthogonally left (right) polarized optical vortices underlying them. All elements of chain reactions consist from loop and chain links when nucleated singularities annihilated directly or with alien singularities in 1:9 ratio. The topological reason of statistics was established by low probability of far enough separation of born singularities pair from existing neighbor singularities during loop trajectories. Topology of developing speckle field was measured and analyzed by dynamic stokes polarimetry with few seconds' resolution. The hierarchy of singularities govern scenario of tangled chain reactions was defined. The useful space-time data about peculiarities of optical damage evolution were obtained from existence and parameters of 'islands of stability' in developing speckle fields.

  16. Exciton size and quantum transport in nanoplatelets.

    PubMed

    Pelzer, Kenley M; Darling, Seth B; Gray, Stephen K; Schaller, Richard D

    2015-12-14

    Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport.

  17. Exciton size and quantum transport in nanoplatelets

    NASA Astrophysics Data System (ADS)

    Pelzer, Kenley M.; Darling, Seth B.; Gray, Stephen K.; Schaller, Richard D.

    2015-12-01

    Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport.

  18. Exciton size and quantum transport in nanoplatelets

    SciTech Connect

    Pelzer, Kenley M. Gray, Stephen K.; Darling, Seth B.; Schaller, Richard D.

    2015-12-14

    Two-dimensional nanoplatelets (NPLs) are an exciting class of materials with promising optical and energy transport properties. The possibility of efficient energy transport between nanoplatelets raises questions regarding the nature of energy transfer in these thin, laterally extended systems. A challenge in understanding exciton transport is the uncertainty regarding the size of the exciton. Depending on the material and defects in the nanoplatelet, an exciton could plausibly extend over an entire plate or localize to a small region. The variation in possible exciton sizes raises the question how exciton size impacts the efficiency of transport between nanoplatelet structures. Here, we explore this issue using a quantum master equation approach. This method goes beyond the assumptions of Förster theory to allow for quantum mechanical effects that could increase energy transfer efficiency. The model is extremely flexible in describing different systems, allowing us to test the effect of varying the spatial extent of the exciton. We first discuss qualitative aspects of the relationship between exciton size and transport and then conduct simulations of exciton transport between NPLs for a range of exciton sizes and environmental conditions. Our results reveal that exciton size has a strong effect on energy transfer efficiency and suggest that manipulation of exciton size may be useful in designing NPLs for energy transport.

  19. Excitons at the (001) surface of anatase: Spatial behavior and optical signatures

    SciTech Connect

    Giorgi, Giacomo; Yamashita, Koichi; Palummo, Maurizia; Chiodo, Letizia

    2011-08-15

    Within an ab initio study, based on the application of Many-Body Perturbation Theory approaches on top of ground-state Density Functional Theory calculations, we study the optical behavior of the TiO{sub 2} anatase (001) surface. We focus on the (1 x 1) and the (1 x 4) reconstructions, both experimentally observed, which reveal a different optical response and an anisotropy, in the (001) plane, not present in the bulk phase. The determination of the spatial behavior of the electron-hole photoexcited couple provides a possible explanation of the observed enhanced photocatalytic activity of TiO{sub 2} anatase nanostructures with a high percentage of (001)-(1 x 1) exposed facets.

  20. Driven optical matter: Dynamics of electrodynamically coupled nanoparticles in an optical ring vortex

    NASA Astrophysics Data System (ADS)

    Figliozzi, Patrick; Sule, Nishant; Yan, Zijie; Bao, Ying; Burov, Stanislav; Gray, Stephen K.; Rice, Stuart A.; Vaikuntanathan, Suriyanarayanan; Scherer, Norbert F.

    2017-02-01

    To date investigations of the dynamics of driven colloidal systems have focused on hydrodynamic interactions and often employ optical (laser) tweezers for manipulation. However, the optical fields that provide confinement and drive also result in electrodynamic interactions that are generally neglected. We address this issue with a detailed study of interparticle dynamics in an optical ring vortex trap using 150-nm diameter Ag nanoparticles. We term the resultant electrodynamically interacting nanoparticles a driven optical matter system. We also show that a superior trap is created by using a Au nanoplate mirror in a retroreflection geometry, which increases the electric field intensity, the optical drive force, and spatial confinement. Using nanoparticles versus micron sized colloids significantly reduces the surface hydrodynamic friction allowing us to access small values of optical topological charge and drive force. We quantify a further 50% reduction of hydrodynamic friction when the nanoparticles are driven over the Au nanoplate mirrors versus over a mildly electrostatically repulsive glass surface. Further, we demonstrate through experiments and electrodynamics-Langevin dynamics simulations that the optical drive force and the interparticle interactions are not constant around the ring for linearly polarized light, resulting in a strong position-dependent variation in the nanoparticle velocity. The nonuniformity in the optical drive force is also manifest as an increase in fluctuations of interparticle separation, or effective temperature, as the optical driving force is increased. Finally, we resolve an open issue in the literature on periodic modulation of interparticle separation with comparative measurements of driven 300-nm-diameter polystyrene beads that also clearly reveal the significance of electrodynamic forces and interactions in optically driven colloidal systems. Therefore, the modulations in the optical forces and electrodynamic interactions

  1. Distributed fibre optic strain measurements on a driven pile

    NASA Astrophysics Data System (ADS)

    Woschitz, Helmut; Monsberger, Christoph; Hayden, Martin

    2016-05-01

    In civil engineering pile systems are used in unstable areas as a foundation of buildings or other structures. Among other parameters, the load capacity of the piles depends on their length. A better understanding of the mechanism of load-transfer to the soil would allow selective optimisation of the system. Thereby, the strain variations along the loaded pile are of major interest. In this paper, we report about a field trial using an optical backscatter reflectometer for distributed fibre-optic strain measurements along a driven pile. The most significant results gathered in a field trial with artificial pile loadings are presented. Calibration results show the performance of the fibre-optic system with variations in the strain-optic coefficient.

  2. Theoretical Investigation Optical Properties of Si12C12 Clusters and Oligomers having Potential as Excitonic Materials

    NASA Astrophysics Data System (ADS)

    Duan, Xiaofeng; Burggraf, Larry

    2015-03-01

    SiC clusters may have potential in 2-D exciton circuits. We determined the most stable SinCn isomer structures (n <=12) out of hundreds to thousands isomers using a method combining Stochastic Potential Surface Search and Pseududopotential Plane-Wave Density Functional Theory Car-Parinello Molecular Dynamics simulated annealing (PSPW-CPMD-SA). Four low-energy Si12C12 isomer structures are discussed to illustrate the varying optical properties of clusters with structures: i) cage type with C- and Si- segregations, ii) symmetric type formed having π-stacked C aromatic rings and exterior Si regions, iii) nearly planar bowl with C fullerene fragment surrounded by Si atoms, and iv) symmetrical SiC cluster having alternate SiC bonding in the structure. We employed B3LYP and PBE0 functionals and both cc-pVTZ and aug-cc-pVTZ basis sets to perform TDDFT calculations of excitation energies and photo-absorption spectra to show how structure and bonding patterns affect photo excitations in different types of SiC clusters. The electron and the hole charge distribution patterns in excitation were calculated for major photoabsorption transitions, reported for the most stable isomer, closo Si12C12. To understand electric field effects we also calculated dynamical polarizabilities for all the four structures using Coupled Perturbed Hartree-Fock (CPHF) at B3LYP/aug-cc-pVTZ and PBE0/aug-cc-pVTZ level of theory. We gratefully acknowledge support from the Air Force Office of Scientific Research in a program managed by Dr Michael Berman.

  3. Suppression of quantum decoherence via infrared-driven coherent exciton-plasmon coupling: Undamped field and Rabi oscillations

    SciTech Connect

    Sadeghi, S. M.; Patty, K. D.

    2014-02-24

    We show that when a semiconductor quantum dot is in the vicinity of a metallic nanoparticle and driven by a mid-infrared laser field, its coherent dynamics caused by interaction with a visible laser field can become free of quantum decoherence. We demonstrate that this process, which can offer undamped Rabi and field oscillations, is the result of coherent normalization of the “effective” polarization dephasing time of the quantum dot (T{sub 2}{sup *}). This process indicates formation of infrared-induced coherently forced oscillations, which allows us to control the value of T{sub 2}{sup *} using the infrared laser. The results offer decay-free ultrafast modulation of the effective field experienced by the quantum dot when neither the visible laser field nor the infrared laser changes with time.

  4. Properties of Type-II ZnTe/ZnSe Submonolayer Quantum Dots Studied via Excitonic Aharonov- Bohm Effect and Polarized Optical Spectroscopy

    NASA Astrophysics Data System (ADS)

    Ji, Haojie

    In this thesis I develop understanding of the fundamental physical and material properties of type-II ZnTe/ZnSe submonolayer quantum dots (QDs), grown via combination of molecular beam epitaxy (MBE) and migration enhanced epitaxy (MEE). I use magneto-photoluminescence, including excitonic Aharonov-Bohm (AB) effect and polarized optical spectroscopy as the primary tools in this work. I present previous studies as well as the background of optical and magneto-optical processes in semiconductor nanostructures and introduce the experimental methods in Chapters 1 - 3. In Chapter 4 I focus on the excitonic AB effect in the type-II QDs. I develop a lateral tightly-bound exciton model for ZnTe/ZnSe type-II QDs, using analytical methods and numerical calculations. This explained the magneto-PL observation and allowed for establishing the size and density of the QDs in each sample based on the results of PL and magneto-PL measurements. For samples with larger QDs, I observe behaviors that fall between properties of quantum-dot and quantum-well-like systems due to increased QD densities and their type-II nature. Finally, the decoherence mechanisms of the AB excitons are investigated via the temperature dependent studies of the magneto-PL. It is determined that the AB exciton decoherence is due to transport-like (acoustic phonon) scattering of the electrons moving in the ZnSe barriers, but with substantially smaller magnitude of electron-phonon coupling constant due to relatively strong electron-hole coupling within these type-II QDs. In Chapter 5 I discuss the results of circularly polarized magneto-PL measurements. A model with ultra-long spin-flip time of holes confined to submonolayer QDs is proposed. The g-factor of type-II excitons was extracted from the Zeeman splitting and the g-factor of electrons was obtained by fitting the temperature dependence of the degree of circular polarization (DCP), from which g-factor of holes confined within ZnTe QDs was found. It is shown

  5. Optical magnon sidebands and exciton dynamics in the two-dimensional antiferromagnet BaMnF4 at T=2 K

    NASA Astrophysics Data System (ADS)

    Moncorgé, R.; Jacquier, B.; Mahiou, R.; Uihlein, C.

    1984-07-01

    Our previous observations of pure exciton and magnon-assisted optical transitions in the 4T1(4G) state of Mn2+ ions in the layer-type antiferromagnet BaMnF4 are analyzed in detail. The magnon density of states and the line profiles of the one-magnon sidebands are calculated with the adoption of a model in which only exchange interactions within the sheets perpendicular to the b axis are considered. The fits to the data allow us to obtain values of the exchange integrals J1 and J2 between near-neighbor Mn2+ ions along the a and c directions of the crystal in very close agreement with those obtained from neutron-diffraction data. In addition, distortion of the magnetic structure by the application of a strong magnetic field is shown to act directly on the exciton dynamics; it confirms its quasilocalized character at zero field as well as the exchange nature of the interionic interactions which allow the exciton to migrate in the crystal when the external field is switched on.

  6. Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides

    DOE PAGES

    Selig, Malte; Berghäuser, Gunnar; Raja, Archana; ...

    2016-11-07

    Atomically thin transition metal dichalcogenides are direct-gap semiconductors with strong light–matter and Coulomb interactions. The latter accounts for tightly bound excitons, which dominate their optical properties. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in the optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. We investigate the microscopic origin of the excitonic coherence lifetime in two representative materials (WS2 and MoSe2) through a study combining microscopic theory with spectroscopic measurements. We also show that the excitonic coherence lifetimemore » is determined by phonon-induced intravalley scattering and intervalley scattering into dark excitonic states. Particularly, we identify exciton relaxation processes involving phonon emission into lower-lying dark states that are operative at all temperatures, in WS2.« less

  7. Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides

    SciTech Connect

    Selig, Malte; Berghäuser, Gunnar; Raja, Archana; Nagler, Philipp; Schüller, Christian; Heinz, Tony F.; Korn, Tobias; Chernikov, Alexey; Malic, Ermin; Knorr, Andreas

    2016-11-07

    Atomically thin transition metal dichalcogenides are direct-gap semiconductors with strong light–matter and Coulomb interactions. The latter accounts for tightly bound excitons, which dominate their optical properties. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in the optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. We investigate the microscopic origin of the excitonic coherence lifetime in two representative materials (WS2 and MoSe2) through a study combining microscopic theory with spectroscopic measurements. We also show that the excitonic coherence lifetime is determined by phonon-induced intravalley scattering and intervalley scattering into dark excitonic states. Particularly, we identify exciton relaxation processes involving phonon emission into lower-lying dark states that are operative at all temperatures, in WS2.

  8. Excitonic linewidth and coherence lifetime in monolayer transition metal dichalcogenides

    PubMed Central

    Selig, Malte; Berghäuser, Gunnar; Raja, Archana; Nagler, Philipp; Schüller, Christian; Heinz, Tony F.; Korn, Tobias; Chernikov, Alexey; Malic, Ermin; Knorr, Andreas

    2016-01-01

    Atomically thin transition metal dichalcogenides are direct-gap semiconductors with strong light–matter and Coulomb interactions. The latter accounts for tightly bound excitons, which dominate their optical properties. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in the optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. Here, we investigate the microscopic origin of the excitonic coherence lifetime in two representative materials (WS2 and MoSe2) through a study combining microscopic theory with spectroscopic measurements. We show that the excitonic coherence lifetime is determined by phonon-induced intravalley scattering and intervalley scattering into dark excitonic states. In particular, in WS2, we identify exciton relaxation processes involving phonon emission into lower-lying dark states that are operative at all temperatures. PMID:27819288

  9. Optical conductivity renormalization of graphene on SrTiO 3 due to resonant excitonic effects mediated by Ti 3 d orbitals

    NASA Astrophysics Data System (ADS)

    Gogoi, Pranjal Kumar; Trevisanutto, Paolo E.; Yang, Ming; Santoso, Iman; Asmara, Teguh Citra; Terentjevs, Aleksandrs; Della Sala, Fabio; Breese, Mark B. H.; Venkatesan, T.; Feng, Yuan Ping; Loh, Kian Ping; Neto, Antonio H. Castro; Rusydi, Andrivo

    2015-01-01

    We present evidence of a drastic renormalization of the optical conductivity of graphene on SrTiO 3 resulting in almost full transparency in the ultraviolet region. These findings are attributed to resonant excitonic effects further supported by ab initio Bethe-Salpeter equation and density functional theory calculations. The (π ,π *) orbitals of graphene and Ti-3 d t2 g orbitals of SrTiO 3 are strongly hybridized and the interactions of electron-hole states residing in those orbitals play dominant role in the graphene optical conductivity. These interactions are present much below the optical band gap of bulk SrTiO 3. These results open a possibility of manipulating interaction strengths in graphene via d orbitals, which could be crucial for optical applications.

  10. Observation and simulation of an optically driven micromotor

    NASA Astrophysics Data System (ADS)

    Metzger, N. K.; Mazilu, M.; Kelemen, L.; Ormos, P.; Dholakia, K.

    2011-04-01

    In the realm of low Reynolds number flow there is a need to find methods to pump, move and mix minute amounts of analyte. Interestingly, micro-devices performing such actuation can be initiated by means of the light-matter interaction. Light induced forces and torques are exerted on such micro-objects, which are then driven by the optical gradient or scattering force. Here, different driving geometries can be realized to harness the light induced force. For example, the scattering force enables micro-gears to be operated in a tangential setup where the micromotor rotors are in line with an optical waveguide. The operational geometry we investigate has the advantage that it reduces the complexity of the driving of such a device in a microfluidic environment by delivering the actuating light by means of a waveguide or fiber optic. In this paper we explore the case of a micromotor being driven by a fiber optically delivered light beam. We experimentally investigate how the driving light interacts with and diffracts from the motor, utilizing two-photon imaging. The micromotor rotation rate dependence on the light field parameters is explored. Additionally, a theoretical model based on the paraxial approximation is used to simulate the torque and predict the rotation rate of such a device and compare it with experiment. The results presented show that our model can be used to optimize the micromotor performance and some example motor designs are evaluated.

  11. Spatially indirect excitons in coupled quantum wells

    SciTech Connect

    Lai, Chih-Wei Eddy

    2004-03-01

    observed. The spatial and energy distributions of optically active excitons were used as thermodynamic quantities to construct a phase diagram of the exciton system, demonstrating the existence of distinct phases. Optical and electrical properties of the CQW sample were examined thoroughly to provide deeper understanding of the formation mechanisms of these cold exciton systems. These insights offer new strategies for producing cold exciton systems, which may lead to opportunities for the realization of BEC in solid-state systems.

  12. Controlled wave-packet manipulation with driven optical lattices

    SciTech Connect

    Arlinghaus, Stephan; Holthaus, Martin

    2011-12-15

    Motivated by recent experimental progress achieved with ultracold atoms in kilohertz-driven optical lattices, we provide a theoretical discussion of mechanisms governing the response of a particle in a cosine lattice potential to strong forcing pulses with smooth envelope. Such pulses effectuate adiabatic motion of a wave packet's momentum distribution on quasienergy surfaces created by spatiotemporal Bloch waves. Deviations from adiabaticity can then be deliberately exploited for exerting coherent control and for reaching target states which may not be accessible by other means. As one particular example, we consider an analog of the {pi} pulses known from optical resonance. We also suggest adapting further techniques previously developed for controlling atomic and molecular dynamics by laser pulses to the coherent control of matter waves in shaken optical lattices.

  13. Optical density of states in ultradilute GaAsN alloy: Coexistence of free excitons and impurity band of localized and delocalized states

    SciTech Connect

    Bhuyan, Sumi; Pal, Bipul; Bansal, Bhavtosh; Das, Sanat K.; Dhar, Sunanda

    2014-07-14

    Optically active states in liquid phase epitaxy-grown ultra-dilute GaAsN are studied. The feature-rich low temperature photoluminescence spectrum has contributions from excitonic band states of the GaAsN alloy, and two types of defect states—localized and extended. The degree of delocalization for extended states both within the conduction and defect bands, characterized by the electron temperature, is found to be similar. The degree of localization in the defect band is analyzed by the strength of the phonon replicas. Stronger emission from these localized states is attributed to their giant oscillator strength.

  14. Contrastive analysis of multiple exciton generation theories

    NASA Astrophysics Data System (ADS)

    Tan, Hengyu; Chang, Qing

    2015-10-01

    Multiple exciton generation (MEG) is an effect that semiconductor nanocrystals (NCs) quantum dots (QDs) generate multiple excitons (electron-hole pairs) through absorbing a single high energy photon. It can translate the excess photon energy of bandgap (Eg) into new excitons instead of heat loss and improve the photovoltaic performance of solar cells. However, the theories of MEG are not uniform. The main MEG theories can be divided into three types. The first is impact ionization. It explains MEG through a conventional way that a photogenerated exciton becomes multiple excitons by Coulomb interactions between carriers. The Second is coherent superposition of excitonic states. Multiple excitons are generated by the coherent superposition of single photogenerated exciton state with enough excess momentum and the two-exciton state with the same momentum. The third is excitation via virtual excitonic states. The nanocrystals vacuum generates a virtual biexciton by coulomb coupling between two valence band electrons. The virtual biexciton absorbing a photon with an intraband optical transition is converted into a real biexciton. This paper describes the MEG influence on solar photoelectric conversion efficiency, concludes and analyzes the fundamentals of different MEG theories, the MEG experimental measure, their merits and demerits, calculation methods of generation efficiency.

  15. Exciton spin dynamics in GaSe

    SciTech Connect

    Tang, Yanhao; Xie, Wei; McGuire, John A. Lai, Chih Wei; Mandal, Krishna C.

    2015-09-21

    We analyze exciton spin dynamics in GaSe under nonresonant circularly polarized optical pumping with an exciton spin-flip rate-equation model. The model reproduces polarized time-dependent photoluminescence measurements in which the initial circular polarization approaches unity even when pumping with 0.15 eV excess energy. At T = 10 K, the exciton spin relaxation exhibits a biexponential decay with sub-20 ps and >500 ps time constants, which are also reproduced by the rate-equation model assuming distinct spin-relaxation rates for hot (nonequilibrium) and cold band-edge excitons.

  16. Magneto-optical spectrum and the effective excitonic Zeeman splitting energies of Mn and Co-doped CdSe nanowires

    SciTech Connect

    Xiong, Wen; Chen, Wensuo

    2013-12-21

    The electronic structure of Mn and Co-doped CdSe nanowires are calculated based on the six-band k·p effective-mass theory. Through the calculation, it is found that the splitting energies of the degenerate hole states in Mn-doped CdSe nanowires are larger than that in Co-doped CdSe nanowires when the concentration of these two kinds of magnetic ions is the same. In order to analysis the magneto-optical spectrum of Mn and Co-doped CdSe nanowires, the four lowest electron states and the four highest hole states are sorted when the magnetic field is applied, and the 10 lowest optical transitions between the conduction subbands and the valence subbands at the Γ point in Mn and Co-doped CdSe nanowires are shown in the paper, it is found that the order of the optical transitions at the Γ point almost do not change although two different kinds of magnetic ions are doped in CdSe nanowires. Finally, the effective excitonic Zeeman splitting energies at the Γ point are found to increase almost linearly with the increase of the concentration of the magnetic ions and the magnetic field; meanwhile, the giant positive effective excitonic g factors in Mn and Co-doped CdSe nanowires are predicted based on our theoretical calculation.

  17. Single-mode tunable laser emission in the single-exciton regime from colloidal nanocrystals.

    PubMed

    Grivas, Christos; Li, Chunyong; Andreakou, Peristera; Wang, Pengfei; Ding, Ming; Brambilla, Gilberto; Manna, Liberato; Lagoudakis, Pavlos

    2013-01-01

    Whispering-gallery-mode resonators have been extensively used in conjunction with different materials for the development of a variety of photonic devices. Among the latter, hybrid structures, consisting of dielectric microspheres and colloidal core/shell semiconductor nanocrystals as gain media, have attracted interest for the development of microlasers and studies of cavity quantum electrodynamic effects. Here we demonstrate single-exciton, single-mode, spectrally tuned lasing from ensembles of optical antenna-designed, colloidal core/shell CdSe/CdS quantum rods deposited on silica microspheres. We obtain single-exciton emission by capitalizing on the band structure of the specific core/shell architecture that strongly localizes holes in the core, and the two-dimensional quantum confinement of electrons across the elongated shell. This creates a type-II conduction band alignment driven by coulombic repulsion that eliminates non-radiative multi-exciton Auger recombination processes, thereby inducing a large exciton-bi-exciton energy shift. Their ultra-low thresholds and single-mode, single-exciton emission make these hybrid lasers appealing for various applications, including quantum information processing.

  18. The dynamics of radiation-driven, optically thick winds

    NASA Astrophysics Data System (ADS)

    Shen, Rong-Feng; Nakar, Ehud; Piran, Tsvi

    2016-06-01

    Recent observation of some luminous transient sources with low colour temperatures suggests that the emission is dominated by optically thick winds driven by super-Eddington accretion. We present a general analytical theory of the dynamics of radiation pressure-driven, optically thick winds. Unlike the classical adiabatic stellar wind solution whose dynamics are solely determined by the sonic radius, here the loss of the radiation pressure due to photon diffusion also plays an important role. We identify two high mass-loss rate regimes (dot{M} > L_Edd/c^2). In the large total luminosity regime, the solution resembles an adiabatic wind solution. Both the radiative luminosity, L, and the kinetic luminosity, Lk, are super-Eddington with L < Lk and L ∝ L_k^{1/3}. In the lower total luminosity regime, most of the energy is carried out by the radiation with Lk < L ≈ LEdd. In a third, low mass-loss regime (dot{M} < L_Edd/c^2), the wind becomes optically thin early on and, unless gas pressure is important at this stage, the solution is very different from the adiabatic one. The results are independent from the energy generation mechanism at the foot of the wind; therefore, they are applicable to a wide range of mass ejection systems, from black hole accretion, to planetary nebulae, and to classical novae.

  19. Optical Frequency Domain Visualization of Electron Beam Driven Plasma Wakefields

    NASA Astrophysics Data System (ADS)

    Zgadzaj, Rafal; Downer, M. C.; Muggli, Patric; Yakimenko, Vitaly; Babzien, Marcus; Kusche, Karl; Fedurin, Mikhail

    2010-11-01

    Beam-driven plasma wakefield accelerators (PWFA), such as the ``plasma afterburner,'' are a promising approach for significantly increasing the particle energies of conventional accelerators. The study and optimization of PWFA would benefit from an experimental correlation between the parameters of the drive bunch, the accelerated bunch and the corresponding, accelerating plasma wave structure. However, the plasma wave structure has not yet been observed directly in PWFA. We will report our current work on noninvasive optical Frequency Domain Interferometric (FDI) and Holographic (FDH) visualization of beam-driven plasma waves. Both techniques employ two laser pulses (probe and reference) co-propagating with the particle drive-beam and its plasma wake. The reference pulse precedes the drive bunch, while the probe overlaps the plasma wave and maps its longitudinal and transverse structure. The experiment is being developed at the BNL/ATF Linac to visualize wakes generated by two and multi-bunch drive beams.

  20. Emergent Cometlike Swarming of Optically Driven Thermally Active Colloids

    NASA Astrophysics Data System (ADS)

    Cohen, Jack A.; Golestanian, Ramin

    2014-02-01

    We propose a simple system of optically driven colloids that convert light into heat and move in response to self-generated and collectively generated thermal gradients. We show that the system exhibits self-organization into a moving cometlike swarm and characterize the structure and response of the swarm to a light-intensity-dependent external tuning parameter. We observe many interesting features in this nonequilibrium system including circulation and evaporation, intensity-dependent shape, density and temperature fluctuations, and ejection of hot colloids from the swarm tip.

  1. Excitons in Time-Dependent Density-Functional Theory.

    PubMed

    Ullrich, Carsten A; Yang, Zeng-hui

    2016-01-01

    This chapter gives an overview of the description of the optical and dielectric properties of bulk insulators and semiconductors in time-dependent density-functional theory (TDDFT), with an emphasis on excitons. We review the linear-response formalism for periodic solids, discuss excitonic exchange-correlation kernels, calculate exciton binding energies for various materials, and compare the treatment of excitons with TDDFT and with the Bethe-Salpeter equation.

  2. Optical Frequency Domain Visualization of Electron Beam Driven Plasma Wakefields

    SciTech Connect

    Zgadzaj, Rafal; Downer, Michael C.; Muggli, Patric; Yakimenko, Vitaly; Kusche, Karl; Fedurin, Michhail; Babzien, Marcus

    2010-11-04

    Bunch driven plasma wakefield accelerators (PWFA), such as the 'plasma afterburner', are a promising emerging method for significantly increasing the energy output of conventional particle accelerators. The study and optimization of this method would benefit from an experimental correlation of the drive bunch parameters and the accelerated particle parameters with the corresponding plasma wave structure. However, the plasma wave structure has not been observed directly so far. We will report ongoing development of a noninvasive optical Frequency Domain Interferometric (FDI) and Holographic (FDH) diagnostics of bunch driven plasma wakes. Both FDI and FDH have been previously demonstrated in the case of laser driven wakes. These techniques employ two laser pulses co-propagating with the drive particle bunch and the trailing plasma wave. One pulse propagates ahead of the drive bunch and serves as a reference, while the second is overlapped with the plasma wave and probes its structure. The multi-shot FDI and single-shot FDH diagnostics permit direct noninvasive observation of longitudinal and transverse structure of the plasma wakes. The experiment is being developed at the 70 MeV Linac in the Accelerator Test Facility at Brookhaven National Laboratory to visualize wakes generated by two and multi-bunch drive beams.

  3. Optical Frequency Domain Visualization of Electron Beam Driven Plasma Wakefields

    NASA Astrophysics Data System (ADS)

    Zgadzaj, Rafal; Downer, Michael C.; Muggli, Patric; Yakimenko, Vitaly; Kusche, Karl; Fedurin, Michhail; Babzien, Marcus

    2010-11-01

    Bunch driven plasma wakefield accelerators (PWFA), such as the "plasma afterburner," are a promising emerging method for significantly increasing the energy output of conventional particle accelerators [1]. The study and optimization of this method would benefit from an experimental correlation of the drive bunch parameters and the accelerated particle parameters with the corresponding plasma wave structure. However, the plasma wave structure has not been observed directly so far. We will report ongoing development of a noninvasive optical Frequency Domain Interferometric (FDI) [2] and Holographic (FDH) [3] diagnostics of bunch driven plasma wakes. Both FDI and FDH have been previously demonstrated in the case of laser driven wakes. These techniques employ two laser pulses co-propagating with the drive particle bunch and the trailing plasma wave. One pulse propagates ahead of the drive bunch and serves as a reference, while the second is overlapped with the plasma wave and probes its structure. The multi-shot FDI and single-shot FDH diagnostics permit direct noninvasive observation of longitudinal and transverse structure of the plasma wakes. The experiment is being developed at the 70 MeV Linac in the Accelerator Test Facility at Brookhaven National Laboratory to visualize wakes generated by two [4] and multi-bunch [5] drive beams.

  4. Optically driven micropump with a twin spiral microrotor.

    PubMed

    Maruo, Shoji; Takaura, Akira; Saito, Yohei

    2009-10-12

    An optically driven micropump that employs viscous drag exerted on a spinning microrotor with left- and right-handed spiral blades on its rotational axis has been developed using two-photon microfabrication. It was demonstrated that the twin spiral microrotor provides a higher rotation speed than a single spiral microrotor. The rotation speed reached 560 rpm at a laser power of 500 mW. The twin spiral microrotor was also applied to a viscous micropump with a U-shaped microchannel. To pump fluid, the twin spiral microrotor located at the corner of the U-shaped microchannel was rotated by focusing a laser beam. The flow field inside the U-shaped microchannel was analyzed using the finite element method (FEM) based on the Navier-Stokes equation to optimize the shape of the microchannel. It was confirmed that the rotation of the twin spiral microrotor generated a unidirectional laminar flow. Finally, a tandem micropump using two twin spiral microrotors was driven by a dual optical trapping system using a spatial light modulation technique.

  5. Theory of core excitons

    SciTech Connect

    Dow, J. D.; Hjalmarson, H. P.; Sankey, O. F.; Allen, R. E.; Buettner, H.

    1980-01-01

    The observation of core excitons with binding energies much larger than those of the valence excitons in the same material has posed a long-standing theoretical problem. A proposed solution to this problem is presented, and Frenkel excitons and Wannier excitons are shown to coexist naturally in a single material. (GHT)

  6. Excitations, optical absorption spectra, and optical excitonic gaps of heterofullerenes: I. C60, C59N+ and C48N12

    SciTech Connect

    Xie, R; Bryant, G W; Sun, G; C.Nicklaus, M; Heringer, D; Frauenheim, T; Manaa, M R; Smith, Jr., V H; Araki, Y; Ito, O

    2003-10-02

    Low-energy excitations and optical absorption spectrum of C{sub 60} are computed by using time-dependent (TD) Hartree-Fock (HF), TD-density functional theory (TD-DFT), TD-DFT-based tight-binding (TD-DFT-TB) and a semiempirical ZINDO method. A detailed comparison of experiment and theory for the excitation energies, optical gap and absorption spectrum of C{sub 60} is presented. It is found that electron correlations and collective effects of exciton pairs play important roles in assigning accurately the spectral features of C{sub 60} and the TD-DFT method with non-hybrid functionals or a local spin density approximation leads to more accurate excitation energies than with hybrid functionals. The level of agreement between theory and experiment for C{sub 60} justifies similar calculations of the excitations and optical absorption spectrum of a monomeric azafullerene cation C{sub 59}N{sup +} exhibits distinguishing spectral features different from C{sub 60}: (1) the first singlet is dipole-allowed and the optical gap is redshifted by 1.44 eV; (2) several weaker absorption maxima occur in the visible region; (3) the transient triplet-triplet absorption at 1.60 eV (775 nm) is much broader and the decay of the triplet state is much faster. The calculated spectra of C{sub 59}N{sup +} characterize and explain well our measured ultraviolet-visible (UV-vis) and transient absorption spectra of the carborane anion salt [C{sub 59}N][Ag(CB{sub 11}H{sub 6}Cl{sub 6}){sub 2}]. For the most stable isomer of C{sub 48}N{sub 12}, we predict that the first singlet is dipole-allowed, the optical gap is redshifted by 1.22 eV relative to that of C{sub 60}, and optical absorption maxima occur at 585, 528, 443, 363, 340, 314 and 303 nm. We point out that the characterization of the UV-vis and transient absorption spectra of C{sub 48}N{sub 12} isomers is helpful in distinguishing the isomer structures required for applications in molecular electronics. For C{sub 59}N{sup +} and C{sub 48}N

  7. Optically driven Archimedes micro-screws for micropump application.

    PubMed

    Lin, Chih-Lang; Vitrant, Guy; Bouriau, Michel; Casalegno, Roger; Baldeck, Patrice L

    2011-04-25

    Archimedes micro-screws have been fabricated by three-dimensional two-photon polymerization using a Nd:YAG Q-switched microchip laser at 532nm. Due to their small sizes they can be easily manipulated, and made to rotate using low power optical tweezers. Rotation rates up to 40 Hz are obtained with a laser power of 200 mW, i.e. 0.2 Hz/mW. A photo-driven micropump action in a microfluidic channel is demonstrated with a non-optimized flow rate of 6 pL/min. The optofluidic properties of such type of Archimedes micro-screws are quantitatively described by the conservation of momentum that occurs when the laser photons are reflected on the helical micro-screw surface.

  8. Intrinsic optical bistability in a strongly driven Rydberg ensemble

    NASA Astrophysics Data System (ADS)

    de Melo, Natalia R.; Wade, Christopher G.; Šibalić, Nikola; Kondo, Jorge M.; Adams, Charles S.; Weatherill, Kevin J.

    2016-06-01

    We observe and characterize intrinsic optical bistability in a dilute Rydberg vapor. The bistability is characterized by sharp jumps between states of low and high Rydberg occupancy with jump-up and -down positions displaying hysteresis depending on the direction in which the control parameter is changed. We find that the shift in frequency of the jump point scales with the fourth power of the principal quantum number. Also, the width of the hysteresis window increases with increasing principal quantum number, before reaching a peak and then closing again. The experimental results are consistent with predictions from a simple theoretical model based on semiclassical Maxwell-Bloch equations including the effects of interaction-induced broadening and level shifts. These results provide insight into the dynamics of driven dissipative systems.

  9. Exciton-Exciton Interaction in KMnF3

    NASA Astrophysics Data System (ADS)

    Strauss, E.; Maniscalco, W. J.; Yen, W. M.; Kellner, U. C.; Gerhardt, V.

    1980-03-01

    Exciton dynamics in KMnF3 at exciton densities up to 1016 cm-3 are examined with time-resolved emission spectroscopy. The exciton emission line shifts and broadens with increasing exciton density. A nonlinear exciton decay channel is observed. These effects are found to be consistent with an exciton-exciton process. The shift scales with the exciton density and suggests that the effect is dominated by pairwise interactions up to the densities reached in these experiments.

  10. Free-exciton states in crystalline GaTe

    NASA Astrophysics Data System (ADS)

    Wan, J. Z.; Brebner, J. L.; Leonelli, R.

    1995-12-01

    Polarized properties of both the singlet and triplet ground exciton states in the photoluminescence and transmission spectra of crystalline GaTe are explained based on the possible symmetry properties of the energy band edge of GaTe. Some experimental results about excited exciton states in GaTe are presented and discussed. The energy positions of exciton series in GaTe follow the three-dimensional direct allowed Wannier exciton formula just as in the the other III-VI layered compounds of GaSe and InSe. The nonthermalized, ``hot'' nature of excitons inside GaTe under higher optical excitation intensities is also discussed.

  11. Exciton dynamics in a single layer MoS2

    NASA Astrophysics Data System (ADS)

    Kim, Jonghwan; Hong, Xiaoping; Shi, Sufei; Jin, Chenhao; Sun, Yinghui; Wang, Feng

    2014-03-01

    In a low dimensional semiconductor, exciton plays a crucial role in the optical property. Recently, a single layer of MoS2 has attracted significant attention due to its unique excitonic features. For example, exciton in MoS2 is predicted to have order of magnitude larger binding energy than conventional direct band gap material. For deeper understanding on such properties, however, it is important to understand how exciton is formed and decays in time domain. Our work on exciton dynamics in MoS2 by pump probe spectroscopy will be presented with control of both power and wavelength.

  12. Exact matter-wave vortices in a driven optical lattice

    NASA Astrophysics Data System (ADS)

    Deng, Yan; Hai, Wenhua; Zhou, Zheng

    2013-07-01

    We investigate vortex dynamics of a periodically driven Bose-Einstein condensate confined in a spatially two-dimensional optical lattice. An exact Floquet solution of the Gross-Pitaevskii equation is obtained for a certain parameter region which can be divided into the phase-jumping and phase-continuing regions. In the former region, the exact solution can describe spatiotemporal evolution of multiple vortices. For a small ratio of driving strength to optical lattice depth the vortices keep nearly unmoved. With the increase of the ratio, the vortices undergo an effective interaction and periodically evolve along some fixed circular orbits that leads the vortex dipoles and quadrupoles to produce and break alternatively. There is a critical ratio in the phase-jumping region beyond which the vortices generate and melt periodically. In the phase-continuing region, the condensate in the exact Floquet state evolves periodically without zero-density nodes. It is numerically demonstrated that the exact solution is stable under an initial perturbation for both parameter regions, except for a subregion of the phase-jumping region in which stability of the condensate is lost. However, the solution is structurally stable under a small parameter perturbation only for the phase-continuing region, while for the whole phase-jumping region the structural stability is destroyed. The results suggest a scheme for creating and controlling matter-wave vortices.

  13. Role of the Zhang-Rice-like exciton in optical absorption spectra of CuGeO3 and CuGe1-xSixO3 single crystals

    NASA Astrophysics Data System (ADS)

    Pagliara, S.; Parmigiani, F.; Galinetto, P.; Revcolevschi, A.; Samoggia, G.

    2002-07-01

    The optical excitations in pure and Si-substituted CuGeO3 are investigated in the 15-300 K temperature range. A structure, detected for the E field parallel to the c axis of the crystals at about 3.2 eV, is due to the formation of a Zhang-Rice-like (ZR) exciton. The weak oscillator strength is due to the low mobility of the ZR-like exciton between the edge-sharing CuO4 plaquettes of CuGeO3. The temperature dependence of the ZR-like energy is explained by a renormalization effect involving phonon-electron interactions or changes in spin ordering, while the increase of ZR-like exciton intensity with the Si content is argued to arise from a different hybridization of the Si-O bond with respect to Ge-O.

  14. Boosting Hot-Electron Generation: Exciton Dissociation at the Order-Disorder Interfaces in Polymeric Photocatalysts.

    PubMed

    Wang, Hui; Sun, Xianshun; Li, Dandan; Zhang, Xiaodong; Chen, Shichuan; Shao, Wei; Tian, Yupeng; Xie, Yi

    2017-02-15

    Excitonic effects, arising from the Coulomb interactions between photogenerated electrons and holes, dominate the optical excitation properties of semiconductors, whereas their influences on photocatalytic processes have seldom been discussed. In view of the competitive generation of excitons and hot carriers, exciton dissociation is proposed as an alternative strategy for hot-carrier harvesting in photocatalysts. Herein, by taking heptazine-based melon as an example, we verified that enhanced hot-carrier generation could be obtained in semicrystalline polymeric photocatalysts, which is ascribed to the accelerated exciton dissociation at the abundant order-disorder interfaces. Moreover, driven by the accompanying electron injection toward ordered chains and hole blocking in disordered chains, semicrystalline heptazine-based melon showed an ∼7-fold promotion in electron concentration with respect to its pristine counterpart. Benefiting from these, the semicrystalline sample exhibited dramatic enhancements in electron-involved photocatalytic processes, such as superoxide radical production and selective alcohol oxidation. This work brightens excitonic aspects for the design of advanced photocatalysts.

  15. Single-mode tunable laser emission in the single-exciton regime from colloidal nanocrystals

    PubMed Central

    Grivas, Christos; Li, Chunyong; Andreakou, Peristera; Wang, Pengfei; Ding, Ming; Brambilla, Gilberto; Manna, Liberato; Lagoudakis, Pavlos

    2013-01-01

    Whispering-gallery-mode resonators have been extensively used in conjunction with different materials for the development of a variety of photonic devices. Among the latter, hybrid structures, consisting of dielectric microspheres and colloidal core/shell semiconductor nanocrystals as gain media, have attracted interest for the development of microlasers and studies of cavity quantum electrodynamic effects. Here we demonstrate single-exciton, single-mode, spectrally tuned lasing from ensembles of optical antenna-designed, colloidal core/shell CdSe/CdS quantum rods deposited on silica microspheres. We obtain single-exciton emission by capitalizing on the band structure of the specific core/shell architecture that strongly localizes holes in the core, and the two-dimensional quantum confinement of electrons across the elongated shell. This creates a type-II conduction band alignment driven by coulombic repulsion that eliminates non-radiative multi-exciton Auger recombination processes, thereby inducing a large exciton–bi-exciton energy shift. Their ultra-low thresholds and single-mode, single-exciton emission make these hybrid lasers appealing for various applications, including quantum information processing. PMID:23974520

  16. Optical spectroscopy and system-bath interactions in molecular aggregates with full configuration interaction Frenkel exciton model

    NASA Astrophysics Data System (ADS)

    Seibt, Joachim; Sláma, Vladislav; Mančal, Tomáš

    2016-12-01

    Standard application of the Frenkel exciton model neglects resonance coupling between collective molecular aggregate states with different number of excitations. These inter-band coupling terms are, however, of the same magnitude as the intra-band coupling between singly excited states. We systematically derive the Frenkel exciton model from quantum chemical considerations, and identify it as a variant of the configuration interaction method. We discuss all non-negligible couplings between collective aggregate states, and provide compact formulae for their calculation. We calculate absorption spectra of molecular aggregate of carotenoids and identify significant band shifts as a result of inter-band coupling. The presence of inter-band coupling terms requires renormalization of the system-bath coupling with respect to standard formulation, but renormalization effects are found to be weak. We present detailed discussion of molecular dimer and calculate its time-resolved two-dimensional Fourier transformed spectra to find weak but noticeable effects of peak amplitude redistribution due to inter-band coupling.

  17. Optical modeling of induction-linac driven free-electron lasers

    SciTech Connect

    Scharlemann, E.T.; Fawley, W.M.

    1986-03-31

    The free-electron laser (FEL) simulation code FRED, developed at Lawrence Livermore National Laboratory (LLNL) primarily to model single-pass FEL amplifiers driven by induction linear accelerators, is described. The main emphasis is on the modeling of optical propagation in the laser and on the differences between the requirements for modeling rf-linac-driven vs. induction-linac-driven FELs. Examples of optical guiding and mode cleanup are presented for a 50 ..mu..m FEL.

  18. Bright and dark excitons in semiconductor carbon nanotubes

    SciTech Connect

    Tretiak, Sergei

    2008-01-01

    We report electronic structure calculations of finite-length semiconducting carbon nanotubes using the time dependent density functional theory (TD-DFT) and the time dependent Hartree Fock (TD-HF) approach coupled with semiempirical AM1 and ZINDO Hamiltonians. We specifically focus on the energy splitting, relative ordering, and localization properties of the optically active (bright) and optically forbidden (dark) states from the lowest excitonic band of the nanotubes. These excitonic states are very important in competing radiative and non-radiative processes in these systems. Our analysis of excitonic transition density matrices demonstrates that pure DFT functionals overdelocalize excitons making an electron-hole pair unbound; consequently, excitonic features are not presented in this method. In contrast, the pure HF and A111 calculations overbind excitons inaccurately predicting the lowest energy state as a bright exciton. Changing AM1 with ZINDO Hamiltonian in TD-HF calculations, predicts the bright exciton as the second state after the dark one. However, in contrast to AM1 calculations, the diameter dependence of the excitation energies obtained by ZINDO does not follow the experimental trends. Finally, the TD-DFT approach incorporating hybrid functions with a moderate portion of the long-range HF exchange, such as B3LYP, has the most generality and predictive capacity providing a sufficiently accurate description of excitonic structure in finite-size nanotubes. These methods characterize four important lower exciton bands. The lowest state is dark, the upper band is bright, and the two other dark and nearly degenerate excitons lie in-between. Although the calculated energy splittings between the lowest dark and the bright excitons are relatively large ({approx}0.1 eV), the dense excitonic manifold below the bright exciton allows for fast non-radiative relaxation leasing to the fast population of the lowest dark exciton. This rationalizes the low

  19. Excitons in the Fractional Quantum Hall Effect

    DOE R&D Accomplishments Database

    Laughlin, R. B.

    1984-09-01

    Quasiparticles of charge 1/m in the Fractional Quantum Hall Effect form excitons, which are collective excitations physically similar to the transverse magnetoplasma oscillations of a Wigner crystal. A variational exciton wavefunction which shows explicitly that the magnetic length is effectively longer for quasiparticles than for electrons is proposed. This wavefunction is used to estimate the dispersion relation of these excitons and the matrix elements to generate them optically out of the ground state. These quantities are then used to describe a type of nonlinear conductivity which may occur in these systems when they are relatively clean.

  20. Radiative lifetime of excitons in carbon nanotubes.

    PubMed

    Perebeinos, Vasili; Tersoff, J; Avouris, Phaedon

    2005-12-01

    We calculate the radiative lifetime and energy bandstructure of excitons in semiconducting carbon nanotubes within a tight-binding approach including the electron-hole correlations via the Bethe-Salpeter equation. In the limit of rapid interband thermalization, the radiative decay rate is maximized at intermediate temperatures and decreases at low temperature because the lowest-energy excitons are optically forbidden. The intrinsic phonons cannot scatter excitons between optically active and forbidden bands, so sample-dependent extrinsic effects that break the symmetries can play a central role. We calculate the diameter-dependent energy splittings between singlet and triplet excitons of different symmetries and the resulting dependence of radiative lifetime on temperature and tube diameter.

  1. Simulation of Multi-Dimensional Signals in the Optical Domain: Quantum-Classical Feedback in Nonlinear Exciton Propagation.

    PubMed

    Richter, Martin; Fingerhut, Benjamin P

    2016-07-12

    We present an algorithm for the simulation of nonlinear 2D spectra of molecular systems in the UV-vis spectral region from atomistic molecular dynamics trajectories subject to nonadiabatic relaxation. We combine the nonlinear exciton propagation (NEP) protocol, that relies on a quasiparticle approach with the surface hopping methodology to account for quantum-classical feedback during the dynamics. Phenomena, such as dynamic Stokes shift due to nuclear relaxation, spectral diffusion, and population transfer among electronic states, are thus naturally included and benchmarked on a model of two electronic states coupled to a harmonic coordinate and a classical heatbath. The capabilities of the algorithm are further demonstrated for the bichromophore diphenylmethane that is described in a fully microscopic fashion including all 69 classical nuclear degrees of freedom. We demonstrate that simulated 2D signals are especially sensitive to the applied theoretical approximations (i.e., choice of active space in the CASSCF method) where population dynamics appears comparable.

  2. Coupled exciton-photon Bose condensate in path integral formalism

    NASA Astrophysics Data System (ADS)

    Elistratov, A. A.; Lozovik, Yu. E.

    2016-03-01

    We study the behavior of exciton polaritons in an optical microcavity with an embedded semiconductor quantum well. We use a two-component exciton-photon approach formulated in terms of path integral formalism. In order to describe spatial distributions of the exciton and photon condensate densities, the two coupled equations of the Gross-Pitaevskii type are derived. For a homogeneous system, we find the noncondensate photon and exciton spectra, calculate the coefficients of transformation from the exciton-photon basis to the lower-upper polariton basis, and obtain the exciton and photon occupation numbers of the lower and upper polariton branches for nonzero temperatures. For an inhomogeneous system, the set of coupled equations of the Bogoliubov-de Gennes type is derived. The equations govern the spectra and spatial distributions of noncondensate photons and excitons.

  3. A silicon-nanowire memory driven by optical gradient force induced bistability

    SciTech Connect

    Dong, B.; Cai, H. Gu, Y. D.; Kwong, D. L.; Chin, L. K.; Ng, G. I.; Ser, W.; Huang, J. G.; Yang, Z. C.; Liu, A. Q.

    2015-12-28

    In this paper, a bistable optical-driven silicon-nanowire memory is demonstrated, which employs ring resonator to generate optical gradient force over a doubly clamped silicon-nanowire. Two stable deformation positions of a doubly clamped silicon-nanowire represent two memory states (“0” and “1”) and can be set/reset by modulating the light intensity (<3 mW) based on the optical force induced bistability. The time response of the optical-driven memory is less than 250 ns. It has applications in the fields of all optical communication, quantum computing, and optomechanical circuits.

  4. Exciton Resonances in Novel Silicon Carbide Polymers

    NASA Astrophysics Data System (ADS)

    Burggraf, Larry; Duan, Xiaofeng

    2015-05-01

    A revolutionary technology transformation from electronics to excitionics for faster signal processing and computing will be advantaged by coherent exciton transfer at room temperature. The key feature required of exciton components for this technology is efficient and coherent transfer of long-lived excitons. We report theoretical investigations of optical properties of SiC materials having potential for high-temperature excitonics. Using Car-Parinello simulated annealing and DFT we identified low-energy SiC molecular structures. The closo-Si12C12 isomer, the most stable 12-12 isomer below 1100 C, has potential to make self-assembled chains and 2-D nanostructures to construct exciton components. Using TDDFT, we calculated the optical properties of the isomer as well as oligomers and 2-D crystal formed from the isomer as the monomer unit. This molecule has large optical oscillator strength in the visible. Its high-energy and low-energy transitions (1.15 eV and 2.56 eV) are nearly pure one-electron silicon-to-carbon transitions, while an intermediate energy transition (1.28 eV) is a nearly pure carbon-to-silicon one-electron charge transfer. These results are useful to describe resonant, coherent transfer of dark excitons in the nanostructures. Research supported by the Air Force Office of Scientific Research.

  5. Zero-gap semiconductor to excitonic insulator transition in Ta2NiSe5

    NASA Astrophysics Data System (ADS)

    Lu, Y. F.; Kono, H.; Larkin, T. I.; Rost, A. W.; Takayama, T.; Boris, A. V.; Keimer, B.; Takagi, H.

    2017-02-01

    The excitonic insulator is a long conjectured correlated electron phase of narrow-gap semiconductors and semimetals, driven by weakly screened electron-hole interactions. Having been proposed more than 50 years ago, conclusive experimental evidence for its existence remains elusive. Ta2NiSe5 is a narrow-gap semiconductor with a small one-electron bandgap EG of <50 meV. Below TC=326 K, a putative excitonic insulator is stabilized. Here we report an optical excitation gap Eop ~0.16 eV below TC comparable to the estimated exciton binding energy EB. Specific heat measurements show the entropy associated with the transition being consistent with a primarily electronic origin. To further explore this physics, we map the TC-EG phase diagram tuning EG via chemical and physical pressure. The dome-like behaviour around EG~0 combined with our transport, thermodynamic and optical results are fully consistent with an excitonic insulator phase in Ta2NiSe5.

  6. Zero-gap semiconductor to excitonic insulator transition in Ta2NiSe5

    PubMed Central

    Lu, Y. F.; Kono, H.; Larkin, T. I.; Rost, A. W.; Takayama, T.; Boris, A. V.; Keimer, B.; Takagi, H.

    2017-01-01

    The excitonic insulator is a long conjectured correlated electron phase of narrow-gap semiconductors and semimetals, driven by weakly screened electron–hole interactions. Having been proposed more than 50 years ago, conclusive experimental evidence for its existence remains elusive. Ta2NiSe5 is a narrow-gap semiconductor with a small one-electron bandgap EG of <50 meV. Below TC=326 K, a putative excitonic insulator is stabilized. Here we report an optical excitation gap Eop ∼0.16 eV below TC comparable to the estimated exciton binding energy EB. Specific heat measurements show the entropy associated with the transition being consistent with a primarily electronic origin. To further explore this physics, we map the TC–EG phase diagram tuning EG via chemical and physical pressure. The dome-like behaviour around EG∼0 combined with our transport, thermodynamic and optical results are fully consistent with an excitonic insulator phase in Ta2NiSe5. PMID:28205553

  7. Optical properties of ionized donor-bound excitons confined in strained wurtzite ZnO/MgxZn1-xO quantum dots

    NASA Astrophysics Data System (ADS)

    Dongmei, Zheng; Zongchi, Wang; Boqi, Xiao

    2015-03-01

    Within the framework of the effective-mass approximation and the dipole approximation, considering the three-dimensional confinement of the electron and hole and the strong built-in electric field (BEF) in strained wurtzite ZnO/Mg0.25Zn0.75O quantum dots (QDs), the optical properties of ionized donor-bound excitons (D+, X) are investigated theoretically using a variational method. The computations are performed in the case of finite band offset. Numerical results indicate that the optical properties of (D+, X) complexes sensitively depend on the donor position, the QD size and the BEF. The binding energy of (D+, X) complexes is larger when the donor is located in the vicinity of the left interface of the QDs, and it decreases with increasing QD size. The oscillator strength reduces with an increase in the dot height and increases with an increase in the dot radius. Furthermore, when the QD size decreases, the absorption peak intensity shows a marked increment, and the absorption coefficient peak has a blueshift. The strong BEF causes a redshift of the absorption coefficient peak and causes the absorption peak intensity to decrease remarkably. The physical reasons for these relationships have been analyzed in depth. Project supported by the National Natural Science Foundation for Young Scientists of China (No. 11102100), the Program for New Century Excellent Talents in Fujian Province University (No. JA14285) and the Program for Young Top-Notch Innovative Talents of Fujian Province of China.

  8. Fine structure of the exciton electroabsorption in semiconductor superlattices

    NASA Astrophysics Data System (ADS)

    Monozon, B. S.; Schmelcher, P.

    2017-02-01

    Wannier-Mott excitons in a semiconductor layered superlattice (SL) are investigated analytically for the case that the period of the superlattice is much smaller than the 2D exciton Bohr radius. Additionally we assume the presence of a longitudinal external static electric field directed parallel to the SL axis. The exciton states and the optical absorption coefficient are derived in the tight-binding and adiabatic approximations. Strong and weak electric fields providing spatially localized and extended electron and hole states, respectively, are studied. The dependencies of the exciton states and the exciton absorption spectrum on the SL parameters and the electric field strength are presented in an explicit form. We focus on the fine structure of the ground quasi-2D exciton level formed by the series of closely spaced energy levels adjacent from the high frequencies. These levels are related to the adiabatically slow relative exciton longitudinal motion governed by the potential formed by the in-plane exciton state. It is shown that the external electric fields compress the fine structure energy levels, decrease the intensities of the corresponding optical peaks and increase the exciton binding energy. A possible experimental study of the fine structure of the exciton electroabsorption is discussed.

  9. Exciton radiative lifetime in transition metal dichalcogenide monolayers

    NASA Astrophysics Data System (ADS)

    Robert, C.; Lagarde, D.; Cadiz, F.; Wang, G.; Lassagne, B.; Amand, T.; Balocchi, A.; Renucci, P.; Tongay, S.; Urbaszek, B.; Marie, X.

    2016-05-01

    We have investigated the exciton dynamics in transition metal dichalcogenide monolayers using time-resolved photoluminescence experiments performed with optimized time resolution. For MoS e2 monolayer, we measure τrad0=1.8 ±0.2 ps at T =7 K that we interpret as the intrinsic radiative recombination time. Similar values are found for WS e2 monolayers. Our detailed analysis suggests the following scenario: at low temperature (T ≲50 K ), the exciton oscillator strength is so large that the entire light can be emitted before the time required for the establishment of a thermalized exciton distribution. For higher lattice temperatures, the photoluminescence dynamics is characterized by two regimes with very different characteristic times. First the photoluminescence intensity drops drastically with a decay time in the range of the picosecond driven by the escape of excitons from the radiative window due to exciton-phonon interactions. Following this first nonthermal regime, a thermalized exciton population is established gradually yielding longer photoluminescence decay times in the nanosecond range. Both the exciton effective radiative recombination and nonradiative recombination channels including exciton-exciton annihilation control the latter. Finally the temperature dependence of the measured exciton and trion dynamics indicates that the two populations are not in thermodynamical equilibrium.

  10. (Gene sequencing by scanning molecular exciton microscopy)

    SciTech Connect

    Not Available

    1991-01-01

    This report details progress made in setting up a laboratory for optical microscopy of genes. The apparatus including a fluorescence microscope, a scanning optical microscope, various spectrometers, and supporting computers is described. Results in developing photon and exciton tips, and in preparing samples are presented. (GHH)

  11. Excitons in boron nitride single layer

    NASA Astrophysics Data System (ADS)

    Galvani, Thomas; Paleari, Fulvio; Miranda, Henrique P. C.; Molina-Sánchez, Alejandro; Wirtz, Ludger; Latil, Sylvain; Amara, Hakim; Ducastelle, François

    2016-09-01

    Boron nitride single layer belongs to the family of two-dimensional materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed in the bulk and still stronger effects are predicted for single layers. We present here a detailed study of these properties by combining ab initio calculations and a tight-binding Wannier analysis in both real and reciprocal space. Due to the simplicity of the band structure with single valence (π ) and conduction (π*) bands the tight-binding analysis becomes quasiquantitative with only two adjustable parameters and provides tools for a detailed analysis of the exciton properties. Strong deviations from the usual hydrogenic model are evidenced. The ground-state exciton is not a genuine Frenkel exciton, but a very localized tightly bound one. The other ones are similar to those found in transition-metal dichalcogenides and, although more localized, can be described within a Wannier-Mott scheme.

  12. Exciton absorption in narrow armchair graphene nanoribbons

    NASA Astrophysics Data System (ADS)

    Monozon, B. S.; Schmelcher, P.

    2016-11-01

    We develop an analytical approach to the exciton optical absorption for narrow gap armchair graphene nanoribbons (AGNR). We focus on the regime of dominant size quantization in combination with the attractive electron-hole interaction. An adiabatic separation of slow and fast motions leads via the two-body Dirac equation to the isolated and coupled subband approximations. Discrete and continuous exciton states are in general coupled and form quasi-Rydberg series of purely discrete and resonance type character. The corresponding oscillator strengths and widths are derived. We show that the exciton peaks are blue-shifted, become broader and increase in magnitude upon narrowing the ribbon. At the edge of a subband the singularity related to the 1D density of states is transformed into finite absorption via the presence of the exciton. Our analytical results are in good agreement with those obtained by other methods including numerical approaches. Estimates of the expected experimental values are provided for realistic AGNR.

  13. Optical and transport properties correlation driven by amorphous/crystalline disorder in InP nanowires

    NASA Astrophysics Data System (ADS)

    Kamimura, H.; Gouveia, R. C.; Carrocine, S. C.; Souza, L. D.; Rodrigues, A. D.; Teodoro, M. D.; Marques, G. E.; Leite, E. R.; Chiquito, A. J.

    2016-11-01

    Indium phosphide nanowires with a single crystalline zinc-blend core and polycrystalline/amorphous shell were grown from a reliable route without the use of hazardous precursors. The nanowires are composed by a crystalline core covered by a polycrystalline shell, presenting typical lengths larger than 10 μm and diameters of 80-90 nm. Raman spectra taken from as-grown nanowires exhibited asymmetric line shapes with broadening towards higher wave numbers which can be attributed to phonon localization effects. It was found that optical phonons in the nanowires are localized in regions with average size of 3 nm, which seems to have the same order of magnitude of grain sizes in the polycrystalline shell. Regardless of the fact that the nanowires exhibit a crystalline core, any considerable degree of disorder can lead to a localized behaviour of carriers. In consequence, the variable range hopping was observed as the main transport instead of the usual thermal excitation mechanisms. Furthermore the hopping length was ten times smaller than nanowire cross-sections, confirming that the nanostructures do behave as a 3D system. Accordingly, the V-shape observed in PL spectra clearly demonstrates a very strong influence of the potential fluctuations on the exciton optical recombination. Such fluctuations can still be observed at low temperature regime, confirming that the amorphous/polycrystalline shell of the nanowires affects the exciton recombination in every laser power regime tested.

  14. Strong charge-transfer excitonic effects and the Bose-Einstein exciton condensate in graphane.

    PubMed

    Cudazzo, Pierluigi; Attaccalite, Claudio; Tokatly, Ilya V; Rubio, Angel

    2010-06-04

    Using first principles many-body theory methods (GW+Bethe-Salpeter equation) we demonstrate that the optical properties of graphane are dominated by localized charge-transfer excitations governed by enhanced electron correlations in a two-dimensional dielectric medium. Strong electron-hole interaction leads to the appearance of small radius bound excitons with spatially separated electron and hole, which are localized out of plane and in plane, respectively. The presence of such bound excitons opens the path towards an excitonic Bose-Einstein condensate in graphane that can be observed experimentally.

  15. Application of metal-doped organic layer both as exciton blocker and optical spacer for organic photovoltaic devices

    NASA Astrophysics Data System (ADS)

    Chan, M. Y.; Lai, S. L.; Lau, K. M.; Lee, C. S.; Lee, S. T.

    2006-10-01

    An effective optical spacer based on doping of ytterbium (Yb) metal into bathophenanthroline (BPhen) has been developed for applications in organic photovoltaic (OPV) devices. Utilizing Yb:BPhen as an optical spacer in standard copper phthalocyanine/C60 photovoltaic devices, power efficiency can be increased by four times to 3.42%. Ultraviolet photoemission spectroscopy measurements reveal that the good electron transport between C60 and Yb:BPhen is mainly related to the suitable energy level alignment at the interface. Combining with its high optical transparency and electrical conductivity, the Yb:BPhen film provides a useful means for maximizing the power conversion efficiency of OPV devices.

  16. Lowest energy Frenkel and charge transfer exciton intermixing in one-dimensional copper phthalocyanine molecular lattice

    NASA Astrophysics Data System (ADS)

    Bondarev, I. V.; Popescu, A.; Younts, R. A.; Hoffman, B.; McAfee, T.; Dougherty, D. B.; Gundogdu, K.; Ade, H. W.

    2016-11-01

    We report the results of the combined experimental and theoretical studies of the low-lying exciton states in crystalline copper phthalocyanine. We derive the eigen energy spectrum for the two lowest intramolecular Frenkel excitons coupled to the intermolecular charge transfer exciton state and compare it with temperature dependent optical absorption spectra measured experimentally, to obtain the parameters of the Frenkel-charge-transfer exciton intermixing. The two Frenkel exciton states are spaced apart by 0.26 eV, and the charge transfer exciton state is 50 meV above the lowest Frenkel exciton. Both Frenkel excitons are strongly mixed with the charge transfer exciton, showing the coupling constant 0.17 eV which agrees with earlier experimental measurements. These results can be used for the proper interpretation of the physical properties of crystalline phthalocyanines.

  17. A new class of collective excitations: Exciton strings

    NASA Astrophysics Data System (ADS)

    Mazumdar, S.; Guo, F.; Meissner, K.; Fluegel, B.; Peyghambarian, N.

    1996-06-01

    Optical excitation in a strongly neutral quasi-one-dimensional mixed-stack charge-transfer solid results in an exciton state, in which the electron and the hole are bound by electrostatic Coulomb interactions that are large compared to the one-electron hopping. We present a joint theoretical-experimental demonstration of a new class of collective excitations, multiexcitons or exciton strings, consisting of a string of several (more than two) bound excitons, in a prototype neutral charge-transfer solid. The stability of the multiexciton states arise from the combined effects of one dimensionality and strong Coulomb interactions. Theoretically, we show that in narrow band one-dimensional semiconductors with long range Coulomb interactions, the occurrence of stable 2-exciton string (biexciton) necessarily implies stable higher multiexcitons. Experimentally, evidence for the multiexciton strings is demonstrated by femtosecond pump-probe spectroscopy of anthracene pyromellitic acid dianhydride. Excellent qualitative agreement is found between the calculated and the measured differential transmission spectra. Photoinduced absorptions to the 2-exciton string at low pump intensity and to the 3-exciton string at high pump intensity are observed, in agreement with the theory of excited state absorption. The 2-exciton string is confirmed also by a direct two-photon absorption measurement. The binding energies of the 2-exciton and the 3-exciton strings are obtained from the experimental data. The larger binding energy of the 3-exciton is in agreement with theory.

  18. Excitonic Stark effect in MoS2 monolayers

    NASA Astrophysics Data System (ADS)

    Scharf, Benedikt; Frank, Tobias; Gmitra, Martin; Fabian, Jaroslav; Žutić, Igor; Perebeinos, Vasili

    2016-12-01

    We theoretically investigate excitons in MoS2 monolayers in an applied in-plane electric field. Tight-binding and Bethe-Salpeter equation calculations predict a quadratic Stark shift, of the order of a few meV for fields of 10 V/μ m , in the linear absorption spectra. The spectral weight of the main exciton peaks decreases by a few percent with an increasing electric field due to the exciton field ionization into free carriers as reflected in the exciton wave functions. Subpicosecond exciton decay lifetimes at fields of a few tens of V/μ m could be utilized in solar energy harvesting and photodetection. We find simple scaling relations of the exciton binding, radius, and oscillator strength with the dielectric environment and an electric field, which provides a path to engineering the MoS2 electro-optical response.

  19. Excitonic gap formation and condensation in the bilayer graphene structure

    NASA Astrophysics Data System (ADS)

    Apinyan, V.; Kopeć, T. K.

    2016-09-01

    We have studied the excitonic gap formation in the Bernal Stacked, bilayer graphene (BLG) structures at half-filling. Considering the local Coulomb interaction between the layers, we calculate the excitonic gap parameter and we discuss the role of the interlayer and intralayer Coulomb interactions and the interlayer hopping on the excitonic pair formation in the BLG. Particularly, we predict the origin of excitonic gap formation and condensation, in relation to the furthermost interband optical transition spectrum. The general diagram of excitonic phase transition is given, explaining different interlayer correlation regimes. The temperature dependence of the excitonic gap parameter is shown and the role of the chemical potential, in the BLG, is discussed in details.

  20. Dirac cones and Dirac saddle points of bright excitons in monolayer transition metal dichalcogenides.

    PubMed

    Yu, Hongyi; Liu, Gui-Bin; Gong, Pu; Xu, Xiaodong; Yao, Wang

    2014-05-12

    In monolayer transition metal dichalcogenides, tightly bound excitons have been discovered with a valley pseudospin optically addressable through polarization selection rules. Here, we show that this valley pseudospin is strongly coupled to the exciton centre-of-mass motion through electron-hole exchange. This coupling realizes a massless Dirac cone with chirality index I = 2 for excitons inside the light cone, that is, bright excitons. Under moderate strain, the I = 2 Dirac cone splits into two degenerate I = 1 Dirac cones, and saddle points with a linear Dirac spectrum emerge. After binding an extra electron, the charged exciton becomes a massive Dirac particle associated with a large valley Hall effect protected from intervalley scattering. Our results point to unique opportunities to study Dirac physics, with exciton's optical addressability at specifiable momentum, energy and pseudospin. The strain-tunable valley-orbit coupling also implies new structures of exciton condensates, new functionalities of excitonic circuits and mechanical control of valley pseudospin.

  1. Optically induced excitonic electroabsorption in a periodically delta-doped InGaAs/GaAs multiple quantum well structure

    NASA Technical Reports Server (NTRS)

    Larsson, A.; Maserjian, J.

    1991-01-01

    Large optically induced Stark shifts have been observed in a periodically delta-doped InGaAs/GaAs multiple quantum well structure. With an excitation intensity of 10 mW/sq cm, an absolute quantum well absorption change of 7000/cm was measured with a corresponding differential absorption change as high as 80 percent. The associated maximum change in the quantum well refractive index is 0.04. This material is promising for device development for all-optical computing and signal processing.

  2. Excitons and multi-excitons in single CdTe quantum dots probed by near-field spectroscopy

    NASA Astrophysics Data System (ADS)

    Brun, M.; Huant, S.; Woehl, J. C.; Motte, J.-F.; Marsal, L.; Mariette, H.

    2002-03-01

    A near-field optical spectroscopy study of a single CdTe/ZnTe quantum dot at low temperatures is presented. While the photoluminescence spectrum at low excitation power reveals only one single sharp peak due to the radiative recombination of excitons (X) in the single dot, several additional sharp peaks appear with increasing excitation density. The dominant features are ascribed to exciton complexes and charged exciton complexes such as negatively charged excitons (X -), neutral (2X and 3X) and negative (2X - and 3X -) biexcitons and triexcitons. Exciton charging arises due to efficient hole trapping by residual acceptors in the barrier material. This partly inhibits the formation of biexcitons and triexcitons. A spectral feature appearing close to the X - peak is tentatively assigned to X 2- negative excitons. This feature is found to shift to the red with increasing power: two possible explanations for this unexpected behaviour are proposed.

  3. Determination of lateral size distribution of type-II ZnTe/ZnSe stacked submonolayer quantum dots via spectral analysis of optical signature of the Aharanov-Bohm excitons

    SciTech Connect

    Ji, Haojie; Dhomkar, Siddharth; Roy, Bidisha; Kuskovsky, Igor L.; Shuvayev, Vladimir; Deligiannakis, Vasilios; Tamargo, Maria C.; Ludwig, Jonathan; Smirnov, Dmitry; Wang, Alice

    2014-10-28

    For submonolayer quantum dot (QD) based photonic devices, size and density of QDs are critical parameters, the probing of which requires indirect methods. We report the determination of lateral size distribution of type-II ZnTe/ZnSe stacked submonolayer QDs, based on spectral analysis of the optical signature of Aharanov-Bohm (AB) excitons, complemented by photoluminescence studies, secondary-ion mass spectroscopy, and numerical calculations. Numerical calculations are employed to determine the AB transition magnetic field as a function of the type-II QD radius. The study of four samples grown with different tellurium fluxes shows that the lateral size of QDs increases by just 50%, even though tellurium concentration increases 25-fold. Detailed spectral analysis of the emission of the AB exciton shows that the QD radii take on only certain values due to vertical correlation and the stacked nature of the QDs.

  4. Multiple exciton generation and recombination in carbon nanotubes and nanocrystals.

    PubMed

    Kanemitsu, Yoshihiko

    2013-06-18

    Semiconducting nanomaterials such as single-walled carbon nanotubes (SWCNTs) and nanocrystals (NCs) exhibit unique size-dependent quantum properties. They have therefore attracted considerable attention from the viewpoints of fundamental physics and functional device applications. SWCNTs and NCs also provide an excellent new stage for experimental studies of many-body effects of electrons and excitons on optical processes in nanomaterials. In this Account, we discuss multiple exciton generation and recombination in SWCNTs and NCs for next-generation photovoltaics. Strongly correlated ensembles of conduction-band electrons and valence-band holes in semiconductors are complex quantum systems that exhibit unique optical phenomena. In bulk crystals, the carrier recombination dynamics can be described by a simple model, which includes the nonradiative single-carrier trapping rate, the radiative two-carrier recombination rate, and the nonradiative three-carrier Auger recombination rate. The nonradiative Auger recombination rate determines the carrier recombination dynamics at high carrier density and depends on the spatial localization of carriers in two-dimensional quantum wells. The Auger recombination and multiple exciton generation rates can be advantageously manipulated by nanomaterials with designated energy structures. In addition, SWCNTs and NCs show quantized recombination dynamics of multiple excitons and carriers. In one-dimensional SWCNTs, excitons have large binding energies and are very stable at room temperature. The extremely rapid Auger recombination between excitons determines the photoluminescence (PL) intensity, the PL linewidth, and the PL lifetime. SWCNTs can undergo multiple exciton generation, while strong exciton-exciton interactions and complicated exciton structures affect the quantized Auger rate and the multiple exciton generation efficiency. Interestingly, in zero-dimensional NC quantum dots, quantized Auger recombination causes unique

  5. Near-field optically driven Brownian motors (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Wu, Shao-Hua; Huang, Ningfeng; Jaquay, Eric; Povinelli, Michelle L.

    2016-09-01

    Brownian ratchets are of fundamental interest in fields from statistical physics to molecular motors. The realization of Brownian ratchets in engineered systems opens up the potential to harness thermal energy for directed motion, with applications in transport and sorting of nanoparticles. Implementations based on optical traps provide a high degree of tunability along with precise spatiotemporal control. Near-field optical methods provide particular flexibility and ease of on-chip integration with other microfluidic components. Here, we demonstrate the first all-optical, near-field Brownian ratchet. Our approach uses an asymmetrically patterned photonic crystal and yields an ultra-stable trap stiffness of 253.6 pN/nm-W, 100x greater than conventional optical tweezers. By modulating the laser power, optical ratcheting with transport speed of 1 micron/s can be achieved, allowing a variety of dynamical lab-on-a-chip applications. The resulting transport speed matches well with the theoretical prediction.

  6. Two-Exciton and Exciton-Magnon Bands in DIMANGANESE(+) Magnets.

    NASA Astrophysics Data System (ADS)

    Darwish, Saqer Mohammed

    The temperature dependence of several exciton -magnon and two-exciton bands in the optical absorption spectra of three antiferromagnets have been studied using a Cary 14 spectrophotometer in conjunction with an Air Product Displex, closed-cycle helium refrigerator. The three antiferromagnets with their T_{ rm N} are: KMnF_3, T_{rm N} = 88.3 K; RbMnF_3, T_{ rm N} = 82.6 K; and MnF_2 , T_{rm N} = 67.3 K. In this work the temperature dependence (10 ^circK to 300^ circK) of the line position E, the oscillator strength f, and the half-width of half maximum delta, for several of these bands were measured. For the two-exciton bands f increases where as for the exciton-magnon and exciton-magnon-phonon bands f decreases as the temperature is lowered through T _{rm N}. The temperature dependence of f for the two-exciton bands in the three antiferromagnets agrees well with the theoretical predictions of Fujiwara et al. For the exciton-magnon bands, f increases with increasing T up to T_{rm N} and then remains essentially constant above T_{rm N}, in reasonable agreement with the theory of Shinagawa and Tanabe. For the exciton-magnon-phonon bands, a slight increase in f above T_{rm N} is believed to be due to the role of a phonon. The temperature dependence of the line positions E(T) is also different for the exciton-magnon and two-exciton bands. The exciton-magnon or exciton-magnon-phonon bands undergo a blue shift in E(T) as the temperature is lowered through T_{rm N}. This is semiquantitatively understood in terms of the exchange field using the molecular field theory of Yen et al. On the other hand, most of the two-exciton bands do not show any anomaly in E(T) below T_{rm N}. Instead their line positions are described well by the Einstein-type relation E(T) = E(O) + A ^{*}/ (exp(T^ {*}/T) - 1), where T^{*} represents an odd symmetry phonon with frequency upsilon * = kT^{*} /h. Above T_{rm N} , the exciton-magnon-phonon bands also follow the same equation. From these fits

  7. Arbitrary GRIN component fabrication in optically driven diffusive photopolymers.

    PubMed

    Urness, Adam C; Anderson, Ken; Ye, Chungfang; Wilson, William L; McLeod, Robert R

    2015-01-12

    We introduce a maskless lithography tool and optically-initiated diffusive photopolymer that enable arbitrary two-dimensional gradient index (GRIN) polymer lens profiles. The lithography tool uses a pulse-width modulated deformable mirror device (DMD) to control the 8-bit gray-scale intensity pattern on the material. The custom polymer responds with a self-developing refractive index profile that is non-linear with optical dose. We show that this nonlinear material response can be corrected with pre-compensation of the intensity pattern to yield high fidelity, optically induced index profiles. The process is demonstrated with quadratic, millimeter aperture GRIN lenses, Zernike polynomials and GRIN Fresnel lenses.

  8. An effective magnetic field from optically driven phonons

    NASA Astrophysics Data System (ADS)

    Nova, T. F.; Cartella, A.; Cantaluppi, A.; Först, M.; Bossini, D.; Mikhaylovskiy, R. V.; Kimel, A. V.; Merlin, R.; Cavalleri, A.

    2016-10-01

    Light fields at terahertz and mid-infrared frequencies allow for the direct excitation of collective modes in condensed matter, which can be driven to large amplitudes. For example, excitation of the crystal lattice has been shown to stimulate insulator-metal transitions, melt magnetic order or enhance superconductivity. Here, we generalize these ideas and explore the simultaneous excitation of more than one lattice mode, which are driven with controlled relative phases. This nonlinear mode mixing drives rotations as well as displacements of the crystal-field atoms, mimicking the application of a magnetic field and resulting in the excitation of spin precession in the rare-earth orthoferrite ErFeO3. Coherent control of lattice rotations may become applicable to other interesting problems in materials research--for example, as a way to affect the topology of electronic phases.

  9. Micro-Ball-Lens Optical Switch Driven by SMA Actuator

    NASA Technical Reports Server (NTRS)

    Yang, Eui-Hyeok

    2003-01-01

    The figure is a simplified cross section of a microscopic optical switch that was partially developed at the time of reporting the information for this article. In a fully developed version, light would be coupled from an input optical fiber to one of two side-by-side output optical fibers. The optical connection between the input and the selected output fiber would be made via a microscopic ball lens. Switching of the optical connection from one output fiber to another would be effected by using a pair of thin-film shape-memory-alloy (SMA) actuators to toggle the lens between two resting switch positions. There are many optical switches some made of macroscopic parts by conventional fabrication techniques and some that are microfabricated and, hence, belong to the class of microelectromechanical systems (MEMS). Conventionally fabricated optical switches tend to be expensive. MEMS switches can be mass-produced at relatively low cost, but their attractiveness has been diminished by the fact that, heretofore, MEMS switches have usually been found to exhibit high insertion losses. The present switch is intended to serve as a prototype of low-loss MEMS switches. In addition, this is the first reported SMA-based optical switch. The optical fibers would be held in V grooves in a silicon frame. The lens would have a diameter of 1 m; it would be held by, and positioned between, the SMA actuators, which would be made of thin films of TiNi alloy. Although the SMA actuators are depicted here as having simple shapes for the sake of clarity of illustration, the real actuators would have complex, partly net-like shapes. With the exception of the lens and the optical fibers, the SMA actuators and other components of the switch would be made by microfabrication techniques. The components would be assembled into a sandwich structure to complete the fabrication of the switch. To effect switching, an electric current would be passed through one of the SMA actuators to heat it above

  10. Exciton management in organic photovoltaic multidonor energy cascades.

    PubMed

    Griffith, Olga L; Forrest, Stephen R

    2014-05-14

    Multilayer donor regions in organic photovoltaics show improved power conversion efficiency when arranged in decreasing exciton energy order from the anode to the acceptor interface. These so-called "energy cascades" drive exciton transfer from the anode to the dissociating interface while reducing exciton quenching and allowing improved overlap with the solar spectrum. Here we investigate the relative importance of exciton transfer and blocking in a donor cascade employing diphenyltetracene (D1), rubrene (D2), and tetraphenyldibenzoperiflanthene (D3) whose optical gaps monotonically decrease from D1 to D3. In this structure, D1 blocks excitons from quenching at the anode, D2 accepts transfer of excitons from D1 and blocks excitons at the interface between D2 and D3, and D3 contributes the most to the photocurrent due to its strong absorption at visible wavelengths, while also determining the open circuit voltage. We observe singlet exciton Förster transfer from D1 to D2 to D3 consistent with cascade operation. The power conversion efficiency of the optimized cascade OPV with a C60 acceptor layer is 7.1 ± 0.4%, which is significantly higher than bilayer devices made with only the individual donors. We develop a quantitative model to identify the dominant exciton processes that govern the photocurrent generation in multilayer organic structures.

  11. Optical response in a laser-driven quantum pseudodot system

    NASA Astrophysics Data System (ADS)

    Kilic, D. Gul; Sakiroglu, S.; Ungan, F.; Yesilgul, U.; Kasapoglu, E.; Sari, H.; Sokmen, I.

    2017-03-01

    We investigate theoretically the intense laser-induced optical absorption coefficients and refractive index changes in a two-dimensional quantum pseudodot system under an uniform magnetic field. The effects of non-resonant, monochromatic intense laser field upon the system are treated within the framework of high-frequency Floquet approach in which the system is supposed to be governed by a laser-dressed potential. Linear and nonlinear absorption coefficients and relative changes in the refractive index are obtained by means of the compact-density matrix approach and iterative method. The results of numerical calculations for a typical GaAs quantum dot reveal that the optical response depends strongly on the magnitude of external magnetic field and characteristic parameters of the confinement potential. Moreover, we have demonstrated that the intense laser field modifies the confinement and thereby causes remarkable changes in the linear and nonlinear optical properties of the system.

  12. Fabrication 3D buried channel optical waveguide modulators on field-driven ion exchange process

    NASA Astrophysics Data System (ADS)

    Zhou, Zigang; Chen, Wenqiang; Zhu, Li; Li, Jing; Luo, Xiaoying

    2010-10-01

    A high electric field technique was developed to fabricate buried optical waveguide modulator on K9 optical glass. The 80V voltage was applied on the glass to accelerate the field-driven ion exchange process by expeditiously replacing host sodium ions in the glass with silver ions. As a result, the optical loss for optical waveguide modulator was measured using the edge coupling technique with a 0.6328μm He-Ne laser. Loss of 0.20 dB/cm was obtained for channel waveguides of 25μm in depth, relatively low for waveguides of such depth at red wavelength.

  13. Microgravity-Driven Optic Nerve/Sheath Biomechanics Simulations

    NASA Technical Reports Server (NTRS)

    Ethier, C. R.; Feola, A.; Myers, J. G.; Nelson, E.; Raykin, J.; Samuels, B.

    2016-01-01

    Visual Impairment and Intracranial Pressure (VIIP) syndrome is a concern for long-duration space flight. Current thinking suggests that the ocular changes observed in VIIP syndrome are related to cephalad fluid shifts resulting in altered fluid pressures [1]. In particular, we hypothesize that increased intracranial pressure (ICP) drives connective tissue remodeling of the posterior eye and optic nerve sheath (ONS). We describe here finite element (FE) modeling designed to understand how altered pressures, particularly altered ICP, affect the tissues of the posterior eye and optic nerve sheath (ONS) in VIIP. METHODS: Additional description of the modeling methodology is provided in the companion IWS abstract by Feola et al. In brief, a geometric model of the posterior eye and optic nerve, including the ONS, was created and the effects of fluid pressures on tissue deformations were simulated. We considered three ICP scenarios: an elevated ICP assumed to occur in chronic microgravity, and ICP in the upright and supine positions on earth. Within each scenario we used Latin hypercube sampling (LHS) to consider a range of ICPs, ONH tissue mechanical properties, intraocular pressures (IOPs) and mean arterial pressures (MAPs). The outcome measures were biomechanical strains in the lamina cribrosa, optic nerve and retina; here we focus on peak values of these strains, since elevated strain alters cell phenotype and induce tissue remodeling. In 3D, the strain field can be decomposed into three orthogonal components, denoted as first, second and third principal strains. RESULTS AND CONCLUSIONS: For baseline material properties, increasing ICP from 0 to 20 mmHg significantly changed strains within the posterior eye and ONS (Fig. 1), indicating that elevated ICP affects ocular tissue biomechanics. Notably, strains in the lamina cribrosa and retina became less extreme as ICP increased; however, within the optic nerve, the occurrence of such extreme strains greatly increased as

  14. Excitonic luminescence upconversion in a two-dimensional semiconductor

    DOE PAGES

    Jones, Aaron M.; Yu, Hongyi; Schaibley, John R.; ...

    2015-12-21

    Photon upconversion is an elementary light-matter interaction process in which an absorbed photon is re-emitted at higher frequency after extracting energy from the medium. Furthermore, this phenomenon lies at the heart of optical refrigeration in solids(1), where upconversion relies on anti-Stokes processes enabled either by rare-earth impurities(2) or exciton-phonon coupling(3). We demonstrate a luminescence upconversion process from a negatively charged exciton to a neutral exciton resonance in monolayer WSe2, producing spontaneous anti-Stokes emission with an energy gain of 30 meV. Polarization-resolved measurements find this process to be valley selective, unique to monolayer semiconductors(4). Since the charged exciton binding energy(5) closelymore » matches the 31 meV A(1)' optical phonon(6-9), we ascribe the spontaneous excitonic anti-Stokes to doubly resonant Raman scattering, where the incident and outgoing photons are in resonance with the charged and neutral excitons, respectively. Additionally, we resolve a charged exciton doublet with a 7 meV splitting, probably induced by exchange interactions, and show that anti-Stokes scattering is efficient only when exciting the doublet peak resonant with the phonon, further confirming the excitonic doubly resonant picture.« less

  15. Excitonic luminescence upconversion in a two-dimensional semiconductor

    SciTech Connect

    Jones, Aaron M.; Yu, Hongyi; Schaibley, John R.; Yan, Jiaqiang; Mandrus, David G.; Taniguchi, Takashi; Watanabe, Kenji; Dery, Hanan; Yao, Wang; Xu, Xiaodong

    2015-12-21

    Photon upconversion is an elementary light-matter interaction process in which an absorbed photon is re-emitted at higher frequency after extracting energy from the medium. Furthermore, this phenomenon lies at the heart of optical refrigeration in solids(1), where upconversion relies on anti-Stokes processes enabled either by rare-earth impurities(2) or exciton-phonon coupling(3). We demonstrate a luminescence upconversion process from a negatively charged exciton to a neutral exciton resonance in monolayer WSe2, producing spontaneous anti-Stokes emission with an energy gain of 30 meV. Polarization-resolved measurements find this process to be valley selective, unique to monolayer semiconductors(4). Since the charged exciton binding energy(5) closely matches the 31 meV A(1)' optical phonon(6-9), we ascribe the spontaneous excitonic anti-Stokes to doubly resonant Raman scattering, where the incident and outgoing photons are in resonance with the charged and neutral excitons, respectively. Additionally, we resolve a charged exciton doublet with a 7 meV splitting, probably induced by exchange interactions, and show that anti-Stokes scattering is efficient only when exciting the doublet peak resonant with the phonon, further confirming the excitonic doubly resonant picture.

  16. Quantized Vortices and Four-Component Superfluidity of Semiconductor Excitons

    NASA Astrophysics Data System (ADS)

    Anankine, Romain; Beian, Mussie; Dang, Suzanne; Alloing, Mathieu; Cambril, Edmond; Merghem, Kamel; Carbonell, Carmen Gomez; Lemaître, Aristide; Dubin, François

    2017-03-01

    We study spatially indirect excitons of GaAs quantum wells, confined in a 10 μ m electrostatic trap. Below a critical temperature of about 1 K, we detect macroscopic spatial coherence and quantized vortices in the weak photoluminescence emitted from the trap. These quantum signatures are restricted to a narrow range of density, in a dilute regime. They manifest the formation of a four-component superfluid, made by a low population of optically bright excitons coherently coupled to a dominant fraction of optically dark excitons.

  17. Exciton storage in a nanoscale Aharonov-Bohm ring with electric field tuning.

    PubMed

    Fischer, Andrea M; Campo, Vivaldo L; Portnoi, Mikhail E; Römer, Rudolf A

    2009-03-06

    We study analytically the optical properties of a simple model for an electron-hole pair on a ring subjected to perpendicular magnetic flux and in-plane electric field. We show how to tune this excitonic system from optically active to optically dark as a function of these external fields. Our results offer a simple mechanism for exciton storage and readout.

  18. Exciton Storage in a Nanoscale Aharonov-Bohm Ring with Electric Field Tuning

    SciTech Connect

    Fischer, Andrea M.; Roemer, Rudolf A.; Campo, Vivaldo L. Jr.; Portnoi, Mikhail E.

    2009-03-06

    We study analytically the optical properties of a simple model for an electron-hole pair on a ring subjected to perpendicular magnetic flux and in-plane electric field. We show how to tune this excitonic system from optically active to optically dark as a function of these external fields. Our results offer a simple mechanism for exciton storage and readout.

  19. Probing excitonic dark states in single-layer tungsten disulphide

    NASA Astrophysics Data System (ADS)

    Ye, Ziliang; Cao, Ting; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G.; Zhang, Xiang

    2014-09-01

    Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors, exhibiting a number of intriguing optical phenomena such as valley-selective circular dichroism, doping-dependent charged excitons and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects. In particular, a recent study based on a GW plus Bethe-Salpeter equation (GW-BSE) approach, which employed many-body Green's-function methodology to address electron-electron and electron-hole interactions, theoretically predicted a diversity of strongly bound excitons. Here we report experimental evidence of a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (~0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many

  20. Optical field emission from resonant gold nanorods driven by femtosecond mid-infrared pulses

    SciTech Connect

    Kusa, F.; Echternkamp, K. E.; Herink, G.; Ropers, C.; Ashihara, S.

    2015-07-15

    We demonstrate strong-field photoelectron emission from gold nanorods driven by femtosecond mid-infrared optical pulses. The maximum photoelectron yield is reached at the localized surface plasmon resonance, indicating that the photoemission is governed by the resonantly-enhanced optical near-field. The wavelength- and field-dependent photoemission yield allows for a noninvasive determination of local field enhancements, and we obtain intensity enhancement factors close to 1300, in good agreement with finite-difference time domain computations.

  1. Optically Driven Spin Based Quantum Dots for Quantum Computing

    DTIC Science & Technology

    2008-01-01

    system approach to quantum optics, Lecture Notes in Physics (Springer, Berlin, 1993). [5] H. M. Wiseman and G. J. Milburn, Phys. Rev. Lett. 70, 548 (1993...Electrical Engineering and Computer Science Department of Physics Harrison M. Randall Laboratory of Physics The University of Michigan Ann Arbor, MI...48109 Phone: 734-764-4469 Email: dst@umich.edu Co-Principal Investigator: L.J. Sham Department of Physics The University of California – San

  2. Driven optical lattices as strong-field simulators

    SciTech Connect

    Arlinghaus, Stephan; Holthaus, Martin

    2010-06-15

    We argue that ultracold atoms in strongly shaken optical lattices can be subjected to conditions similar to those experienced by electrons in laser-irradiated crystalline solids, but without introducing secondary polarization effects. As a consequence, one can induce nonperturbative multiphoton-like resonances due to the mutual penetration of ac-Stark-shifted Bloch bands. These phenomena can be detected with a combination of currently available laboratory techniques.

  3. How to Draw Energy Level Diagrams in Excitonic Solar Cells.

    PubMed

    Zhu, X-Y

    2014-07-03

    Emerging photovoltaic devices based on molecular and nanomaterials are mostly excitonic in nature. The initial absorption of a photon in these materials creates an exciton that can subsequently dissociate in each material or at their interfaces to give charge carriers. Any attempt at mechanistic understanding of excitonic solar cells must start with drawing energy level diagrams. This seemingly elementary exercise, which is described in textbooks for inorganic solar cells, has turned out to be a difficult subject in the literature. The problem stems from conceptual confusion of single-particle energy with quasi-particle energy and the misleading practice of mixing the two on the same energy level diagram. Here, I discuss how to draw physically accurate energy diagrams in excitonic solar cells using only single-particle energies (ionization potentials and electron affinities) of both ground and optically excited states. I will briefly discuss current understanding on the electronic energy landscape responsible for efficient charge separation in excitonic solar cells.

  4. Exciton band structure in two-dimensional materials

    NASA Astrophysics Data System (ADS)

    Cudazzo, Pier Luigi; Sponza, Lorenzo; Giorgetti, Christine; Reining, Lucia; Sottile, Francesco; Gatti, Matteo

    In low-dimensional materials the screening of the Coulomb interaction is strongly reduced. As a consequence, the binding energy of both Wannier and Frenkel excitons in the optical spectra is large and comparable in size. Therefore, contrarily to bulk materials, it cannot serve as a criterion to distinguish different kinds of excitons. Here we demonstrate that the exciton band structure, which can be accessed experimentally, instead provides a powerful way to identify the exciton character. By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane and single-layer hexagonal BN, we draw a general picture of the exciton dispersion in two-dimensional materials, highlighting the different role played by the exchange electron-hole interaction and by the hopping terms related to the electronic band structure.

  5. Radiative properties of multicarrier bound excitons in GaAs

    NASA Astrophysics Data System (ADS)

    Karin, Todd; Barbour, Russell J.; Santori, Charles; Yamamoto, Yoshihisa; Hirayama, Yoshiro; Fu, Kai-Mei C.

    2015-04-01

    Excitons in semiconductors can have multiple lifetimes due to spin-dependent oscillator strengths and interference between different recombination pathways. In addition, strain and symmetry effects can further modify lifetimes via the removal of degeneracies. We present a convenient formalism for predicting the optical properties of k =0 excitons with an arbitrary number of charge carriers in different symmetry environments. Using this formalism, we predict three distinct lifetimes for the neutral acceptor bound exciton in GaAs, and confirm this prediction through polarization dependent and time-resolved photoluminescence experiments. We find the acceptor bound-exciton lifetimes to be To×(1 ,3 ,3/4 ) , where To=(0.61 ±0.12 ) ns . Furthermore, we provide an estimate of the intralevel and interlevel exciton spin-relaxation rates.

  6. Robust intravascular optical coherence elastography driven by acoustic radiation pressure

    NASA Astrophysics Data System (ADS)

    van Soest, Gijs; Bouchard, Richard R.; Mastik, Frits; de Jong, Nico; van der Steen, Anton F. W.

    2007-07-01

    High strain spots in the vessel wall indicate the presence of vulnerable plaques. The majority of acute cardiovascular events are preceded by rupture of such a plaque in a coronary artery. Intracoronary optical coherence tomography (OCT) can be extended, in principle, to an elastography technique, mapping the strain in the vascular wall. However, the susceptibility of OCT to frame-to-frame decorrelation, caused by tissue and catheter motion, inhibits reliable tissue displacement tracking and has to date obstructed the development of OCT-based intravascular elastography. We introduce a new technique for intravascular optical coherence elastography, which is robust against motion artifacts. Using acoustic radiation force, we apply a pressure to deform the tissue synchronously with the line scan rate of the OCT instrument. Radial tissue displacement can be tracked based on the correlation between adjacent lines, instead of subsequent frames in conventional elastography. The viability of the method is demonstrated with a simulation study. The root mean square (rms) error of the displacement estimate is 0.55 μm, and the rms error of the strain is 0.6%. It is shown that high-strain spots in the vessel wall, such as observed at the sites of vulnerable atherosclerotic lesions, can be detected with the technique. Experiments to realize this new elastographic method are presented. Simultaneous optical and ultrasonic pulse-echo tracking demonstrate that the material can be put in a high-frequency oscillatory motion with an amplitude of several micrometers, more than sufficient for accurate tracking with OCT. The resulting data are used to optimize the acoustic pushing sequence and geometry.

  7. Permanent Rabi oscillations in coupled exciton-photon systems with PT-symmetry.

    PubMed

    Chestnov, Igor Yu; Demirchyan, Sevak S; Alodjants, Alexander P; Rubo, Yuri G; Kavokin, Alexey V

    2016-01-21

    We propose a physical mechanism which enables permanent Rabi oscillations in driven-dissipative condensates of exciton-polaritons in semiconductor microcavities subjected to external magnetic fields. The method is based on stimulated scattering of excitons from the incoherent reservoir. We demonstrate that permanent non-decaying oscillations may appear due to the parity-time symmetry of the coupled exciton-photon system realized in a specific regime of pumping to the exciton state and depletion of the reservoir. At non-zero exciton-photon detuning, robust permanent Rabi oscillations occur with unequal amplitudes of exciton and photon components. Our predictions pave way to realization of integrated circuits based on exciton-polariton Rabi oscillators.

  8. Permanent Rabi oscillations in coupled exciton-photon systems with PT -symmetry

    PubMed Central

    Chestnov, Igor Yu.; Demirchyan, Sevak S.; Alodjants, Alexander P.; Rubo, Yuri G.; Kavokin, Alexey V.

    2016-01-01

    We propose a physical mechanism which enables permanent Rabi oscillations in driven-dissipative condensates of exciton-polaritons in semiconductor microcavities subjected to external magnetic fields. The method is based on stimulated scattering of excitons from the incoherent reservoir. We demonstrate that permanent non-decaying oscillations may appear due to the parity-time symmetry of the coupled exciton-photon system realized in a specific regime of pumping to the exciton state and depletion of the reservoir. At non-zero exciton-photon detuning, robust permanent Rabi oscillations occur with unequal amplitudes of exciton and photon components. Our predictions pave way to realization of integrated circuits based on exciton-polariton Rabi oscillators. PMID:26790534

  9. Self-organized optical device driven by motor proteins

    PubMed Central

    Aoyama, Susumu; Shimoike, Masahiko; Hiratsuka, Yuichi

    2013-01-01

    Protein molecules produce diverse functions according to their combination and arrangement as is evident in a living cell. Therefore, they have a great potential for application in future devices. However, it is currently very difficult to construct systems in which a large number of different protein molecules work cooperatively. As an approach to this challenge, we arranged protein molecules in artificial microstructures and assembled an optical device inspired by a molecular system of a fish melanophore. We prepared arrays of cell-like microchambers, each of which contained a scaffold of microtubule seeds at the center. By polymerizing tubulin from the fixed microtubule seeds, we obtained radially arranged microtubules in the chambers. We subsequently prepared pigment granules associated with dynein motors and attached them to the radial microtubule arrays, which made a melanophore-like system. When ATP was added to the system, the color patterns of the chamber successfully changed, due to active transportation of pigments. Furthermore, as an application of the system, image formation on the array of the optical units was performed. This study demonstrates that a properly designed microstructure facilitates arrangement and self-organization of molecules and enables assembly of functional molecular systems. PMID:24065817

  10. Phonon-Induced Dephasing of Excitons in Semiconductor Quantum Dots: Multiple Exciton Generation, Fission, and Luminescence

    NASA Astrophysics Data System (ADS)

    Madrid, Angeline; Kim, Hyeon-Deuk; Habenicht, Bradley; Prezhdo, Oleg

    2010-03-01

    Phonon-induced dephasing processes that govern optical line widths, multiple exciton (ME) generation (MEG), and ME fission (MEF) in semiconductor quantum dots (QDs) are investigated by ab initio molecular dynamics simulation. Using Si QDs as an example, we propose that MEF occurs by phonon-induced dephasing and, for the first time, estimate its time scale to be 100 fs. In contrast, luminescence and MEG dephasing times are all sub-10 fs. Generally, dephasing is faster for higher-energy and higher-order excitons and increased temperatures. MEF is slow because it is facilitated only by low-frequency acoustic modes. Luminescence and MEG couple to both acoustic and optical modes of the QD, as well as ligand vibrations. The detailed atomistic simulation of the dephasing processes advances understanding of exciton dynamics in QDs and other nanoscale materials.

  11. A Modular, IGBT Driven, Ignitron Switched, Optically Controlled Power Supply

    NASA Astrophysics Data System (ADS)

    Carroll, Evan; von der Linden, Jens; You, Setthivoine

    2013-10-01

    An experiment to investigate the dynamics of canonical flux tubes at the University of Washington uses two high energy pulsed power supplies to generate and sustain the plasma discharge. A modular 240 μF , 12 kV DC capacitor based power supply, discharged by ignitron, has been developed specifically for this application. Design considerations include minimizing inductance, rapid switching, fast rise times, and electrically isolated control. An optically coupled front panel and fast IGBT ignitron drive circuit, sequenced manually or by software, control the charge and discharge of the power supply. A complete, sequenced charge/discharge has been successfully tested with a dummy load, producing a peak current of 100 kA and a rise time of 25 μs . This work was sponsored in part by the US DOE Grant DE-SC0010340.

  12. The implementation of Grover's algorithm in optically driven quantum dots

    NASA Astrophysics Data System (ADS)

    Yin, W.; Liang, J. Q.; Yan, Q. W.

    2006-11-01

    In this paper, we study the implementation of Grover's algorithm using the system of three identical quantum dots (QDs) coupled by a multi-frequency optical field. Our result shows that increasing the electric field strength A speeds up the oscillations of the occupations of the excited states rather than increasing the occupation probabilities of those states. The larger the detuning of the field from resonance, the fewer the states which can be used as qubits. Compared with a multi-frequency external field, a single-frequency external field will generate much lower amplitudes of the excited states under the same coupling strength A and interdot Coulomb interaction V. However, when the three quantum dots are coupled with a single-frequency external field, these amplitudes increase on increasing the coupling strength A or decreasing the interdot Coulomb interaction V.

  13. Optical position feedback for electrostatically driven MOEMS scanners

    NASA Astrophysics Data System (ADS)

    Tortschanoff, A.; Baumgart, M.; Frank, A.; Wildenhain, M.; Sandner, T.; Schenk, H.; Kenda, A.

    2012-03-01

    For MOEMS devices which do not have intrinsic on-chip feedback, position information can be provided with optical methods, most simply by using a reflection from the backside of a MOEMS scanner. Measurement of timing signals using fast differential photodiodes can be used for resonant scanner mirrors performing sinusoidal motion with large amplitude. While this approach provides excellent accuracy it cannot be directly extended to arbitrary trajectories or static deflection angles. Another approach is based on the measurement of the position of the reflected laser beam with a quadrant diode. In this work, we present position sensing devices based on either principle and compare both approaches showing first experimental results from the implemented devices

  14. Diamagnetic excitons in semiconductors (Review)

    NASA Astrophysics Data System (ADS)

    Seisyan, R. P.

    2016-05-01

    Optical properties of semiconductor crystals in the presence of a high magnetic field have been considered. The field turn-on gives rise to oscillations of the optical-absorption edge or, more specifically, the formation of a complex absorption spectrum with a periodic structure, referred to as the spectrum of "diamagnetic excitons." Such spectra appear a source of the most accurate knowledge about the band structure of semiconductors. Moreover, these spectra can be used for simulating the low-dimensional state in semiconductors and possible interpretation of the emission spectra of neutron stars. The proposed analytical review is based on extensive experimental and theoretical data contained mostly in cited original works of the author with colleagues.

  15. All-optically driven system in ultrasonic wave-based structural health monitoring

    NASA Astrophysics Data System (ADS)

    Bi, Siwen; Wu, Nan; Zhou, Jingcheng; Zhang, Haifeng; Wang, Xingwei

    2016-04-01

    Ultrasonic wave based structural health monitoring (SHM) is an innovative method for nondestructive detection and an area of growing interest. This is due to high demands for wireless detection in the field of structural engineering. Through optically exciting and detecting ultrasonic waves, electrical wire connections can be avoided, and non-contact SHM can be achieved. With the combination of piezoelectric transducer (PZT) (which possesses high heat resistance) and the noncontact detection, this system has a broad range of applications, even in extreme conditions. This paper reports an all-optically driven SHM system. The resonant frequencies of the PZT transducers are sensitive to a variety of structural damages. Experimental results have verified the feasibility of the all-optically driven SHM system.

  16. Preliminary study of lever-based optical driven micro-actuator

    NASA Astrophysics Data System (ADS)

    Lin, Chih-Lang; Li, Yi-Hsiung; Lin, Chin-Te; Chiang, Chia-Chin; Liu, Yi-Jui; Chung, Tien-Tung; Baldeck, Patrice L.

    2012-04-01

    This study presents a novel type of optically driven lever-based micro-actuator fabricated using two-photon polymerization 3D-microfabrication technique. The lever is composed of a beam, an arch, and a sphere. First, optical tweezers is applied on the spheres to demonstrate the actuation of the lever. A spring is jointed at the lever for verifying the induced forces. Under the dragging by laser focusing, the lever simultaneously turns and results a torque like a mechanical arm. Then, the demonstration of a photo-driven micro-transducer with a mechanical arm and a gear is preformed. The experimental result indicates that our design enables precise manipulation of the mirco-actuator by optical tweezers at micron scale. This study provides a possibility for driving micron-sized structured mechanisms, such as connecting rods, valves. It is expected to contribute on the investigation of "Lab-on-a-chip".

  17. Identification of singlet and triplet states of negatively charged excitons in CdTe-based quantum wells

    NASA Astrophysics Data System (ADS)

    Astakhov, G. V.; Yakovlev, D. R.; Crooker, S. A.; Ossau, W.; Christianen, P. C. M.; Rudenkov, V. V.; Karczewski, G.; Wojtowicz, T.; Kossut, J.

    2004-02-01

    We present comprehensive study of negatively charged exciton in high magnetic fields for filling factors < 1. In magneto-optical spectra the fine structure was found to be contributed by neutral exciton and different a set of bound states of charged exciton. These states were identified due to their unique polarization properties charecteristics in emission and absorption spectra.

  18. Visible Light Driven Photocatalytic Reactor Based on Micro-structured Polymer Optical Fiber Preform

    NASA Astrophysics Data System (ADS)

    Li, Dong-Dong; She, Jiang-Bo; Wang, Chang-Shun; Peng, Bo

    2014-05-01

    A novel visible light driven photocatalytic reactor with 547 pieces of Ag/AgBr-film-modified capillaries is reported and it is derived from a microstructured polymer optical fiber (MPOF) preform. The MPOF preform not only plays the role of a light-transmitting media, but it is also a Ag/AgBr supporting and waste-water pipe to supply the photocatalytic degradation of dyes solute. The photocatalytic reactor has such a large surface area for Ag/AgBr loading, which is a visible light driven photocatalyst that photodegradation efficiency is enhanced.

  19. Transport of indirect excitons in high magnetic fields

    NASA Astrophysics Data System (ADS)

    Kuznetsova, Y. Y.; Dorow, C. J.; Calman, E. V.; Butov, L. V.; Wilkes, J.; Muljarov, E. A.; Campman, K. L.; Gossard, A. C.

    2017-03-01

    We present spatially and spectrally resolved photoluminescence measurements of indirect excitons in high magnetic fields. Long indirect exciton lifetimes give the opportunity to measure magnetoexciton transport by optical imaging. Indirect excitons formed from electrons and holes at zeroth Landau levels (0e-0h indirect magnetoexcitons) travel over large distances and form a ring emission pattern around the excitation spot. In contrast, the spatial profiles of 1e-1h and 2e-2h indirect magnetoexciton emission closely follow the laser excitation profile. The 0e-0h indirect magnetoexciton transport distance reduces with increasing magnetic field. These effects are explained in terms of magnetoexciton energy relaxation and effective mass enhancement.

  20. Electrical Activation of Dark Excitonic States in Carbon Nanotubes

    NASA Astrophysics Data System (ADS)

    Uda, Takushi; Yoshida, Masahiro; Ishii, Akihiro; Kato, Yuichiro K.

    Electrical activation of optical transitions to parity-forbidden dark excitonic states in individual carbon nanotubes is reported. We examine electric field effects on various excitonic states by simultaneously measuring both photocurrent and photoluminescence. As the applied field increases, we observe an emergence of new absorption peaks in the excitation spectra. From the diameter dependence of the energy separation between the new peaks and the ground state of E11 excitons, we attribute the peaks to the dark excited states which became optically active due to the applied field. A simple field-induced exciton dissociation model is introduced to explain the photocurrent threshold fields, and the edge of the E11 continuum states have been identified using this model. Work supported by JSPS (KAKENHI 24340066, 26610080), MEXT (Photon Frontier Network Program, Nanotechnology Platform), Canon Foundation, and Asahi Glass Foundation.

  1. Anomalous magnetization of a carbon nanotube as an excitonic insulator

    NASA Astrophysics Data System (ADS)

    Rontani, Massimo

    2014-11-01

    We show theoretically that an undoped carbon nanotube might be an excitonic insulator—the long-sought phase of matter proposed by Keldysh, Kohn, and others fifty years ago. We predict that the condensation of triplet excitons, driven by intervalley exchange interaction, spontaneously occurs at equilibrium if the tube radius is sufficiently small. The signatures of exciton condensation are its sizable contributions to both the energy gap and the magnetic moment per electron. The increase of the gap might have already been measured, albeit with a different explanation [V. V. Deshpande, B. Chandra, R. Caldwell, D. S. Novikov, J. Hone, and M. Bockrath, Science 323, 106 (2009), 10.1126/science.1165799]. The enhancement of the quasiparticle magnetic moment is a pair-breaking effect that counteracts the weak paramagnetism of the ground-state condensate of excitons. This property could rationalize the anomalous magnitude of magnetic moments recently observed in different devices close to charge neutrality.

  2. Deformation Measurement of a Driven Pile Using Distributed Fibre-optic Sensing

    NASA Astrophysics Data System (ADS)

    Monsberger, Christoph; Woschitz, Helmut; Hayden, Martin

    2016-03-01

    New developments in distributed fibre-optic sensing allow the measurement of strain with a very high precision of about 1 µm / m and a spatial resolution of 10 millimetres or even better. Thus, novel applications in several scientific fields may be realised, e. g. in structural monitoring or soil and rock mechanics. Especially due to the embedding capability of fibre-optic sensors, fibre-optic systems provide a valuable extension to classical geodetic measurement methods, which are limited to the surface in most cases. In this paper, we report about the application of an optical backscatter reflectometer for deformation measurements along a driven pile. In general, pile systems are used in civil engineering as an efficient and economic foundation of buildings and other structures. Especially the length of the piles is crucial for the final loading capacity. For optimization purposes, the interaction between the driven pile and the subsurface material is investigated using pile testing methods. In a field trial, we used a distributed fibre-optic sensing system for measuring the strain below the surface of an excavation pit in order to derive completely new information. Prior to the field trial, the fibre-optic sensor was investigated in the laboratory. In addition to the results of these lab studies, we briefly describe the critical process of field installation and show the most significant results from the field trial, where the pile was artificially loaded up to 800 kN. As far as we know, this is the first time that the strain is monitored along a driven pile with such a high spatial resolution.

  3. Coherent control of a strongly driven silicon vacancy optical transition in diamond

    PubMed Central

    Zhou, Yu; Rasmita, Abdullah; Li, Ke; Xiong, Qihua; Aharonovich, Igor; Gao, Wei-bo

    2017-01-01

    The ability to prepare, optically read out and coherently control single quantum states is a key requirement for quantum information processing. Optically active solid-state emitters have emerged as promising candidates with their prospects for on-chip integration as quantum nodes and sources of coherent photons connecting these nodes. Under a strongly driving resonant laser field, such quantum emitters can exhibit quantum behaviour such as Autler–Townes splitting and the Mollow triplet spectrum. Here we demonstrate coherent control of a strongly driven optical transition in silicon vacancy centre in diamond. Rapid optical detection of photons enabled the observation of time-resolved coherent Rabi oscillations and the Mollow triplet spectrum. Detection with a probing transition further confirmed Autler–Townes splitting generated by a strong laser field. The coherence time of the emitted photons is comparable to its lifetime and robust under a very strong driving field, which is promising for the generation of indistinguishable photons. PMID:28218237

  4. Coherent control of a strongly driven silicon vacancy optical transition in diamond

    NASA Astrophysics Data System (ADS)

    Zhou, Yu; Rasmita, Abdullah; Li, Ke; Xiong, Qihua; Aharonovich, Igor; Gao, Wei-Bo

    2017-02-01

    The ability to prepare, optically read out and coherently control single quantum states is a key requirement for quantum information processing. Optically active solid-state emitters have emerged as promising candidates with their prospects for on-chip integration as quantum nodes and sources of coherent photons connecting these nodes. Under a strongly driving resonant laser field, such quantum emitters can exhibit quantum behaviour such as Autler-Townes splitting and the Mollow triplet spectrum. Here we demonstrate coherent control of a strongly driven optical transition in silicon vacancy centre in diamond. Rapid optical detection of photons enabled the observation of time-resolved coherent Rabi oscillations and the Mollow triplet spectrum. Detection with a probing transition further confirmed Autler-Townes splitting generated by a strong laser field. The coherence time of the emitted photons is comparable to its lifetime and robust under a very strong driving field, which is promising for the generation of indistinguishable photons.

  5. Theory of Enhanced Interlayer Tunneling in Optically Driven High-T_{c} Superconductors.

    PubMed

    Okamoto, Jun-Ichi; Cavalleri, Andrea; Mathey, Ludwig

    2016-11-25

    Motivated by recent pump-probe experiments indicating enhanced coherent c-axis transport in underdoped YBCO, we study Josephson junctions periodically driven by optical pulses. We propose a mechanism for this observation by demonstrating that a parametrically driven Josephson junction shows an enhanced imaginary part of the low-frequency conductivity when the driving frequency is above the plasma frequency, implying an effectively enhanced Josephson coupling. We generalize this analysis to a bilayer system of Josephson junctions modeling YBCO. Again, the Josephson coupling is enhanced when the pump frequency is blue detuned to either of the two plasma frequencies of the material. We show that the emergent driven state is a genuine, nonequilibrium superconducting state, in which equilibrium relations between the Josephson coupling, current fluctuations, and the critical current no longer hold.

  6. Theory of Enhanced Interlayer Tunneling in Optically Driven High-Tc Superconductors

    NASA Astrophysics Data System (ADS)

    Okamoto, Jun-ichi; Cavalleri, Andrea; Mathey, Ludwig

    2016-11-01

    Motivated by recent pump-probe experiments indicating enhanced coherent c -axis transport in underdoped YBCO, we study Josephson junctions periodically driven by optical pulses. We propose a mechanism for this observation by demonstrating that a parametrically driven Josephson junction shows an enhanced imaginary part of the low-frequency conductivity when the driving frequency is above the plasma frequency, implying an effectively enhanced Josephson coupling. We generalize this analysis to a bilayer system of Josephson junctions modeling YBCO. Again, the Josephson coupling is enhanced when the pump frequency is blue detuned to either of the two plasma frequencies of the material. We show that the emergent driven state is a genuine, nonequilibrium superconducting state, in which equilibrium relations between the Josephson coupling, current fluctuations, and the critical current no longer hold.

  7. Grouping of optic flow stimuli during binocular rivalry is driven by monocular information.

    PubMed

    Holten, Vivian; Stuit, Sjoerd M; Verstraten, Frans A J; van der Smagt, Maarten J

    2016-10-01

    During binocular rivalry, perception alternates between two dissimilar images, presented dichoptically. Although binocular rivalry is thought to result from competition at a local level, neighboring image parts with similar features tend to be perceived together for longer durations than image parts with dissimilar features. This simultaneous dominance of two image parts is called grouping during rivalry. Previous studies have shown that this grouping depends on a shared eye-of-origin to a much larger extent than on image content, irrespective of the complexity of a static image. In the current study, we examine whether grouping of dynamic optic flow patterns is also primarily driven by monocular (eye-of-origin) information. In addition, we examine whether image parameters, such as optic flow direction, and partial versus full visibility of the optic flow pattern, affect grouping durations during rivalry. The results show that grouping of optic flow is, as is known for static images, primarily affected by its eye-of-origin. Furthermore, global motion can affect grouping durations, but only under specific conditions. Namely, only when the two full optic flow patterns were presented locally. These results suggest that grouping during rivalry is primarily driven by monocular information even for motion stimuli thought to rely on higher-level motion areas.

  8. Proposal for dark exciton based chemical sensors

    NASA Astrophysics Data System (ADS)

    Feierabend, Maja; Berghäuser, Gunnar; Knorr, Andreas; Malic, Ermin

    2017-03-01

    The rapidly increasing use of sensors throughout different research disciplines and the demand for more efficient devices with less power consumption depends critically on the emergence of new sensor materials and novel sensor concepts. Atomically thin transition metal dichalcogenides have a huge potential for sensor development within a wide range of applications. Their optimal surface-to-volume ratio combined with strong light-matter interaction results in a high sensitivity to changes in their surroundings. Here, we present a highly efficient sensing mechanism to detect molecules based on dark excitons in these materials. We show that the presence of molecules with a dipole moment transforms dark states into bright excitons, resulting in an additional pronounced peak in easy accessible optical spectra. This effect exhibits a huge potential for sensor applications, since it offers an unambiguous optical fingerprint for the detection of molecules--in contrast to common sensing schemes relying on small peak shifts and intensity changes.

  9. Two-dimensional coherent spectroscopy of excitons, biexcitons, and exciton-polaritons

    NASA Astrophysics Data System (ADS)

    Bristow, Alan D.

    2016-10-01

    Semiconductors systems exhibiting excitonic properties are discussed in terms of their coherent response, which is extracted using two-dimensional coherent spectroscopy. This control method allows for separation of quantum pathways that comprise the optical response, such as interactions between excitons, their dephasing rates, the effects of many-body interactions and the role of structure on the microscopic electronic environment. Additional controls, such as polarization can be used to further distinguish biexcitons and suppress many-body interactions. These result are compared and contrasted with those from a semiconductor microcavity where the excitons form polaritonic modes due to normal-mode splitting. Rephrasing spectra map the detuning dependence of the exciton-polariton branches. Increasing the detuning shifts all features to higher energy and the expected anti-crossing is observed. An isolated biexciton is seen only at negative detuning, separated by a binding energy. For positive detuning, the spectral weight of the off-diagonal features swap, as the lower polariton branch and biexciton come into resonance. This indicates that the off-diagonal features are sensitive to the interactions of the exciton-polaritons and other resonances in the system.

  10. Nano-stepper-driven optical shutter for applications in free-space micro-optics

    NASA Astrophysics Data System (ADS)

    Zawadzka, Justyna; Li, Lijie; Unamuno, Anartz; Uttamchandani, Deepak G.

    2002-09-01

    In this paper we report a simple design of a micro-optical shutter/attenuator. The standard MUMPS process was used to fabricate the device. A vertically erected, gold-coated, 200x300 mm side length micro-mirror was precisely placed between the end faces of two closely spaced optical fibers. The position of the micro-mirror with respect to the optical fiber end face was controlled by a nano-stepping motor array. Optical and mechanical tests were performed on the device. A 1.55 mm laser beam was sent along the optical fiber. When the micro-mirror was removed from the front of the fiber, the coupling efficiency between two fibers was -10 dBm. Once the micro-mirror was placed in the optical path the coupling efficiency dropped to -51.5 dBm. The best attenuation was obtained when the micro-mirror blocked the whole cross-section of the laser beam diameter. It is evident that the device can operate as a high precision fiber optic attenuator or shutter.

  11. Singlet exciton fission photovoltaics.

    PubMed

    Lee, Jiye; Jadhav, Priya; Reusswig, Philip D; Yost, Shane R; Thompson, Nicholas J; Congreve, Daniel N; Hontz, Eric; Van Voorhis, Troy; Baldo, Marc A

    2013-06-18

    Singlet exciton fission, a process that generates two excitons from a single photon, is perhaps the most efficient of the various multiexciton-generation processes studied to date, offering the potential to increase the efficiency of solar devices. But its unique characteristic, splitting a photogenerated singlet exciton into two dark triplet states, means that the empty absorption region between the singlet and triplet excitons must be filled by adding another material that captures low-energy photons. This has required the development of specialized device architectures. In this Account, we review work to develop devices that harness the theoretical benefits of singlet exciton fission. First, we discuss singlet fission in the archetypal material, pentacene. Pentacene-based photovoltaic devices typically show high external and internal quantum efficiencies. They have enabled researchers to characterize fission, including yield and the impact of competing loss processes, within functional devices. We review in situ probes of singlet fission that modulate the photocurrent using a magnetic field. We also summarize studies of the dissociation of triplet excitons into charge at the pentacene-buckyball (C60) donor-acceptor interface. Multiple independent measurements confirm that pentacene triplet excitons can dissociate at the C60 interface despite their relatively low energy. Because triplet excitons produced by singlet fission each have no more than half the energy of the original photoexcitation, they limit the potential open circuit voltage within a solar cell. Thus, if singlet fission is to increase the overall efficiency of a solar cell and not just double the photocurrent at the cost of halving the voltage, it is necessary to also harvest photons in the absorption gap between the singlet and triplet energies of the singlet fission material. We review two device architectures that attempt this using long-wavelength materials: a three-layer structure that uses

  12. Artificially Constructed Plasmarons and Plasmon-Exciton Molecules in 2D Metals

    NASA Astrophysics Data System (ADS)

    Zhuravlev, A. S.; Kuznetsov, V. A.; Kulik, L. V.; Bisti, V. E.; Kirpichev, V. E.; Kukushkin, I. V.; Schmult, S.

    2016-11-01

    Resonant optical excitation was used to create a macroscopic nonequilibrium ensemble of "dark" excitons with an unprecedented long lifetime in a two-dimensional electron system placed in a quantizing magnetic field. Exotic three-particle and four-particle states, plasmarons and plasmon-exciton molecules, coupled with the surrounding electrons through the collective plasma oscillations are engineered. Plasmarons and plasmon-exciton molecules are manifested as new features in the recombination spectra of nonequilibrium systems.

  13. Control of Exciton Photon Coupling in Nano-structures

    NASA Astrophysics Data System (ADS)

    Liu, Xiaoze

    In this thesis, we study the interaction of excitons with photons and plasmons and methods to control and enhance this interaction. This study is categorized in three parts: light-matter interaction in microcavity structures, direct dipole-dipole interactions, and plasmon-exciton interaction in metal-semiconductor systems. In the microcavity structures, the light-matter interactions become significant when the excitonic energy is in resonance with microcavity photons. New hybrid quantum states named polariton states will be formed if the strong coupling regime is achieved, where the interaction rate is faster than the average decay rate of the excitons and photons. Polaritons have been investigated in zinc oxide (ZnO) nanoparticles based microcavity at room temperature and stimulated emission of the polaritons has also been observed with a low optical pump threshold. Exictons in organic semiconductors (modeled as Frenkel excitons) are tightly bound to molecular sites, and differ considerably from loosely bound hydrogen atom-like inorganic excitons (modeled as Wannier-Mott excitons). This fundamental difference results in distinct optoelectronic properties. Not only strongly coupled to Wannier-Mott excitons in ZnO, the microcavity photons have also been observed to be simultaneously coupled to Frenkel excitons in 3,4,7,8-naphthalene tetracarboxylic dianhydride (NTCDA). The photons here act like a glue combining Wannier-Mott and Frenkel excitons into new hybrid polaritons taking the best from both constituents. Two-dimensional (2D) excitons in monolayer transition metal dichalcogenides (TMDs) have emerged as a new and fascinating type of Wannier-Mott-like excitons due to direct bandgap transition, huge oscillator strength and large binding energy. Monolayer molybdenum disulfide (MoS2) has been incorporated into the microcavity structure and 2D exciton-polaritons have been observed for the first time with directional emission in the strong coupling regime. Valley

  14. Optically controlled dense current structures driven by relativistic plasma aperture-induced diffraction

    NASA Astrophysics Data System (ADS)

    Gonzalez-Izquierdo, Bruno; Gray, Ross J.; King, Martin; Dance, Rachel J.; Wilson, Robbie; McCreadie, John; Butler, Nicholas M. H.; Capdessus, Remi; Hawkes, Steve; Green, James S.; Borghesi, Marco; Neely, David; McKenna, Paul

    2016-05-01

    The collective response of charged particles to intense fields is intrinsic to plasma accelerators and radiation sources, relativistic optics and many astrophysical phenomena. Here we show that a relativistic plasma aperture is generated in thin foils by intense laser light, resulting in the fundamental optical process of diffraction. The plasma electrons collectively respond to the resulting laser near-field diffraction pattern, producing a beam of energetic electrons with a spatial structure that can be controlled by variation of the laser pulse parameters. It is shown that static electron-beam and induced-magnetic-field structures can be made to rotate at fixed or variable angular frequencies depending on the degree of ellipticity in the laser polarization. The concept is demonstrated numerically and verified experimentally, and is an important step towards optical control of charged particle dynamics in laser-driven dense plasma sources.

  15. Optically driven self-oscillations of a silica nanospike at low gas pressures

    NASA Astrophysics Data System (ADS)

    Xie, Shangran; Pennetta, Riccardo; Noskov, Roman E.; Russell, Philip St. J.

    2016-09-01

    We report light-driven instability and optomechanical self-oscillation of a fused silica "nanospike" at low gas pressures. The nanospike (tip diameter 400 nm), fabricated by thermally tapering and HF-etching a single mode fiber (SMF), was set pointing at the endface of a hollow-core photonic crystal fiber (HC-PCF) into the field created by the fundamental optical mode emerging from the HC-PCF. At low pressures, the nanospike became unstable and began to self-oscillate for optical powers above a certain threshold, acting like a phonon laser or "phaser". Because the nanospike is robustly connected to the base, direct measurement of the temporal dynamics of the instability is possible. The experiment sheds light on why particles escape from optical traps at low pressures.

  16. Negative activation energy and dielectric signatures of excitons and excitonic Mott transitions in quantum confined laser structures

    NASA Astrophysics Data System (ADS)

    Bhunia, Amit; Bansal, Kanika; Henini, Mohamed; Alshammari, Marzook S.; Datta, Shouvik

    2016-10-01

    Mostly, optical spectroscopies are used to investigate the physics of excitons, whereas their electrical evidences are hardly explored. Here, we examined a forward bias activated differential capacitance response of GaInP/AlGaInP based multi-quantum well laser diodes to trace the presence of excitons using electrical measurements. Occurrence of "negative activation energy" after light emission is understood as thermodynamical signature of steady state excitonic population under intermediate range of carrier injections. Similar corroborative results are also observed in an InGaAs/GaAs quantum dot laser structure grown by molecular beam epitaxy. With increasing biases, the measured differential capacitance response slowly vanishes. This represents gradual Mott transition of an excitonic phase into an electron-hole plasma in a GaInP/AlGaInP laser diode. This is further substantiated by more and more exponentially looking shapes of high energy tails in electroluminescence spectra with increasing forward bias, which originates from a growing non-degenerate population of free electrons and holes. Such an experimental correlation between electrical and optical properties of excitons can be used to advance the next generation excitonic devices.

  17. Excitonic effects in oxyhalide scintillating host compounds

    SciTech Connect

    Shwetha, G.; Kanchana, V.; Valsakumar, M. C.

    2014-10-07

    Ab-initio calculations based on density functional theory have been performed to study the electronic, optical, mechanical, and vibrational properties of scintillator host compounds YOX (X = F, Cl, Br, and I). Semiempirical dispersion correction schemes are used to find the effect of van der Waals forces on these layered compounds and we found this effect to be negligible except for YOBr. Calculations of phonons and elastic constants showed that all the compounds studied here are both dynamically and mechanically stable. YOF and YOI are found to be indirect band gap insulators while YOCl and YOBr are direct band gap insulators. The band gap is found to decrease as we move from fluorine to iodine, while the calculated refractive index shows the opposite trend. As the band gap decreases on going down the periodic table from YOF to YOI, the luminescence increases. The excitonic binding energy calculated, within the effective mass approximation, is found to be more for YOF than the remaining compounds, suggesting that the excitonic effect to be more in YOF than the other compounds. The optical properties are calculated within the Time-Dependent Density Functional Theory (TDDFT) and compared with results obtained within the random phase approximation. The TDDFT calculations, using the newly developed bootstrap exchange-correlation kernel, showed significant excitonic effects in all the compounds studied here.

  18. Interlayer Exciton Optoelectronics in a 2D Heterostructure p-n Junction.

    PubMed

    Ross, Jason S; Rivera, Pasqual; Schaibley, John; Lee-Wong, Eric; Yu, Hongyi; Taniguchi, Takashi; Watanabe, Kenji; Yan, Jiaqiang; Mandrus, David; Cobden, David; Yao, Wang; Xu, Xiaodong

    2017-02-08

    Semiconductor heterostructures are backbones for solid-state-based optoelectronic devices. Recent advances in assembly techniques for van der Waals heterostructures have enabled the band engineering of semiconductor heterojunctions for atomically thin optoelectronic devices. In two-dimensional heterostructures with type II band alignment, interlayer excitons, where Coulomb bound electrons and holes are confined to opposite layers, have shown promising properties for novel excitonic devices, including a large binding energy, micron-scale in-plane drift-diffusion, and a long population and valley polarization lifetime. Here, we demonstrate interlayer exciton optoelectronics based on electrostatically defined lateral p-n junctions in a MoSe2-WSe2 heterobilayer. Applying a forward bias enables the first observation of electroluminescence from interlayer excitons. At zero bias, the p-n junction functions as a highly sensitive photodetector, where the wavelength-dependent photocurrent measurement allows the direct observation of resonant optical excitation of the interlayer exciton. The resulting photocurrent amplitude from the interlayer exciton is about 200 times smaller than the resonant excitation of intralayer exciton. This implies that the interlayer exciton oscillator strength is 2 orders of magnitude smaller than that of the intralayer exciton due to the spatial separation of electron and hole to the opposite layers. These results lay the foundation for exploiting the interlayer exciton in future 2D heterostructure optoelectronic devices.

  19. Interlayer Exciton Optoelectronics in a 2D Heterostructure p–n Junction

    NASA Astrophysics Data System (ADS)

    Ross, Jason S.; Rivera, Pasqual; Schaibley, John; Lee-Wong, Eric; Yu, Hongyi; Taniguchi, Takashi; Watanabe, Kenji; Yan, Jiaqiang; Mandrus, David; Cobden, David; Yao, Wang; Xu, Xiaodong

    2017-02-01

    Semiconductor heterostructures are backbones for solid state based optoelectronic devices. Recent advances in assembly techniques for van der Waals heterostructures has enabled the band engineering of semiconductor heterojunctions for atomically thin optoelectronic devices. In two-dimensional heterostructures with type II band alignment, interlayer excitons, where Coulomb-bound electrons and holes are confined to opposite layers, have shown promising properties for novel excitonic devices, including a large binding energy, micron-scale in-plane drift-diffusion, and long population and valley polarization lifetime. Here, we demonstrate interlayer exciton optoelectronics based on electrostatically defined lateral p-n junctions in a MoSe2-WSe2 heterobilayer. Applying a forward bias enables the first observation of electroluminescence from interlayer excitons. At zero bias, the p-n junction functions as a highly sensitive photodetector, where the wavelength-dependent photocurrent measurement allows the direct observation of resonant optical excitation of the interlayer exciton. The resulting photocurrent amplitude from the interlayer exciton is about 200 times smaller compared to the resonant excitation of intralayer exciton. This implies that the interlayer exciton oscillator strength is two orders of magnitude smaller than that of the intralayer exciton due to the spatial separation of electron and hole to opposite layers. These results lay the foundation for exploiting the interlayer exciton in future 2D heterostructure optoelectronic devices.

  20. Optically driven Archimedes micro-screws for micropump applications: multiple blade design

    NASA Astrophysics Data System (ADS)

    Baldeck, Patrice L.; Lin, Chih-Lang; Lin, Yu-Sheng; Lin, Chin-Te; Chung, Tien-Tung; Bouriau, Michel; Vitrant, Guy

    2011-10-01

    We study the rotation of photo-driven Archimedes screw with multiple blades. The micron-sized Archimedes screws are readily made by the two-photon polymerization technique. Free-floating screws that are trapped by optical tweezers align in the laser irradiation direction, and rotate spontaneously. In this study we demonstrate that the rotation speeds of two-blade-screws is twice the rotation speed of one-blade-screw. However, more complex 3-blade-screws rotate slower than 2-blade-screws due to their limited geometry resolution at this micron scale.

  1. Characterization of laser-driven shock waves in solids using a fiber optic pressure probe

    DOE PAGES

    Cranch, Geoffrey A.; Lunsford, Robert; Grun, Jacob; ...

    2013-11-08

    Measurement of laser-driven shock wave pressure in solid blocks of polymethyl methacrylate is demonstrated using fiber optic pressure probes. Three probes based on a fiber Fabry–Perot, fiber Bragg grating, and interferometric fiber tip sensor are tested and compared. Shock waves are generated using a high-power laser focused onto a thin foil target placed in close proximity to the test blocks. The fiber Fabry–Perot sensor appears capable of resolving the shock front with a rise time of 91 ns. As a result, the peak pressure is estimated, using a separate shadowgraphy measurement, to be 3.4 GPa.

  2. Non-adiabatic effects on the optical response of driven systems

    NASA Astrophysics Data System (ADS)

    Fregoso, Benjamin M.; Kolodrubetz, Michael; Moore, Joel

    Periodically driven systems have received renewed interest due to their capacity to engineer non-trivial effective Hamiltonians. A characteristic of such systems is how they respond to weak periodicity-breaking drive, as for example when a laser is pulsed instead of continuous wave. We develop semi-classical equations of motion of a wave packet in the presence of electric and magnetic fields which are turned on non-adiabatically. We then show the emergence of significant corrections to electronic collective excitations and optical responses of topological insulator surface states, Weyl metals and semiconductor mono-chalcogenides.

  3. Dynamic modeling of optically pumped electrically driven terahertz quantum cascade lasers

    NASA Astrophysics Data System (ADS)

    Hamadou, A.; Thobel, J.-L.; Lamari, S.

    2017-03-01

    Based on our four-level rate equations model, we analyze through numerical simulations the dynamics of the electron density, population inversion and terahertz intensity present within the cavity of a mid-infrared optically pumped electrically driven THz quantum cascade laser. We find in particular that the mid-infrared pump intensity influences significantly the dynamical behavior of the present device. Moreover, compared to its homologue, the conventional electrically injected THz quantum cascade laser, this system presents much faster dynamics. In addition, within the premises of our model, we derive in the most general case the equation that allows for the determination of the turn-on delay time tth.

  4. Relaxation of hot excitons in CdZnSe/ZnSe quantum wells and quantum dots

    NASA Astrophysics Data System (ADS)

    Spiegel, R.; Bacher, G.; Breitwieser, O.; Forchel, A.; Jobst, B.; Hommel, D.; Landwehr, G.

    1998-05-01

    The relaxation dynamics of hot excitons was studied in (Zn,Cd)Se/ZnSe quantum wells and quantum dots. A fast population of the radiative excitonic ground state occurs for an excitation excess energy corresponding to an integer number of optical phonon energies. This is indicated by a spectrally narrow photoluminescence peak observed immediately after the exciting laser pulse. Spatial diffusion of excitons, controlled by the interaction between excitons and acoustic phonons, causes a distinct linewidth broadening with increasing delay time in quantum wells. In contrast, this process is found to be strongly suppressed in quantum dots.

  5. Anomalous Photoluminescence of Weakly Confined Excitons including Radiative Correction in Nano-to-Bulk Crossover Regime

    NASA Astrophysics Data System (ADS)

    Matsuda, Takuya; Yokoshi, Nobuhiko; Ishihara, Hajime

    2015-06-01

    We develop a theoretical formalism to calculate photoluminescence (PL) spectrum of weakly confined excitons incorporating the microscopic nonlocal optical response. The nonlocality is caused by the center-of-mass (c. m.) motion of exciton and becomes remarkable in nano-to-bulk crossover regime. The theory successfully explains the characteristics of recently observed peculiar PL spectra in high quality CuCl films [5], wherein the signals appear at the exciton states with the very large radiative corrections not only for the lowest level but also for the higher ones including non-dipole types of excitons.

  6. Exciton coherence in highly disordered solid DBN: "band-to-band" ODMR transitions in zero field

    NASA Astrophysics Data System (ADS)

    Tegelaar, P. M. H. L.; Glasbeek, M.; Zewail, A. H.

    1986-08-01

    Optically detected ELDOR is used to study the 2 | E| band-to-band zero-field spin transition of quasi-one-dimensional triplet excitons in doped 1,4-dibromonaphthalene (16% deutero in proto). Results from line-narrowing experiments provide evidence for a model which correlates homogeneous spin dephasing to the dispersion in the ODMR frequencies of coherent excitons.

  7. Control of excitons in multi-layer van der Waals heterostructures

    NASA Astrophysics Data System (ADS)

    Calman, E. V.; Dorow, C. J.; Fogler, M. M.; Butov, L. V.; Hu, S.; Mishchenko, A.; Geim, A. K.

    2016-03-01

    We report an experimental study of excitons in a double quantum well van der Waals heterostructure made of atomically thin layers of MoS2 and hexagonal boron nitride. The emission of neutral and charged excitons is controlled by gate voltage, temperature, and both the helicity and the power of optical excitation.

  8. Evidence of Hybrid Excitons in Weakly Interacting Nanopeapods

    PubMed Central

    2013-01-01

    Nanopeapods, consisting of optically active π-conjugated molecules encapsulated inside of the cavity of carbon nanotubes, exhibit efficient photon emission in the visible spectral range. Combining optical experiments with ab initio theory, we show that the puzzling features observed in photoluminescence spectra are of excitonic nature. The subunits though being van der Waals bound are demonstrated to interact in the excited state, giving rise to the formation of hybrid excitons. We rationalize why this many-body effect makes such nanohybrids useful for optoelectronic devices. PMID:23991266

  9. Many-body effects and excitonic features in 2D biphenylene carbon

    SciTech Connect

    Lüder, Johann Puglia, Carla; Eriksson, Olle; Sanyal, Biplab; Brena, Barbara; Ottosson, Henrik

    2016-01-14

    The remarkable excitonic effects in low dimensional materials in connection to large binding energies of excitons are of great importance for research and technological applications such as in solar energy and quantum information processing as well as for fundamental investigations. In this study, the unique electronic and excitonic properties of the two dimensional carbon network biphenylene carbon were investigated with GW approach and the Bethe-Salpeter equation accounting for electron correlation effects and electron-hole interactions, respectively. Biphenylene carbon exhibits characteristic features including bright and dark excitons populating the optical gap of 0.52 eV and exciton binding energies of 530 meV as well as a technologically relevant intrinsic band gap of 1.05 eV. Biphenylene carbon’s excitonic features, possibly tuned, suggest possible applications in the field of solar energy and quantum information technology in the future.

  10. Two-dimensional excitons in three-dimensional hexagonal boron nitride

    SciTech Connect

    Cao, X. K.; Lin, J. Y. Jiang, H. X.; Clubine, B.; Edgar, J. H.

    2013-11-04

    The recombination processes of excitons in hexagonal boron nitride (hBN) have been probed using time-resolved photoluminescence. It was found that the theory for two-dimensional (2D) exciton recombination describes well the exciton dynamics in three-dimensional hBN. The exciton Bohr radius and binding energy deduced from the temperature dependent exciton recombination lifetime is around 8 Å and 740 meV, respectively. The effective masses of electrons and holes in 2D hBN deduced from the generalized relativistic dispersion relation of 2D systems are 0.54m{sub o}, which are remarkably consistent with the exciton reduced mass deduced from the experimental data. Our results illustrate that hBN represents an ideal platform to study the 2D optical properties as well as the relativistic properties of particles in a condensed matter system.

  11. Many-body effects and excitonic features in 2D biphenylene carbon

    NASA Astrophysics Data System (ADS)

    Lüder, Johann; Puglia, Carla; Ottosson, Henrik; Eriksson, Olle; Sanyal, Biplab; Brena, Barbara

    2016-01-01

    The remarkable excitonic effects in low dimensional materials in connection to large binding energies of excitons are of great importance for research and technological applications such as in solar energy and quantum information processing as well as for fundamental investigations. In this study, the unique electronic and excitonic properties of the two dimensional carbon network biphenylene carbon were investigated with GW approach and the Bethe-Salpeter equation accounting for electron correlation effects and electron-hole interactions, respectively. Biphenylene carbon exhibits characteristic features including bright and dark excitons populating the optical gap of 0.52 eV and exciton binding energies of 530 meV as well as a technologically relevant intrinsic band gap of 1.05 eV. Biphenylene carbon's excitonic features, possibly tuned, suggest possible applications in the field of solar energy and quantum information technology in the future.

  12. Many-body effects and excitonic features in 2D biphenylene carbon.

    PubMed

    Lüder, Johann; Puglia, Carla; Ottosson, Henrik; Eriksson, Olle; Sanyal, Biplab; Brena, Barbara

    2016-01-14

    The remarkable excitonic effects in low dimensional materials in connection to large binding energies of excitons are of great importance for research and technological applications such as in solar energy and quantum information processing as well as for fundamental investigations. In this study, the unique electronic and excitonic properties of the two dimensional carbon network biphenylene carbon were investigated with GW approach and the Bethe-Salpeter equation accounting for electron correlation effects and electron-hole interactions, respectively. Biphenylene carbon exhibits characteristic features including bright and dark excitons populating the optical gap of 0.52 eV and exciton binding energies of 530 meV as well as a technologically relevant intrinsic band gap of 1.05 eV. Biphenylene carbon's excitonic features, possibly tuned, suggest possible applications in the field of solar energy and quantum information technology in the future.

  13. Optical diagnostics of turbulent mixing in explosively-driven shock tube

    NASA Astrophysics Data System (ADS)

    Anderson, James; Hargather, Michael

    2016-11-01

    Explosively-driven shock tube experiments were performed to investigate the turbulent mixing of explosive product gases and ambient air. A small detonator initiated Al / I2O5 thermite, which produced a shock wave and expanding product gases. Schlieren and imaging spectroscopy were applied simultaneously along a common optical path to identify correlations between turbulent structures and spatially-resolved absorbance. The schlieren imaging identifies flow features including shock waves and turbulent structures while the imaging spectroscopy identifies regions of iodine gas presence in the product gases. Pressure transducers located before and after the optical diagnostic section measure time-resolved pressure. Shock speed is measured from tracking the leading edge of the shockwave in the schlieren images and from the pressure transducers. The turbulent mixing characteristics were determined using digital image processing. Results show changes in shock speed, product gas propagation, and species concentrations for varied explosive charge mass. Funded by DTRA Grant HDTRA1-14-1-0070.

  14. High-Q silica zipper cavity for optical radiation pressure driven MOMS switch

    SciTech Connect

    Tetsumoto, Tomohiro; Tanabe, Takasumi

    2014-07-15

    We design a silica zipper cavity that has high optical and mechanical Q (quality factor) values and demonstrate numerically the feasibility of a radiation pressure driven micro opto-mechanical system (MOMS) directional switch. The silica zipper cavity has an optical Q of 4.0 × 10{sup 4} and an effective mode volume V{sub mode} of 0.67λ{sup 3} when the gap between two cavities is 34 nm. The mechanical Q (Q{sub m}) is determined by thermo-elastic damping and is 2.0 × 10{sup 6} in a vacuum at room temperature. The opto-mechanical coupling rate g{sub OM} is as high as 100 GHz/nm, which allows us to move the directional cavity-waveguide system and switch 1550-nm light with 770-nm light by controlling the radiation pressure.

  15. Optical silencing of C. elegans cells with light-driven proton pumps.

    PubMed

    Okazaki, Ayako; Takahashi, Megumi; Toyoda, Naoya; Takagi, Shin

    2014-08-01

    Recent development of optogenetic techniques, which utilize light-driven ion channels or ion pumps for controlling the activity of excitable cells, has greatly facilitated the investigation of nervous systems in vivo. A new generation of optical silencers includes outward-directed proton pumps, such as Arch, which have several advantages over currently widely used halorhodopsin (NpHR). These advantages include the resistance to inactivation during prolonged illumination and the ability to generate a larger optical current from low intensity light. C. elegans, with its small transparent body and well-characterized neural circuits, is especially suitable for optogenetic analyses. In this article, we will outline the practical aspects of using of Arch and other proton pumps as optogenetic tools in C. elegans.

  16. Reconfigurable exciton-plasmon interconversion for nanophotonic circuits.

    PubMed

    Lee, Hyun Seok; Luong, Dinh Hoa; Kim, Min Su; Jin, Youngjo; Kim, Hyun; Yun, Seokjoon; Lee, Young Hee

    2016-11-28

    The recent challenges for improving the operation speed of nanoelectronics have motivated research on manipulating light in on-chip integrated circuits. Hybrid plasmonic waveguides with low-dimensional semiconductors, including quantum dots and quantum wells, are a promising platform for realizing sub-diffraction limited optical components. Meanwhile, two-dimensional transition metal dichalcogenides (TMDs) have received broad interest in optoelectronics owing to tightly bound excitons at room temperature, strong light-matter and exciton-plasmon interactions, available top-down wafer-scale integration, and band-gap tunability. Here, we demonstrate principal functionalities for on-chip optical communications via reconfigurable exciton-plasmon interconversions in ∼200-nm-diameter Ag-nanowires overlapping onto TMD transistors. By varying device configurations for each operation purpose, three active components for optical communications are realized: field-effect exciton transistors with a channel length of ∼32 μm, field-effect exciton multiplexers transmitting multiple signals through a single NW and electrical detectors of propagating plasmons with a high On/Off ratio of∼190. Our results illustrate the unique merits of two-dimensional semiconductors for constructing reconfigurable device architectures in integrated nanophotonic circuits.

  17. Reconfigurable exciton-plasmon interconversion for nanophotonic circuits

    PubMed Central

    Lee, Hyun Seok; Luong, Dinh Hoa; Kim, Min Su; Jin, Youngjo; Kim, Hyun; Yun, Seokjoon; Lee, Young Hee

    2016-01-01

    The recent challenges for improving the operation speed of nanoelectronics have motivated research on manipulating light in on-chip integrated circuits. Hybrid plasmonic waveguides with low-dimensional semiconductors, including quantum dots and quantum wells, are a promising platform for realizing sub-diffraction limited optical components. Meanwhile, two-dimensional transition metal dichalcogenides (TMDs) have received broad interest in optoelectronics owing to tightly bound excitons at room temperature, strong light-matter and exciton-plasmon interactions, available top-down wafer-scale integration, and band-gap tunability. Here, we demonstrate principal functionalities for on-chip optical communications via reconfigurable exciton-plasmon interconversions in ∼200-nm-diameter Ag-nanowires overlapping onto TMD transistors. By varying device configurations for each operation purpose, three active components for optical communications are realized: field-effect exciton transistors with a channel length of ∼32 μm, field-effect exciton multiplexers transmitting multiple signals through a single NW and electrical detectors of propagating plasmons with a high On/Off ratio of∼190. Our results illustrate the unique merits of two-dimensional semiconductors for constructing reconfigurable device architectures in integrated nanophotonic circuits. PMID:27892463

  18. Reconfigurable exciton-plasmon interconversion for nanophotonic circuits

    NASA Astrophysics Data System (ADS)

    Lee, Hyun Seok; Luong, Dinh Hoa; Kim, Min Su; Jin, Youngjo; Kim, Hyun; Yun, Seokjoon; Lee, Young Hee

    2016-11-01

    The recent challenges for improving the operation speed of nanoelectronics have motivated research on manipulating light in on-chip integrated circuits. Hybrid plasmonic waveguides with low-dimensional semiconductors, including quantum dots and quantum wells, are a promising platform for realizing sub-diffraction limited optical components. Meanwhile, two-dimensional transition metal dichalcogenides (TMDs) have received broad interest in optoelectronics owing to tightly bound excitons at room temperature, strong light-matter and exciton-plasmon interactions, available top-down wafer-scale integration, and band-gap tunability. Here, we demonstrate principal functionalities for on-chip optical communications via reconfigurable exciton-plasmon interconversions in ~200-nm-diameter Ag-nanowires overlapping onto TMD transistors. By varying device configurations for each operation purpose, three active components for optical communications are realized: field-effect exciton transistors with a channel length of ~32 μm, field-effect exciton multiplexers transmitting multiple signals through a single NW and electrical detectors of propagating plasmons with a high On/Off ratio of~190. Our results illustrate the unique merits of two-dimensional semiconductors for constructing reconfigurable device architectures in integrated nanophotonic circuits.

  19. Two-dimensional exciton properties in monolayer semiconducting phosphorus allotropes.

    PubMed

    Villegas, Cesar E P; Rodin, A S; Carvalho, Alexandra; Rocha, A R

    2016-10-12

    Excitons play a key role in technological applications since they have a strong influence on determining the efficiency of photovoltaic devices. Recently, it has been shown that the allotropes of phosphorus possess an optical band gap that can be tuned over a wide range of values including the near-infrared and visible spectra, which would make them promising candidates for optoelectronic applications. In this work we carry out ab initio many-body perturbation theory calculations to study the excitonic effects on the optical properties of two-dimensional phosphorus allotropes: the case of blue and black monolayers. We elucidate the most relevant optical transitions, exciton binding energy spectrum as well as real-space exciton distribution, particularly focusing on the absorption spectrum dependence on the incident light polarization. In addition, based on our results, we use a set of effective hydrogenic models, in which the electron-hole Coulomb interaction is included to estimate exciton binding energies and radii. Our results show an excellent agreement between the many-body methodology and the effective models.

  20. Probing excitonic dark states in single-layer tungsten disulphide.

    PubMed

    Ye, Ziliang; Cao, Ting; O'Brien, Kevin; Zhu, Hanyu; Yin, Xiaobo; Wang, Yuan; Louie, Steven G; Zhang, Xiang

    2014-09-11

    Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors, exhibiting a number of intriguing optical phenomena such as valley-selective circular dichroism, doping-dependent charged excitons and strong photocurrent responses. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects. In particular, a recent study based on a GW plus Bethe-Salpeter equation (GW-BSE) approach, which employed many-body Green's-function methodology to address electron-electron and electron-hole interactions, theoretically predicted a diversity of strongly bound excitons. Here we report experimental evidence of a series of excitonic dark states in single-layer WS2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (∼0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many

  1. Controlling exciton photophysics in single-walled carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Sarpkaya, Ibrahim

    Single-walled carbon nanotubes (SWCNTs) have been studied extensively by scientists and engineers due to their unique mechanical, optical, electronic and thermal properties that make them attractive for both fundamental research and device applications. Specifically, important optical properties of SWCNTs such as formation of strongly bound excitons (electron-hole pairs), being stable at room temperature, and bandgap-tunable light emission from visible to telecom wavelengths make them a promising material for optoelectronic and nanophotonic devices. However, the photophysics of excitons in SWCNTs is not yet fully understood and is largely affected by detrimental extrinsic effects, which give rise to strongly reduced device performance. This dissertation demonstrates novel methods and techniques to better understand and to control the photophysics of excitons in SWCNTs. The first part presents novel ways to completely remove detrimental spectral diffusion and blinking in the optical emission of surfactant dispersed SWCNTs on millisecond time scales and also demonstrates 50-fold enhanced exciton emission. Furthermore, pronounced photon antibunching is observed for the first time under resonant excitation. The demonstrated single photon emission is promising for applications in quantum cryptography, while the achieved stable long term emission is important for optoelectronic devices. The second part demonstrates a new regime of intrinsic exciton photophysics in ultra-clean SWCNTs that is characterized by ultra-narrow exciton linewidth and prolonged emission times up to 18 ns. These lifetimes are two orders of magnitude better than prior measurements and in agreement with values predicted by theorists a decade ago. Moreover, I measure for the first time exciton decoherence times of individual nanotubes in the time-domain and demonstrate fourfold prolonged values up to 2 ps compared to previous ensemble studies. Finally, I demonstrate a novel method which controls

  2. Exciton energy-momentum map of hexagonal boron nitride

    NASA Astrophysics Data System (ADS)

    Fugallo, Giorgia; Aramini, Matteo; Koskelo, Jaakko; Watanabe, Kenji; Taniguchi, Takashi; Hakala, Mikko; Huotari, Simo; Gatti, Matteo; Sottile, Francesco

    2015-10-01

    Understanding and controlling the way excitons propagate in solids is a key for tailoring materials with improved optoelectronic properties. A fundamental step in this direction is the determination of the exciton energy-momentum dispersion. Here, thanks to the solution of the parameter-free Bethe-Salpeter equation (BSE), we draw and explain the exciton energy-momentum map of hexagonal boron nitride (h-BN) in the first three Brillouin zones. We show that h-BN displays strong excitonic effects not only in the optical spectra at vanishing momentum q , as previously reported, but also at large q . We validate our theoretical predictions by assessing the calculated exciton map by means of an inelastic x-ray scattering (IXS) experiment. Moreover, we solve the discrepancies between previous experimental data and calculations, proving then that the BSE is highly accurate through the whole momentum range. Therefore, these results put forward the combination BSE and IXS as the tool of choice for addressing the exciton dynamics in complex materials.

  3. Exciton Band Structure in Two-Dimensional Materials

    NASA Astrophysics Data System (ADS)

    Cudazzo, Pierluigi; Sponza, Lorenzo; Giorgetti, Christine; Reining, Lucia; Sottile, Francesco; Gatti, Matteo

    2016-02-01

    Low-dimensional materials differ from their bulk counterparts in many respects. In particular, the screening of the Coulomb interaction is strongly reduced, which can have important consequences such as the significant increase of exciton binding energies. In bulk materials the binding energy is used as an indicator in optical spectra to distinguish different kinds of excitons, but this is not possible in low-dimensional materials, where the binding energy is large and comparable in size for excitons of very different localization. Here we demonstrate that the exciton band structure, which can be accessed experimentally, instead provides a powerful way to identify the exciton character. By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane and single-layer hexagonal boron nitride, we draw a general picture of the exciton dispersion in two-dimensional materials, highlighting the different role played by the exchange electron-hole interaction and by the electronic band structure. Our interpretation is substantiated by a prediction for phosphorene.

  4. Exciton Band Structure in Two-Dimensional Materials.

    PubMed

    Cudazzo, Pierluigi; Sponza, Lorenzo; Giorgetti, Christine; Reining, Lucia; Sottile, Francesco; Gatti, Matteo

    2016-02-12

    Low-dimensional materials differ from their bulk counterparts in many respects. In particular, the screening of the Coulomb interaction is strongly reduced, which can have important consequences such as the significant increase of exciton binding energies. In bulk materials the binding energy is used as an indicator in optical spectra to distinguish different kinds of excitons, but this is not possible in low-dimensional materials, where the binding energy is large and comparable in size for excitons of very different localization. Here we demonstrate that the exciton band structure, which can be accessed experimentally, instead provides a powerful way to identify the exciton character. By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane and single-layer hexagonal boron nitride, we draw a general picture of the exciton dispersion in two-dimensional materials, highlighting the different role played by the exchange electron-hole interaction and by the electronic band structure. Our interpretation is substantiated by a prediction for phosphorene.

  5. Two-dminensional exciton states in monolayer semiconducting phosphorus alotropes

    NASA Astrophysics Data System (ADS)

    Rocha, Alexandre R.; Villegas, Cesar E. P.

    During the last decade, novel two-dimensional (2D) semiconducting materials have been synthesized and characterised. As a result, there have been several theoretical and experimental proposals to incorporate 2D materials for designing next generation electronic and optoelectronics devices. In particular, it has been demonstrated that light absorption in phosphorus-based monolayers can span the whole visible spectrum, suggesting they could be used for optolectronic applications. A key ingredient for optolectronic applications is the presence of excitons and their subsequent diffusion along a donor material. This is influenced by the character of the different excitations taking place, as well as, the exciton binding energy. Therefore, In this work we use accurate many-body corrected density functional theory by means of GW-BSE methodology to elucidate the most important optical transitions, exciton energy spectrum as well as exciton extension in different types of phosphorene materials. In addition, we solve the Schrodinger equation for different 2D screened potentials and estimate the 2D exciton energy levels and radius extension. Finally, in order to assess further studies based on these systems, we provide a simple analityc expression for estimating 2D exciton energy levels. Research funded by FAPESP-Brazil.

  6. Transport of dipolar excitons in (Al,Ga)N/GaN quantum wells

    NASA Astrophysics Data System (ADS)

    Fedichkin, F.; Andreakou, P.; Jouault, B.; Vladimirova, M.; Guillet, T.; Brimont, C.; Valvin, P.; Bretagnon, T.; Dussaigne, A.; Grandjean, N.; Lefebvre, P.

    2015-05-01

    We investigate the transport of dipolar indirect excitons along the growth plane of polar (Al,Ga)N/GaN quantum well structures by means of spatially and time-resolved photoluminescence spectroscopy. The transport in these strongly disordered quantum wells is activated by dipole-dipole repulsion. The latter induces an emission blue shift that increases linearly with exciton density, whereas the radiative recombination rate increases exponentially. Under continuous, localized excitation, we observe continuously decreasing emission energy, as excitons propagate away from the excitation spot. This corresponds to a steady-state gradient of exciton density, measured over several tens of micrometers. Time-resolved microphotoluminescence experiments provide information on the dynamics of recombination and transport of dipolar excitons. We account for the ensemble of experimental results by solving the nonlinear drift-diffusion equation. Quantitative analysis suggests that in such structures, exciton propagation on the scale of 10 to 20 μ m is mainly driven by diffusion, rather than by drift, due to the strong disorder and the presence of nonradiative defects. Secondary exciton creation, most probably by the intense higher-energy luminescence, guided along the sample plane, is shown to contribute to the exciton emission pattern on the scale up to 100 μ m . The exciton propagation length is strongly temperature dependent, the emission being quenched beyond a critical distance governed by nonradiative recombination.

  7. Optical design of the Fresnel lens for LED-driven flashlight.

    PubMed

    Chen, Yi-Cheng; Nian, Shih-Chih; Huang, Ming-Shyan

    2016-02-01

    The Fresnel lens is composed of micrometer-sized v-groove structures that determine the maximum illuminance and brightness uniformity of LED-driven flashlights, which are used in high-quality photography. The fabrication quality of the microstructures and the accuracy of the geometrical curvature of the Fresnel lens affect the optical characteristics of the emitted light traveling through the lens, which in turn determines the maximum illuminance and brightness uniformity. This paper presents a systematic design procedure for fabricating the Fresnel lens and investigates the influence of geometrical design and fabrication process on optical performance. The optical analysis was performed using the commercial software TracePro. The results revealed that a small tip radius of the v-groove microstructure facilitates brightness uniformity. Furthermore, both the simulation and the experimental results revealed that Fresnel lenses fabricated through injection molding or injection compression molding have either errors of microstructure height more than 3%-6% or curvature errors higher than 6%, which would affect the optical performance, especially the brightness uniformity.

  8. Magnetic edge-state excitons in zigzag graphene nanoribbons.

    PubMed

    Yang, Li; Cohen, Marvin L; Louie, Steven G

    2008-10-31

    We present first-principles calculations of the optical properties of zigzag-edged graphene nanoribbons (ZGNRs) employing the GW-Bethe-Salpeter equation approach with the spin interaction included. Optical response of the ZGNRs is found to be dominated by magnetic edge-state-derived excitons with large binding energy. The absorption spectrum is composed of a characteristic series of exciton states, providing a possible signature for identifying the ZGNRs. The edge-state excitons are charge-transfer excitations with the excited electron and hole located on opposite edges; they moreover induce a spin transfer across the ribbon, resulting in a photoreduction of the magnetic ordering. These novel characteristics are potentially useful in the applications.

  9. Anisotropy-Induced Quantum Interference and Population Trapping between Orthogonal Quantum Dot Exciton States in Semiconductor Cavity Systems

    NASA Astrophysics Data System (ADS)

    Hughes, Stephen; Agarwal, Girish S.

    2017-02-01

    We describe how quantum dot semiconductor cavity systems can be engineered to realize anisotropy-induced dipole-dipole coupling between orthogonal dipole states in a single quantum dot. Quantum dots in single-mode cavity structures as well as photonic crystal waveguides coupled to spin states or linearly polarized excitons are considered. We demonstrate how the dipole-dipole coupling can control the radiative decay rate of excitons and form pure entangled states in the long time limit. We investigate both field-free entanglement evolution and coherently pumped exciton regimes, and show how a double-field pumping scenario can completely eliminate the decay of coherent Rabi oscillations and lead to population trapping. In the Mollow triplet regime, we explore the emitted spectra from the driven dipoles and show how a nonpumped dipole can take on the form of a spectral triplet, quintuplet, or a singlet, which has applications for producing subnatural linewidth single photons and more easily accessing regimes of high-field quantum optics and cavity-QED.

  10. Valley excitons in two-dimensional semiconductors

    SciTech Connect

    Yu, Hongyi; Cui, Xiaodong; Xu, Xiaodong; Yao, Wang

    2014-12-30

    Monolayer group-VIB transition metal dichalcogenides have recently emerged as a new class of semiconductors in the two-dimensional limit. The attractive properties include: the visible range direct band gap ideal for exploring optoelectronic applications; the intriguing physics associated with spin and valley pseudospin of carriers which implies potentials for novel electronics based on these internal degrees of freedom; the exceptionally strong Coulomb interaction due to the two-dimensional geometry and the large effective masses. The physics of excitons, the bound states of electrons and holes, has been one of the most actively studied topics on these two-dimensional semiconductors, where the excitons exhibit remarkably new features due to the strong Coulomb binding, the valley degeneracy of the band edges, and the valley dependent optical selection rules for interband transitions. Here we give a brief overview of the experimental and theoretical findings on excitons in two-dimensional transition metal dichalcogenides, with focus on the novel properties associated with their valley degrees of freedom.

  11. Valley excitons in two-dimensional semiconductors

    DOE PAGES

    Yu, Hongyi; Cui, Xiaodong; Xu, Xiaodong; ...

    2014-12-30

    Monolayer group-VIB transition metal dichalcogenides have recently emerged as a new class of semiconductors in the two-dimensional limit. The attractive properties include: the visible range direct band gap ideal for exploring optoelectronic applications; the intriguing physics associated with spin and valley pseudospin of carriers which implies potentials for novel electronics based on these internal degrees of freedom; the exceptionally strong Coulomb interaction due to the two-dimensional geometry and the large effective masses. The physics of excitons, the bound states of electrons and holes, has been one of the most actively studied topics on these two-dimensional semiconductors, where the excitons exhibitmore » remarkably new features due to the strong Coulomb binding, the valley degeneracy of the band edges, and the valley dependent optical selection rules for interband transitions. Here we give a brief overview of the experimental and theoretical findings on excitons in two-dimensional transition metal dichalcogenides, with focus on the novel properties associated with their valley degrees of freedom.« less

  12. Giant optical enhancement of strain gradient in ferroelectric BiFeO3 thin films and its physical origin

    PubMed Central

    Li, Yuelin; Adamo, Carolina; Chen, Pice; Evans, Paul G.; Nakhmanson, Serge M.; Parker, William; Rowland, Clare E.; Schaller, Richard D.; Schlom, Darrell G.; Walko, Donald A.; Wen, Haidan; Zhang, Qingteng

    2015-01-01

    Through mapping of the spatiotemporal strain profile in ferroelectric BiFeO3 epitaxial thin films, we report an optically initiated dynamic enhancement of the strain gradient of 105–106 m−1 that lasts up to a few ns depending on the film thickness. Correlating with transient optical absorption measurements, the enhancement of the strain gradient is attributed to a piezoelectric effect driven by a transient screening field mediated by excitons. These findings not only demonstrate a new possible way of controlling the flexoelectric effect, but also reveal the important role of exciton dynamics in photostriction and photovoltaic effects in ferroelectrics. PMID:26586421

  13. Giant optical enhancement of strain gradient in ferroelectric BiFeO3 thin films and its physical origin

    DOE PAGES

    Li, Yuelin; Adamo, C.; Chen, Pice; ...

    2015-11-20

    Through mapping of the spatiotemporal strain profile in ferroelectric BiFeO3 epitaxial thin films, we report an optically initiated dynamic enhancement of the strain gradient of 105–106 m-1 that lasts up to a few ns depending on the film thickness. Correlating with transient optical absorption measurements, the enhancement of the strain gradient is attributed to a piezoelectric effect driven by a transient screening field mediated by excitons. In conclusion, these findings not only demonstrate a new possible way of controlling the flexoelectric effect, but also reveal the important role of exciton dynamics in photostriction and photovoltaic effects in ferroelectrics.

  14. Exciton diffusion, end quenching, and exciton-exciton annihilation in individual air-suspended carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Ishii, A.; Yoshida, M.; Kato, Y. K.

    2015-03-01

    Luminescence properties of carbon nanotubes are strongly affected by exciton diffusion, which plays an important role in various nonradiative decay processes. Here we perform photoluminescence microscopy on hundreds of individual air-suspended carbon nanotubes to elucidate the interplay between exciton diffusion, end quenching, and exciton-exciton annihilation processes. A model derived from random-walk theory as well as Monte Carlo simulations are utilized to analyze nanotube length dependence and excitation power dependence of emission intensity. We have obtained the values of exciton diffusion length and absorption cross section for different chiralities, and diameter-dependent photoluminescence quantum yields have been observed. The simulations have also revealed the nature of a one-dimensional coalescence process, and an analytical expression for the power dependence of emission intensity is given.

  15. Exciton-photon correlations in bosonic condensates of exciton-polaritons.

    PubMed

    Kavokin, Alexey V; Sheremet, Alexandra S; Shelykh, Ivan A; Lagoudakis, Pavlos G; Rubo, Yuri G

    2015-07-08

    Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the rate of incoherent exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers.

  16. A Self-Driven and Adaptive Adjusting Teaching Learning Method for Optimizing Optical Multicast Network Throughput

    NASA Astrophysics Data System (ADS)

    Liu, Huanlin; Xu, Yifan; Chen, Yong; Zhang, Mingjia

    2016-09-01

    With the development of one point to multiple point applications, network resources become scarcer and wavelength channels become more crowded in optical networks. To improve the bandwidth utilization, the multicast routing algorithm based on network coding can greatly increase the resource utilization, but it is most difficult to maximize the network throughput owing to ignoring the differences between the multicast receiving nodes. For making full use of the destination nodes' receives ability to maximize optical multicast's network throughput, a new optical multicast routing algorithm based on teaching-learning-based optimization (MR-iTLBO) is proposed in the paper. In order to increase the diversity of learning, a self-driven learning method is adopted in MR-iTLBO algorithm, and the mutation operator of genetic algorithm is introduced to prevent the algorithm into a local optimum. For increasing learner's learning efficiency, an adaptive learning factor is designed to adjust the learning process. Moreover, the reconfiguration scheme based on probability vector is devised to expand its global search capability in MR-iTLBO algorithm. The simulation results show that performance in terms of network throughput and convergence rate has been improved significantly with respect to the TLBO and the variant TLBO.

  17. Exciton-vibrational coupling in the dynamics and spectroscopy of Frenkel excitons in molecular aggregates

    NASA Astrophysics Data System (ADS)

    Schröter, M.; Ivanov, S. D.; Schulze, J.; Polyutov, S. P.; Yan, Y.; Pullerits, T.; Kühn, O.

    2015-03-01

    The influence of exciton-vibrational coupling on the optical and transport properties of molecular aggregates is an old problem that gained renewed interest in recent years. On the experimental side, various nonlinear spectroscopic techniques gave insight into the dynamics of systems as complex as photosynthetic antennae. Striking evidence was gathered that in these protein-pigment complexes quantum coherence is operative even at room temperature conditions. Investigations were triggered to understand the role of vibrational degrees of freedom, beyond that of a heat bath characterized by thermal fluctuations. This development was paralleled by theory, where efficient methods emerged, which could provide the proper frame to perform non-Markovian and non-perturbative simulations of exciton-vibrational dynamics and spectroscopy. This review summarizes the state of affairs of the theory of exciton-vibrational interaction in molecular aggregates and photosynthetic antenna complexes. The focus is put on the discussion of basic effects of exciton-vibrational interaction from the stationary and dynamics points of view. Here, the molecular dimer plays a prominent role as it permits a systematic investigation of absorption and emission spectra by numerical diagonalization of the exciton-vibrational Hamiltonian in a truncated Hilbert space. An extension to larger aggregates, having many coupled nuclear degrees of freedom, becomes possible with the Multi-Layer Multi-Configuration Time-Dependent Hartree (ML-MCTDH) method for wave packet propagation. In fact it will be shown that this method allows one to approach the limit of almost continuous spectral densities, which is usually the realm of density matrix theory. Real system-bath situations are introduced for two models, which differ in the way strongly coupled nuclear coordinates are treated, as a part of the relevant system or the bath. A rather detailed exposition of the Hierarchy Equations Of Motion (HEOM) method will be

  18. Exciton-phonon system on a star graph: A perturbative approach.

    PubMed

    Yalouz, Saad; Pouthier, Vincent

    2016-05-01

    Based on the operatorial formulation of the perturbation theory, the properties of an exciton coupled with optical phonons on a star graph are investigated. Within this method, the dynamics is governed by an effective Hamiltonian, which accounts for exciton-phonon entanglement. The exciton is dressed by a virtual phonon cloud whereas the phonons are clothed by virtual excitonic transitions. In spite of the coupling with the phonons, it is shown that the energy spectrum of the dressed exciton resembles that of a bare exciton. The only differences originate in a polaronic mechanism that favors an energy shift and a decay of the exciton hopping constant. By contrast, the motion of the exciton allows the phonons to propagate over the graph so that the dressed normal modes drastically differ from the localized modes associated to bare phonons. They define extended vibrations whose properties depend on the state occupied by the exciton that accompanies the phonons. It is shown that the phonon frequencies, either red shifted or blue shifted, are very sensitive to the model parameter in general, and to the size of the graph in particular.

  19. Exciton-phonon system on a star graph: A perturbative approach

    NASA Astrophysics Data System (ADS)

    Yalouz, Saad; Pouthier, Vincent

    2016-05-01

    Based on the operatorial formulation of the perturbation theory, the properties of an exciton coupled with optical phonons on a star graph are investigated. Within this method, the dynamics is governed by an effective Hamiltonian, which accounts for exciton-phonon entanglement. The exciton is dressed by a virtual phonon cloud whereas the phonons are clothed by virtual excitonic transitions. In spite of the coupling with the phonons, it is shown that the energy spectrum of the dressed exciton resembles that of a bare exciton. The only differences originate in a polaronic mechanism that favors an energy shift and a decay of the exciton hopping constant. By contrast, the motion of the exciton allows the phonons to propagate over the graph so that the dressed normal modes drastically differ from the localized modes associated to bare phonons. They define extended vibrations whose properties depend on the state occupied by the exciton that accompanies the phonons. It is shown that the phonon frequencies, either red shifted or blue shifted, are very sensitive to the model parameter in general, and to the size of the graph in particular.

  20. Magnons and Phonons Optically Driven out of Local Equilibrium in a Magnetic Insulator

    NASA Astrophysics Data System (ADS)

    An, Kyongmo; Olsson, Kevin S.; Weathers, Annie; Sullivan, Sean; Chen, Xi; Li, Xiang; Marshall, Luke G.; Ma, Xin; Klimovich, Nikita; Zhou, Jianshi; Shi, Li; Li, Xiaoqin

    2016-09-01

    The coupling and possible nonequilibrium between magnons and other energy carriers have been used to explain several recently discovered thermally driven spin transport and energy conversion phenomena. Here, we report experiments in which local nonequilibrium between magnons and phonons in a single crystalline bulk magnetic insulator, Y3Fe5O12 , has been created optically within a focused laser spot and probed directly via micro-Brillouin light scattering. Through analyzing the deviation in the magnon number density from the local equilibrium value, we obtain the diffusion length of thermal magnons. By explicitly establishing and observing local nonequilibrium between magnons and phonons, our studies represent an important step toward a quantitative understanding of various spin-heat coupling phenomena.

  1. Spatially resolved optical-emission spectroscopy of a radio-frequency driven iodine plasma source

    NASA Astrophysics Data System (ADS)

    Dedrick, James; Doyle, Scott; Grondein, Pascaline; Aanesland, Ane

    2016-09-01

    Iodine is of interest for potential use as a propellant for spacecraft propulsion, and has become attractive as a replacement to xenon due to its similar mass and ionisation potential. Optical emission spectroscopy has been undertaken to characterise the emission from a low-pressure, radio-frequency driven inductively coupled plasma source operating in iodine with respect to axial distance across its transverse magnetic filter. The results are compared with axial profiles of the electron temperature and density for identical source conditions, and the spatial distribution of the emission intensity is observed to be closely correlated with the electron temperature. This work has been done within the LABEX Plas@Par project, and received financial state aid managed by the ``Agence Nationale de la Recherche'', as part of the ``Programme d'Investissements d'Avenir'' under the reference ANR-11-IDEX-0004-02.

  2. Optical measurement of the deformation of giant lipid vesicles driven by a micropipet electrode.

    PubMed

    Lee, Chau-Hwang; Chang, Yu-Fen; Tsai, Chia-Hsuan; Wang, Po-Hsiang

    2005-08-02

    We investigate the deformation of giant lipid vesicles driven by a micropipet electrode by use of differential confocal microscopy. This optical technique provides nanometer depth resolution without mechanical contact and hence prevents large tension or perforation of the soft membrane. For dipalmitoyl phosphatidylcholine (DPPC) membranes in the gel phase, we observed deformations of several hundreds of nanometers when the driving voltage was about 0.1 V. The voltage and frequency responses of the vesicle deformation can be explained by the balance between the electroosmotic force inside the micropipet and the membrane tension. We also used DPPC:cholesterol vesicles to check the validity of this model. In the fluid phase, however, the deformation is independent of the modulation signal because micrometer-scale thermal fluctuations dominate the membrane motion.

  3. Geometric deformable model driven by CoCRFs: application to optical coherence tomography.

    PubMed

    Tsechpenakis, Gabriel; Lujan, Brandon; Martinez, Oscar; Gregori, Giovanni; Rosenfeld, Philip J

    2008-01-01

    We present a geometric deformable model driven by dynamically updated probability fields. The shape is defined with the signed distance function, and the internal (smoothness) energy consists of a C1 continuity constraint, a shape prior, and a term that forces the zero-level of the shape distance function towards a connected form. The image probability fields are estimated by our collaborative Conditional Random Field (CoCRF), which is updated during the evolution in an active learning manner: it infers class posteriors in pixels or regions with feature ambiguities by assessing the joint appearance of neighboring sites and using the classification confidence. We apply our method to Optical Coherence Tomography fundus images for the segmentation of geographic atrophies in dry age-related macular degeneration of the human eye.

  4. High-performance genetically targetable optical neural silencing by light-driven proton pumps.

    PubMed

    Chow, Brian Y; Han, Xue; Dobry, Allison S; Qian, Xiaofeng; Chuong, Amy S; Li, Mingjie; Henninger, Michael A; Belfort, Gabriel M; Lin, Yingxi; Monahan, Patrick E; Boyden, Edward S

    2010-01-07

    The ability to silence the activity of genetically specified neurons in a temporally precise fashion would provide the opportunity to investigate the causal role of specific cell classes in neural computations, behaviours and pathologies. Here we show that members of the class of light-driven outward proton pumps can mediate powerful, safe, multiple-colour silencing of neural activity. The gene archaerhodopsin-3 (Arch) from Halorubrum sodomense enables near-100% silencing of neurons in the awake brain when virally expressed in the mouse cortex and illuminated with yellow light. Arch mediates currents of several hundred picoamps at low light powers, and supports neural silencing currents approaching 900 pA at light powers easily achievable in vivo. Furthermore, Arch spontaneously recovers from light-dependent inactivation, unlike light-driven chloride pumps that enter long-lasting inactive states in response to light. These properties of Arch are appropriate to mediate the optical silencing of significant brain volumes over behaviourally relevant timescales. Arch function in neurons is well tolerated because pH excursions created by Arch illumination are minimized by self-limiting mechanisms to levels comparable to those mediated by channelrhodopsins or natural spike firing. To highlight how proton pump ecological and genomic diversity may support new innovation, we show that the blue-green light-drivable proton pump from the fungus Leptosphaeria maculans (Mac) can, when expressed in neurons, enable neural silencing by blue light, thus enabling alongside other developed reagents the potential for independent silencing of two neural populations by blue versus red light. Light-driven proton pumps thus represent a high-performance and extremely versatile class of 'optogenetic' voltage and ion modulator, which will broadly enable new neuroscientific, biological, neurological and psychiatric investigations.

  5. Towards optical polarization control of laser-driven proton acceleration in foils undergoing relativistic transparency

    PubMed Central

    Gonzalez-Izquierdo, Bruno; King, Martin; Gray, Ross J.; Wilson, Robbie; Dance, Rachel J.; Powell, Haydn; Maclellan, David A.; McCreadie, John; Butler, Nicholas M. H.; Hawkes, Steve; Green, James S.; Murphy, Chris D.; Stockhausen, Luca C.; Carroll, David C.; Booth, Nicola; Scott, Graeme G.; Borghesi, Marco; Neely, David; McKenna, Paul

    2016-01-01

    Control of the collective response of plasma particles to intense laser light is intrinsic to relativistic optics, the development of compact laser-driven particle and radiation sources, as well as investigations of some laboratory astrophysics phenomena. We recently demonstrated that a relativistic plasma aperture produced in an ultra-thin foil at the focus of intense laser radiation can induce diffraction, enabling polarization-based control of the collective motion of plasma electrons. Here we show that under these conditions the electron dynamics are mapped into the beam of protons accelerated via strong charge-separation-induced electrostatic fields. It is demonstrated experimentally and numerically via 3D particle-in-cell simulations that the degree of ellipticity of the laser polarization strongly influences the spatial-intensity distribution of the beam of multi-MeV protons. The influence on both sheath-accelerated and radiation pressure-accelerated protons is investigated. This approach opens up a potential new route to control laser-driven ion sources. PMID:27624920

  6. Towards optical polarization control of laser-driven proton acceleration in foils undergoing relativistic transparency

    NASA Astrophysics Data System (ADS)

    Gonzalez-Izquierdo, Bruno; King, Martin; Gray, Ross J.; Wilson, Robbie; Dance, Rachel J.; Powell, Haydn; MacLellan, David A.; McCreadie, John; Butler, Nicholas M. H.; Hawkes, Steve; Green, James S.; Murphy, Chris D.; Stockhausen, Luca C.; Carroll, David C.; Booth, Nicola; Scott, Graeme G.; Borghesi, Marco; Neely, David; McKenna, Paul

    2016-09-01

    Control of the collective response of plasma particles to intense laser light is intrinsic to relativistic optics, the development of compact laser-driven particle and radiation sources, as well as investigations of some laboratory astrophysics phenomena. We recently demonstrated that a relativistic plasma aperture produced in an ultra-thin foil at the focus of intense laser radiation can induce diffraction, enabling polarization-based control of the collective motion of plasma electrons. Here we show that under these conditions the electron dynamics are mapped into the beam of protons accelerated via strong charge-separation-induced electrostatic fields. It is demonstrated experimentally and numerically via 3D particle-in-cell simulations that the degree of ellipticity of the laser polarization strongly influences the spatial-intensity distribution of the beam of multi-MeV protons. The influence on both sheath-accelerated and radiation pressure-accelerated protons is investigated. This approach opens up a potential new route to control laser-driven ion sources.

  7. Exciton Scattering approach for conjugated macromolecules: from electronic spectra to electron-phonon coupling

    NASA Astrophysics Data System (ADS)

    Tretiak, Sergei

    2014-03-01

    The exciton scattering (ES) technique is a multiscale approach developed for efficient calculations of excited-state electronic structure and optical spectra in low-dimensional conjugated macromolecules. Within the ES method, the electronic excitations in the molecular structure are attributed to standing waves representing quantum quasi-particles (excitons), which reside on the graph. The exciton propagation on the linear segments is characterized by the exciton dispersion, whereas the exciton scattering on the branching centers is determined by the energy-dependent scattering matrices. Using these ES energetic parameters, the excitation energies are then found by solving a set of generalized ``particle in a box'' problems on the graph that represents the molecule. All parameters can be extracted from quantum-chemical computations of small molecular fragments and tabulated in the ES library for further applications. Subsequently, spectroscopic modeling for any macrostructure within considered molecular family could be performed with negligible numerical effort. The exciton scattering properties of molecular vertices can be further described by tight-binding or equivalently lattice models. The on-site energies and hopping constants are obtained from the exciton dispersion and scattering matrices. Such tight-binding model approach is particularly useful to describe the exciton-phonon coupling, energetic disorder and incoherent energy transfer in large branched conjugated molecules. Overall the ES applications accurately reproduce the optical spectra compared to the reference quantum chemistry results, and make possible to predict spectra of complex macromolecules, where conventional electronic structure calculations are unfeasible.

  8. Influence of Exciton Lifetime on Charge Carrier Dynamics in an Organic Heterostructure

    SciTech Connect

    Agrawal, Kanika L.; Sykes, Matthew E.; An, Kwang Hyup; Frieberg, Bradley; Green, P. F.; Shtein, Max

    2013-03-18

    Interactions between charge carriers and excitons, as well as between excitons and optical cavity modes in organic optoelectronic devices are fundamental to their operational limits and chief in preventing the realization of certain phenomena, such as electrically pumped organic lasing. We uncovered a previously unreported phenomenon, wherein optical cavity-modulated exciton decay rate leads to a concomitant modulation in the electrical current of an archetypal NPD/Alq₃ organic light emitting device operated in forward bias. The magnitude of this variation is sensitive to the local dielectric environment of the device and is found to be as large as 15%.

  9. Crossed excitons in a semiconductor nanostructure of mixed dimensionality

    SciTech Connect

    Owschimikow, Nina Kolarczik, Mirco; Kaptan, Yücel I.; Grosse, Nicolai B.; Woggon, Ulrike

    2014-09-08

    Semiconductor systems of reduced dimensionality, e.g., quantum dots or quantum wells, display a characteristic spectrum of confined excitons. Combining several of these systems may lead to the formation of “crossed” excitons, and thus new equilibrium states and scattering channels. We derive gain excitation spectra from two-color pump-probe experiments on an In(Ga)As based quantum dot semiconductor optical amplifier by analyzing the amplitudes of the traces. This grants access to the quantum dot response, even in the presence of strong absorption by the surroundings at the excitation energy. The gain excitation spectra yield evidence of crossed quantum dot-bulk states.

  10. Light propagation in tunable exciton-polariton one-dimensional photonic crystals

    NASA Astrophysics Data System (ADS)

    Sedov, E. S.; Cherotchenko, E. D.; Arakelian, S. M.; Kavokin, A. V.

    2016-09-01

    Simulations of propagation of light beams in specially designed multilayer semiconductor structures (one-dimensional photonic crystals) with embedded quantum wells reveal characteristic optical properties of resonant hyperbolic metamaterials. A strong dependence of the refraction angle and the optical beam spread on the exciton radiative lifetime is revealed. We demonstrate the strong negative refraction of light and the control of the group velocity of light by an external bias through its effect upon the exciton radiative properties.

  11. A VHF driven coaxial atmospheric air plasma: electrical and optical characterization

    NASA Astrophysics Data System (ADS)

    Byrns, Brandon; Wooten, Daniel; Lindsay, Alexander; Shannon, Steven

    2012-05-01

    A coaxially driven VHF plasma source for atmospheric air plasmas has been built and characterized. Electrical and optical characterization of this source present a unique operating regime when compared to state of the art atmospheric systems such as dielectric barrier discharge, pulsed dc, microwave, or ac blown arc discharges. The discharge does not appear to produce streamers or arcs, but instead remains as a steady-state glow located at the end of the inner coaxial power feed. Plasma impedance was determined by comparing the loaded and unloaded impedance of the coaxial source RF input; this termination impedance was combined with a simple high-frequency global model to estimate an electron density of approximately 1011 cm-3 at 400 W delivered power in air. Optical emission characterization of the source shows a monotonic increase in emission with respect with power; the relative intensity of the peaks from excited species, however, remains constant over a power range from 300 to 600 W. This unique source geometry presents a possible pathway for high gas throughput, large area, high power density processes such as surface modification, air purification, media removal and chemical surface treatment.

  12. Charged two-exciton emission from a single semiconductor nanocrystal

    SciTech Connect

    Hu, Fengrui; Zhang, Qiang; Zhang, Chunfeng; Wang, Xiaoyong; Xiao, Min

    2015-03-30

    Here, we study the photoluminescence (PL) time trajectories of single CdSe/ZnS nanocrystals (NCs) as a function of the laser excitation power. At the low laser power, the PL intensity of a single NC switches between the “on” and “off” levels arising from the neutral and positively charged single excitons, respectively. With the increasing laser power, an intermediate “grey” level is formed due to the optical emission from a charged multiexciton state composed of two excitons and an extra electron. Both the inter-photon correlation and the PL decay measurements demonstrate that lifetime-indistinguishable photon pairs are emitted from this negatively charged two-exciton state.

  13. Quantum confinement-induced tunable exciton states in graphene oxide

    PubMed Central

    Lee, Dongwook; Seo, Jiwon; Zhu, Xi; Lee, Jiyoul; Shin, Hyeon-Jin; Cole, Jacqueline M.; Shin, Taeho; Lee, Jaichan; Lee, Hangil; Su, Haibin

    2013-01-01

    Graphene oxide has recently been considered to be a potential replacement for cadmium-based quantum dots due to its expected high fluorescence. Although previously reported, the origin of the luminescence in graphene oxide is still controversial. Here, we report the presence of core/valence excitons in graphene-based materials, a basic ingredient for optical devices, induced by quantum confinement. Electron confinement in the unreacted graphitic regions of graphene oxide was probed by high resolution X-ray absorption near edge structure spectroscopy and first-principles calculations. Using experiments and simulations, we were able to tune the core/valence exciton energy by manipulating the size of graphitic regions through the degree of oxidation. The binding energy of an exciton in highly oxidized graphene oxide is similar to that in organic electroluminescent materials. These results open the possibility of graphene oxide-based optoelectronic device technology. PMID:23872608

  14. Huge excitonic effects in layered hexagonal boron nitride.

    PubMed

    Arnaud, B; Lebègue, S; Rabiller, P; Alouani, M

    2006-01-20

    The all-electron GW approximation energy band gap of bulk hexagonal boron nitride is shown to be of indirect type. The resulting computed in-plane polarized optical spectrum, obtained by solving the Bethe-Salpeter equation for the electron-hole two-particle Green function, is in excellent agreement with experiment and has a strong anisotropy compared to out-of-plane polarized spectrum. A detailed analysis of the excitonic structures within the band gap shows that the low-lying excitons belong to the Frenkel class and are tightly confined within the layers. The calculated exciton binding energy is much larger than that obtained by Watanabe et al. [Nat. Mater. 3, 404 (2004).] based on a Wannier model assuming h-BN to be a direct-band-gap semiconductor.

  15. Multiple beats of weakly confined excitons with inverted selection rule

    NASA Astrophysics Data System (ADS)

    Yasuda, Hideki; Ishihara, Hajime

    2009-05-01

    The phenomenon of multiple beats (MBs) arising from nondipole-type excitons weakly confined in a thin film is theoretically elucidated using a nonlocal transient-response theory. Kojima previously demonstrated for a GaAs thin film that the degenerate four-wave mixing signals from the quantized levels of the center-of-mass motion of excitons exhibit complex interference between beats under femtosecond-order pulse incidence [Kojima , J. Phys. Soc. Jpn. 77, 044701 (2008)]. This leads to an ultrafast optical response on the order of femtoseconds. This effect occurs in a size region beyond the long-wavelength approximation regime due to the resonant enhancement of the internal field, wherein the usual dipole selection rule is violated. Our analysis of MBs employs a model of the nonlocal multilevel system that considers the spatial interplay between excitonic waves and the radiation field to elucidate the mechanism behind the observed ultrafast response.

  16. Computational study of exciton generation in suspended carbon nanotube transistors.

    PubMed

    Koswatta, Siyuranga O; Perebeinos, Vasili; Lundstrom, Mark S; Avouris, Phaedon

    2008-06-01

    Optical emission from carbon nanotube transistors (CNTFETs) has recently attracted significant attention due to its potential applications. In this paper, we use a self-consistent numerical solution of the Boltzmann transport equation in the presence of both phonon and exciton scattering to present a detailed study of the operation of a partially suspended CNTFET light emitter, which has been discussed in a recent experiment. We determine the energy distribution of hot carriers in the CNTFET and, as reported in the experiment, observe localized generation of excitons near the trench-substrate junction and an exponential increase in emission intensity with a linear increase in current versus gate voltage. We further provide detailed insight into device operation and propose optimization schemes for efficient exciton generation; a deeper trench increases the generation efficiency, and use of high-k substrate oxides could lead to even larger enhancements.

  17. Indirect optical absorption and origin of the emission from β-FeSi2 nanoparticles: Bound exciton (0.809 eV) and band to acceptor impurity (0.795 eV) transitions

    NASA Astrophysics Data System (ADS)

    Lang, R.; Amaral, L.; Meneses, E. A.

    2010-05-01

    We investigated the optical absorption of the fundamental band edge and the origin of the emission from β-FeSi2 nanoparticles synthesized by ion-beam-induced epitaxial crystallization of Fe+ implanted SiO2/Si(100) followed by thermal annealing. From micro-Raman scattering and transmission electron microscopy measurements it was possible to attest the formation of strained β-FeSi2 nanoparticles and its structural quality. The optical absorption near the fundamental gap edge of β-FeSi2 nanoparticles evaluated by spectroscopic ellipsometry showed a step structure characteristic of an indirect fundamental gap material. Photoluminescence spectroscopy measurements at each synthesis stage revealed complex emissions in the 0.7-0.9 eV spectral region, with different intensities and morphologies strongly dependent on thermal treatment temperature. Spectral deconvolution into four transition lines at 0.795, 0.809, 0.851, and 0.873 eV was performed. We concluded that the emission at 0.795 eV may be related to a radiative direct transition from the direct conduction band to an acceptor level and that the emission at 0.809 eV derives from a recombination of an indirect bound exciton to this acceptor level of β-FeSi2. Emissions 0.851 and 0.873 eV were confirmed to be typical dislocation-related photoluminescence centers in Si. From the energy balance we determined the fundamental indirect and direct band gap energies to be 0.856 and 0.867 eV, respectively. An illustrative energy band diagram derived from a proposed model to explain the possible transition processes involved is presented.

  18. Self-trapped exciton and core-valence luminescence in BaF{sub 2} nanoparticles

    SciTech Connect

    Vistovskyy, V. V. Zhyshkovych, A. V.; Chornodolskyy, Ya. M.; Voloshinovskii, A. S.; Myagkota, O. S.; Gloskovskii, A.; Gektin, A. V.; Vasil'ev, A. N.; Rodnyi, P. A.

    2013-11-21

    The influence of the BaF{sub 2} nanoparticle size on the intensity of the self-trapped exciton luminescence and the radiative core-valence transitions is studied by the luminescence spectroscopy methods using synchrotron radiation. The decrease of the self-trapped exciton emission intensity at energies of exciting photons in the range of optical exciton creation (hν ≤ E{sub g}) is less sensitive to the reduction of the nanoparticle sizes than in the case of band-to-band excitation, where excitons are formed by the recombination way. The intensity of the core-valence luminescence shows considerably weaker dependence on the nanoparticle sizes in comparison with the intensity of self-trapped exciton luminescence. The revealed regularities are explained by considering the relationship between nanoparticle size and photoelectron or photohole thermalization length as well as the size of electronic excitations.

  19. Effect of periodic potential on exciton states in semiconductor carbon nanotubes

    SciTech Connect

    Roslyak, Oleksiy; Piryatinski, Andrei

    2016-05-28

    Here we develop a theoretical background to treat exciton states in semiconductor single-walled carbon nanotubes (SWCNTs) in the presence of a periodic potential induced by a surface acoustic wave (SAW) propagating along SWCNT. The formalism accounts for the electronic band splitting into the Floquet subbands induced by the Bragg scattering on the SAW potential. Optical transitions between the Floquet states and correlated electron–hole pairs (excitons) are numerically examined. Formation of new van Hove singularities within the edges of Floquet sub-bands and associated transfer of the exciton oscillator strengths resulting in the photoluminescence quenching are predicted. The simulations demonstrate the exciton energy red Stark shift and reduction in the exciton binding energy. We provide comparison of our results with reported theoretical and experimental studies.

  20. Frenkel-like Wannier-Mott excitons in few-layer Pb I2

    NASA Astrophysics Data System (ADS)

    Toulouse, Alexis S.; Isaacoff, Benjamin P.; Shi, Guangsha; Matuchová, Marie; Kioupakis, Emmanouil; Merlin, Roberto

    2015-04-01

    Optical measurements and first-principles calculations of the band structure and exciton states in direct-gap bulk and few-layer Pb I2 indicate that the n =1 exciton is Frenkel-like in nature in that its energy exhibits a weak dependence on thickness down to atomic-length scales. Results reveal large increases in the gap and exciton binding energy with a decreasing number of layers and a transition of the fundamental gap, which becomes indirect for one and two monolayers. Calculated values are in reasonable agreement with a particle-in-a-box model relying on the Wannier-Mott theory of exciton formation. General arguments and existing data suggest that the Frenkel-like character of the lowest exciton is a universal feature of wide-gap layered semiconductors whose effective masses and dielectric constants give bulk Bohr radii that are on the order of the layer spacing.

  1. Effect of periodic potential on exciton states in semiconductor carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Roslyak, Oleksiy; Piryatinski, Andrei

    2016-12-01

    We develop a theoretical background to treat exciton states in semiconductor single-walled carbon nanotubes (SWCNTs) in the presence of a periodic potential induced by a surface acoustic wave (SAW) propagating along SWCNT. The formalism accounts for the electronic band splitting into the Floquet sub-bands induced by the Bragg scattering on the SAW potential. Optical transitions between the Floquet states and correlated electron-hole pairs (excitons) are numerically examined. Formation of new van Hove singularities within the edges of Floquet sub-bands and associated transfer of the exciton oscillator strengths resulting in the photoluminescence quenching are predicted. The simulations demonstrate the exciton energy red Stark shift and reduction in the exciton binding energy. Comparison of our results with reported theoretical and experimental studies is provided.

  2. Giant permanent dipole moment of two-dimensional excitons bound to a single stacking fault

    NASA Astrophysics Data System (ADS)

    Karin, Todd; Linpeng, Xiayu; Glazov, M. M.; Durnev, M. V.; Ivchenko, E. L.; Harvey, Sarah; Rai, Ashish K.; Ludwig, Arne; Wieck, Andreas D.; Fu, Kai-Mei C.

    2016-07-01

    We investigate the magneto-optical properties of excitons bound to single stacking faults in high-purity GaAs. We find that the two-dimensional stacking fault potential binds an exciton composed of an electron and a heavy hole, and we confirm a vanishing in-plane hole g -factor, consistent with the atomic-scale symmetry of the system. The unprecedented homogeneity of the stacking-fault potential leads to ultranarrow photoluminescence emission lines (with a full width at half-maximum ≲80 μ eV ) and reveals a large magnetic nonreciprocity effect that originates from the magneto-Stark effect for mobile excitons. These measurements unambiguously determine the direction and magnitude of the giant electric dipole moment (≳e ×10 nm ) of the stacking-fault exciton, making stacking faults a promising new platform to study interacting excitonic gases.

  3. Recent developments in laser-driven and hollow-core fiber optic gyroscopes

    NASA Astrophysics Data System (ADS)

    Digonnet, M. J. F.; Chamoun, J. N.

    2016-05-01

    Although the fiber optic gyroscope (FOG) continues to be a commercial success, current research efforts are endeavoring to improve its precision and broaden its applicability to other markets, in particular the inertial navigation of aircraft. Significant steps in this direction are expected from the use of (1) laser light to interrogate the FOG instead of broadband light, and (2) a hollow-core fiber (HCF) in the sensing coil instead of a conventional solid-core fiber. The use of a laser greatly improves the FOG's scale-factor stability and eliminates the source excess noise, while an HCF virtually eliminates the Kerr-induced drift and significantly reduces the thermal and Faraday-induced drifts. In this paper we present theoretical evidence that in a FOG with a 1085-m coil interrogated with a laser, the two main sources of noise and drift resulting from the use of coherent light can be reduced below the aircraft-navigation requirement by using a laser with a very broad linewidth, in excess of 40 GHz. We validate this concept with a laser broadened with an external phase modulator driven with a pseudo-random bit sequence at 2.8 GHz. This FOG has a measured noise of 0.00073 deg/√h, which is 30% below the aircraft-navigation requirement. Its measured drift is 0.03 deg/h, the lowest reported for a laser-driven FOG and only a factor of 3 larger than the navigation-grade specification. To illustrate the potential benefits of a hollow-core fiber in the FOG, this review also summarizes the previously reported performance of an experimental FOG utilizing 235 m of HCF and interrogated with broadband light.

  4. Exciton size and binding energy limitations in one-dimensional organic materials

    SciTech Connect

    Kraner, S. Koerner, C.; Leo, K.; Scholz, R.; Plasser, F.

    2015-12-28

    In current organic photovoltaic devices, the loss in energy caused by the charge transfer step necessary for exciton dissociation leads to a low open circuit voltage, being one of the main reasons for rather low power conversion efficiencies. A possible approach to avoid these losses is to tune the exciton binding energy to a value of the order of thermal energy, which would lead to free charges upon absorption of a photon, and therefore increase the power conversion efficiency towards the Shockley-Queisser limit. We determine the size of the excitons for different organic molecules and polymers by time dependent density functional theory calculations. For optically relevant transitions, the exciton size saturates around 0.7 nm for one-dimensional molecules with a size longer than about 4 nm. For the ladder-type polymer poly(benzimidazobenzophenanthroline), we obtain an exciton binding energy of about 0.3 eV, serving as a lower limit of the exciton binding energy for the organic materials investigated. Furthermore, we show that charge transfer transitions increase the exciton size and thus identify possible routes towards a further decrease of the exciton binding energy.

  5. Identification of a triplet pair intermediate in singlet exciton fission in solution.

    PubMed

    Stern, Hannah L; Musser, Andrew J; Gelinas, Simon; Parkinson, Patrick; Herz, Laura M; Bruzek, Matthew J; Anthony, John; Friend, Richard H; Walker, Brian J

    2015-06-23

    Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into two spin-triplet excitons. This exciton multiplication mechanism offers an attractive route to solar cells that circumvent the single-junction Shockley-Queisser limit. Most theoretical descriptions of singlet fission invoke an intermediate state of a pair of spin-triplet excitons coupled into an overall spin-singlet configuration, but such a state has never been optically observed. In solution, we show that the dynamics of fission are diffusion limited and enable the isolation of an intermediate species. In concentrated solutions of bis(triisopropylsilylethynyl)[TIPS]--tetracene we find rapid (<100 ps) formation of excimers and a slower (∼ 10 ns) break up of the excimer to two triplet exciton-bearing free molecules. These excimers are spectroscopically distinct from singlet and triplet excitons, yet possess both singlet and triplet characteristics, enabling identification as a triplet pair state. We find that this triplet pair state is significantly stabilized relative to free triplet excitons, and that it plays a critical role in the efficient endothermic singlet fission process.

  6. Exciton states in a circular graphene quantum dot: Magnetic field induced intravalley to intervalley transition

    NASA Astrophysics Data System (ADS)

    Li, L. L.; Zarenia, M.; Xu, W.; Dong, H. M.; Peeters, F. M.

    2017-01-01

    The magnetic-field dependence of the energy spectrum, wave function, binding energy, and oscillator strength of exciton states confined in a circular graphene quantum dot (CGQD) is obtained within the configuration interaction method. We predict that (i) excitonic effects are very significant in the CGQD as a consequence of a combination of geometric confinement, magnetic confinement, and reduced screening; (ii) two types of excitons (intravalley and intervalley) are present in the CGQD because of the valley degree of freedom in graphene; (iii) the intravalley and intervalley exciton states display different magnetic-field dependencies due to the different electron-hole symmetries of the single-particle energy spectra; (iv) with increasing magnetic field, the exciton ground state in the CGQD undergoes an intravalley to intervalley transition accompanied by a change of angular momentum; (v) the exciton binding energy does not increase monotonically with the magnetic field due to the competition between geometric and magnetic confinements; and (vi) the optical transitions of the intervalley and intravalley excitons can be tuned by the magnetic field, and valley-dependent excitonic transitions can be realized in a CGQD.

  7. Radiation effects from first principles : the role of excitons in electronic-excited processes.

    SciTech Connect

    Wong, Bryan Matthew

    2009-09-01

    Electron-hole pairs, or excitons, are created within materials upon optical excitation or irradiation with X-rays/charged particles. The ability to control and predict the role of excitons in these energetically-induced processes would have a tremendous impact on understanding the effects of radiation on materials. In this report, the excitonic effects in large cycloparaphenylene carbon structures are investigated using various first-principles methods. These structures are particularly interesting since they allow a study of size-scaling properties of excitons in a prototypical semi-conducting material. In order to understand these properties, electron-hole transition density matrices and exciton binding energies were analyzed as a function of size. The transition density matrices allow a global view of electronic coherence during an electronic excitation, and the exciton binding energies give a quantitative measure of electron-hole interaction energies in these structures. Based on overall trends in exciton binding energies and their spatial delocalization, we find that excitonic effects play a vital role in understanding the unique photoinduced dynamics in these systems.

  8. Exciton dispersion in molecular solids.

    PubMed

    Cudazzo, Pierluigi; Sottile, Francesco; Rubio, Angel; Gatti, Matteo

    2015-03-25

    The investigation of the exciton dispersion (i.e. the exciton energy dependence as a function of the momentum carried by the electron-hole pair) is a powerful approach to identify the exciton character, ranging from the strongly localised Frenkel to the delocalised Wannier-Mott limiting cases. We illustrate this possibility at the example of four prototypical molecular solids (picene, pentacene, tetracene and coronene) on the basis of the parameter-free solution of the many-body Bethe-Salpeter equation. We discuss the mixing between Frenkel and charge-transfer excitons and the origin of their Davydov splitting in the framework of many-body perturbation theory and establish a link with model approaches based on molecular states. Finally, we show how the interplay between the electronic band dispersion and the exchange electron-hole interaction plays a fundamental role in setting the nature of the exciton. This analysis has a general validity holding also for other systems in which the electron wavefunctions are strongly localized, as in strongly correlated insulators.

  9. Exciton dispersion in molecular solids

    NASA Astrophysics Data System (ADS)

    Cudazzo, Pierluigi; Sottile, Francesco; Rubio, Angel; Gatti, Matteo

    2015-03-01

    The investigation of the exciton dispersion (i.e. the exciton energy dependence as a function of the momentum carried by the electron-hole pair) is a powerful approach to identify the exciton character, ranging from the strongly localised Frenkel to the delocalised Wannier-Mott limiting cases. We illustrate this possibility at the example of four prototypical molecular solids (picene, pentacene, tetracene and coronene) on the basis of the parameter-free solution of the many-body Bethe-Salpeter equation. We discuss the mixing between Frenkel and charge-transfer excitons and the origin of their Davydov splitting in the framework of many-body perturbation theory and establish a link with model approaches based on molecular states. Finally, we show how the interplay between the electronic band dispersion and the exchange electron-hole interaction plays a fundamental role in setting the nature of the exciton. This analysis has a general validity holding also for other systems in which the electron wavefunctions are strongly localized, as in strongly correlated insulators.

  10. Coherent Exciton Dynamics in the Presence of Underdamped Vibrations.

    PubMed

    Dijkstra, Arend G; Wang, Chen; Cao, Jianshu; Fleming, Graham R

    2015-02-19

    Recent ultrafast optical experiments show that excitons in large biological light-harvesting complexes are coupled to molecular vibration modes. These high-frequency vibrations will not only affect the optical response, but also drive the exciton transport. Here, using a model dimer system, the frequency of the underdamped vibration is shown to have a strong effect on the exciton dynamics such that quantum coherent oscillations in the system can be present even in the case of strong noise. Two mechanisms are identified to be responsible for the enhanced transport efficiency: critical damping due to the tunable effective strength of the coupling to the bath, and resonance coupling where the vibrational frequency coincides with the energy gap in the system. The interplay of these two mechanisms determines parameters responsible for the most efficient transport, and these optimal control parameters are comparable to those in realistic light-harvesting complexes. Interestingly, oscillations in the excitonic coherence at resonance are suppressed in comparison to the case of an off-resonant vibration.

  11. Exciton transport, charge extraction, and loss mechanisms in organic photovoltaics

    NASA Astrophysics Data System (ADS)

    Scully, Shawn Ryan

    Organic photovoltaics have attracted significant interest over the last decade due to their promise as clean low-cost alternatives to large-scale electric power generation such as coal-fired power, natural gas, and nuclear power. Many believe power conversion efficiency targets of 10-15% must be reached before commercialization is possible. Consequently, understanding the loss mechanisms which currently limit efficiencies to 4-5% is crucial to identify paths to reach higher efficiencies. In this work, we investigate the dominant loss mechanisms in some of the leading organic photovoltaic architectures. In the first class of architectures, which include planar heterojunctions and bulk heterojunctions with large domains, efficiencies are primarily limited by the distance photogenerated excitations (excitons) can be transported (termed the exciton diffusion length) to a heterojunction where the excitons may dissociate. We will discuss how to properly measure the exciton diffusion length focusing on the effects of optical interference and of energy transfer when using fullerenes as quenching layers and show how this explains the variety of diffusion lengths reported for the same material. After understanding that disorder and defects limit exciton diffusion lengths, we suggest some approaches to overcome this. We then extensively investigate the use of long-range resonant energy transfer to increase exciton harvesting. Using simulations and experiments as support, we discuss how energy transfer can be engineered into architectures to increase the distance excitons can be harvested. In an experimental model system, DOW Red/PTPTB, we will show how the distance excitons are harvested can be increased by almost an order of magnitude up to 27 nm from a heterojunction and give design rules and extensions of this concept for future architectures. After understanding exciton harvesting limitations we will look at other losses that are present in planar heterojunctions. One of

  12. A motor-driven ventricular assist device controlled with an optical encoder system.

    PubMed

    Nakamura, T; Hayashi, K; Yamane, H

    1993-01-01

    An electric motor-driven ventricular assist device has been developed for long-term use inside the body. The system is composed of a pusher-plate-type blood pump and an actuator consisting of an electrical motor and a ball screw. Cyclic change of the direction of motor rotation makes a back-and-forth axial movement of the ball screw shaft. The shaft, which is detached from the pump diaphragm, pushes the diaphragm via a pusher plate to eject blood during systole; blood is sucked by the diaphragm resilience during diastole. Using the output signals from a newly designed, incremental-type, miniature optical rotary encoder mounted inside the actuator, the input voltage of the motor is optimally controlled referring to the phase difference between the current position of the moving rotor and the electrical reference signal of the rotation generated by a microprocessor-based controller. In vitro performance tests indicated that the system fulfills required specifications. The maximum efficiency was 11%, which was about twice as high as that obtained with the previous open-loop prototype system. In the air, the surface temperature of the actuator elevated to 20 degrees C above the room temperature. An acute in vivo test showed its feasibility as a left ventricular assist device. Analysis of the energy loss in each component of the system indicated that redesign and precise assembly of the mechanical parts could increase the system efficiency.

  13. Multichannel emission spectrometer for high dynamic range optical pyrometry of shock-driven materials

    NASA Astrophysics Data System (ADS)

    Bassett, Will P.; Dlott, Dana D.

    2016-10-01

    An emission spectrometer (450-850 nm) using a high-throughput, high numerical aperture (N.A. = 0.3) prism spectrograph with stepped fiberoptic coupling, 32 fast photomultipliers and thirty-two 1.25 GHz digitizers is described. The spectrometer can capture single-shot events with a high dynamic range in amplitude and time (nanoseconds to milliseconds or longer). Methods to calibrate the spectrometer and verify its performance and accuracy are described. When a reference thermal source is used for calibration, the spectrometer can function as a fast optical pyrometer. Applications of the spectrometer are illustrated by using it to capture single-shot emission transients from energetic materials or reactive materials initiated by kmṡs-1 impacts with laser-driven flyer plates. A log (time) data analysis method is used to visualize multiple kinetic processes resulting from impact initiation of HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) or a Zr/CuO nanolaminate thermite. Using a gray body algorithm to interpret the spectral radiance from shocked HMX, a time history of temperature and emissivity was obtained, which could be used to investigate HMX hot spot dynamics. Finally, two examples are presented showing how the spectrometer can avoid temperature determination errors in systems where thermal emission is accompanied by atomic or molecular emission lines.

  14. Tuning excitons in monolayer and few-layer MoS2

    NASA Astrophysics Data System (ADS)

    Qiu, Diana Y.; da Jornada, Felipe H.; Louie, Steven G.

    2014-03-01

    Our recent ab initio GW-BSE calculations showed that monolayer MoS2 is a computationally challenging system, requiring a large number of empty bands and very fine k-point sampling to converge its quasiparticle band structure and optical properties. Careful convergence of a GW-BSE calculation reveals that MoS2 has a large number of bound excitons with varying k-space characteristics. Specifically, there are two series of excitons: a low-energy series with k-space wavefunctions localized at the K/K' valleys in the Brillouin zone and a higher energy series localized in a ring around the Γ point. There is very little hybridization between these two exciton series in monolayer MoS2, but changes in electronic structure and screening due to additional layers, strain, or doping can lead to changes in exciton binding energies, character, and hybridization. Thus, we have carried out ab initio GW-BSE calculations to study the excitonic properties of few-layer MoS2. We find that layering and straining MoS2 systematically changes the exciton binding energies, the peak positions and amount of absorbance in the optical spectrum, and the character and hybridization of the excitons near Γ. This work was supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231.

  15. Excitonic properties of semiconducting monolayer and bilayer MoT e2

    NASA Astrophysics Data System (ADS)

    Robert, C.; Picard, R.; Lagarde, D.; Wang, G.; Echeverry, J. P.; Cadiz, F.; Renucci, P.; Högele, A.; Amand, T.; Marie, X.; Gerber, I. C.; Urbaszek, B.

    2016-10-01

    MoT e2 belongs to the semiconducting transition-metal dichalcogenide family with certain properties differing from the other well-studied members (Mo ,W ) (S,Se ) 2 . The optical band gap is in the near-infrared region, and both monolayers and bilayers may have a direct optical band gap. We first simulate the single-particle band structure of both monolayer and bilayer MoT e2 with density-functional-theory-G W calculations. We find a direct (indirect) electronic band gap for the monolayer (bilayer). By solving in addition the Bethe-Salpeter equation, we find similar energies for the direct exciton transitions in monolayers and bilayers. We then study the optical properties by means of photoluminescence (PL) excitation, reflectivity, time-resolved PL, and power-dependent PL spectroscopy. With differential reflectivity, we find a similar oscillator strength for the optical transition observed in PL in both monolayers and bilayers suggesting a direct transition in both cases. We identify the same energy for the B -exciton state in the monolayer and the bilayer. Following circularly polarized excitation, we do not find any exciton polarization for a large range of excitation energies. At low temperatures (T =10 K ) , we measure similar PL decay times on the order of 4 ps for both monolayer and bilayer excitons with a slightly longer one for the bilayer. Finally, we observe a reduction of the exciton-exciton annihilation contribution to the nonradiative recombination in bilayers.

  16. Excitonic polaritons in Fibonacci quasicrystals.

    PubMed

    Hendrickson, J; Richards, B C; Sweet, J; Khitrova, G; Poddubny, A N; Ivchenko, E L; Wegener, M; Gibbs, H M

    2008-09-29

    The fabrication and characterization of light-emitting one-dimensional photonic quasicrystals based on excitonic resonances is reported. The structures consist of high-quality GaAs/AlGaAs quantum wells grown by molecular-beam epitaxy with wavelength-scale spacings satisfying a Fibonacci sequence. The polaritonic (resonant light-matter coupling) effects and light emission originate from the quantum well excitonic resonances. Measured reflectivity spectra as a function of detuning between emission and Bragg wavelength are in good agreement with excitonic polariton theory. Photoluminescence experiments show that active photonic quasicrystals, unlike photonic crystals, can be good light emitters: While their long-range order results in a stopband similar to that of photonic crystals, the lack of periodicity results in strong emission.

  17. Determination of the Exciton Binding Energy in CdSe Quantum Dots

    SciTech Connect

    Meulenberg, R; Lee, J; Wolcott, A; Zhang, J; Terminello, L; van Buuren, T

    2009-10-27

    The exciton binding energy (EBE) in CdSe quantum dots (QDs) has been determined using x-ray spectroscopy. Using x-ray absorption and photoemission spectroscopy, the conduction band (CB) and valence band (VB) edge shifts as a function of particle size have been determined and combined to obtain the true band gap of the QDs (i.e. without and exciton). These values can be compared to the excitonic gap obtained using optical spectroscopy to determine the EBE. The experimental EBE results are compared with theoretical calculations on the EBE and show excellent agreement.

  18. Exciton-photon correlations in bosonic condensates of exciton-polaritons

    PubMed Central

    Kavokin, Alexey V.; Sheremet, Alexandra S.; Shelykh, Ivan A.; Lagoudakis, Pavlos G.; Rubo, Yuri G.

    2015-01-01

    Exciton-polaritons are mixed light-matter quasiparticles. We have developed a statistical model describing stochastic exciton-photon transitions within a condensate of exciton polaritons. We show that the exciton-photon correlator depends on the rate of incoherent exciton-photon transformations in the condensate. We discuss implications of this effect for the quantum statistics of photons emitted by polariton lasers. PMID:26153979

  19. Significant Lowering Optical Loss of Electrodes via using Conjugated Polyelectrolytes Interlayer for Organic Laser in Electrically Driven Device Configuration

    NASA Astrophysics Data System (ADS)

    Yi, Jianpeng; Niu, Qiaoli; Xu, Weidong; Hao, Lin; Yang, Lei; Chi, Lang; Fang, Yueting; Huang, Jinjin; Xia, Ruidong

    2016-05-01

    One of the challenges toward electrically driven organic lasers is the huge optical loss associated with the contact of electrodes and organic gain medium in device. We demonstrated a significant reduction of the optical loss by using our newly developed conjugated polyelectrolytes (CPE) PPFN+Br- as interlayer between gain medium and electrode. The optically pumped amplified spontaneous emission (ASE) was observed at very low threshold for PFO as optical gain medium and up to 37 nm thick CPE as interlayer in device configuration, c.f., a 5.7-fold ASE threshold reduction from pump energy 150 μJ/cm2 for ITO/PFO to 26.3 μJ/cm2 for ITO/PPFN+Br-/PFO. Furthermore, ASE narrowing displayed at pump energy up to 61.8 μJ/cm2 for device ITO/PEDOT:PSS/PFO/PPFN+Br-/Ag, while no ASE was observed for the reference devices without CPE interlayer at pump energy up to 240 μJ/cm2. The optically pumped lasing operation has also been achieved at threshold up to 45 μJ/cm2 for one-dimensional distributed feedback laser fabricated on ITO etched grating in devices with CPE interlayer, demonstrating a promising device configuration for addressing the challenge of electrically driven organic lasers.

  20. Significant Lowering Optical Loss of Electrodes via using Conjugated Polyelectrolytes Interlayer for Organic Laser in Electrically Driven Device Configuration

    PubMed Central

    Yi, Jianpeng; Niu, Qiaoli; Xu, Weidong; Hao, Lin; Yang, Lei; Chi, Lang; Fang, Yueting; Huang, Jinjin; Xia, Ruidong

    2016-01-01

    One of the challenges toward electrically driven organic lasers is the huge optical loss associated with the contact of electrodes and organic gain medium in device. We demonstrated a significant reduction of the optical loss by using our newly developed conjugated polyelectrolytes (CPE) PPFN+Br− as interlayer between gain medium and electrode. The optically pumped amplified spontaneous emission (ASE) was observed at very low threshold for PFO as optical gain medium and up to 37 nm thick CPE as interlayer in device configuration, c.f., a 5.7-fold ASE threshold reduction from pump energy 150 μJ/cm2 for ITO/PFO to 26.3 μJ/cm2 for ITO/PPFN+Br−/PFO. Furthermore, ASE narrowing displayed at pump energy up to 61.8 μJ/cm2 for device ITO/PEDOT:PSS/PFO/PPFN+Br−/Ag, while no ASE was observed for the reference devices without CPE interlayer at pump energy up to 240 μJ/cm2. The optically pumped lasing operation has also been achieved at threshold up to 45 μJ/cm2 for one-dimensional distributed feedback laser fabricated on ITO etched grating in devices with CPE interlayer, demonstrating a promising device configuration for addressing the challenge of electrically driven organic lasers. PMID:27165729

  1. High-gain optical Cherenkov oscillator driven by low-voltage electron beam

    SciTech Connect

    Smetanin, I.V.; Oraevsky, A.N.

    1995-12-31

    A novel scheme of high-gain optical (from IR up to UV) Cherenkov-type oscillator driven by low-voltage high-current electron beam is proposed in the present report. In the scheme discussed the magnetized electron beam propagates above the surface of absorbing medium of complex dielectric susceptibility {epsilon}{omega} = {epsilon}{sub 1}({omega}) + i{epsilon}{sub 2}({omega}), {epsilon}{sub 2}>0. We have found that at frequencies {omega} that {beta}{sup 2}> 2{epsilon}{sub 1}/{vert_bar}{epsilon}{vert_bar}{sup 2} ({beta} = v/c, v is the electron velocity), an amplification of co-propagating slow surface electromagnetic wave is possible. In contrast to the conventional Cherenkov oscillators, the absorption condition {epsilon}2>0 is crucial for the gain, which is absent for transparent medium. The physics of this amplification effect is analogous to that of electron beam dissipative instability. The wavelength generated is determined here by dielectric properties of the surface, and does not depend strongly on electron energy. Thus it is possible to use rather compact low voltage ({le} 1MeV) high-current accelerators as drivers. Optimum oscillation conditions are found to be at frequencies near the resonance absorption lines of surface material (i.e. from IR up to UV). The gain up to {approximately}0.5cm{sup -1} in the near IR ({approximately}10THz, SrF{sub 2} absorption line) is possible for 250keV high current (density {approximately}10{sup 12}cm{sup -3}) electron beam.

  2. Three dimensional data-driven multi scale atomic representation of optical coherence tomography.

    PubMed

    Kafieh, Raheleh; Rabbani, Hossein; Selesnick, Ivan

    2015-05-01

    In this paper, we discuss about applications of different methods for decomposing a signal over elementary waveforms chosen in a family called a dictionary (atomic representations) in optical coherence tomography (OCT). If the representation is learned from the data, a nonparametric dictionary is defined with three fundamental properties of being data-driven, applicability on 3D, and working in multi-scale, which make it appropriate for processing of OCT images. We discuss about application of such representations including complex wavelet based K-SVD, and diffusion wavelets on OCT data. We introduce complex wavelet based K-SVD to take advantage of adaptability in dictionary learning methods to improve the performance of simple dual tree complex wavelets in speckle reduction of OCT datasets in 2D and 3D. The algorithm is evaluated on 144 randomly selected slices from twelve 3D OCTs taken by Topcon 3D OCT-1000 and Cirrus Zeiss Meditec. Improvement of contrast to noise ratio (CNR) (from 0.9 to 11.91 and from 3.09 to 88.9, respectively) is achieved. Furthermore, two approaches are proposed for image segmentation using diffusion. The first method is designing a competition between extended basis functions at each level and the second approach is defining a new distance for each level and clustering based on such distances. A combined algorithm, based on these two methods is then proposed for segmentation of retinal OCTs, which is able to localize 12 boundaries with unsigned border positioning error of 9.22 ±3.05 μm, on a test set of 20 slices selected from 13 3D OCTs.

  3. Measurement of exciton correlations using electrostatic lattices

    NASA Astrophysics Data System (ADS)

    Remeika, M.; Leonard, J. R.; Dorow, C. J.; Fogler, M. M.; Butov, L. V.; Hanson, M.; Gossard, A. C.

    2015-09-01

    We present a method for determining correlations in a gas of indirect excitons in a semiconductor quantum well structure. The method involves subjecting the excitons to a periodic electrostatic potential that causes modulations of the exciton density and photoluminescence (PL). Experimentally measured amplitudes of energy and intensity modulations of exciton PL serve as an input to a theoretical estimate of the exciton correlation parameter and temperature. We also present a proof-of-principle demonstration of the method for determining the correlation parameter and discuss how its accuracy can be improved.

  4. Relative Ordering between Bright and Dark Excitons in Single-walled Carbon Nanotubes

    PubMed Central

    Zhou, Weihang; Nakamura, Daisuke; Liu, Huaping; Kataura, Hiromichi; Takeyama, Shojiro

    2014-01-01

    The ordering and relative energy splitting between bright and dark excitons are critical to the optical properties of single-walled carbon nanotubes (SWNTs), as they eventually determine the radiative and non-radiative recombination processes of generated carriers. In this work, we report systematic high-field magneto-optical study on the relative ordering between bright and dark excitons in SWNTs. We identified the relative energy position of the dark exciton unambiguously by brightening it in ultra-high magnetic field. The bright-dark excitonic ordering was found to depend not only on the tube structure, but also on the type of transitions. For the 1st sub-band transition, the bright exciton appears to be higher in energy than its dark counterpart for any chiral species and is robust against environmental effect. While for the 2nd sub-band, their relative ordering was found to be chirality-sensitive: the bright exciton can be either higher or lower than the dark one, depending on the specific nanotube structures. These findings provide new clues for engineering the optical and electronic properties of SWNTs. PMID:25385545

  5. Relative ordering between bright and dark excitons in single-walled carbon nanotubes.

    PubMed

    Zhou, Weihang; Nakamura, Daisuke; Liu, Huaping; Kataura, Hiromichi; Takeyama, Shojiro

    2014-11-11

    The ordering and relative energy splitting between bright and dark excitons are critical to the optical properties of single-walled carbon nanotubes (SWNTs), as they eventually determine the radiative and non-radiative recombination processes of generated carriers. In this work, we report systematic high-field magneto-optical study on the relative ordering between bright and dark excitons in SWNTs. We identified the relative energy position of the dark exciton unambiguously by brightening it in ultra-high magnetic field. The bright-dark excitonic ordering was found to depend not only on the tube structure, but also on the type of transitions. For the 1(st) sub-band transition, the bright exciton appears to be higher in energy than its dark counterpart for any chiral species and is robust against environmental effect. While for the 2(nd) sub-band, their relative ordering was found to be chirality-sensitive: the bright exciton can be either higher or lower than the dark one, depending on the specific nanotube structures. These findings provide new clues for engineering the optical and electronic properties of SWNTs.

  6. Atomistic theory of excitonic fine structure in InAs/InP nanowire quantum dot molecules

    NASA Astrophysics Data System (ADS)

    Świderski, M.; Zieliński, M.

    2017-03-01

    Nanowire quantum dots have peculiar electronic and optical properties. In this work we use atomistic tight binding to study excitonic spectra of artificial molecules formed by a double nanowire quantum dot. We demonstrate a key role of atomistic symmetry and nanowire substrate orientation rather than cylindrical shape symmetry of a nanowire and a molecule. In particular for [001 ] nanowire orientation we observe a nonvanishing bright exciton splitting for a quasimolecule formed by two cylindrical quantum dots of different heights. This effect is due to interdot coupling that effectively reduces the overall symmetry, whereas single uncoupled [001 ] quantum dots have zero fine structure splitting. We found that the same double quantum dot system grown on [111 ] nanowire reveals no excitonic fine structure for all considered quantum dot distances and individual quantum dot heights. Further we demonstrate a pronounced, by several orders of magnitude, increase of the dark exciton optical activity in a quantum dot molecule as compared to a single quantum dot. For [111 ] systems we also show spontaneous localization of single particle states in one of nominally identical quantum dots forming a molecule, which is mediated by strain and origins from the lack of the vertical inversion symmetry in [111 ] nanostructures of overall C3 v symmetry. Finally, we study lowering of symmetry due to alloy randomness that triggers nonzero excitonic fine structure and the dark exciton optical activity in realistic nanowire quantum dot molecules of intermixed composition.

  7. Control of Exciton Valley Coherence in Transition Metal Dichalcogenide Monolayers

    NASA Astrophysics Data System (ADS)

    Wang, G.; Marie, X.; Liu, B. L.; Amand, T.; Robert, C.; Cadiz, F.; Renucci, P.; Urbaszek, B.

    2016-10-01

    The direct gap interband transitions in transition metal dichalcogenide monolayers are governed by chiral optical selection rules. Determined by laser helicity, optical transitions in either the K+ or K- valley in momentum space are induced. Linearly polarized laser excitation prepares a coherent superposition of valley states. Here, we demonstrate the control of the exciton valley coherence in monolayer WSe2 by tuning the applied magnetic field perpendicular to the monolayer plane. We show rotation of this coherent superposition of valley states by angles as large as 30° in applied fields up to 9 T. This exciton valley coherence control on the ps time scale could be an important step towards complete control of qubits based on the valley degree of freedom.

  8. Excitonic absorption intensity of semiconducting and metallic carbon nanotubes.

    PubMed

    Verdenhalven, Eike; Malić, Ermin

    2013-06-19

    The knowledge of the intrinsic absorption intensity of each carbon nanotube is of crucial importance for the optical assignment of nanotube species and the estimation of their abundance in a sample. Based on a microscopic approach, we calculate excitonic absorption spectra for a variety of semiconducting and metallic nanotubes, revealing a clear diameter, chirality, and family dependence of the absorption intensity. In particular, we also study the appearance of excited excitonic transitions, which are shown to be well pronounced for semiconducting nanotubes, reaching intensities of up to 10% of the main transition. We find that nanotubes with large diameters show the most pronounced absorption intensities, confirming well the experimentally observed trend. Depending on the CNT family and transition, the absorption is enhanced or reduced with the chiral angle. This behavior reflects well the qualitative chirality dependence of the analytically derived optical matrix element.

  9. Ultrafast exciton dissociation at donor/acceptor interfaces

    NASA Astrophysics Data System (ADS)

    Grancini, G.; Fazzi, D.; Binda, M.; Maiuri, M.; Petrozza, A.; Criante, L.; Perissinotto, S.; Egelhaaf, H.-J.; Brida, D.; Cerullo, G.; Lanzani, G.

    2013-09-01

    Charge generation at donor/acceptor interface is a highly debated topic in the organic photovoltaics (OPV) community. The primary photoexcited state evolution happens in few femtosecond timescale, thus making very intriguing their full understanding. In particular charge generation is believed to occur in < 200 fs, but no clear picture emerged so far. In this work we reveal for the first time the actual charge generation mechanism following in real time the exciton dissociation mechanism by means of sub-22 fs pump-probe spectroscopy. We study a low-band-gap polymer: fullerene interface as an ideal system for OPV. We demonstrate that excitons dissociation leads, on a timescale of 20-50 fs, to two byproducts: bound interfacial charge transfer states (CTS) and free charges. The branching ratio of their formation depends on the excess photon energy provided. When high energy singlet polymer states are excited, well above the optical band gap, an ultrafast hot electron transfer happens between the polymer singlet state and the interfacial hot CTS* due to the high electronic coupling between them. Hot exciton dissociation prevails then on internal energy dissipation that occurs within few hundreds of fs. By measuring the internal quantum efficiency of a prototypical device a rising trend with energy is observed, thus indicating that hot exciton dissociation effectively leads to a higher fraction of free charges.

  10. Exciton-polariton condensation in transition metal dichalcogenide bilayer heterostructure

    NASA Astrophysics Data System (ADS)

    Lee, Ki Hoon; Jeong, Jae-Seung; Min, Hongki; Chung, Suk Bum

    For the bilayer heterostructure system in an optical microcavity, the interplay of the Coulomb interaction and the electron-photon coupling can lead to the emergence of quasiparticles consisting of the spatially indirect exciton and cavity photons known as dipolariton, which can form the Bose-Einstein condensate above a threshold density. Additional physics comes into play when each layer of the bilayer system consists of the transition metal dichalcogenide (TMD) monolayer. The TMD monolayer band structure in the low energy spectrum has two valley components with nontrivial Berry phase, which gives rise to a selection rule in the exciton-polariton coupling, e.g. the exciton from one (the other) valley can couple only to the clockwise (counter-clockwise) polarized photon. We investigate possible condensate phases of exciton-polariton in the bilayer TMD microcavity changing relevant parameters such as detuning, excitation density and interlayer distance. This work was supported in part by the Institute for Basic Science of Korea (IBS) under Grant IBS-R009-Y1 and by the National Research Foundation of Korea (NRF) under the Basic Science Research Program Grant No. 2015R1D1A1A01058071.

  11. Strong quantum coherence between Fermi liquid Mahan excitons

    SciTech Connect

    Paul, J.; Stevens, C. E.; Liu, C.; Dey, P.; McIntyre, C.; Turkowski, V.; Reno, J. L.; Hilton, D. J.; Karaiskaj, D.

    2016-04-14

    In modulation doped quantum wells, the excitons are formed as a result of the interactions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the so-called “Mahan excitons.” The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence destroyed as a result of the screening and electron-electron interactions. Surprisingly, we observe strong quantum coherence between the heavy hole and light hole excitons. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum two-dimensional Fourier transform spectra. Theoretical simulations based on the optical Bloch equations where many-body effects are included phenomenologically reproduce well the experimental spectra. Furthermore, time-dependent density functional theory calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system.

  12. Femtosecond studies of exciton dynamics in a novel main chain chiral conjugated poly(arylenevinylene)

    NASA Astrophysics Data System (ADS)

    Zhang, J. Z.; Kreger, M. A.; Hu, Q.-S.; Vitharana, D.; Pu, L.; Brock, P. J.; Scott, J. C.

    1997-03-01

    The formation and decay dynamics of photogenerated excitons in an optically active poly(arylenevinylene), PAV, in solution have been studied using femtosecond transient absorption spectroscopy. Photoexcitation initially creates hot excitons which quickly (<200 fs) relax geometrically towards the equilibrium position in the excited state. The exciton subsequently decays following a double exponential with time constants of 6.5 and 420 ps in toluene. The decays become faster (5 and 250 ps) in pyridine, indicating a dependence of the relaxation process on the solvent environment. The fast decay is attributed to vibrational relaxation and internal conversion (recombination) of the exciton from the excited to the ground electronic state through tunneling or thermal-activated barrier crossing before thermalization. The slow decay is assigned to conversion of the thermalized exciton to the ground state through both radiative and nonradiative pathways. Anisotropy decay shows a fast component (6 ps in toluene and 10 ps in pyridine) and an offset which persists up to 650 ps. Possible explanations for the fast decay include internal conversion, vibrational relaxation, conformational change, and exciton migration. The offset may decay on a longer time scale through local reorientation of the conjugation segments, exciton migration, or rotational diffusion of the polymer. Comparison to a well-studied system, MEH-PPV [poly(2-methoxy, 5-(2-ethylhexoxy)-p-phenylenevinylene], provides further insight into the relaxation mechanism of photoexcitations in this PAV polymer.

  13. Exciton luminescence from Cu2SnS3 bulk crystals

    NASA Astrophysics Data System (ADS)

    Aihara, Naoya; Matsumoto, Yusuke; Tanaka, Kunihiko

    2016-02-01

    The optical properties of Cu2SnS3 (CTS) bulk crystals grown by chemical vapor transport were studied by photoluminescence (PL) spectroscopy. The PL spectra from the CTS bulk crystals were analyzed as a function of excitation power and temperature. The main phase of the as-grown samples was determined to be monoclinic CTS by Raman spectroscopy. The observed PL spectra from the CTS bulk crystals were composed of peaks corresponding to free-exciton, two bound-excitons, and donor-acceptor pair recombination luminescence. The peak energies for the free-exciton and two bound-exciton emissions were 0.9317, 0.9291, and 0.9260 eV, respectively, at temperature of 4.2 K. The bound-exciton luminescence was not observed above 30 K. The thermal activation energies for the free-exciton and two bound-exciton emissions were 6.5, 4.8, and 5.2 meV, respectively. The fundamental band gap in the CTS bulk crystals was expected to be ca. 0.94 eV.

  14. Excitons and Davydov splitting in sexithiophene from first-principles many-body Green's function theory.

    PubMed

    Leng, Xia; Yin, Huabing; Liang, Dongmei; Ma, Yuchen

    2015-09-21

    Organic semiconductors have promising and broad applications in optoelectronics. Understanding their electronic excited states is important to help us control their spectroscopic properties and performance of devices. There have been a large amount of experimental investigations on spectroscopies of organic semiconductors, but theoretical calculation from first principles on this respect is still limited. Here, we use density functional theory (DFT) and many-body Green's function theory, which includes the GW method and Bethe-Salpeter equation, to study the electronic excited-state properties and spectroscopies of one prototypical organic semiconductor, sexithiophene. The exciton energies of sexithiophene in both the gas and bulk crystalline phases are very sensitive to the exchange-correlation functionals used in DFT for ground-state structure relaxation. We investigated the influence of dynamical screening in the electron-hole interaction on exciton energies, which is found to be very pronounced for triplet excitons and has to be taken into account in first principles calculations. In the sexithiophene single crystal, the energy of the lowest triplet exciton is close to half the energy of the lowest singlet one. While lower-energy singlet and triplet excitons are intramolecular Frenkel excitons, higher-energy excitons are of intermolecular charge-transfer type. The calculated optical absorption spectra and Davydov splitting are in good agreement with experiments.

  15. Excitons and Davydov splitting in sexithiophene from first-principles many-body Green's function theory

    NASA Astrophysics Data System (ADS)

    Leng, Xia; Yin, Huabing; Liang, Dongmei; Ma, Yuchen

    2015-09-01

    Organic semiconductors have promising and broad applications in optoelectronics. Understanding their electronic excited states is important to help us control their spectroscopic properties and performance of devices. There have been a large amount of experimental investigations on spectroscopies of organic semiconductors, but theoretical calculation from first principles on this respect is still limited. Here, we use density functional theory (DFT) and many-body Green's function theory, which includes the GW method and Bethe-Salpeter equation, to study the electronic excited-state properties and spectroscopies of one prototypical organic semiconductor, sexithiophene. The exciton energies of sexithiophene in both the gas and bulk crystalline phases are very sensitive to the exchange-correlation functionals used in DFT for ground-state structure relaxation. We investigated the influence of dynamical screening in the electron-hole interaction on exciton energies, which is found to be very pronounced for triplet excitons and has to be taken into account in first principles calculations. In the sexithiophene single crystal, the energy of the lowest triplet exciton is close to half the energy of the lowest singlet one. While lower-energy singlet and triplet excitons are intramolecular Frenkel excitons, higher-energy excitons are of intermolecular charge-transfer type. The calculated optical absorption spectra and Davydov splitting are in good agreement with experiments.

  16. Exciton-exciton scattering: Composite boson versus elementary boson

    NASA Astrophysics Data System (ADS)

    Combescot, M.; Betbeder-Matibet, O.; Combescot, R.

    2007-05-01

    This paper shows the necessity of introducing a quantum object, the “coboson,” to properly describe, through a fermion scheme, any composite particle, such as the exciton, which is made of two fermions. Although commonly dealt with as elementary bosons, these composite bosons—cobosons in short—differ from them due to their composite nature which makes the handling of their many-body effects quite different from the existing treatments valid for elementary bosons. As a direct consequence of this composite nature, there is no correct way to describe the interaction between cobosons as a potential V . This is rather dramatic because, with the Hamiltonian not written as H=H0+V , all the usual approaches to many-body effects fail. In particular, the standard form of the Fermi golden rule, written in terms of V , cannot be used to obtain the transition rates of two cobosons. To get them, we have had to construct an unconventional expression for this Fermi golden rule in which H only appears. Making use of this expression, we give here a detailed calculation of the time evolution of two excitons. We compare the results of this exact approach with the ones obtained by using an effective bosonic Hamiltonian in which the excitons are considered as elementary bosons with effective scatterings between them, these scatterings resulting from an elaborate mapping between the two-fermion space and the ideal boson space. We show that the relation between the inverse lifetime and the sum of the transition rates for elementary bosons differs from the one of the composite bosons by a factor of 1/2 , so that it is impossible to find effective scatterings between bosonic excitons giving these two physical quantities correctly, whatever the mapping from composite bosons to elementary bosons is. The present paper thus constitutes a strong mathematical proof that, in spite of a widely spread belief, we cannot forget the composite nature of these cobosons, even in the extremely low

  17. Exciton-polariton state in nanocrystalline SiC films

    NASA Astrophysics Data System (ADS)

    Semenov, A. V.; Lopin, A. V.

    2016-05-01

    We studied the features of optical absorption in the films of nanocrystalline SiC (nc-SiC) obtained on the sapphire substrates by the method of direct ion deposition. The optical absorption spectra of the films with a thickness less than ~500 nm contain a maximum which position and intensity depend on the structure and thickness of the nc-SiC films. The most intense peak at 2.36 eV is observed in the nc-SiC film with predominant 3C-SiC polytype structure and a thickness of 392 nm. Proposed is a resonance absorption model based on excitation of exciton polaritons in a microcavity. In the latter, under the conditions of resonance, there occurs strong interaction between photon modes of light with λph=521 nm and exciton of the 3С polytype with an excitation energy of 2.36 eV that results in the formation of polariton. A mismatch of the frequencies of photon modes of the cavity and exciton explains the dependence of the maximum of the optical absorption on the film thickness.

  18. Exciton dynamics of C60-based single-photon emitters explored by Hanbury Brown–Twiss scanning tunnelling microscopy

    PubMed Central

    Merino, P.; Große, C.; Rosławska, A.; Kuhnke, K.; Kern, K.

    2015-01-01

    Exciton creation and annihilation by charges are crucial processes for technologies relying on charge-exciton-photon conversion. Improvement of organic light sources or dye-sensitized solar cells requires methods to address exciton dynamics at the molecular scale. Near-field techniques have been instrumental for this purpose; however, characterizing exciton recombination with molecular resolution remained a challenge. Here, we study exciton dynamics by using scanning tunnelling microscopy to inject current with sub-molecular precision and Hanbury Brown–Twiss interferometry to measure photon correlations in the far-field electroluminescence. Controlled injection allows us to generate excitons in solid C60 and let them interact with charges during their lifetime. We demonstrate electrically driven single-photon emission from localized structural defects and determine exciton lifetimes in the picosecond range. Monitoring lifetime shortening and luminescence saturation for increasing carrier injection rates provides access to charge-exciton annihilation dynamics. Our approach introduces a unique way to study single quasi-particle dynamics on the ultimate molecular scale. PMID:26416705

  19. Ultrahigh resolution endoscopic spectral domain optical coherence tomography with a tiny rotary probe driven by a hollow ultrasonic motor

    NASA Astrophysics Data System (ADS)

    Zhang, Ning; Chen, Tianyuan; Huo, Tiancheng; Wang, Chengming; Zheng, Jing-gao; Zhou, Tieying; Xue, Ping

    2013-03-01

    This paper proposes a novel rotary endoscopic probe for spectral-domain optical coherence tomography (SD-OCT). The probe with a large N.A. objective lens is driven by an ultra-small hollow rectangular ultrasonic motor for circular scanning. Compared to the conventional driven techniques, the hollow ultrasonic motor enables the fiber to pass through its inside. Therefore the fiber, the objective lens and the motor are all at the same side. This enables 360 degree unobstructed imaging without any shadow resulted from power wire as in the conventional motor-driven endoscopic OCT. Moreover, it shortens the length of the rigid tip and enhances the flexibility of the probe. Meanwhile, the ultrasonic motor is robust, simple, quiet and of high torque, very suitable for OCT endoscopic probe. The side length of the motor is 0.7 mm with 5mm in length. The outer diameter of the probe is 1.5mm. A significant improvement in the lateral resolution is demonstrated due to the novel design of the objective lens. A right-angle lens is utilized instead of the traditional right-angle prism as the last optics close to the sample, leading to a reduction of the working distance and an enlargement of the N.A. of the objective lens. It is demonstrated that the endoscopic SD-OCT system achieves an axial resolution of ~7μm, a lateral resolution of ~6μm and a SNR of ~96dB.

  20. Entropy-based measures of in vivo cilia-driven microfluidic mixing derived from quantitative optical imaging

    NASA Astrophysics Data System (ADS)

    Chandrasekera, Kenny; Jonas, Stephan; Bhattacharya, Dipankan; Khokha, Mustafa; Choma, Michael A.

    2012-02-01

    Motile cilia are cellular organelles that project from different epithelial surfaces including respiratory epithelium. They generate directional fluid flow that removes harmful pathogens and particulate matter from the respiratory system. While it has been known that primary ciliary dyskinesia increases the risk of recurrent pulmonary infections, there is now heightened interest in understanding the role that cilia play in a wide-variety of respiratory diseases. Different optical imaging technologies are being investigated to visualize cilia-driven fluid flow, and quantitative image analysis is used to generate measures of ciliary performance. Here, we demonstrate the quantification of in vivo cilia-driven microfluidic mixing using spatial and temporal measures of Shannon information entropy. Using videomicroscopy, we imaged in vivo cilia-driven fluid flow generated by the epidermis of the Xenopus tropicalis embryo. Flow was seeded with either dyes or microparticles. Both spatial and temporal measures of entropy show significant levels of mixing, with maximum entropy measures of ~6.5 (out of a possible range of 0 to 8). Spatial entropy measures showed localization of mixing "hot-spots" and "cold-spots" and temporal measures showed mixing throughout.In sum, entropy-based measures of microfluidic mixing can characterize in vivo cilia-driven fluid flow and hold the potential for better characterization of ciliary dysfunction.

  1. Exciton Relaxation and Electron Transfer Dynamics of Semiconductor Quantum Dots

    NASA Astrophysics Data System (ADS)

    Liu, Cunming

    Quantum dots (QDs), also referred to as colloidal semiconductor nanocrystals, exhibit unique electronic and optical properties arising from their three-dimensional confinement and strongly enhanced coulomb interactions. Developing a detailed understanding of the exciton relaxation dynamics within QDs is important not only for sake of exploring the fundamental physics of quantum confinement processes, but also for their applications. Ultrafast transient absorption (TA) spectroscopy, as a powerful tool to explore the relaxation dynamics of excitons, was employed to characterize the hot single/multiexciton relaxation dynamics at the first four exciton states of CdSe/CdZnS QDs. We observed for the first time that the hot hole can relax through two possible pathways: Intraband multiple phonon coupling and intrinsic defect trapping, with a lifetime of ˜7 ps. Additionally, an ultra-short component of ˜ 8 ps, directly associated with the Auger recombination of highly energetic exciton states, was discovered. After exploring the exciton relaxation inside QDs, ultrafast TA spectroscopy was further applied to study the electron transferring outside from QDs. By using a brand-new photocatalytic system consisting of CdSe QDs and Ni-dihydrolipoic acid (Ni-DHLA) catalyst, which has represented a robust photocatalysis of H2 from water, the photoinduced electron transfer (ET) dynamics between QD and the catalyst, one of most important steps during H2 generation, was studied. We found smaller bare CdSe QDs exhibit a better ET performance and CdS shelling on the bare QDs leads to worsen the ET. The calculations of effective mass approximation (EMA) and Marcus theory show the ET process is mainly dominated by driving force, electronic coupling strength and reorganization energy between QD and the catalyst.

  2. [Gene sequencing by scanning molecular exciton microscopy]. Progress report, October 1, 1990--September 30, 1991

    SciTech Connect

    Not Available

    1991-12-31

    This report details progress made in setting up a laboratory for optical microscopy of genes. The apparatus including a fluorescence microscope, a scanning optical microscope, various spectrometers, and supporting computers is described. Results in developing photon and exciton tips, and in preparing samples are presented. (GHH)

  3. Effect of periodic potential on exciton states in semiconductor carbon nanotubes

    DOE PAGES

    Roslyak, Oleksiy; Piryatinski, Andrei

    2016-05-28

    Here we develop a theoretical background to treat exciton states in semiconductor single-walled carbon nanotubes (SWCNTs) in the presence of a periodic potential induced by a surface acoustic wave (SAW) propagating along SWCNT. The formalism accounts for the electronic band splitting into the Floquet subbands induced by the Bragg scattering on the SAW potential. Optical transitions between the Floquet states and correlated electron–hole pairs (excitons) are numerically examined. Formation of new van Hove singularities within the edges of Floquet sub-bands and associated transfer of the exciton oscillator strengths resulting in the photoluminescence quenching are predicted. The simulations demonstrate the excitonmore » energy red Stark shift and reduction in the exciton binding energy. We provide comparison of our results with reported theoretical and experimental studies.« less

  4. Quantum dot-like excitonic behavior in individual single walled-carbon nanotubes

    PubMed Central

    Wang, Xu; Alexander-Webber, Jack A.; Jia, Wei; Reid, Benjamin P. L.; Stranks, Samuel D.; Holmes, Mark J.; Chan, Christopher C. S.; Deng, Chaoyong; Nicholas, Robin J.; Taylor, Robert A.

    2016-01-01

    Semiconducting single-walled carbon nanotubes are one-dimensional materials with great prospects for applications such as optoelectronic and quantum information devices. Yet, their optical performance is hindered by low fluorescent yield. Highly mobile excitons interacting with quenching sites are attributed to be one of the main non-radiative decay mechanisms that shortens the exciton lifetime. In this paper we report on time-integrated photoluminescence measurements on individual polymer wrapped semiconducting carbon nanotubes. An ultra narrow linewidth we observed demonstrates intrinsic exciton dynamics. Furthermore, we identify a state filling effect in individual carbon nanotubes at cryogenic temperatures as previously observed in quantum dots. We propose that each of the CNTs is segmented into a chain of zero-dimensional states confined by a varying local potential along the CNT, determined by local environmental factors such as the amount of polymer wrapping. Spectral diffusion is also observed, which is consistent with the tunneling of excitons between these confined states. PMID:27849046

  5. Quantum dot-like excitonic behavior in individual single walled-carbon nanotubes

    NASA Astrophysics Data System (ADS)

    Wang, Xu; Alexander-Webber, Jack A.; Jia, Wei; Reid, Benjamin P. L.; Stranks, Samuel D.; Holmes, Mark J.; Chan, Christopher C. S.; Deng, Chaoyong; Nicholas, Robin J.; Taylor, Robert A.

    2016-11-01

    Semiconducting single-walled carbon nanotubes are one-dimensional materials with great prospects for applications such as optoelectronic and quantum information devices. Yet, their optical performance is hindered by low fluorescent yield. Highly mobile excitons interacting with quenching sites are attributed to be one of the main non-radiative decay mechanisms that shortens the exciton lifetime. In this paper we report on time-integrated photoluminescence measurements on individual polymer wrapped semiconducting carbon nanotubes. An ultra narrow linewidth we observed demonstrates intrinsic exciton dynamics. Furthermore, we identify a state filling effect in individual carbon nanotubes at cryogenic temperatures as previously observed in quantum dots. We propose that each of the CNTs is segmented into a chain of zero-dimensional states confined by a varying local potential along the CNT, determined by local environmental factors such as the amount of polymer wrapping. Spectral diffusion is also observed, which is consistent with the tunneling of excitons between these confined states.

  6. External pumping of hybrid nanostructures in microcavity with Frenkel and Wannier-Mott excitons

    NASA Astrophysics Data System (ADS)

    Dubovskiy, O. A.; Agranovich, V. M.

    2016-09-01

    The exciton-exciton interaction in hybrid nanostructures with resonating Frenkel and Wannier-Mott excitons was investigated in many publications. In microcavity the hybrid nanostructures can be exposed to different types of optical pumping, the most common one being pumping through one of the microcavity side. However, not investigated and thus never been discussed the hybrid excitons generation by pumping of confined quantum wells from the side of empty microcavity without nanostructures in a wave guided configuration. Here, we consider the hybrid excitations in cavity with organic and inorganic quantum wells and with different types of pumping from external source. The frequency dependence for intensity of excitations in hybrid structure is also investigated. The results may be used for search of most effective fluorescence and relaxation processes. The same approach may be used when both quantum wells are organic or inorganic.

  7. Excitonic resonances in thin films of WSe2: from monolayer to bulk material

    NASA Astrophysics Data System (ADS)

    Arora, Ashish; Koperski, Maciej; Nogajewski, Karol; Marcus, Jacques; Faugeras, Clément; Potemski, Marek

    2015-06-01

    We present optical spectroscopy (photoluminescence and reflectance) studies of thin layers of the transition metal dichalcogenide WSe2, with thickness ranging from mono- to tetra-layer and in the bulk limit. The investigated spectra show the evolution of excitonic resonances as a function of layer thickness, due to changes in the band structure and, importantly, due to modifications of the strength of Coulomb interactions as well. The observed temperature-activated energy shift and broadening of the fundamental direct exciton are well accounted for by standard formalisms used for conventional semiconductors. A large increase of the photoluminescence yield with temperature is observed in a WSe2 monolayer, indicating the existence of competing radiative channels. The observation of absorption-type resonances due to both neutral and charged excitons in the WSe2 monolayer is reported and the effect of the transfer of oscillator strength from charged to neutral excitons upon an increase of temperature is demonstrated.

  8. Multiple-exciton generation in lead selenide nanorod solar cells with external quantum efficiencies exceeding 120%

    PubMed Central

    Davis, Nathaniel J. L. K.; Böhm, Marcus L.; Tabachnyk, Maxim; Wisnivesky-Rocca-Rivarola, Florencia; Jellicoe, Tom C.; Ducati, Caterina; Ehrler, Bruno; Greenham, Neil C.

    2015-01-01

    Multiple-exciton generation—a process in which multiple charge-carrier pairs are generated from a single optical excitation—is a promising way to improve the photocurrent in photovoltaic devices and offers the potential to break the Shockley–Queisser limit. One-dimensional nanostructures, for example nanorods, have been shown spectroscopically to display increased multiple exciton generation efficiencies compared with their zero-dimensional analogues. Here we present solar cells fabricated from PbSe nanorods of three different bandgaps. All three devices showed external quantum efficiencies exceeding 100% and we report a maximum external quantum efficiency of 122% for cells consisting of the smallest bandgap nanorods. We estimate internal quantum efficiencies to exceed 150% at relatively low energies compared with other multiple exciton generation systems, and this demonstrates the potential for substantial improvements in device performance due to multiple exciton generation. PMID:26411283

  9. Photon echo in exciton-plasmon nanomaterials: A time-dependent signature of strong coupling.

    PubMed

    Blake, Adam; Sukharev, Maxim

    2017-02-28

    We investigate the dynamics of the photon echo exhibited by exciton-plasmon systems under strong coupling conditions. Using a self-consistent model based on coupled Maxwell-Bloch equations, we investigate the femtosecond time dynamics of ensembles of interacting molecules optically coupled to surface plasmon supporting materials. It is shown that observed photon echoes under a two pulse pump-probe sequence are highly dependent on various material parameters such as molecular concentration and periodicity. Simulations of photon echoes in exciton-plasmon materials reveal a unique signature of the strong exciton-plasmon coupling, namely, a double-peak structure in spectra of recorded echo signals. This phenomenon is shown to be related to hybrid states (upper and lower polaritons) in exciton-plasmon systems under strong coupling conditions. It is also demonstrated that the double-peak echo is highly sensitive to mild deviations of the coupling from resonant conditions making it a great tool for ultrafast probes.

  10. Multiple-exciton generation in lead selenide nanorod solar cells with external quantum efficiencies exceeding 120.

    PubMed

    Davis, Nathaniel J L K; Böhm, Marcus L; Tabachnyk, Maxim; Wisnivesky-Rocca-Rivarola, Florencia; Jellicoe, Tom C; Ducati, Caterina; Ehrler, Bruno; Greenham, Neil C

    2015-09-28

    Multiple-exciton generation-a process in which multiple charge-carrier pairs are generated from a single optical excitation-is a promising way to improve the photocurrent in photovoltaic devices and offers the potential to break the Shockley-Queisser limit. One-dimensional nanostructures, for example nanorods, have been shown spectroscopically to display increased multiple exciton generation efficiencies compared with their zero-dimensional analogues. Here we present solar cells fabricated from PbSe nanorods of three different bandgaps. All three devices showed external quantum efficiencies exceeding 100% and we report a maximum external quantum efficiency of 122% for cells consisting of the smallest bandgap nanorods. We estimate internal quantum efficiencies to exceed 150% at relatively low energies compared with other multiple exciton generation systems, and this demonstrates the potential for substantial improvements in device performance due to multiple exciton generation.

  11. Chaos control and synchronization in Bragg acousto-optic bistable systems driven by a separate chaotic system.

    PubMed

    Wang, Rong; Gao, Jin-Yue

    2005-09-01

    In this paper we propose a new scheme to achieve chaos control and synchronization in Bragg acousto-optic bistable systems. In the scheme, we use the output of one system to drive two identical chaotic systems. Using the maximal conditional Lyapunov exponent (MCLE) as the criterion, we analyze the conditions for realizing chaos synchronization. Numerical calculation shows that the two identical systems in chaos with negative MCLEs and driven by a chaotic system can go into chaotic synchronization whether or not they were in chaos initially. The two systems can go into different periodic states from chaos following an inverse period-doubling bifurcation route as well when driven by a periodic system.

  12. Exciton-polariton integrated circuits

    NASA Astrophysics Data System (ADS)

    Liew, T. C. H.; Kavokin, A. V.; Ostatnický, T.; Kaliteevski, M.; Shelykh, I. A.; Abram, R. A.

    2010-07-01

    We show that logical signals encoded in bistable states in semiconductor microcavities can be generated and controlled electronically by exploiting the electrical sensitivity of Tamm-plasmon-exciton-polariton modes. The signals can be transported along polariton neurons, created with a patterned metal surface. Using the Gross-Pitaevskii equations, we simulate an electrically controlled transistor and find that high repetition rates (10 GHz) are possible.

  13. Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe2.

    PubMed

    Steinleitner, Philipp; Merkl, Philipp; Nagler, Philipp; Mornhinweg, Joshua; Schüller, Christian; Korn, Tobias; Chernikov, Alexey; Huber, Rupert

    2017-03-08

    Many of the fundamental optical and electronic properties of atomically thin transition metal dichalcogenides are dominated by strong Coulomb interactions between electrons and holes, forming tightly bound atom-like states called excitons. Here, we directly trace the ultrafast formation of excitons by monitoring the absolute densities of bound and unbound electron-hole pairs in single monolayers of WSe2 on a diamond substrate following femtosecond nonresonant optical excitation. To this end, phase-locked mid-infrared probe pulses and field-sensitive electro-optic sampling are used to map out the full complex-valued optical conductivity of the nonequilibrium system and to discern the hallmark low-energy responses of bound and unbound pairs. While the spectral shape of the infrared response immediately after above-bandgap injection is dominated by free charge carriers, up to 60% of the electron-hole pairs are bound into excitons already on a subpicosecond time scale, evidencing extremely fast and efficient exciton formation. During the subsequent recombination phase, we still find a large density of free carriers in addition to excitons, indicating a nonequilibrium state of the photoexcited electron-hole system.

  14. CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES: Intraband Dynamics and Terahertz Emission in Biased GaAs/In0.53Ga0.47As Semiconductor Superlattices

    NASA Astrophysics Data System (ADS)

    Li, Min; Mi, Xian-Wu

    2009-12-01

    Using an excitonic basis, we investigate the intraband polarization, optical absorption spectra, and terahertz emission of semiconductor superlattice with the density matrix theory. The excitonic Bloch oscillation is driven by the dc and ac electric fields. The slow variation in the intraband polarization depends on the ac electric field frequency. The intraband polarization increases when the ac electric field frequency is below the Bloch frequency. When the ac electric field frequency is above the Bloch frequency, the intraband polarization downwards and its intensity decreases. The satellite structures in the optical absorption spectra are presented. Due to excitonic dynamic localization, the emission lines of terahertz shift in different ac electric field and dc electric field.

  15. Exciton Seebeck effect in molecular systems.

    PubMed

    Yan, Yun-An; Cai, Shaohong

    2014-08-07

    We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect.

  16. Exciton Seebeck effect in molecular systems

    SciTech Connect

    Yan, Yun-An; Cai, Shaohong

    2014-08-07

    We investigate the exciton dynamics under temperature difference with the hierarchical equations of motion. Through a nonperturbative simulation of the transient absorption of a heterogeneous trimer model, we show that the temperature difference causes exciton population redistribution and affects the exciton transfer time. It is found that one can reproduce not only the exciton population redistribution but also the change of the exciton transfer time induced by the temperature difference with a proper tuning of the site energies of the aggregate. In this sense, there exists a site energy shift equivalence for any temperature difference in a broad range. This phenomenon is similar to the Seebeck effect as well as spin Seebeck effect and can be named as exciton Seebeck effect.

  17. Coupled plasmon-exciton hybrid excitations in colloidal gold nanorods coated with J-aggregated dye molecules (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    Simon, Thomas; Melnikau, Dimitry; Sanchez-Iglesia, Ana; Grzelczak, Marek; Liz-Marzán, Luis M.; Rakovich, Yury P.; Feldmann, Jochen; Urban, Alexander S.

    2016-09-01

    Strong coupling of plasmons and excitons can form hybrid states, the so called "plexcitons". Especially with molecular J-aggregated dye molecules strong interaction becomes observable even under ambient conditions. In our work we investigate the nature of plexcitonic states formed in gold nanorods coated with a cyanine dye by transient absorption spectroscopy. We demonstrate that plexcitons show tunable plasmonic and excitonic non-linear optical behaviour. Our experimental data can be explained on the basis of a simple Lorentz oscillator model. We find that both the quality factor and the coupling strength between plasmons and excitons can be optically manipulated on an ultrashort time scale. T. Simon, D. Melnikau, A. Sánchez-Iglesias, M. Grzelczak, L. Liz-Marzán, Y. Rakovich, J. Feldmann and A. Urban, Exploring the optical non-linearities of plasmon-exciton hybrid resonances in coupled colloidal nanostructures, submitted (2016)

  18. Low noise and low drift in a laser-driven fiber optic gyroscope with a 1-km coil

    NASA Astrophysics Data System (ADS)

    Chamoun, J. N.; Evans, A.; Mosca, F. A.; Digonnet, M. J. F.

    2014-05-01

    We report an experimental fiber optic gyroscope (FOG) utilizing a 1085-m coil of 8-cm diameter driven with a laser of 10- MHz linewidth, with a record rotation-rate noise as low as 0.2 deg/h/√Hz and a drift below 0.038 deg/h. Simulations and comparison to the measured performance of a similar 150-m FOG show that the noise is limited approximately equally by coherent backscattering and polarization coupling in the sensing coil, and unaffected by the Kerr effect.

  19. Laser-optical path to nuclear energy without radioactivity: Fusion of hydrogen-boron by nonlinear force driven plasma blocks

    NASA Astrophysics Data System (ADS)

    Hora, H.; Miley, G. H.; Ghoranneviss, M.; Malekynia, B.; Azizi, N.

    2009-10-01

    Anomalous interaction of terawatt-picosecond laser pulses allows side-on ignition of solid state density fusion fuel with the unexpected possibility of igniting uncompressed hydrogen-boron p- 11B. Suppression of relativistic self-focusing by using very clean laser pulses with an extremely high contrast ratio is essential to achieve ignition thresholds only ten times more difficult than fusion of deuterium-tritium (DT). This opens the possibility for laser driven fusion energy without neutrons and less radioactivity than from burning coal. The complex nonlinear optical properties involved are elaborated.

  20. Splitting between bright and dark excitons in transition metal dichalcogenide monolayers

    NASA Astrophysics Data System (ADS)

    Echeverry, J. P.; Urbaszek, B.; Amand, T.; Marie, X.; Gerber, I. C.

    2016-03-01

    The optical properties of transition metal dichalcogenide monolayers such as the two-dimensional semiconductors MoS2 and WSe2 are dominated by excitons, Coulomb bound electron-hole pairs. The light emission yield depends on whether the electron-hole transitions are optically allowed (bright) or forbidden (dark). By solving the Bethe-Salpeter equation on top of G W wave functions in density functional theory calculations, we determine the sign and amplitude of the splitting between bright and dark exciton states. We evaluate the influence of the spin-orbit coupling on the optical spectra and clearly demonstrate the strong impact of the intra-valley Coulomb exchange term on the dark-bright exciton fine structure splitting.

  1. Distinct exciton dissociation behavior of organolead trihalide perovskite and excitonic semiconductors studied in the same system.

    PubMed

    Hu, Miao; Bi, Cheng; Yuan, Yongbo; Xiao, Zhengguo; Dong, Qingfeng; Shao, Yuchuan; Huang, Jinsong

    2015-05-13

    The nonexcitonic character for organometal trihalide perovskites is demonstrated by examining the field-dependent exciton dissociation behavior. It is found that photogenerated excitons can be effectively dissociated into free charges inside perovskite without the assistance of charge extraction layer or external field, which is a stark contrast to the charge-separation behavior in excitonic materials in the same photovoltaic operation system.

  2. Distinct exciton dissociation behavior of organolead trihalide perovskite and excitonic semiconductors studied in a same device

    DOE PAGES

    Hu, Miao; Bi, Cheng; Yuan, Yongbo; ...

    2015-01-15

    The nonexcitonic character for organometal trihalide perovskites is demonstrated by examining the field-dependent exciton dissociation behavior. Moreover, it is found that photogenerated excitons can be effectively dissociated into free charges inside perovskite without the assistance of charge extraction layer or external field, which is a stark contrast to the charge-separation behavior in excitonic materials in the same photovoltaic operation system.

  3. Ground state energy of N Frenkel excitons

    NASA Astrophysics Data System (ADS)

    Pogosov, W.; Combescot, M.

    2009-03-01

    By using the composite many-body theory for Frenkel excitons we have recently developed, we here derive the ground state energy of N Frenkel excitons in the Born approximation through the Hamiltonian mean value in a state made of N identical Q = 0 excitons. While this quantity reads as a density expansion in the case of Wannier excitons, due to many-body effects induced by fermion exchanges between N composite particles, we show that the Hamiltonian mean value for N Frenkel excitons only contains a first order term in density, just as for elementary bosons. Such a simple result comes from a subtle balance, difficult to guess a priori, between fermion exchanges for two or more Frenkel excitons appearing in Coulomb term and the ones appearing in the N exciton normalization factor - the cancellation being exact within terms in 1/Ns where Ns is the number of atomic sites in the sample. This result could make us naively believe that, due to the tight binding approximation on which Frenkel excitons are based, these excitons are just bare elementary bosons while their composite nature definitely appears at various stages in the precise calculation of the Hamiltonian mean value.

  4. Ballistic spin transport in exciton gases

    NASA Astrophysics Data System (ADS)

    Kavokin, A. V.; Vladimirova, M.; Jouault, B.; Liew, T. C. H.; Leonard, J. R.; Butov, L. V.

    2013-11-01

    Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. The challenges for the realization of long-range electron spin transport include rapid spin relaxation due to electron scattering. Scattering and, in turn, spin relaxation can be effectively suppressed in excitonic devices where the spin currents are carried by electrically neutral bosonic quasiparticles: excitons or exciton-polaritons. They can form coherent quantum liquids that carry spins over macroscopic distances. The price to pay is a finite lifetime of the bosonic spin carriers. We present the theory of exciton ballistic spin transport which may be applied to a range of systems supporting bosonic spin transport, in particular to indirect excitons in coupled quantum wells. We describe the effect of spin-orbit interaction for the electron and the hole on the exciton spin, account for the Zeeman effect induced by external magnetic fields and long-range and short-range exchange splittings of the exciton resonances. We also consider exciton transport in the nonlinear regime and discuss the definitions of the exciton spin current, polarization current, and spin conductivity.

  5. Linewidths in excitonic absorption spectra of cuprous oxide

    NASA Astrophysics Data System (ADS)

    Schweiner, Frank; Main, Jörg; Wunner, Günter

    2016-02-01

    We present a theoretical calculation of the absorption spectrum of cuprous oxide (Cu2O ) based on the general theory developed by Y. Toyozawa. An inclusion not only of acoustic phonons but also of optical phonons and of specific properties of the excitons in Cu2O like the central-cell corrections for the 1 S exciton allows us to calculate the experimentally observed linewidths in experiments by T. Kazimierczuk et al. [T. Kazimierczuk, D. Fröhlich, S. Scheel, H. Stolz, and M. Bayer, Nature (London) 514, 343 (2014), 10.1038/nature13832] within the same order of magnitude, which demonstrates a clear improvement in comparison to earlier work on this topic. We also discuss a variety of further effects, which explain the still observable discrepancy between theory and experiment but can hardly be included in theoretical calculations.

  6. Quantitative characterization of exciton from GW+Bethe-Salpeter calculation

    NASA Astrophysics Data System (ADS)

    Hirose, Daichi; Noguchi, Yoshifumi; Sugino, Osamu

    2017-01-01

    We propose a method of classifying excitons into local-, Rydberg-, or charge transfer-type as a step toward enabling a data-driven material design of organic solar cells. The classification method is based on the first-principles many-body theory and improves over the conventional method based on state-by-state visualization of the one-electron wave functions. In our method, the exciton wave function is calculated within the level of the GW+Bethe-Salpeter equation, which is used to obtain two dimensionless parameters for the automatic classification. We construct criteria for exciton classification from experiences with a model molecule, dipeptide. Then we check the validity of our method using a model β-dipeptide which has a geometry and an excitation spectrum similar to the model dipeptide. In addition, we test the effectiveness of the method using porphyrin molecules, or P1TA and P2TA, for which the conventional method is hampered by the strong state hybridization associated with excitation. We find that our method works successfully for P1TA, but the analysis of P2TA is hindered by its centrosymmetry.

  7. Terahertz spectroscopy of two-dimensional electron-hole pairs: probing Mott physics of magneto-excitons

    NASA Astrophysics Data System (ADS)

    Zhang, Qi; Gao, Weilu; Watson, John; Manfra, Michael; Kono, Junichiro

    2015-03-01

    Density-dependent Coulomb interactions can drive electron-hole (e - h) pairs in semiconductors through an excitonic Mott transition from an excitonic gas into an e - h plasma. Theoretical studies suggest that these interactions can be strongly modified by an external magnetic field, including the absence of inter-exciton interactions in the high magnetic field limit in two dimensions, due to an e - h charge symmetry, which results in ultrastable magneto-excitons. Here, we present a systematic experimental study of e - h pairs in photo-excited undoped GaAs quantum wells in magnetic fields with ultrafast terahertz spectroscopy. We simultaneously monitored the dynamics of the intraexcitonic 1 s-2 p transition (which splits into 1 s-2p+ and 1 s-2p- transitions in a magnetic field) and the cyclotron resonance of unbound electrons and holes up to 10 Tesla. We found that the 1 s-2p- absorption feature is robust at high magnetic fields even under high excitation fluences, indicating magnetically enhanced stability of excitons. We will discuss the Mott physics of magneto-excitons as a function of temperature, e - h pair density, optical pump delay time, as well as magnetic field, and also compare two-dimensional excitons in GaAs quantum wells with three-dimensional excitons in bulk GaAs.

  8. Features of exciton dynamics in molecular nanoclusters (J-aggregates): Exciton self-trapping (Review Article)

    NASA Astrophysics Data System (ADS)

    Malyukin, Yu. V.; Sorokin, A. V.; Semynozhenko, V. P.

    2016-06-01

    We present thoroughly analyzed experimental results that demonstrate the anomalous manifestation of the exciton self-trapping effect, which is already well-known in bulk crystals, in ordered molecular nanoclusters called J-aggregates. Weakly-coupled one-dimensional (1D) molecular chains are the main structural feature of J-aggregates, wherein the electron excitations are manifested as 1D Frenkel excitons. According to the continuum theory of Rashba-Toyozawa, J-aggregates can have only self-trapped excitons, because 1D excitons must adhere to barrier-free self-trapping at any exciton-phonon coupling constant g = ɛLR/2β, wherein ɛLR is the lattice relaxation energy, and 2β is the half-width of the exciton band. In contrast, very often only the luminescence of free, mobile excitons would manifest in experiments involving J-aggregates. Using the Urbach rule in order to analyze the low-frequency region of the low-temperature exciton absorption spectra has shown that J-aggregates can have both a weak (g < 1) and a strong (g > 1) exciton-phonon coupling. Moreover, it is experimentally demonstrated that under certain conditions, the J-aggregate excited state can have both free and self-trapped excitons, i.e., we establish the existence of a self-trapping barrier for 1D Frenkel excitons. We demonstrate and analyze the reasons behind the anomalous existence of both free and self-trapped excitons in J-aggregates, and demonstrate how exciton-self trapping efficiency can be managed in J-aggregates by varying the values of g, which is fundamentally impossible in bulk crystals. We discuss how the exciton-self trapping phenomenon can be used as an alternate interpretation of the wide band emission of some J-aggregates, which has thus far been explained by the strongly localized exciton model.

  9. From Si nanowires to porous silicon: the role of excitonic effects.

    PubMed

    Bruno, Mauro; Palummo, Maurizia; Marini, Andrea; Del Sole, Rodolfo; Ossicini, Stefano

    2007-01-19

    We show that the electronic and optical properties of silicon nanowires, with different size and orientation, are dominated by important many-body effects. The electronic and excitonic gaps, calculated within first principles, agree with the available experimental data. Huge excitonic effects, which depend strongly on wire orientation and size, characterize the optical spectra. Modeling porous silicon as a collection of interacting nanowires, we find an absorption spectrum which is in very good agreement with experimental measurements only when the electron-hole interaction is included.

  10. Controlled exciton transfer between quantum dots with acoustic phonons taken into account

    SciTech Connect

    Golovinski, P. A.

    2015-09-15

    A system of excitons in two quantum dots coupled by the dipole–dipole interaction is investigated. The excitation transfer process controlled by the optical Stark effect at nonresonant frequencies is considered and the effect of the interaction between excitons and acoustic phonons in a medium on this process is taken into account. The system evolution is described using quantum Heisenberg equations. A truncated set of equations is obtained and the transfer dynamics is numerically simulated. High-efficiency picosecond switching of the excitation transfer by a laser pulse with a rectangular envelope is demonstrated. The dependence of picosecond switching on the quantum-dot parameters and optical-pulse length is presented.

  11. Josephson effects in condensates of excitons and exciton polaritons

    SciTech Connect

    Shelykh, I. A.; Solnyshkov, D. D.; Pavlovic, G.; Malpuech, G.

    2008-07-15

    We analyze theoretically the phenomena related to the Josephson effect for exciton and polariton condensates, taking into account their specific spin degrees of freedom. We distinguish between two types of Josephson effects: the extrinsic effect, related to the coherent tunneling of particles with the same spin between two spatially separated potential traps, and the intrinsic effect, related to the 'tunneling' between different spinor components of the condensate within the same trap. We show that the Josephson effect in the nonlinear regime can lead to nontrivial polarization dynamics and produce spontaneous separation of the condensates with opposite polarization in real space.

  12. Polycrystalline magnetic garnet films comprising weakly coupled crystallites for piezoelectrically-driven magneto-optic spatial light modulators

    SciTech Connect

    Mito, S.; Sakurai, H.; Takagi, H.; Inoue, M.; Baryshev, A. V.

    2012-04-01

    We have investigated the magnetization process of the polycrystalline magnetic garnet films in order to determine the most suitable composition of garnet films for piezoelectrically-driven magneto-optic spatial light modulators (MOSLMs). For experiment, the bismuth-dysprosium-aluminum-substituted yttrium iron (Bi{sub 1.3}Dy{sub 0.7}Y{sub 1.0}Fe{sub 3.1}Al{sub 1.9}O{sub 12}) garnet films were deposited by an RF magnetron sputter and annealed at 700 deg. C in air. The annealing time was varied in a range of several minutes to control the grain size. The saturation magnetization, the remanent magnetization and the composition of the fabricated garnet films slightly changed versus the annealing time. Experiments showed that the coercivity and the grain size increased at longer annealing; the coercivity was larger for films with bigger grains. This work shows that garnet films with smaller coercivity are most suitable for controlling the magnetization of garnet and, correspondingly, the magneto-optical rotation of MOSLM pixels driven by piezoelectrics.

  13. Accessing the Dark Exciton States in Semiconducting Single-Walled Carbon Nanotubes with Terahertz Pulses

    NASA Astrophysics Data System (ADS)

    Luo, Liang; Chatzakis, Ioannis; Wang, Jigang; Wang Team, Dr

    2014-03-01

    Singled-walled carbon nanotubes (SWNTs) represent a model system to systematically investigate correlated charge excitation in 1-D limits. One of the most outstanding issues both in fundamental nanotube physics and for their technological development is to detect and understand optically-forbidden, dark collective states. Thus far supporting evidence of dark states has been demonstrated in static magneto-optics and light scattering. However, the unique internal transitions from dark excitonic ground states and their dynamic evolution remain highly elusive. We report our investigation of this problem using optical pump, terahertz probe spectroscopy of (6,5) and (7,5) SWNTs. We measure transient THz conductivity from 0.5-2.5 THz (2-10.5 meV) at low temperature down to 5 K with resonant and off-resonant excitation at the E22 transitions of (6,5) and (7,5) nanotubes. These results reveal, for the first time, dynamics of lowest dark excitons and density-dependent renormalization of these many-particle states. The internal-excitonic spectroscopy with THz pulses represents a fundamentally new spectroscopy tools to study dark excitons and shine new lights on the correlation physics of excitonic ground states.

  14. Electrons, Phonons and Excitons at Semiconductor Surfaces

    NASA Astrophysics Data System (ADS)

    Pollmann, Johannes; Krueger, Peter; Mazur, Albert; Rohlfing, Michael

    We briefly address 'state-of-the-art' ab-initio calculations of basic properties of semiconductor surfaces such as their atomic configuration, electronic structure and surface vibrations, as well as, their optical properties and compare exemplary results with experimental data. The surface structure and the electronic ground state are described within the local density approximation (LDA) of density functional theory (DFT). The description of excited electronic states requires to take dynamical correlations in the many electron system into account, which is achieved to a considerable extent within the GW approximation (GWA) leading to the concept of the quasiparticle bandstructure. Surface phonons are treated from first principles within density functional perturbation theory (DFPT). The theory of optical surface properties and surface excitons, in particular, requires to account for the electron-hole Coulomb correlation which is done in the framework of the Bethe-Salpeter equation (BSE). These methods yield results which are in good agreement with experiment and can significantly contribute to an interpretation of experimental data from high-resolution surface microscopy and spectroscopy thus enhancing our current understanding of semiconductor surfaces.

  15. Consistency Properties of a Chaotic Semiconductor Laser Driven by Optical Feedback

    NASA Astrophysics Data System (ADS)

    Oliver, Neus; Jüngling, Thomas; Fischer, Ingo

    2015-03-01

    We experimentally study consistency properties of a semiconductor laser in response to a coherent optical drive originating from delayed feedback. The laser is connected to a short and a long optical fiber loop, switched such that only one is providing input to the laser at a time. This way, repeating the exact same optical drive twice, we find consistent or inconsistent responses depending on the pump parameter and we relate the kind of response to strong and weak chaos. Moreover, we are able to experimentally determine the sub-Lyapunov exponent, underlying the consistency properties.

  16. 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.

  17. Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy

    NASA Astrophysics Data System (ADS)

    Klots, A. R.; Newaz, A. K. M.; Wang, Bin; Prasai, D.; Krzyzanowska, H.; Lin, Junhao; Caudel, D.; Ghimire, N. J.; Yan, J.; Ivanov, B. L.; Velizhanin, K. A.; Burger, A.; Mandrus, D. G.; Tolk, N. H.; Pantelides, S. T.; Bolotin, K. I.

    2014-10-01

    The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind >= 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. The analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.

  18. Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy

    SciTech Connect

    Klots, A. R.; Newaz, A. K. M.; Wang, Bin; Prasai, D.; Krzyzanowska, H.; Lin, Junhao; Caudel, D.; Ghimire, N. J.; Yan, J.; Ivanov, B. L.; Velizhanin, K. A.; Burger, A.; Mandrus, D. G.; Tolk, N. H.; Pantelides, S. T.; Bolotin, K. I.

    2014-10-16

    The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions of peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. In conclusion, the analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.

  19. Probing excitonic states in suspended two-dimensional semiconductors by photocurrent spectroscopy

    DOE PAGES

    Klots, A. R.; Newaz, A. K. M.; Wang, Bin; ...

    2014-10-16

    The optical response of semiconducting monolayer transition-metal dichalcogenides (TMDCs) is dominated by strongly bound excitons that are stable even at room temperature. However, substrate-related effects such as screening and disorder in currently available specimens mask many anticipated physical phenomena and limit device applications of TMDCs. Here, we demonstrate that that these undesirable effects are strongly suppressed in suspended devices. Extremely robust (photogain > 1,000) and fast (response time < 1 ms) photoresponse allow us to study, for the first time, the formation, binding energies, and dissociation mechanisms of excitons in TMDCs through photocurrent spectroscopy. By analyzing the spectral positions ofmore » peaks in the photocurrent and by comparing them with first-principles calculations, we obtain binding energies, band gaps and spin-orbit splitting in monolayer TMDCs. For monolayer MoS2, in particular, we obtain an extremely large binding energy for band-edge excitons, Ebind ≥ 570 meV. Along with band-edge excitons, we observe excitons associated with a van Hove singularity of rather unique nature. In conclusion, the analysis of the source-drain voltage dependence of photocurrent spectra reveals exciton dissociation and photoconversion mechanisms in TMDCs.« less

  20. Coherently driven semiconductor quantum dot at a telecommunication wavelength.

    PubMed

    Takagi, Hiroyuki; Nakaoka, Toshihiro; Watanabe, Katsuyuki; Kumagai, Naoto; Arakawa, Yasuhiko

    2008-09-01

    We proposed and demonstrate use of optical driving pulses at a telecommunication wavelength for exciton-based quantum gate operation. The exciton in a self-assembled quantum dot is coherently manipulated at 1.3 microm through Rabi oscillation. The telecom-band exciton-qubit system incorporates standard optical fibers and fiber optic devices. The coherent manipulation of the two-level system compatible with flexible and stable fiber network paves the way toward practical optical implementation of quantum information processing devices.

  1. Bound exciton and free exciton states in GaSe thin slab

    PubMed Central

    Wei, Chengrong; Chen, Xi; Li, Dian; Su, Huimin; He, Hongtao; Dai, Jun-Feng

    2016-01-01

    The photoluminescence (PL) and absorption experiments have been performed in GaSe slab with incident light polarized perpendicular to c-axis of sample at 10 K. An obvious energy difference of about 34 meV between exciton absorption peak and PL peak (the highest energy peak) is observed. By studying the temperature dependence of PL and absorption spectra, we attribute it to energy difference between free exciton and bound exciton states, where main exciton absorption peak comes from free exciton absorption, and PL peak is attributed to recombination of bound exciton at 10 K. This strong bound exciton effect is stable up to 50 K. Moreover, the temperature dependence of integrated PL intensity and PL lifetime reveals that a non-radiative process, with activation energy extracted as 0.5 meV, dominates PL emission. PMID:27654064

  2. Bound exciton and free exciton states in GaSe thin slab

    NASA Astrophysics Data System (ADS)

    Wei, Chengrong; Chen, Xi; Li, Dian; Su, Huimin; He, Hongtao; Dai, Jun-Feng

    2016-09-01

    The photoluminescence (PL) and absorption experiments have been performed in GaSe slab with incident light polarized perpendicular to c-axis of sample at 10 K. An obvious energy difference of about 34 meV between exciton absorption peak and PL peak (the highest energy peak) is observed. By studying the temperature dependence of PL and absorption spectra, we attribute it to energy difference between free exciton and bound exciton states, where main exciton absorption peak comes from free exciton absorption, and PL peak is attributed to recombination of bound exciton at 10 K. This strong bound exciton effect is stable up to 50 K. Moreover, the temperature dependence of integrated PL intensity and PL lifetime reveals that a non-radiative process, with activation energy extracted as 0.5 meV, dominates PL emission.

  3. Observation of non-Hermitian degeneracies in a chaotic exciton-polariton billiard

    NASA Astrophysics Data System (ADS)

    Gao, T.; Estrecho, E.; Bliokh, K. Y.; Liew, T. C. H.; Fraser, M. D.; Brodbeck, S.; Kamp, M.; Schneider, C.; Höfling, S.; Yamamoto, Y.; Nori, F.; Kivshar, Y. S.; Truscott, A. G.; Dall, R. G.; Ostrovskaya, E. A.

    2015-10-01

    Exciton-polaritons are hybrid light-matter quasiparticles formed by strongly interacting photons and excitons (electron-hole pairs) in semiconductor microcavities. They have emerged as a robust solid-state platform for next-generation optoelectronic applications as well as for fundamental studies of quantum many-body physics. Importantly, exciton-polaritons are a profoundly open (that is, non-Hermitian) quantum system, which requires constant pumping of energy and continuously decays, releasing coherent radiation. Thus, the exciton-polaritons always exist in a balanced potential landscape of gain and loss. However, the inherent non-Hermitian nature of this potential has so far been largely ignored in exciton-polariton physics. Here we demonstrate that non-Hermiticity dramatically modifies the structure of modes and spectral degeneracies in exciton-polariton systems, and, therefore, will affect their quantum transport, localization and dynamical properties. Using a spatially structured optical pump, we create a chaotic exciton-polariton billiard--a two-dimensional area enclosed by a curved potential barrier. Eigenmodes of this billiard exhibit multiple non-Hermitian spectral degeneracies, known as exceptional points. Such points can cause remarkable wave phenomena, such as unidirectional transport, anomalous lasing/absorption and chiral modes. By varying parameters of the billiard, we observe crossing and anti-crossing of energy levels and reveal the non-trivial topological modal structure exclusive to non-Hermitian systems. We also observe mode switching and a topological Berry phase for a parameter loop encircling the exceptional point. Our findings pave the way to studies of non-Hermitian quantum dynamics of exciton-polaritons, which may uncover novel operating principles for polariton-based devices.

  4. Exciton-polaritons condensate in a microwire

    NASA Astrophysics Data System (ADS)

    Kamoun, O.; Jaziri, S.

    2013-12-01

    Recently, polariton condensation has been claimed in microwires. Numerical solutions of the time-dependent Gross-Pitaevskii equation that describes the behavior of the condensate in a trap and exciton-polariton interaction, have been obtained. In this work we study theoretically exciton-polariton one dimensional condensation in several quantized states.

  5. DNA-mediated excitonic upconversion FRET switching

    DOE PAGES

    Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; ...

    2015-11-17

    Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy up conversion via up conversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based up conversion has been demonstrated, it suffersmore » from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an up conversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy up conversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy up conversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.« less

  6. DNA-mediated excitonic upconversion FRET switching

    SciTech Connect

    Kellis, Donald L.; Rehn, Sarah M.; Cannon, Brittany L.; Davis, Paul H.; Graugnard, Elton; Lee, Jeunghoon; Yurke, Bernard; Knowlton, William B.

    2015-11-17

    Excitonics is a rapidly expanding field of nanophotonics in which the harvesting of photons, ensuing creation and transport of excitons via Förster resonant energy transfer (FRET), and subsequent charge separation or photon emission has led to the demonstration of excitonic wires, switches, Boolean logic and light harvesting antennas for many applications. FRET funnels excitons down an energy gradient resulting in energy loss with each step along the pathway. Conversely, excitonic energy up conversion via up conversion nanoparticles (UCNPs), although currently inefficient, serves as an energy ratchet to boost the exciton energy. Although FRET-based up conversion has been demonstrated, it suffers from low FRET efficiency and lacks the ability to modulate the FRET. We have engineered an up conversion FRET-based switch by combining lanthanide-doped UCNPs and fluorophores that demonstrates excitonic energy up conversion by nearly a factor of 2, an excited state donor to acceptor FRET efficiency of nearly 25%, and an acceptor fluorophore quantum efficiency that is close to unity. These findings offer a promising path for energy up conversion in nanophotonic applications including artificial light harvesting, excitonic circuits, photovoltaics, nanomedicine, and optoelectronics.

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

    SciTech Connect

    Webber, D.; Hacquebard, L.; Hall, K. C.; Liu, X.; Dobrowolska, M.; Furdyna, J. K.

    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.

  8. Strong exciton-localized plasmon coupling in a-Ge24Se76/AuNP heterostructure

    NASA Astrophysics Data System (ADS)

    Sharma, Rituraj; Khan, Pritam; Aneesh, J.; Sangunni, K. S.; Csarnovics, I.; Kokenyesi, S.; Jain, H.; Adarsh, K. V.

    2016-10-01

    Metal nanoparticle-semiconductor interfaces are sites of complex light-matter interactions, in particular, the exciton-plasmon coupling which plays a key role in the optical response of such heterostructures. There exists a pathway of photoinduced charge transfer from the semiconductor to the metal, which can be used to controllably vary the driving forces at the interface that leads to tunable optoelectronic properties. In this letter, we report the observation of a dramatic suppression of plasmonic as well as excitonic absorption in a-Ge24Se76/gold nanoparticle heterostructures by trapped charges. Suppression of the excitonic absorption is strongly correlated with the plasmon wavelength.

  9. Exciton spin coherence in InGaAs/GaAs quantum dots revisited by heterodyne pump-probe experiment

    NASA Astrophysics Data System (ADS)

    Eble, B.; Siarry, B.; Bernardot, F.; Grinberg, P.; Testelin, C.; Lemaître, A.

    2016-10-01

    We demonstrate here the phase control of the neutral exciton quantum beats in InGaAs/GaAs quantum dots. A longitudinal magnetic field is used as a tuning parameter to change the phase of the oscillations in a deterministic way. This effect arises from the competition between the Zeeman splitting and the electron/hole exchange interaction on the exciton dipole symmetry. To explore this mechanism, we have developed a pump-probe setup based on the optical heterodyne detection of the quantum dots reflectivity allowing one to measure the exciton dynamics from a small quantum dots ensemble (˜300).

  10. Excitonic magnetism in d6 perovskites

    NASA Astrophysics Data System (ADS)

    Afonso, J. Fernández; Kuneš, J.

    2017-03-01

    We use the LDA+U method to study the possibility of exciton condensation in perovskites of transition metals with the d6 electronic configuration such as LaCoO3. For realistic interaction parameters we find several distinct solutions exhibiting a spin-triplet exciton condensate, which gives rise to a local spin density distribution while the ordered moments are vanishingly small. Rhombohedral distortion from the ideal cubic structure suppresses the ordered state, contrary to the spin-orbit coupling which enhances the excitonic condensation energy. We explain the trends observed in the numerical simulations with the help of a simplified strong-coupling model. Our results indicate that LaCoO3 is close to the excitonic instability and suggest ways how to achieve the exciton condensation.

  11. Population Pulsation Resonances of Excitons in Monolayer MoSe2 with Sub-1 μeV Linewidths

    DOE PAGES

    Schaibley, John R.; Karin, Todd; Yu, Hongyi; ...

    2015-04-01

    Monolayer transition metal dichalcogenides, a new class of atomically thin semiconductors, possess optically coupled 2D valley excitons. The nature of exciton relaxation in these systems is currently poorly understood. In this paper, we investigate exciton relaxation in monolayer MoSe2 using polarization-resolved coherent nonlinear optical spectroscopy with high spectral resolution. We report strikingly narrow population pulsation resonances with two different characteristic linewidths of 1 and <0.2 μeV at low temperature. These linewidths are more than 3 orders of magnitude narrower than the photoluminescence and absorption linewidth, and indicate that a component of the exciton relaxation dynamics occurs on time scales longermore » than 1 ns. Finally, the ultranarrow resonance (<0.2 μeV) emerges with increasing excitation intensity, and implies the existence of a long-lived state whose lifetime exceeds 6 ns.« less

  12. Tailoring double Fano profiles with plasmon-assisted quantum interference in hybrid exciton-plasmon system

    SciTech Connect

    Zhao, Dongxing; Wu, Jiarui; Gu, Ying Gong, Qihuang

    2014-09-15

    We propose tailoring of the double Fano profiles via plasmon-assisted quantum interference in a hybrid exciton-plasmon system. Tailoring is performed by the interference between two exciton channels interacting with a common localized surface plasmon. Using an applied field of low intensity, the absorption spectrum of the hybrid system reveals a double Fano lineshape with four peaks. For relatively large field intensity, a broad flat window in the absorption spectrum appears which results from the destructive interference between excitons. Because of strong constructive interference, this window vanishes as intensity is further increased. We have designed a nanometer bandpass optical filter for visible light based on tailoring of the optical spectrum. This study provides a platform for quantum interference that may have potential applications in ultracompact tunable quantum devices.

  13. Plasmon-exciton interaction and screening of exciton in ZnO-based thin film on bulk Pt as analyzed by spectroscopic ellipsometry

    NASA Astrophysics Data System (ADS)

    Darma, Yudi; Dimas Satrya, Christoforus; Marlina, Resti; Kurniawan, Robi; Herng, Tun Seng; Ding, Jun; Rusydi, Andrivo

    2017-01-01

    We study plasmon-exciton interaction in ZnO-based thin film on bulk Pt by using high resolution spectroscopic ellipsometry. ZnO films on quartz are used as reference. This study shows the strong electronic interactions between ZnO film and Pt by considering the significant suppression of exciton in ZnO film, in comparison to ZnO film on quartz. We found that plasmon in Pt are responsible to provide transferred electron for electronic blocking of exciton in ZnO film induce by spontaneous recombination from Pt. In the case of Cu doped ZnO film, we confirm screening effects on exciton and a localized interband transition for both systems (ZnO film on Pt and ZnO film on quartz). In Cu-doped ZnO film, electronic blocking of exciton by Pt plasmon is more pronounce rather than screening effect by interband transition. Our results show the importance of plasmon from substrate and doping to modify the optical properties of wide bandgap semiconductor.

  14. On the output characteristics of a semiconductor optical amplifier driven by an ultrafast optical time division multiplexing pulse train

    NASA Astrophysics Data System (ADS)

    Zoiros, K. E.; Chasioti, R.; Koukourlis, C. S.; Houbavlis, T.

    2007-03-01

    A comprehensive theoretical analysis of a semiconductor optical amplifier (SOA) that is subject to an ultrafast optical time division multiplexing pulse stream is presented with the help of a simple but efficient model developed for this purpose. The model combines the necessary set of mathematical equations with the appropriate simplifying assumptions to describe in the time domain gain saturation and recovery for the case of multiple incoming pulses. In this manner, analytical expressions can be obtained for the power and chirp profile of the amplified pulses, essentially extending the work that has been performed for a single pulse only. This allows to identify the critical operational parameters and to investigate and evaluate their effect on these two output characteristics. The derived simulation curves are thoroughly studied to specify the limitations imposed on the SOA small signal gain and carrier lifetime as well as on the full-width at half-maximum (FWHM) and energy of the input pulses and, based on a series of logical arguments, to extract useful rules concerning their selection so as to achieve improved performance with respect to the practical applications of all-optical switching and pulse compression. The obtained results indicate that due to the continuous insertion of pulses, the requirements for the SOA small signal gain and the input pulse energy are stringent than those for the case of isolated pulse amplification. The combination of these two parameters determines also the regime in which the amplifier must be biased to operate in order to ensure distortionless pulse amplification and enhanced chirp for efficient pulse compression and it has been found that low saturation is necessary for the former case whilst heavy saturation for the latter. The scopes of the corresponding requirements for the carrier lifetime and the FWHM are also tight but to a less extent and can be simply satisfied with the available photonics technology. These results

  15. Exciton fine structure and spin relaxation in semiconductor colloidal quantum dots.

    PubMed

    Kim, Jeongho; Wong, Cathy Y; Scholes, Gregory D

    2009-08-18

    Quantum dots (QDs) have discrete quantum states isolated from the environment, making QDs well suited for quantum information processing. In semiconductor QDs, the electron spins can be coherently oriented by photoexcitation using circularly polarized light, creating optical orientation. The optically induced spin orientation could serve as a unit for data storage and processing. Carrier spin orientation is also envisioned to be a key component in a related, though parallel, field of semiconductor spintronics. However, the oriented spin population rapidly loses its coherence by interaction with the environment, thereby erasing the prepared information. Since long-lasting spin orientation is desirable in both areas of investigation, spin relaxation is the central focus of investigation for optimization of device performance. In this Account, we discuss a topic peripherally related to these emerging areas of investigation: exciton fine structure relaxation (EFSR). The radiationless transition occurring in the exciton fine structure not only highlights a novel aspect of QD exciton relaxation but also has implications for carrier spin relaxation in QDs. We focus on examining the EFSR in connection with optical spin orientation and subsequent ultrafast relaxation of electron and hole spin densities in the framework of the exciton fine structure basis. Despite its significance, the study of exciton fine structure in colloidal QDs has been hampered by the experimental challenge arising from inhomogeneous line broadening that obscures the details of closely spaced fine structure states in the frequency domain. In this Account, we show that spin relaxation occurring in the fine structure of CdSe QDs can be probed by a time-domain nonlinear polarization spectroscopy, circumventing the obstacles confronted in the frequency-domain spectroscopy. In particular, by combining polarization sequences of multiple optical pulses with the unique optical selection rules of

  16. Testing of optical diagnostics for ion-beam-driven WDM experiments at NDCX-1

    NASA Astrophysics Data System (ADS)

    Ni, P. A.; Bieniosek, F. M.; Leitner, M.; Weber, C.; Waldron, W. L.

    2009-07-01

    We report on the testing of optical diagnostics developed for warm-dense-matter (WDM) experiments on the neutralized drift compression experiments (NDCX-1) at Lawrence Berkeley National Laboratory (LBNL). The diagnostics consists of a fast optical pyrometer, a streak camera spectrometer, and a Doppler-shift laser interferometer (VISAR). While the NDCX is in the last stage of commissioning for the target experiments, the diagnostics were tested elsewhere in an experiment where an intense laser pulse was used to generate the WDM state in metallic and carbon samples.

  17. Emission energy, exciton dynamics and lasing properties of buckled CdS nanoribbons

    PubMed Central

    Wang, Qi; Sun, Liaoxin; Lu, Jian; Ren, Ming-Liang; Zhang, Tianning; Huang, Yan; Zhou, Xiaohao; Sun, Yan; Zhang, Bo; Chen, Changqing; Shen, Xuechu; Agarwal, Ritesh; Lu, Wei

    2016-01-01

    We report the modulation of emission energy, exciton dynamics and lasing properties in a single buckled CdS nanoribbon (NR) by strain-engineering. Inspired by ordered structure fabrication on elastomeric polymer, we develop a new method to fabricate uniform buckled NRs supported on polydimethylsiloxane (PDMS). Wavy structure, of which compressive and tensile strain periodically varied along the CdS NR, leads to a position-dependent emission energy shift as large as 14 nm in photoluminescence (PL) mapping. Both micro-PL and micro-reflectance reveal the spectral characteristics of broad emission of buckled NR, which can be understood by the discrepancy of strain-induced energy shift of A- and B-exciton of CdS. Furthermore, the dynamics of excitons under tensile strain are also investigated; we find that the B-exciton have much shorter lifetime than that of redshifted A-exciton. In addition, we also present the lasing of buckled CdS NRs, in which the strain-dominated mode selection in multi-mode laser and negligible mode shifts in single-mode laser are clearly observed. Our results show that the strained NRs may serve as new functional optical elements for flexible light emitter or on-chip all-optical devices. PMID:27210303

  18. Well separated trion and neutral excitons on superacid treated MoS2 monolayers

    NASA Astrophysics Data System (ADS)

    Cadiz, Fabian; Tricard, Simon; Gay, Maxime; Lagarde, Delphine; Wang, Gang; Robert, Cedric; Renucci, Pierre; Urbaszek, Bernhard; Marie, Xavier

    2016-06-01

    Developments in optoelectronics and spin-optronics based on transition metal dichalcogenide monolayers (MLs) need materials with efficient optical emission and well-defined transition energies. In as-exfoliated MoS2 MLs, the photoluminescence (PL) spectra even at low temperature consist typically of broad, overlapping contributions from neutral, charged excitons (trions) and localized states. Here, we show that in superacid treated MoS2 MLs, the PL intensity increases by up to 60 times at room temperature. The neutral and charged exciton transitions are spectrally well separated in PL and reflectivity at T = 4 K, with linewidth for the neutral exciton of 15 meV, but both transitions have similar intensities compared to the ones in as-exfoliated MLs at the same temperature. Time resolved experiments uncover picoseconds recombination dynamics analyzed separately for charged and neutral exciton emissions. Using the chiral interband selection rules, we demonstrate optically induced valley polarization for both complexes and valley coherence for only the neutral exciton.

  19. 2D Behaviors of Excitons in Cesium Lead Halide Perovskite Nanoplatelets.

    PubMed

    Li, Jing; Luo, Laihao; Huang, Hongwen; Ma, Chao; Ye, Zhizhen; Zeng, Jie; He, Haiping

    2017-03-16

    Fundamental to understanding and predicting the optoelectronic properties of semiconductors is the basic parameters of excitons such as oscillator strength and exciton binding energy. However, such knowledge of CsPbBr3 perovskite, a promising optoelectronic material, is still unexplored. Here we demonstrate that quasi-two-dimensional (quasi-2D) CsPbBr3 nanoplatelets (NPLs) with 2D exciton behaviors serve as an ideal system for the determination of these parameters. It is found that the oscillator strength of CsPbBr3 NPLs is up to 1.18 × 10(4), higher than that of colloidal II-VI NPLs and epitaxial quantum wells. Furthermore, the exciton binding energy is determined to be of ∼120 meV from either the optical absorption or the photoluminescence analysis, comparable to that reported in colloidal II-VI quantum wells. Our work provides physical understanding of the observed excellent optical properties of CsPbBr3 nanocrystals and would benefit the prediction of high-performance excitonic devices based on such materials.

  20. Emission energy, exciton dynamics and lasing properties of buckled CdS nanoribbons

    NASA Astrophysics Data System (ADS)

    Wang, Qi; Sun, Liaoxin; Lu, Jian; Ren, Ming-Liang; Zhang, Tianning; Huang, Yan; Zhou, Xiaohao; Sun, Yan; Zhang, Bo; Chen, Changqing; Shen, Xuechu; Agarwal, Ritesh; Lu, Wei

    2016-05-01

    We report the modulation of emission energy, exciton dynamics and lasing properties in a single buckled CdS nanoribbon (NR) by strain-engineering. Inspired by ordered structure fabrication on elastomeric polymer, we develop a new method to fabricate uniform buckled NRs supported on polydimethylsiloxane (PDMS). Wavy structure, of which compressive and tensile strain periodically varied along the CdS NR, leads to a position-dependent emission energy shift as large as 14 nm in photoluminescence (PL) mapping. Both micro-PL and micro-reflectance reveal the spectral characteristics of broad emission of buckled NR, which can be understood by the discrepancy of strain-induced energy shift of A- and B-exciton of CdS. Furthermore, the dynamics of excitons under tensile strain are also investigated; we find that the B-exciton have much shorter lifetime than that of redshifted A-exciton. In addition, we also present the lasing of buckled CdS NRs, in which the strain-dominated mode selection in multi-mode laser and negligible mode shifts in single-mode laser are clearly observed. Our results show that the strained NRs may serve as new functional optical elements for flexible light emitter or on-chip all-optical devices.

  1. Exciton band structure in layered MoSe2: from a monolayer to the bulk limit

    NASA Astrophysics Data System (ADS)

    Arora, Ashish; Nogajewski, Karol; Molas, Maciej; Koperski, Maciej; Potemski, Marek

    2015-12-01

    We present the micro-photoluminescence (μPL) and micro-reflectance contrast (μRC) spectroscopy studies on thin films of MoSe2 with layer thicknesses ranging from a monolayer (1L) up to 5L. The thickness dependent evolution of the ground and excited state excitonic transitions taking place at various points of the Brillouin zone is determined. Temperature activated energy shifts and linewidth broadenings of the excitonic resonances in 1L, 2L and 3L flakes are accounted for by using standard formalisms previously developed for semiconductors. A peculiar shape of the optical response of the ground state (A) exciton in monolayer MoSe2 is tentatively attributed to the appearance of a Fano-type resonance. Rather trivial and clearly decaying PL spectra of monolayer MoSe2 with temperature confirm that the ground state exciton in this material is optically bright in contrast to a dark exciton ground state in monolayer WSe2.

  2. Cloud-Driven Changes in Aerosol Optical Properties - Final Technical Report

    SciTech Connect

    Ogren, John A.; Sheridan, Patrick S.; Andrews, Elisabeth

    2007-09-30

    The optical properties of aerosol particles are the controlling factors in determining direct aerosol radiative forcing. These optical properties depend on the chemical composition and size distribution of the aerosol particles, which can change due to various processes during the particles’ lifetime in the atmosphere. Over the course of this project we have studied how cloud processing of atmospheric aerosol changes the aerosol optical properties. A counterflow virtual impactor was used to separate cloud drops from interstitial aerosol and parallel aerosol systems were used to measure the optical properties of the interstitial and cloud-scavenged aerosol. Specifically, aerosol light scattering, back-scattering and absorption were measured and used to derive radiatively significant parameters such as aerosol single scattering albedo and backscatter fraction for cloud-scavenged and interstitial aerosol. This data allows us to demonstrate that the radiative properties of cloud-processed aerosol can be quite different than pre-cloud aerosol. These differences can be used to improve the parameterization of aerosol forcing in climate models.

  3. Optical interferometry diagnostics in laser-driven equation of state experiments

    SciTech Connect

    Cauble, R C; Celliers, P M; Collins, G W; Da Silva, L B; Gold, D M; Kalantar, D H; Remington, B A; Weber, S V

    1999-06-18

    We have developed and tested several optical interferometric diagnostics to measure preheat and shock velocity in high-pressure equation of state experiments on the Nova laser. Theory and practical application of interferometric measurement techniques with illustrative experimental results are presented.

  4. Effects of carbon nanotubes on electro-optical characteristics of liquid crystal cell driven by in-plane field

    NASA Astrophysics Data System (ADS)

    Jeon, Sang Youn; Shin, Seung Hwan; Jeong, Seok Jin; Lee, Seung Hee; Jeong, Seok Ho; Lee, Young Hee; Choi, Hyun Chul; Kim, Kyeong Jin

    2007-03-01

    Homogeneously aligned nematic liquid crystal (LC) cells doped with carbon nanotubes (CNTs) driven by an in-plane field were fabricated and their electro-optic characteristics were investigated. The effective cell retardation values showed no difference between doped and undoped LC cells in the absence of electric field. However, in the presence of electric field, it was smaller in the CNT-doped cell than in the undoped cell, resulting in the decrease of transmittance. Furthermore, the CNT-doped cell exhibited a slight increase in the driving voltage due to the increase of the twist elastic constant (K22) and the decrease in the decay response time due to the decrease in the rotational viscosity (γ) and γ /K22 compared to the undoped cell.

  5. Electro-optic characteristics of 90° twisted nematic liquid crystal display driven by fringe-electric field

    NASA Astrophysics Data System (ADS)

    Song, I. S.; Shin, S. S.; Kim, H. Y.; Song, S. H.; Lee, S. H.

    2004-02-01

    We investigated the electro-optic characteristics of a fringe-field driven twisted nematic (TN) display. In the absence of an electric field, the liquid crystals (LCs) are initially twisted 90° from the top to the bottom substrate under parallel polarizers so that the cell appears to be black. In the presence of a fringe-electric field, the LCs with negative dielectric anisotropy are rotated toward a plane that is almost perpendicular to the horizontal component of the fringe field, above the entire electrode surface. The cell then appears to be white, and shows high transmittance. In addition, the cell displays a wide viewing angle and has excellent color characteristics over a wide viewing range due to almost in-plane switching, unlike a conventional TN device where the LC director tilts upward in only one direction and results in a narrow viewing angle.

  6. Ultrafast ammonia-driven, microwave-assisted synthesis of nitrogen-doped graphene quantum dots and their optical properties

    NASA Astrophysics Data System (ADS)

    Zheng, Binjie; Chen, Yuanfu; Li, Pingjian; Wang, Zegao; Cao, Bingqiang; Qi, Fei; Liu, Jinbo; Qiu, Zhiwen; Zhang, Wanli

    2017-01-01

    For the first time, a facile, ultrafast, ammonia-driven microwave-assisted synthesis of high-quality nitrogen-doped graphene quantum dots (NGQDs) at room temperature and atmospheric pressure is presented. This one-step method is very cheap, environment friendly, and suitable for large-scale production. The as-synthesized NGQDs consisting of one to three graphene monolayers exhibit highly crystalline quality with an average size of 5.3 nm. A new fluorescence (FL) emission peak at 390 nm is observed, which might be attributed to the doped nitrogen atoms into the GQDs. An interesting red-shift is observed by comparing the FL excitation spectra to the UV-visible absorption spectra. Based on the optical properties, the detailed Jablonski diagram representing the energy level structure of NGQDs is derived.

  7. Bright solitons and self-trapping with a Bose-Einstein condensate of atoms in driven tilted optical lattices

    SciTech Connect

    Kolovsky, Andrey R.

    2010-07-15

    We discuss a method for creating bright matter solitons by loading a Bose-Einstein condensate of atoms in a driven tilted optical lattice. It is shown that one can realize the self-focusing regime for the wave-packet dynamics by properly adjusting the phase of the driving field with respect to the phase of Bloch oscillations. If atom-atom interactions are larger than some critical value g{sub min}, this self-focusing regime is followed by the formation of bright solitons. Increasing the interactions above another critical value g{sub max} makes this process unstable. Instead of soliton formation one now meets the phenomenon of incoherent self-trapping. In this regime a fraction of atoms is trapped in incoherent localized wave packets, while the remaining atoms spread ballistically.

  8. Lightwave-driven quasiparticle collisions on a subcycle timescale

    NASA Astrophysics Data System (ADS)

    Langer, F.; Hohenleutner, M.; Schmid, C. P.; Poellmann, C.; Nagler, P.; Korn, T.; Schüller, C.; Sherwin, M. S.; Huttner, U.; Steiner, J. T.; Koch, S. W.; Kira, M.; Huber, R.

    2016-05-01

    Ever since Ernest Rutherford scattered α-particles from gold foils, collision experiments have revealed insights into atoms, nuclei and elementary particles. In solids, many-body correlations lead to characteristic resonances—called quasiparticles—such as excitons, dropletons, polarons and Cooper pairs. The structure and dynamics of quasiparticles are important because they define macroscopic phenomena such as Mott insulating states, spontaneous spin- and charge-order, and high-temperature superconductivity. However, the extremely short lifetimes of these entities make practical implementations of a suitable collider challenging. Here we exploit lightwave-driven charge transport, the foundation of attosecond science, to explore ultrafast quasiparticle collisions directly in the time domain: a femtosecond optical pulse creates excitonic electron-hole pairs in the layered dichalcogenide tungsten diselenide while a strong terahertz field accelerates and collides the electrons with the holes. The underlying dynamics of the wave packets, including collision, pair annihilation, quantum interference and dephasing, are detected as light emission in high-order spectral sidebands of the optical excitation. A full quantum theory explains our observations microscopically. This approach enables collision experiments with various complex quasiparticles and suggests a promising new way of generating sub-femtosecond pulses.

  9. Excitonic polaritons of zinc diarsenide single crystals

    NASA Astrophysics Data System (ADS)

    Syrbu, N. N.; Stamov, I. G.; Zalamai, V. V.; Dorogan, A.

    2017-02-01

    Excitonic polaritons of ZnAs2 single crystals had been investigated. Parameters of singlet excitons with D2bar(z) symmetry and orthoexcitons 2D1bar(y)+D2bar(x) had been determined. Spectral dependencies of ordinary and extraordinary dispersion of refractive index had been calculated using interferential reflection and transmittance spectra. It was shown, that A excitonic series were due to hole (V1) and electron (C1) bands. The values of effective masses of electrons (mc*=0.10 m0) and holes (mv1*=0.89 m0) had been estimated. It was revealed that the hole mass mv1* changes from 1.03 m0 to 0.55 m0 at temperature increasing from 10 K up to 230 K and that the electron mass mc* does not depend on temperature. The integral absorption A (eV cm-1) of the states n=1, 2 and 3 of D2bar(z) excitons depends on the An≈n-3 equality, which it is characteristic for S-type excitonic functions. Temperature dependences of the integral absorption of ground states for D2bar(z) and D2bar(D) excitons differ. The ground states of B and C excitons formed by V3 - C1 and V4 - C1 bands and its parameters had been determined.

  10. High Speed Low Power Nonlinear Optical Signal Processing.

    DTIC Science & Technology

    1986-09-15

    Page 1. INTRODUCTION ............... .............................. 1 2. LINEAR PROPERTIES OF FREE AND BOUND EXCITONS: THEORY ..... .......... 2 2.1...BOUND EXCITONS: THEORY 2.1 EXCITON-POLARITON Some of the largest optical nonlinearities in nature have been observed in direct-gap semiconductors...bands (Figure 1). This leads to three separate intrinsic exciton series , A, B, and C. Series A, occurring at the lowest photon energies, is strongly

  11. Tunable Magnetic Alignment between Trapped Exciton-Polariton Condensates.

    PubMed

    Ohadi, H; Del Valle-Inclan Redondo, Y; Dreismann, A; Rubo, Y G; Pinsker, F; Tsintzos, S I; Hatzopoulos, Z; Savvidis, P G; Baumberg, J J

    2016-03-11

    Tunable spin correlations are found to arise between two neighboring trapped exciton-polariton condensates which spin polarize spontaneously. We observe a crossover from an antiferromagnetic to a ferromagnetic pair state by reducing the coupling barrier in real time using control of the imprinted pattern of pump light. Fast optical switching of both condensates is then achieved by resonantly but weakly triggering only a single condensate. These effects can be explained as the competition between spin bifurcations and spin-preserving Josephson coupling between the two condensates, and open the way to polariton Bose-Hubbard ladders.

  12. Exciton-polariton gap solitons in two-dimensional lattices.

    PubMed

    Cerda-Méndez, E A; Sarkar, D; Krizhanovskii, D N; Gavrilov, S S; Biermann, K; Skolnick, M S; Santos, P V

    2013-10-04

    We report on the two-dimensional gap-soliton nature of exciton-polariton macroscopic coherent phases (PMCP) in a square lattice with a tunable amplitude. The resonantly excited PMCP forms close to the negative mass M point of the lattice band structure with energy within the lattice band gap and its wave function localized within a few lattice periods. The PMCPs are well described as gap solitons resulting from the interplay between repulsive polariton-polariton interactions and effective attractive forces due to the negative mass. The solitonic nature accounts for the reduction of the PMCP coherence length and optical excitation threshold with increasing lattice amplitude.

  13. Exciton recombination dynamics in single ZnO tetrapods

    SciTech Connect

    Fernandes-Silva, Lígia C.; Martín, Maria D.; Meulen, Herko P. van der; Calleja, José M.; Viña, Luis; Klopotowski, Lukasz

    2013-12-04

    We present the optical properties of individual ZnO tetrapods as a function of excitation power and temperature by time-integrated and time-resolved spectroscopy. At 10K, we identify the different excitonic transitions by both their characteristic energy and their excitation power dependence. When we increase the tetrapod temperature we observe that the emission intensity decrease and occur a red shift of the emission energies. Our time-resolved studies confirm the predominance of the radiative recombination at low temperatures (< 45 K). Increasing the temperature opens up the non-radiative channels, which are evidenced by a much faster decay time.

  14. Optically-driven red blood cell rotor in linearly polarized laser tweezers

    NASA Astrophysics Data System (ADS)

    Khan, Manas; Mohanty, Samarendra K.; Sood, A. K.

    2005-11-01

    We have constructed a dual trap optical tweezers set-up around an inverted microscope where both the traps can be independently controlled and manipulated in all the three dimensions. Here we report our observations on rotation of red blood cells (RBCs) in a linearly polarized optical trap. Red blood cells deform and become twisted in hypertonic phosphate buffer saline and when trapped, experience an unbalanced radiation pressure force. The torque generated from the unbalanced force causes the trapped RBC to rotate. Addition of Ca^{++} ions in the solution, keeping the osmolarity same, makes the cell membranes stiffer and the cells deform less. Thus the speed of rotation of the red blood cells can be controlled, as less deformation and in turn less asymmetry in shape produces less torque under the radiation pressure resulting in slower rotation at the same laser power.

  15. Directly driven source of multi-gigahertz, sub-picosecond optical pulses

    DOEpatents

    Messerly, Michael J.; Dawson, Jay W.; Barty, Christopher P.J.; Gibson, David J.; Prantil, Matthew A.; Cormier, Eric

    2015-10-20

    A robust, compact optical pulse train source is described, with the capability of generating sub-picosecond micro-pulse sequences, which can be periodic as well as non-periodic, and at repetition rates tunable over decades of baseline frequencies, from MHz to multi-GHz regimes. The micro-pulses can be precisely controlled and formatted to be in the range of many ps in duration to as short as several fs in duration. The system output can be comprised of a continuous wave train of optical micro-pulses or can be programmed to provide gated bursts of macro-pulses, with each macro-pulse consisting of a specific number of micro-pulses or a single pulse picked from the higher frequency train at a repetition rate lower than the baseline frequency. These pulses could then be amplified in energy anywhere from the nJ to MJ range.

  16. Holographic optical manipulation of motor-driven membranous structures in living NG-108 cells

    NASA Astrophysics Data System (ADS)

    Farré, Arnau; López-Quesada, Carol; Andilla, Jordi; Martín-Badosa, Estela; Montes-Usategui, Mario

    2010-08-01

    Optical tweezer experiments have partially unveiled the mechanical properties of processive motor proteins while driving polystyrene or silica microbeads in vitro. However, the set of forces underlying the more complex transport mechanisms in living samples remains poorly understood. Several studies have shown that optical tweezers are capable of trapping vesicles and organelles in the cytoplasm of living cells, which can be used as handles to mechanically interact with engaged (active) motors, or other components regulating transport. This may ultimately enable the exploration of the mechanics of this trafficking mechanism in vivo. These cell manipulation experiments have been carried out using different strategies to achieve dynamic beam steering capable of trapping these subcellular structures. We report here the first trapping and manipulation, to our knowledge, of such small motor-propelled cargos in living cells using holographic technology.

  17. van der Waals Metal-Organic Framework as an Excitonic Material for Advanced Photonics.

    PubMed

    Milichko, Valentin A; Makarov, Sergey V; Yulin, Alexey V; Vinogradov, Alexandr V; Krasilin, Andrei A; Ushakova, Elena; Dzyuba, Vladimir P; Hey-Hawkins, Evamarie; Pidko, Evgeny A; Belov, Pavel A

    2017-01-23

    Synergistic combination of organic and inorganic nature in van der Waals metal-organic frameworks supports different types of robust excitons that can be effectively and independently manipulated by light at room temperature, and opens new concepts for all-optical data processing and storage.

  18. Aharonov-Bohm interference in neutral excitons: effects of built-in electric fields.

    PubMed

    Teodoro, M D; Campo, V L; Lopez-Richard, V; Marega, E; Marques, G E; Gobato, Y Galvão; Iikawa, F; Brasil, M J S P; Abuwaar, Z Y; Dorogan, V G; Mazur, Yu I; Benamara, M; Salamo, G J

    2010-02-26

    We report a comprehensive discussion of quantum interference effects due to the finite structure of neutral excitons in quantum rings and their first experimental corroboration observed in the optical recombinations. The signatures of built-in electric fields and temperature on quantum interference are demonstrated by theoretical models that describe the modulation of the interference pattern and confirmed by complementary experimental procedures.

  19. Oblique half-solitons and their generation in exciton-polariton condensates

    SciTech Connect

    Flayac, H.; Solnyshkov, D. D.; Malpuech, G.

    2011-05-15

    We describe oblique half-solitons, a new type of topological defects in a two-dimensional spinor Bose-Einstein condensate. A realistic protocol based on the optical spin Hall effect is proposed toward their generation within an exciton-polariton system.

  20. Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence

    NASA Astrophysics Data System (ADS)

    Hallinan, G.; Littlefair, S. P.; Cotter, G.; Bourke, S.; Harding, L. K.; Pineda, J. S.; Butler, R. P.; Golden, A.; Basri, G.; Doyle, J. G.; Kao, M. M.; Berdyugina, S. V.; Kuznetsov, A.; Rupen, M. P.; Antonova, A.

    2015-07-01

    Aurorae are detected from all the magnetized planets in our Solar System, including Earth. They are powered by magnetospheric current systems that lead to the precipitation of energetic electrons into the high-latitude regions of the upper atmosphere. In the case of the gas-giant planets, these aurorae include highly polarized radio emission at kilohertz and megahertz frequencies produced by the precipitating electrons, as well as continuum and line emission in the infrared, optical, ultraviolet and X-ray parts of the spectrum, associated with the collisional excitation and heating of the hydrogen-dominated atmosphere. Here we report simultaneous radio and optical spectroscopic observations of an object at the end of the stellar main sequence, located right at the boundary between stars and brown dwarfs, from which we have detected radio and optical auroral emissions both powered by magnetospheric currents. Whereas the magnetic activity of stars like our Sun is powered by processes that occur in their lower atmospheres, these aurorae are powered by processes originating much further out in the magnetosphere of the dwarf star that couple energy into the lower atmosphere. The dissipated power is at least four orders of magnitude larger than what is produced in the Jovian magnetosphere, revealing aurorae to be a potentially ubiquitous signature of large-scale magnetospheres that can scale to luminosities far greater than those observed in our Solar System. These magnetospheric current systems may also play a part in powering some of the weather phenomena reported on brown dwarfs.

  1. Noise-driven optical absorption coefficients of impurity doped quantum dots

    NASA Astrophysics Data System (ADS)

    Ganguly, Jayanta; Saha, Surajit; Pal, Suvajit; Ghosh, Manas

    2016-01-01

    We make an extensive investigation of linear, third-order nonlinear, and total optical absorption coefficients (ACs) of impurity doped quantum dots (QDs) in presence and absence of noise. The noise invoked in the present study is a Gaussian white noise. The quantum dot is doped with repulsive Gaussian impurity. Noise has been introduced to the system additively and multiplicatively. A perpendicular magnetic field acts as a source of confinement and a static external electric field has been applied. The AC profiles have been studied as a function of incident photon energy when several important parameters such as optical intensity, electric field strength, magnetic field strength, confinement energy, dopant location, relaxation time, Al concentration, dopant potential, and noise strength take on different values. In addition, the role of mode of application of noise (additive/multiplicative) on the AC profiles has also been analyzed meticulously. The AC profiles often consist of a number of interesting observations such as one photon resonance enhancement, shift of AC peak position, variation of AC peak intensity, and bleaching of AC peak. However, presence of noise alters the features of AC profiles and leads to some interesting manifestations. Multiplicative noise brings about more complexity in the AC profiles than its additive counterpart. The observations indeed illuminate several useful aspects in the study of linear and nonlinear optical properties of doped QD systems, specially in presence of noise. The findings are expected to be quite relevant from a technological perspective.

  2. Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence.

    PubMed

    Hallinan, G; Littlefair, S P; Cotter, G; Bourke, S; Harding, L K; Pineda, J S; Butler, R P; Golden, A; Basri, G; Doyle, J G; Kao, M M; Berdyugina, S V; Kuznetsov, A; Rupen, M P; Antonova, A

    2015-07-30

    Aurorae are detected from all the magnetized planets in our Solar System, including Earth. They are powered by magnetospheric current systems that lead to the precipitation of energetic electrons into the high-latitude regions of the upper atmosphere. In the case of the gas-giant planets, these aurorae include highly polarized radio emission at kilohertz and megahertz frequencies produced by the precipitating electrons, as well as continuum and line emission in the infrared, optical, ultraviolet and X-ray parts of the spectrum, associated with the collisional excitation and heating of the hydrogen-dominated atmosphere. Here we report simultaneous radio and optical spectroscopic observations of an object at the end of the stellar main sequence, located right at the boundary between stars and brown dwarfs, from which we have detected radio and optical auroral emissions both powered by magnetospheric currents. Whereas the magnetic activity of stars like our Sun is powered by processes that occur in their lower atmospheres, these aurorae are powered by processes originating much further out in the magnetosphere of the dwarf star that couple energy into the lower atmosphere. The dissipated power is at least four orders of magnitude larger than what is produced in the Jovian magnetosphere, revealing aurorae to be a potentially ubiquitous signature of large-scale magnetospheres that can scale to luminosities far greater than those observed in our Solar System. These magnetospheric current systems may also play a part in powering some of the weather phenomena reported on brown dwarfs.

  3. Analytical formulas for low-fluence non-line-narrowed hole-burned spectra in an excitonically coupled dimer

    NASA Astrophysics Data System (ADS)

    Reppert, Mike; Naibo, Virginia; Jankowiak, Ryszard

    2009-12-01

    We present exact equations for the low-fluence non-line-narrowed (NLN) nonphotochemical hole-burning (NPHB) spectrum of an excitonically coupled dimer (for arbitrary coupling strength) under the assumption that postburn and preburn site energies are independent. The equations provide a transparent view into the contributions of various effects to the NPHB spectrum. It is demonstrated that the NPHB spectrum in dimers is largely dominated by the statistical reshuffling of site energies and by altered excitonic transition energies of both excitonic states (in contrast with only the lowest state). For comparison of these results with those from larger excitonically coupled systems, the low-fluence NLN NPHB spectrum obtained for the CP47 complex (a 16-pigment core antenna complex of Photosystem II) is also calculated using Monte Carlo simulations. In this larger system it is shown that the NPHB spectra for individual excitonic states are not entirely conservative (although the changes in average oscillator strength for the higher excitonic states are in most cases less than 1%), a feature which we argue is due primarily to reordering of the contributions of various pigments to the excitonic states. We anticipate that a better understanding of NPHB spectra obtained for various photosynthetic complexes and their simultaneous fits with other optical spectra (e.g., absorption, emission, and circular dichroism spectra) will provide more insight into the underlying electronic structures of various photosynthetic systems.

  4. Excitons and Davydov splitting in sexithiophene from first-principles many-body Green’s function theory

    SciTech Connect

    Leng, Xia; Yin, Huabing; Liang, Dongmei; Ma, Yuchen

    2015-09-21

    Organic semiconductors have promising and broad applications in optoelectronics. Understanding their electronic excited states is important to help us control their spectroscopic properties and performance of devices. There have been a large amount of experimental investigations on spectroscopies of organic semiconductors, but theoretical calculation from first principles on this respect is still limited. Here, we use density functional theory (DFT) and many-body Green’s function theory, which includes the GW method and Bethe-Salpeter equation, to study the electronic excited-state properties and spectroscopies of one prototypical organic semiconductor, sexithiophene. The exciton energies of sexithiophene in both the gas and bulk crystalline phases are very sensitive to the exchange-correlation functionals used in DFT for ground-state structure relaxation. We investigated the influence of dynamical screening in the electron-hole interaction on exciton energies, which is found to be very pronounced for triplet excitons and has to be taken into account in first principles calculations. In the sexithiophene single crystal, the energy of the lowest triplet exciton is close to half the energy of the lowest singlet one. While lower-energy singlet and triplet excitons are intramolecular Frenkel excitons, higher-energy excitons are of intermolecular charge-transfer type. The calculated optical absorption spectra and Davydov splitting are in good agreement with experiments.

  5. Ultracold Gas of Excitons in Traps

    DTIC Science & Technology

    2012-06-08

    Excitons in a GaAs Quantum-Well Structurewith a Diamond-Shaped Electrostatic Trap, Physical Review Letters, (08 2009): 87403. doi: 2012/06/08 14:38:06 19...Kinetics of the inner ring in the exciton emission pattern in coupled GaAs quantum wells, Physical Review B, (10 2009): 155331. doi: 2012/06/08 14:28:30...17 Sen Yang, , L. V. Butov, , L. S. Levitov, , B. D. Simons,, A. C. Gossard. Exciton front propagation in photoexcited GaAs quantum wells, Physical

  6. Interaction-Driven Topological Insulator in Fermionic Cold Atoms on an Optical Lattice: A Design with a Density Functional Formalism.

    PubMed

    Kitamura, Sota; Tsuji, Naoto; Aoki, Hideo

    2015-07-24

    We design an interaction-driven topological insulator for fermionic cold atoms in an optical lattice; that is, we pose the question of whether we can realize in a continuous space a spontaneous symmetry breaking induced by the interatom interaction into a topological Chern insulator. Such a state, sometimes called a "topological Mott insulator," has yet to be realized in solid-state systems, since this requires, in the tight-binding model, large off-site interactions on top of a small on-site interaction. Here, we overcome the difficulty by introducing a spin-dependent potential, where a spin-selective occupation of fermions in A and B sublattices makes the on-site interaction Pauli forbidden, while a sizeable intersite interaction is achieved by a shallow optical potential with a large overlap between neighboring Wannier orbitals. This puts the system away from the tight-binding model, so that we adopt density functional theory for cold atoms, here extended to accommodate noncollinear spin structures emerging in the topological regime, to quantitatively demonstrate the phase transition to the topological Mott insulator.

  7. Temperature-driven band inversion in Pb0.77Sn0.23Se: Optical and Hall effect studies

    DOE PAGES

    Anand, Naween; Buvaev, Sanal; Hebard, A. F.; ...

    2014-12-23

    Optical and Hall-effect measurements have been performed on single crystals of Pb₀.₇₇Sn₀.₂₃Se, a IV-VI mixed chalcogenide. The temperature dependent (10–300 K) reflectance was measured over 40–7000 cm⁻¹ (5–870 meV) with an extension to 15,500 cm⁻¹ (1.92 eV) at room temperature. The reflectance was fit to the Drude-Lorentz model using a single Drude component and several Lorentz oscillators. The optical properties at the measured temperatures were estimated via Kramers-Kronig analysis as well as by the Drude-Lorentz fit. The carriers were p-type with the carrier density determined by Hall measurements. A signature of valence intraband transition is found in the low-energy opticalmore » spectra. It is found that the valence-conduction band transition energy as well as the free carrier effective mass reach minimum values at 100 K, suggesting temperature-driven band inversion in the material. Thus, density function theory calculation for the electronic band structure also make similar predictions.« less

  8. 20Hz membrane potential oscillations are driven by synaptic inputs in collision-detecting neurons in the frog optic tectum.

    PubMed

    Baranauskas, Gytis; Svirskiene, Natasa; Svirskis, Gytis

    2012-10-24

    Although the firing patterns of collision-detecting neurons have been described in detail in several species, the mechanisms generating responses in these neurons to visual objects on a collision course remain largely unknown. This is partly due to the limited number of intracellular recordings from such neurons, particularly in vertebrate species. By employing patch recordings in a novel integrated frog eye-tectum preparation we tested the hypothesis that OFF retinal ganglion cells were driving the responses to visual objects on a collision course in the frog optic tectum neurons. We found that the majority (22/26) of neurons in layer 6 responding to visual stimuli fitted the definition of η class collision-detectors: they readily responded to a looming stimulus imitating collision but not a receding stimulus (spike count difference ∼10 times) and the spike firing rate peaked after the stimulus visual angle reached a threshold value of ∼20-45°. In the majority of these neurons (15/22) a slow frequency oscillation (f=∼20Hz) of the neuronal membrane potential could be detected in the responses to a simulated collision stimulus, as well as to turning off the lights. Since OFF retinal ganglion cells could produce such oscillations, our observations are in agreement with the hypothesis that 'collision' responses in the frog optic tectum neurons are driven by synaptic inputs from OFF retinal ganglion cells.

  9. Acoustic-excitonic effects in a two-dimensional gas of dipolar excitons

    NASA Astrophysics Data System (ADS)

    Boev, M. V.; Kovalev, V. M.; Chaplik, A. V.

    2016-08-01

    The theory of the interaction of a two-dimensional gas of indirect dipolar excitons with Rayleigh surface elastic waves has been developed. The absorption and renormalization of the phase velocity of a surface wave, as well as the drag of excitons by the surface acoustic wave and the generation of bulk acoustic waves by a twodimensional gas of dipolar excitons irradiated by external electromagnetic radiation, have been considered. These effects have been studied both in a normal phase at high temperatures and in a condensed phase of the exciton gas. The calculations have been performed in the ballistic and diffusion limits for both phases.

  10. Light-hole exciton in a nanowire quantum dot

    NASA Astrophysics Data System (ADS)

    Jeannin, Mathieu; Artioli, Alberto; Rueda-Fonseca, Pamela; Bellet-Amalric, Edith; Kheng, Kuntheak; André, Régis; Tatarenko, Serge; Cibert, Joël; Ferrand, David; Nogues, Gilles

    2017-01-01

    Quantum dots inserted inside semiconductor nanowires are extremely promising candidates as building blocks for solid-state-based quantum computation and communication. They provide very high crystalline and optical properties and offer a convenient geometry for electrical contacting. Having a complete determination and full control of their emission properties is one of the key goals of nanoscience researchers. Here we use strain as a tool to create in a single magnetic nanowire quantum dot a light-hole exciton, an optically active quasiparticle formed from a single electron bound to a single light hole. In this frame, we provide a general description of the mixing within the hole quadruplet induced by strain or confinement. A multi-instrumental combination of cathodoluminescence, polarization-resolved Fourier imaging, and magneto-optical spectroscopy, allows us to fully characterize the hole ground state, including its valence band mixing with heavy-hole states.

  11. Excitons in solids with non-empirical hybrid time-dependent density-functional theory

    NASA Astrophysics Data System (ADS)

    Ullrich, Carsten; Yang, Zeng-Hui; Sottile, Francesco

    2015-03-01

    The Bethe-Salpeter equation (BSE) accurately describes the optical properties of solids, but is computationally expensive. Time-dependent density-functional theory (TDDFT) is more efficient, but standard functionals do not produce excitons in extended systems. We present a new, non-empirical hybrid TDDFT approach whose computational cost is much less than BSE, while the accuracy for both bound excitons and the continuum spectra is comparable to that of the BSE. Good performance is observed for both small-gap semiconductors and large-gap insulators. Work supported by NSF Grant DMR-1408904.

  12. Excitons in solids with time-dependent density-functional theory: the bootstrap kernel and beyond

    NASA Astrophysics Data System (ADS)

    Byun, Young-Moo; Yang, Zeng-Hui; Ullrich, Carsten

    Time-dependent density-functional theory (TDDFT) is an efficient method to describe the optical properties of solids. Lately, a series of bootstrap-type exchange-correlation (xc) kernels have been reported to produce accurate excitons in solids, but different bootstrap-type kernels exist in the literature, with mixed results. In this presentation, we reveal the origin of the confusion and show a new empirical TDDFT xc kernel to compute excitonic properties of semiconductors and insulators efficiently and accurately. Our method can be used for high-throughput screening calculations and large unit cell calculations. Work supported by NSF Grant DMR-1408904.

  13. Plasmonic-exciton coupling in synthesized metal/semiconductor hybrid nanocomposites

    SciTech Connect

    Gadalla, A.; Hamad, D. A.; Mohamed, M. B.

    2015-12-31

    A new method has been developed to grow plasmonic semiconductor nanocomposites of Au/CdSe and Ag/CdSe. Their chemical composition and crystal structure are determined by X-ray diffraction. The collective optical properties of the prepared semiconductor nanohybrid have been measured using spectrophotometer techniques and compared to those of the individual components. The electron transfer processes from CdSe to the gold are faster than that of the silver. Au/CdSe has a strong plasmonic-excitonic coupling, but Ag/CdSe has a weak plasmonic-excitonic coupling.

  14. Type II InAs/GaAsSb quantum dots: Highly tunable exciton geometry and topology

    SciTech Connect

    Llorens, J. M.; Wewior, L.; Cardozo de Oliveira, E. R.; Alén, B.; Ulloa, J. M.; Utrilla, A. D.; Guzmán, A.; Hierro, A.

    2015-11-02

    External control over the electron and hole wavefunctions geometry and topology is investigated in a p-i-n diode embedding a dot-in-a-well InAs/GaAsSb quantum structure with type II band alignment. We find highly tunable exciton dipole moments and largely decoupled exciton recombination and ionization dynamics. We also predicted a bias regime where the hole wavefunction topology changes continuously from quantum dot-like to quantum ring-like as a function of the external bias. All these properties have great potential in advanced electro-optical applications and in the investigation of fundamental spin-orbit phenomena.

  15. Anomalous Metal Phase Emergent on the Verge of an Exciton Mott Transition

    NASA Astrophysics Data System (ADS)

    Sekiguchi, Fumiya; Mochizuki, Toshimitsu; Kim, Changsu; Akiyama, Hidefumi; Pfeiffer, Loren N.; West, Ken W.; Shimano, Ryo

    2017-02-01

    We investigate the exciton Mott transition (EMT) by using optical pump-terahertz probe spectroscopy on GaAs, with realizing the condition of Mott's gedanken experiment by the resonant excitation of 1 s excitons. We show that an anomalous metallic phase emerges on the verge of the EMT as manifested by a peculiar enhancement of the quasiparticle mass and scattering rate. From the temperature and density dependence, the observed anomaly is shown to originate from the electron-hole (e -h ) correlation which becomes prominent at low temperatures, possibly suggesting a precursor of e -h Cooper pairing.

  16. Injected-charge-driven increase in electro-optic effect of KTN crystals

    SciTech Connect

    Toyoda, Seiji; Imai, Tadayuki; Miyazu, Jun; Okabe, Yuichi; Ueno, Masahiro; Kobayashi, Junya

    2014-05-15

    We report a significant increase in the electro-optic (EO) effect of KTa{sub x}Nb{sub 1-x}O{sub 3} (KTN) crystals that we achieved by injecting carriers into them. The dielectric constant of KTN was increased approximately twofold by carrier injection. The EO beam scanning performance was effectively improved by the increase in the EO effect resulting from the increased dielectric constant. The estimated densities of the trapped electrons were as small as 5.8 × 10{sup 20}m{sup -3}. The very small quantity of injected electrons greatly affected the dielectric constant and EO effect of the KTN crystals.

  17. Phase conjugation and adiabatic mode conversion in a driven optical parametric oscillator with orbital angular momentum

    SciTech Connect

    Coutinho dos Santos, B.; Souza, C. E. R.; Dechoum, K.; Khoury, A. Z.

    2007-11-15

    We developed a theoretical model for the spatial mode dynamics of an optical parametric oscillator under injection of orbital angular momentum. This process is interpreted in terms of a Poincare representation of first order spatial modes. The spatial properties of the down-converted fields can be easily understood from their symmetries in this geometric representation. By considering an adiabatic mode conversion of the injected signal, we calculate the evolution of the down-converted beams. A phase conjugation effect is predicted which is a consequence of the symmetry in the Poincare sphere. We also propose an experiment to measure this effect.

  18. Phase Transition of Bosons Driven by a Staggered Gauge Field in AN Optical Lattice

    NASA Astrophysics Data System (ADS)

    Cha, Min-Chul

    2013-06-01

    We have studied the ground state properties of hard-core bosons in a two-leg optical ladder in the presence of uniform and staggered frustrations due to an artificial gauge field. By calculating the ground state via the Lanczos method, we find first-order phase transitions tuned by the staggered gauge field between the Meissner and the vortex states. The momentum distributions show that the Meissner state has edge and staggered currents, while the vortex states have vortex-solid or vortex-glass phases in the presence of a staggered field.

  19. Seeing Many-Body Effects in Single- and Few-Layer Graphene: Observation of Two-Dimensional Saddle-Point Excitons

    DTIC Science & Technology

    2010-01-01

    of the Fermi velocity [3], and of plasmarons [4]. The role of electron-hole interactions, as are relevant for the optical response, has remained...somewhat paradoxical . On the one hand, theoretical studies have predicted strong excitonic corrections to the optical response [5-9], as well as BCS...layer and from the increased density of states at the Fermi energy [31]. Thus, excitonic effects are expected to diminish with increasing layer number

  20. Measuring Exciton Migration in Conjugated Polymer Films with Ultrafast Time Resolved Stimulated Emission Depletion Microscopy

    NASA Astrophysics Data System (ADS)

    Penwell, Samuel

    Conjugated polymers are highly tunable organic semiconductors, which can be solution processed to form thin films, making them prime candidates for organic photovoltaic devices. One of the most important parameters in a conjugated polymer solar cell is the exciton diffusion length, which depends on intermolecular couplings, and is typically on the order of 10 nm. This mean exciton migration can vary dramatically between films and within a single film due to heterogeneities in morphology on length scales of 10's to 100's nm. To study the variability of exciton diffusion and morphology within individual conjugated polymer films, we are adapting stimulated emission depletion (STED) microscopy. STED is typically used in biology with sparse well-engineered fluorescent labels or on NV-centers in diamond. I will, however, describe how we have demonstrated the extension of STED to conjugated polymer films and nanoparticles of MEH-PPV and CN-PPV, despite the presence of two photon absorption, by taking care to first understand the material's photophysical properties. We then further adapt this approach, by introducing a second ultrafast STED pulse at a variable delay. Excitons that migrate away from the initial subdiffraction excitation volume during the ps-ns time delay, are preferentially quenched by the second STED pulse, while those that remain in the initial volume survive. The resulting effect of the second STED pulse is modulated by the degree of migration over the ultrafast time delay, thus providing a new method to study exciton migration. Since this technique utilizes subdiffraction optical excitation and detection volumes with ultrafast time resolution, it provides a means of spatially and temporally resolving measurements of exciton migration on the native length and time scales. In this way, we will obtain a spatiotemporal map of exciton distributions and migration that will help to correlate the energetic landscape to film morphology at the nanoscale.

  1. Large-scale coherence of the bose condensate of spatially indirect excitons

    NASA Astrophysics Data System (ADS)

    Gorbunov, A. V.; Timofeev, V. B.

    2006-11-01

    The Bose condensation of spatially indirect (dipolar) excitons in a wide single quantum well in an electric field transverse to the heterolayers is analyzed. Voltage is applied between a metallic film on the surface (Schottky gate) and a conducting electron layer inside a heterostructure (integrated electrode). The excitation of dipolar excitons and observation of their luminescence are performed through circle windows in a metallic mask 5 μm in diameter. Excitons are collected in a ring lateral trap, which is formed along the window perimeter owing to the strongly inhomogeneous electric field. When the critical condensation conditions in pump and temperature are reached, a narrow line of dipolar excitons corresponding to the exciton condensate appears stepwise in the luminescence spectrum. Under these conditions, a spatially periodic structure of equidistant luminescence spots appears in the luminescence pattern that is observed through a window with a resolution of about 1 μm and is selected by means of an interference filter. An in situ optical Fourier transform of spatially periodic structures from the real space to the k space is derived. The resulting Fourier transforms reproducing the pattern of the luminescence intensity distribution in the far field exhibit the result of the destructive and constructive interference, as well as the fact that the luminescence is directed along the normal to the heterolayers. These results are consequences of the large-scale coherence of the condensed exciton state in the ring lateral trap. Direct measurements of double-beam interference from pairs of luminescence spots in the ring show that the spatial coherence length is no less than 4 μm. Such a large scale means that the experimentally observed periodic luminescence structures are described by a common wavefunction under the condition of the Bose condensation of dipolar excitons.

  2. Tapered Glass-Fiber Microspike: High-Q Flexural Wave Resonator and Optically Driven Knudsen Pump

    NASA Astrophysics Data System (ADS)

    Pennetta, Riccardo; Xie, Shangran; Russell, Philip St. J.

    2016-12-01

    Appropriately designed optomechanical devices are ideal for making ultra-sensitive measurements. Here we report a fused-silica microspike that supports a flexural resonance with a quality factor greater than 100 000 at room temperature in vacuum. Fashioned by tapering single-mode fiber (SMF), it is designed so that the core-guided optical mode in the SMF evolves adiabatically into the fundamental mode of the air-glass waveguide at the tip. The very narrow mechanical linewidth (20 mHz) makes it possible to measure extremely small changes in resonant frequency. In a vacuum chamber at low pressure, the weak optical absorption of the glass is sufficient to create a temperature gradient along the microspike, which causes it to act as a microscopic Knudsen pump, driving a flow of gas molecules towards the tip where the temperature is highest. The result is a circulating molecular flow within the chamber. Momentum exchange between the vibrating microspike and the flowing molecules causes an additional restoring force that can be measured as a tiny shift in the resonant frequency. The effect is strongest when the mean free path of the gas molecules is comparable with the dimensions of the vacuum chamber. The system offers a novel means of monitoring the behavior of weakly absorbing optomechanical sensors operating in vacuum.

  3. Probing dark exciton diffusion using photovoltage

    PubMed Central

    Mullenbach, Tyler K.; Curtin, Ian J.; Zhang, Tao; Holmes, Russell J.

    2017-01-01

    The migration of weakly and non-luminescent (dark) excitons remains an understudied subset of exciton dynamics in molecular thin films. Inaccessible via photoluminescence, these states are often probed using photocurrent methods that require efficient charge collection. Here we probe exciton harvesting in both luminescent and dark materials using a photovoltage-based technique. Transient photovoltage permits a real-time measurement of the number of charges in an organic photovoltaic cell, while avoiding non-geminate recombination losses. The extracted exciton diffusion lengths are found to be similar to those determined using photocurrent. For the luminescent material boron subphthalocyanine chloride, the photovoltage determined diffusion length is less than that extracted from photoluminescence. This indicates that while photovoltage circumvents non-geminate losses, geminate recombination at the donor–acceptor interface remains the primary recombination pathway. Photovoltage thus offers a general approach for extracting a device-relevant diffusion length, while also providing insight in to the dominant carrier recombination pathways. PMID:28128206

  4. Probing dark exciton diffusion using photovoltage

    NASA Astrophysics Data System (ADS)

    Mullenbach, Tyler K.; Curtin, Ian J.; Zhang, Tao; Holmes, Russell J.

    2017-01-01

    The migration of weakly and non-luminescent (dark) excitons remains an understudied subset of exciton dynamics in molecular thin films. Inaccessible via photoluminescence, these states are often probed using photocurrent methods that require efficient charge collection. Here we probe exciton harvesting in both luminescent and dark materials using a photovoltage-based technique. Transient photovoltage permits a real-time measurement of the number of charges in an organic photovoltaic cell, while avoiding non-geminate recombination losses. The extracted exciton diffusion lengths are found to be similar to those determined using photocurrent. For the luminescent material boron subphthalocyanine chloride, the photovoltage determined diffusion length is less than that extracted from photoluminescence. This indicates that while photovoltage circumvents non-geminate losses, geminate recombination at the donor-acceptor interface remains the primary recombination pathway. Photovoltage thus offers a general approach for extracting a device-relevant diffusion length, while also providing insight in to the dominant carrier recombination pathways.

  5. Excitonic susceptibility in near triangular quantum wells

    NASA Astrophysics Data System (ADS)

    Anitha, A.; Arulmozhi, M.

    2017-03-01

    Diamagnetic susceptibility and binding energy of an exciton in a near triangular quantum well, with potential profile proportional to |z|2/3 composed of GaAs/Ga1- x Al x As and ZnO/Zn1- x Mg x O are calculated as a function of the wellwidth and concentration of Al and Mg respectively varying the magnetic field applied along growth direction (i.e. z-axis). Diamagnetic susceptibility of light hole exciton and heavy hole exciton, shows inverse behaviors in the two materials below 20 nm wellwidth and the binding energy of both excitons increases, as the magnetic field increases. The results obtained, are compared with those of quantum wells with varied potential profiles and the experimental results reported in the literature.

  6. Static and dynamic characterization of AlN-driven microcantilevers using optical interference microscopy

    NASA Astrophysics Data System (ADS)

    Krupa, Katarzyna; Józwik, Michał; Gorecki, Christophe; Andrei, Alexandru; Nieradko, Łukasz; Delobelle, Patrick; Hirsinger, Laurent

    2009-02-01

    We have developed an optomechanical methodology, combining interferometric deflection data, the nanoindentation technique and analytical modeling to perform the characterization of piezoelectrically driven microcantilevers operating as MEMS actuators. Here, the association of standard Twyman-Green interferometry (TGI) with time averaged and stroboscopic techniques permits the evaluation of the 3-D out-of-plane deflections of microdevices and provides feedback of measurements that helps us to optimize MEMS structures and improve the reliability and stability of microcantilevers. The goal of the presented study was investigation of high-quality cantilevers composed from silicon beam and a transducer including the aluminum nitride (AlN) layer. It is a material with piezoelectric properties, which can be an alternative for PZT films in micromachining technology. After presenting the fabrication process of the testing devices, the rest of the paper will focus on non-contact measurements of cantilevers deflection by interferometry: static data (e.g., initial shape, deformation, stress) and dynamic parameters of samples (e.g., resonance frequency and amplitude distributions in vibration modes). On the basis of these experimental data, parameters such as piezoelectric coefficient d31 have been calculated taking into account multiple film stacking.

  7. Dynamic Stark effect, light emission, and entanglement generation in a laser-driven quantum optical system

    NASA Astrophysics Data System (ADS)

    Pagel, D.; Alvermann, A.; Fehske, H.

    2017-01-01

    We calculate the emission spectra, the Glauber g(2 ) function, and the entanglement of formation for two-level emitters coupled to a single cavity mode and subject to an external laser excitation. To evaluate these quantities we couple the system to environmental degrees of freedom, which leads to dissipative dynamics. Because of the periodic time dependence of the system Hamiltonian, the coefficients of the Markovian master equation are constant only if Floquet states are used as the computational basis. Studying the emission spectra, we show that the dynamic Stark effect first appears in second order of the laser intensity. For the Glauber function, we find clearly distinguished parameter regimes of super- and sub-Poissonian light emission and explain the additional features appearing for finite laser intensity in terms of the quasienergy spectrum of the driven emitter-cavity system. Finally, we analyze the temperature and emitter-cavity-coupling regimes where entanglement among the emitters is generated and show that the laser excitation leads to a decrease of entanglement.

  8. Magnetically driven middle ear ossicles for optical measurement of vibrations in an ear with opened eardrum

    NASA Astrophysics Data System (ADS)

    Peacock, John; Von Unge, Magnus; Dirckx, Joris

    2012-06-01

    Vibrations of the middle ear ossicles are easily measured by means of laser vibrometry. However, exposing the ossicles requires the removal of the eardrum, with the result that the ossicles can no longer be stimulated acoustically. To overcome this we devised a new set up in which the ossicles can be driven magnetically. After measuring the response of the eardrum to an acoustic signal, we then remove the eardrum and attach a small magnet to the exposed manubrium (the part of the first auditory ossicle, the malleus, which is normally attached to the eardrum). An electromagnetic excitation coil is then used to drive the magnet, and the output to the coil adjusted until the vibration of the manubrium, as measured by the vibrometer, matches that measured in response to the acoustic signal. Such a set-up has uses in research on middle ear mechanics, such as the measurement of non-linearities in their response, as well as applications in the diagnosis of middle ear conditions such as the fixation of the ossicles by otosclerosis, or in chronic otitis media. We describe our set up in which the vibrometer unit is attached to a surgical microscope, offering accurate positioning of the laser beam. We discuss the viability of our method and its future potential by presenting some measurements on artificially fixated ears.

  9. Exciton-polariton wakefields in semiconductor microcavities

    NASA Astrophysics Data System (ADS)

    Terças, H.; Mendonça, J. T.

    2016-02-01

    We consider the excitation of polariton wakefields due to a propagating light pulse in a semiconductor microcavity. We show that two kinds of wakes are possible, depending on the constituents fraction (either exciton or photon) of the polariton wavefunction. The nature of the wakefields (pure excitonic or polaritonic) can be controlled by changing the speed of propagation of the external pump. This process could be used as a diagnostic for the internal parameters of the microcavity.

  10. Exciton properties of selected aromatic hydrocarbon systems

    NASA Astrophysics Data System (ADS)

    Roth, Friedrich; Mahns, Benjamin; Hampel, Silke; Nohr, Markus; Berger, Helmuth; Büchner, Bernd; Knupfer, Martin

    2013-02-01

    We have examined the singlet excitons in two representatives of acene-type (tetracene and pentacene) and phenacene-type (chrysene and picene) molecular crystals, respectively, using electron energy-loss spectroscopy at low temperatures. We show that the excitation spectra of the two hydrocarbon families significantly differ. Moreover, close inspection of the data indicates that there is an increasing importance of charge-transfer excitons at lowest excitation energy with increasing length of the molecules.

  11. Ultrafast exciton dynamics at molecular surfaces

    NASA Astrophysics Data System (ADS)

    Monahan, Nicholas R.

    Further improvements to device performance are necessary to make solar energy conversion a compelling alternative to fossil fuels. Singlet exciton fission and charge separation are two processes that can heavily influence the power conversion efficiency of a solar cell. During exciton fission one singlet excitation converts into two triplet excitons, potentially doubling the photocurrent generated by higher energy photons. There is significant discord over the singlet fission mechanism and of particular interest is whether the process involves a multiexciton intermediate state. I used time-resolved two-photon photoemission to investigate singlet fission in hexacene thin films, a model system with strong electronic coupling. My results indicate that a multiexciton state forms within 40 fs of photoexcitation and loses singlet character on a 280 fs timescale, creating two triplet excitons. This is concordant with the transient absorption spectra of hexacene single crystals and definitively proves that exciton fission in hexacene proceeds through a multiexciton state. This state is likely common to all strongly-coupled systems and my results suggest that a reassessment of the generally-accepted singlet fission mechanism is required. Charge separation is the process of splitting neutral excitons into carriers that occurs at donor-acceptor heterojunctions in organic solar cells. Although this process is essential for device functionality, there are few compelling explanations for why it is highly efficient in certain organic photovoltaic systems. To investigate the charge separation process, I used the model system of charge transfer excitons at hexacene surfaces and time-resolved two-photon photoemission. Charge transfer excitons with sufficient energy spontaneously delocalize, growing from about 14 nm to over 50 nm within 200 fs. Entropy drives this delocalization, as the density of states within the Coulomb potential increases significantly with energy. This charge

  12. Atypical Exciton-Phonon Interactions in WS2 and WSe2 Monolayers Revealed by Resonance Raman Spectroscopy.

    PubMed

    Del Corro, E; Botello-Méndez, A; Gillet, Y; Elias, A L; Terrones, H; Feng, S; Fantini, C; Rhodes, Daniel; Pradhan, N; Balicas, L; Gonze, X; Charlier, J-C; Terrones, M; Pimenta, M A

    2016-04-13

    Resonant Raman spectroscopy is a powerful tool for providing information about excitons and exciton-phonon coupling in two-dimensional materials. We present here resonant Raman experiments of single-layered WS2 and WSe2 using more than 25 laser lines. The Raman excitation profiles of both materials show unexpected differences. All Raman features of WS2 monolayers are enhanced by the first-optical excitations (with an asymmetric response for the spin-orbit related XA and XB excitons), whereas Raman bands of WSe2 are not enhanced at XA/B energies. Such an intriguing phenomenon is addressed by DFT calculations and by solving the Bethe-Salpeter equation. These two materials are very similar. They prefer the same crystal arrangement, and their electronic structure is akin, with comparable spin-orbit coupling. However, we reveal that WS2 and WSe2 exhibit quite different exciton-phonon interactions. In this sense, we demonstrate that the interaction between XC and XA excitons with phonons explains the different Raman responses of WS2 and WSe2, and the absence of Raman enhancement for the WSe2 modes at XA/B energies. These results reveal unusual exciton-phonon interactions and open new avenues for understanding the two-dimensional materials physics, where weak interactions play a key role coupling different degrees of freedom (spin, optic, and electronic).

  13. Relaxation of hot excitons in inhomogeneously broadened CdxZn1-xSe/ZnSe nanostructures

    NASA Astrophysics Data System (ADS)

    Bacher, G.; Spiegel, R.; Kümmell, T.; Breitwieser, O.; Forchel, A.; Jobst, B.; Hommel, D.; Landwehr, G.

    1997-09-01

    The dimensionality dependence of hot exciton relaxation was studied by means of photoluminescence (PL) spectroscopy comparing CdxZn1-xSe/ZnSe quantum wells and quantum dots. In both systems, an efficient population of radiative excitonic ground states via longitudinal-optical (LO) phonon scattering is observed for an excess energy of the exciting laser corresponding to an integer number of LO-phonon energies. Thus, only selected areas of the inhomogeneously broadened systems contribute to the PL spectrum immediately after the exciting laser pulse. This results in a drastical reduction of the PL linewidth. In quantum wells, the redistribution of excitons occurs due to acoustic phonon assisted migration within the CdxZn1-xSe/ZnSe layer. In contrast, this process is found to be substantially suppressed in quantum dots due to the three-dimensional exciton confinement.

  14. Finite-difference time-domain analysis for the dynamics and diffraction of exciton-polaritons.

    PubMed

    Chen, Minfeng; Chang, Yia-Chung; Hsieh, Wen-Feng

    2015-10-01

    We adopted a finite-difference time-domain (FDTD) scheme to simulate the dynamics and diffraction of exciton-polaritons, governed by the coupling of polarization waves with electromagnetic waves. The polarization wave, an approximate solution to the Schrödinger's equation at low frequencies, essentially captures the exciton behavior. Numerical stability of the scheme is analyzed and simple examples are provided to prove its validity. The system considered is both temporally and spatially dispersive, for which the FDTD analysis has attracted less attention in the literature. Here, we demonstrate that the FDTD scheme could be useful for studying the optical response of the exciton-polariton and its dynamics. The diffraction of a polariton wave from a polaritonic grating is also considered, and many sharp resonances are found, which manifest the interference effect of polariton waves. This illustrates that the measurement of transmittance or reflectance near polariton resonance can reveal subwavelength features in semiconductors, which are sensitive to polariton scattering.

  15. Excitonic spectra and energy band structure of ZnAl2Se4 crystals

    NASA Astrophysics Data System (ADS)

    Syrbu, N. N.; Zalamai, V. V.; Tiron, A. V.; Tiginyanu, I. M.

    2015-11-01

    Absorption, reflection and wavelength modulated reflection spectra were investigated in ZnAl2Se4 crystals. The energy positions of ground and excited states for three excitonic series (А, В and С) were determined. The main parameters of excitons and more precise values of energy intervals V1(Γ7)-C1(Γ6), V2(Γ6)-C1(Γ6), and V3(Γ7)-C1(Γ6) were estimated. Values of splitting due to crystal field and spin-orbital interaction were calculated. Effective masses of electrons (mC1∗) and holes (mV1∗, mV2∗, mV3∗) were estimated. Reflection spectra contours in excitonic region were calculated using dispersion equations. Optical functions for E > Eg from measured reflection spectra were assigned on the base of Kramers-Kronig relations.

  16. 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.

  17. Probing ultrafast energy transfer between excitons and plasmons in the ultrastrong coupling regime

    NASA Astrophysics Data System (ADS)

    Balci, Sinan; Kocabas, Coskun; Küçüköz, Betül; Karatay, Ahmet; Akhüseyin, Elif; Gul Yaglioglu, H.; Elmali, Ayhan

    2014-08-01

    We investigate ultrafast energy transfer between excitons and plasmons in ensembles of core-shell type nanoparticles consisting of metal core covered with a concentric thin J-aggregate (JA) shell. The high electric field localization by the Ag nanoprisms and the high oscillator strength of the JAs allow us to probe this interaction in the ultrastrong plasmon-exciton coupling regime. Linear and nonlinear optical properties of the coupled system have been measured using transient absorption spectroscopy revealing that the hybrid system shows half-plasmonic and half-excitonic properties. The tunability of the nanoprism plasmon resonance provides a flexible platform to study the dynamics of the hybrid state in a broad range of wavelengths.

  18. Semiconducting monolayer materials as a tunable platform for excitonic solar cells.

    PubMed

    Bernardi, Marco; Palummo, Maurizia; Grossman, Jeffrey C

    2012-11-27

    The recent advent of two-dimensional monolayer materials with tunable optical properties and high carrier mobility offers renewed opportunities for efficient, ultrathin excitonic solar cells alternative to those based on conjugated polymer and small molecule donors. Using first-principles density functional theory and many-body calculations, we demonstrate that monolayers of hexagonal BN and graphene (CBN) combined with commonly used acceptors such as PCBM fullerene or semiconducting carbon nanotubes can provide excitonic solar cells with tunable absorber gap, donor-acceptor interface band alignment, and power conversion efficiency, as well as novel device architectures. For the case of CBN-PCBM devices, we predict power conversion efficiency limits in the 10-20% range depending on the CBN monolayer structure. Our results demonstrate the possibility of using monolayer materials in tunable, efficient, ultrathin solar cells in which unexplored exciton and carrier transport regimes are at play.

  19. Excitonic effects in two-dimensional semiconductors: Path integral Monte Carlo approach

    SciTech Connect

    Velizhanin, Kirill A.; Saxena, Avadh

    2015-11-01

    The most striking features of novel two-dimensional semiconductors (e.g., transition metal dichalcogenide monolayers or phosphorene) is a strong Coulomb interaction between charge carriers resulting in large excitonic effects. In particular, this leads to the formation of multicarrier bound states upon photoexcitation (e.g., excitons, trions, and biexcitons), which could remain stable at near-room temperatures and contribute significantly to the optical properties of such materials. In our work we have used the path integral Monte Carlo methodology to numerically study properties of multicarrier bound states in two-dimensional semiconductors. Specifically, we have accurately investigated and tabulated the dependence of single-exciton, trion, and biexciton binding energies on the strength of dielectric screening, including the limiting cases of very strong and very weak screening. Our results of this work are potentially useful in the analysis of experimental data and benchmarking of theoretical and computational models.

  20. Excitonic effects in two-dimensional semiconductors: Path integral Monte Carlo approach

    DOE PAGES

    Velizhanin, Kirill A.; Saxena, Avadh

    2015-11-01

    The most striking features of novel two-dimensional semiconductors (e.g., transition metal dichalcogenide monolayers or phosphorene) is a strong Coulomb interaction between charge carriers resulting in large excitonic effects. In particular, this leads to the formation of multicarrier bound states upon photoexcitation (e.g., excitons, trions, and biexcitons), which could remain stable at near-room temperatures and contribute significantly to the optical properties of such materials. In our work we have used the path integral Monte Carlo methodology to numerically study properties of multicarrier bound states in two-dimensional semiconductors. Specifically, we have accurately investigated and tabulated the dependence of single-exciton, trion, and biexcitonmore » binding energies on the strength of dielectric screening, including the limiting cases of very strong and very weak screening. Our results of this work are potentially useful in the analysis of experimental data and benchmarking of theoretical and computational models.« less

  1. Cavity-less on-chip optomechanics using excitonic transitions in semiconductor heterostructures

    PubMed Central

    Okamoto, Hajime; Watanabe, Takayuki; Ohta, Ryuichi; Onomitsu, Koji; Gotoh, Hideki; Sogawa, Tetsuomi; Yamaguchi, Hiroshi

    2015-01-01

    The hybridization of semiconductor optoelectronic devices and nanomechanical resonators provides a new class of optomechanical systems in which mechanical motion can be coupled to light without any optical cavities. Such cavity-less optomechanical systems interconnect photons, phonons and electrons (holes) in a highly integrable platform, opening up the development of functional integrated nanomechanical devices. Here we report on a semiconductor modulation-doped heterostructure–cantilever hybrid system, which realizes efficient cavity-less optomechanical transduction through excitons. The opto-piezoelectric backaction from the bound electron–hole pairs enables us to probe excitonic transition simply with a sub-nanowatt power of light, realizing high-sensitivity optomechanical spectroscopy. Detuning the photon energy from the exciton resonance results in self-feedback cooling and amplification of the thermomechanical motion. This cavity-less on-chip coupling enables highly tunable and addressable control of nanomechanical resonators, allowing high-speed programmable manipulation of nanomechanical devices and sensor arrays. PMID:26477487

  2. Plasmon-excitonic scattering of light from a nanoparticle located near a quantum well

    NASA Astrophysics Data System (ADS)

    Kosobukin, V. A.

    2015-07-01

    A solution is presented for the problem of resonant elastic scattering of polarized light from a nanoparticle and a quantum well located near semiconductor surface. Coupling between surface plasmons of the metal particle and quasi-2D excitons of the quantum well is taken into account. The problem is solved by the Green's functions technique treating the resonant polarization response of particle and quantum well in a self-consistent approximation. The effective polarizability is found for a metal nanoparticle of ellipsoidal shape with account of dynamical effect of "image" charges caused by semiconductor surface and quantum well. Spectra are numerically calculated for a model structure "metal-semiconductor" including a silver nanoparticle and a quantum well AlGaAs/GaAs. Appearance of exciton-plasmon interaction in the resonant scattering of light is interpreted as an enhancement by surface plasmons of the optical response due to quantum-well quasi-2D excitons.

  3. An exciton-polariton laser based on biologically produced fluorescent protein

    PubMed Central

    Dietrich, Christof P.; Steude, Anja; Tropf, Laura; Schubert, Marcel; Kronenberg, Nils M.; Ostermann, Kai; Höfling, Sven; Gather, Malte C.

    2016-01-01

    Under adequate conditions, cavity polaritons form a macroscopic coherent quantum state, known as polariton condensate. Compared to Wannier-Mott excitons in inorganic semiconductors, the localized Frenkel excitons in organic emitter materials show weaker interaction with each other but stronger coupling to light, which recently enabled the first realization of a polariton condensate at room temperature. However, this required ultrafast optical pumping, which limits the applications of organic polariton condensates. We demonstrate room temperature polariton condensates of cavity polaritons in simple laminated microcavities filled with biologically produced enhanced green fluorescent protein (eGFP). The unique molecular structure of eGFP prevents exciton annihilation even at high excitation densities, thus facilitating polariton condensation under conventional nanosecond pumping. Condensation is clearly evidenced by a distinct threshold, an interaction-induced blueshift of the condensate, long-range coherence, and the presence of a second threshold at higher excitation density that is associated with the onset of photon lasing. PMID:27551686

  4. Long-range excitons in conjugated polymers with ring torsions: poly( para-phenylene) and polyaniline

    NASA Astrophysics Data System (ADS)

    Harigaya, Kikuo

    1998-08-01

    Ring torsion effects on the optical excitation properties of poly( para-phenylene) (PPP) and polyaniline (PAN) are investigated by extending the Shimoi-Abe model (Shimoi Y and Abe S 1996 Synth. Met. 78 219). The model is solved using the intermediate-exciton formalism. Long-range excitons are characterized, and the long-range component of the oscillator strengths is calculated. We find that ring torsions affect the long-range excitons in PAN more readily than those in PPP, due to the larger torsion angle of PAN and the large number of bonds whose hopping integrals are modulated by torsions. Next, ring torsional disorder effects simulated by the Gaussian distribution function are analysed. The long-range component of the total oscillator strengths after sample averaging is nearly independent of the disorder strength in the PPP case, while that in the PAN case decreases readily as the disorder becomes stronger.

  5. Tiny endoscopic optical coherence tomography probe driven by a miniaturized hollow ultrasonic motor

    NASA Astrophysics Data System (ADS)

    Chen, Tianyuan; Zhang, Ning; Huo, Tiancheng; Wang, Chengming; Zheng, Jing-gao; Zhou, Tieying; Xue, Ping

    2013-08-01

    We present an endoscopic probe for optical coherence tomography (OCT) equipped with a miniaturized hollow ultrasonic motor that rotates the objective lens and provides an internal channel for the fiber to pass through, enabling 360 deg unobstructed circumferential scanning. This probe has an outer diameter of 1.5 mm, which is ultra-small for motorized probes with an unobstructed view in distal scanning endoscopic OCT. Instead of a mirror or prism, a customized aspheric right-angle lens is utilized, leading to an enlargement of the numerical aperture and thus high transverse resolution. Spectral-domain OCT imaging of bio-tissue and a phantom are demonstrated with resolution of 7.5 μm(axial)×6.6 μm(lateral) and sensitivity of 96 dB.

  6. Electro-optical properties of low viscosity driven holographic polymer dispersed liquid crystals

    NASA Astrophysics Data System (ADS)

    Moon, K. R.; Bae, S. Y.; Kim, B. K.

    2015-04-01

    Relative diffraction efficiency (RDE), operating voltage, and response times are most important performance characteristics of holographic polymer dispersed liquid crystals (HPDLC). Two types of triallyl isocyanurate (TI) having different structures were incorporated into the conventional transmission grating of HPDLC. Premix viscosity decreased by 13-18% with up to 3% TI, beyond which it increased. TI eliminated induction period and augmented initial grating formation rate at all contents. Saturation RDE increased over 200% while threshold voltage and rise time decreased to about half and 2/3, respectively up to 3% TI, beyond which the tendencies were reversed. Among the two TIs, low viscosity monomer (TA) showed high RDE, while high miscibility monomer (TE) low characteristic voltages and short response times. It is concluded that grating formation is largely favored by low viscosity, while interface tensions and electro-optical performances by miscibility at similar viscosities.

  7. Self-ordered stationary states of driven quantum degenerate gases in optical resonators

    NASA Astrophysics Data System (ADS)

    Sandner, R. M.; Niedenzu, W.; Piazza, F.; Ritsch, H.

    2015-09-01

    We study the role of quantum statistics in the self-ordering of ultracold bosons and fermions moving inside an optical resonator with transverse coherent pumping. For few particles we numerically compute the nonequilibrium dynamics of the density matrix towards the self-ordered stationary state of the coupled atom-cavity system. We include quantum fluctuations of the particles and the cavity field. These fluctuations in conjunction with cavity cooling determine the stationary distribution of the particles, which exhibits a transition from a homogeneous to a spatially ordered phase with the appearance of a superradiant scattering peak in the cavity output spectrum. While the ordering threshold is generally lower for bosons, we confirm the recently predicted zero pump strength threshold for superradiant scattering for fermions when the cavity photon momentum coincides with twice the Fermi momentum.

  8. The effective excitonic g factors of Mn-doped InAs nanowires

    NASA Astrophysics Data System (ADS)

    Xiong, Wen

    2017-04-01

    Based on the derived eight-band k · p Hamiltonian, the electronic structures of Mn-doped InAs nanowires in the magnetic field are calculated. We find the lowest optical transition will be split to four individual transitions when the magnetic field is applied along z axis, and two of them are σ polarized light. Furthermore, the Zeeman splitting energy at the Γ point of two σ polarized light will increase nonlinearly as the increase of the magnetic field. Additionally, an effective excitonic g factor at the Γ point is defined, and the effective excitonic g factors will decrease greatly with the increase of the radius of nanowires and the decrease of the concentration of manganese ions, while the effective excitonic g factors decrease slightly when the magnetic field increases. Interestingly, the effective excitonic g factors can experience a substantial decrease when the temperature increases from 10 K to 100 K and is almost not affected when the temperature varies from 100 K to 300 K. Therefore, we can infer that large effective excitonic g factors can be obtained when small radius of nanowires, high concentration of manganese ions and low temperature are satisfied.

  9. Intrinsic radiative lifetime derived via absorption cross section of one-dimensional excitons

    PubMed Central

    Chen, Shaoqiang; Yoshita, Masahiro; Ishikawa, Akira; Mochizuki, Toshimitsu; Maruyama, Shun; Akiyama, Hidefumi; Hayamizu, Yuhei; Pfeiffer, Loren N.; West, Ken W.

    2013-01-01

    Intrinsic radiative lifetime is an essential physical property of low-dimensional excitons that represents their optical transition rate and wavefunction, which directly measures the probability of finding an electron and a hole at the same position in an exciton. However, the conventional method that is used to determine this property via measuring the temperature-dependent photoluminescence (PL) decay time involves uncertainty due to various extrinsic contributions at high temperatures. Here, we propose an alternative method to derive the intrinsic radiative lifetime via temperature-independent measurement of the absorption cross section and transformation using Einstein's A-B-coefficient equations derived for low-dimensional excitons. We experimentally verified our approach for one-dimensional (1D) excitons in high-quality 14 × 6 nm2 quantum wires by comparing it to the conventional approach. Both independent evaluations showed good agreement with each other and with theoretical predictions. This approach opens a promising path to studying low-dimensional exciton physics. PMID:23736905

  10. Exciton dynamics in a site-controlled quantum dot coupled to a photonic crystal cavity

    NASA Astrophysics Data System (ADS)

    Jarlov, C.; Lyasota, A.; Ferrier, L.; Gallo, P.; Dwir, B.; Rudra, A.; Kapon, E.

    2015-11-01

    Exciton and cavity mode (CM) dynamics in site-controlled pyramidal quantum dots (QDs), integrated with linear photonic crystal membrane cavities, are investigated for a range of temperatures and photo-excitation power levels. The absence of spurious multi-excitonic effects, normally observed in similar structures based on self-assembled QDs, permits the observation of effects intrinsic to two-level systems embedded in a solid state matrix and interacting with optical cavity modes. The coupled exciton and CM dynamics follow the same trend, indicating that the CM is fed only by the exciton transition. The Purcell reduction of the QD and CM decay times is reproduced well by a theoretical model that includes exciton linewidth broadening and temperature dependent non-radiative processes, from which we extract a Purcell factor of 17 ± 5. For excitation powers above QD saturation, we show the influence of quantum wire barrier states at short delay time, and demonstrate the absence of multiexcitonic background emission.

  11. Measuring Exciton Diffusion in Conjugated Polymer Films with Super-resolution Microscopy

    NASA Astrophysics Data System (ADS)

    Penwell, Samuel; Ginsberg, Lucas; Noriega Manez, Rodrigo; Ginsberg, Naomi

    2015-03-01

    Conjugated polymers are highly tunable organic semiconductors, which can be solution processed to form thin films, making them prime candidates for organic photovoltaic devices. One of the most important parameters in a conjugated polymer solar cell is the exciton diffusion length, which depends on intermolecular couplings, and is typically on the order of 10 nm. This mean exciton migration can vary dramatically between films and within a single film due to heterogeneities in morphology on length scales of 10's to 100's nm. To study the variability of exciton diffusion and morphology within individual conjugated polymer films, we are adapting stimulated emission depletion microscopy. STED is typically used in biology with well-engineered fluorescent labels or on NV-centers in diamond. I will, however, describe how we have demonstrated STED in conjugated polymer films of MEH-PPV and CN-PPV by taking care to first understand the film's photophysical properties. This new approach provides a way to study exciton diffusion by utilizing subdiffraction optical excitation volumes. In this way, we will obtain a spatiotemporal map of exciton distributions that will help to correlate the energetic landscape to film morphology at the nanoscale. This research is supported in part by the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under Contract No. DE-AC05-06.

  12. Characterizing and tuning excitons in monolayer and few-layer MoS 2

    NASA Astrophysics Data System (ADS)

    Qiu, Diana Y.; da Jornada, Felipe H.; Louie, Steven G.

    2015-03-01

    We use the GW-BSE method to study excitons arising from transitions in different regions of momentum space in mono- and few-layer MoS2 and consider mechanisms to fundamentally change the features and character of the optical spectra. Our calculations show that sharp spatial variations in dielectric screening make 2D systems, such as MoS2 , computationally challenging, requiring very fine k-space sampling to resolve the structure of excitonic wave functions and converge binding energies. In highly converged calculations, we identify a series of excitons arising from transitions at the K/K' valleys in the Brillouin zone, a higher energy series arising from transitions in the valley of a Mexican hat potential centered at the Γ point, and transitions at the indirect gap from Γ to Λ in few-layer MoS2 . As layer number changes, these states, which have varying character, momentum-space structure and real-space locations, are affected differently by changes in confinement and hybridization. By tuning layer number and strain, we find that we not only can tune the excitation energies but can also change the relative energies of the various excitonic series, allowing for movement of the lowest energy exciton between different regions of the Brillouin zone This work was supported by NSF Grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231.

  13. Nonanalyticity, Valley Quantum Phases, and Massless Excitons in Monolayer Transition Metal Dichalcogenides

    NASA Astrophysics Data System (ADS)

    Qiu, Diana Y.; Cao, Ting; Louie, Steven G.

    Exciton dispersion as a function of center-of-mass momentum Q is essential to the understanding of exciton dynamics, relaxation, and condensation. We use the ab initio GW-Bethe-Salpeter equation(GW-BSE) method to calculate the dispersion of excitons in monolayer MoS2 and find a nonanalytic lightlike dispersion. This behavior arises from the interplay of an unusual | Q | -term in both the intra- and intervalley exchange of the electron-hole interaction, which concurrently gives rise to a valley quantum phase of winding number two. We have derived a simple, effective Hamiltonian and analytic solutions, which quantitatively describe this physics, and we predict that signatures of this unusual dispersion can be measured with a linearly polarized optical beam tilted away from normal incidence. The existence of a nonanalytic exciton dispersion can be generalized to other 2D semiconductors with excitons whose amplitudes are localized in a small region of the Brillioun zone. This work was supported by NSF Grant No. DMR15-1508412 and U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by NERSC and XSEDE.

  14. Modeling of multi-exciton transient absorption spectra of protochlorophyllide aggregates in aqueous solution.

    PubMed

    Sytina, Olga A; Novoderezhkin, Vladimir I; van Grondelle, Rienk; Groot, Marie Louise

    2011-11-03

    Protochlorophyllide (Pchlide) is a natural porphyrin, a precursor of chlorophyll, synthesized by plants for its photosynthetic apparatus. The pigment spontaneously forms aggregates when dissolved in neat water solution. We present here calculations of the transient absorption spectra and its comprising components (ground-state bleach, stimulated emission, and excited-state absorption) for a strongly excitonically coupled linear chain of four Pchlide chromophores, using exciton theory with phenomenological Gaussian line shapes and without energetic disorder. A refined multiexciton model that includes static disorder is applied to fit the experimental power-dependent transient absorption spectra of aqueous protochlorophyllide and the kinetics for delay times up to 20 ps after photoexcitation. We show that population up to the 4-exciton manifold is sufficient to explain the pronounced saturation of the bleaching and the shape changes in the instantaneous, t = 0.2 ps transient spectra when the pulse energy is increased from 10 to 430 nJ per pulse. The decay of the multiexciton manifold is relatively slow and is preceded by a spectroscopically distinct process. We suggest that the exciton states in the Pchlide aggregates are mixed with charge-transfer states (CTS) and that the population and repopulation of the CTS coupled to the exciton states explains the relatively slow decay of the multiexciton manifold. The relevance of our results to the optical properties and dynamics of natural photosynthetic complexes and the possible physical origin of CTS formation are discussed.

  15. Exciton dynamics in a site-controlled quantum dot coupled to a photonic crystal cavity

    SciTech Connect

    Jarlov, C. Lyasota, A.; Ferrier, L.; Gallo, P.; Dwir, B.; Rudra, A.; Kapon, E.

    2015-11-09

    Exciton and cavity mode (CM) dynamics in site-controlled pyramidal quantum dots (QDs), integrated with linear photonic crystal membrane cavities, are investigated for a range of temperatures and photo-excitation power levels. The absence of spurious multi-excitonic effects, normally observed in similar structures based on self-assembled QDs, permits the observation of effects intrinsic to two-level systems embedded in a solid state matrix and interacting with optical cavity modes. The coupled exciton and CM dynamics follow the same trend, indicating that the CM is fed only by the exciton transition. The Purcell reduction of the QD and CM decay times is reproduced well by a theoretical model that includes exciton linewidth broadening and temperature dependent non-radiative processes, from which we extract a Purcell factor of 17 ± 5. For excitation powers above QD saturation, we show the influence of quantum wire barrier states at short delay time, and demonstrate the absence of multiexcitonic background emission.

  16. Self-organization of plasmonic and excitonic nanoparticles into resonant chiral supraparticle assemblies.

    PubMed

    Hu, Tao; Isaacoff, Benjamin P; Bahng, Joong Hwan; Hao, Changlong; Zhou, Yunlong; Zhu, Jian; Li, Xinyu; Wang, Zhenlong; Liu, Shaoqin; Xu, Chuanlai; Biteen, Julie S; Kotov, Nicholas A

    2014-12-10

    Chiral nanostructures exhibit strong coupling to the spin angular momentum of incident photons. The integration of metal nanostructures with semiconductor nanoparticles (NPs) to form hybrid plasmon-exciton nanoscale assemblies can potentially lead to plasmon-induced optical activity and unusual chiroptical properties of plasmon-exciton states. Here we investigate such effects in supraparticles (SPs) spontaneously formed from gold nanorods (NRs) and chiral CdTe NPs. The geometry of this new type of self-limited nanoscale superstructures depends on the molar ratio between NRs and NPs. NR dimers surrounded by CdTe NPs were obtained for the ratio NR/NP = 1:15, whereas increasing the NP content to a ratio of NR/NP = 1:180 leads to single NRs in a shell of NPs. The SPs based on NR dimers exhibit strong optical rotatory activity associated in large part with their twisted scissor-like geometry. The preference for a specific nanoscale enantiomer is attributed to the chiral interactions between CdTe NP in the shell. The SPs based on single NRs also yield surprising chiroptical activity at the frequency of the longitudinal mode of NRs. Numerical simulations reveal that the origin of this chiroptical band is the cross talk between the longitudinal and the transverse plasmon modes, which makes both of them coupled with the NP excitonic state. The chiral SP NR-NP assemblies combine the optical properties of excitons and plasmons that are essential for chiral sensing, chiroptical memory, and chiral catalysis.

  17. Exploring ultrafast dynamics of excitons and multiexcitons in "giant" nanocrystal quantum dots

    NASA Astrophysics Data System (ADS)

    Sampat, Siddharth

    In this work, we have performed extensive time resolved photoluminescence (PL) studies to further the understanding of charge dynamics in semiconductor nanocrystal quantum dots (QDs). Recent developments in QD synthesis have introduced a new set of QD known as "giant" quantum dots (gQDs) that consist of a CdSe core coated with up to 19 monolayers of a CdS shell. The thick shell layer is grown using a SILAR method resulting in a defect free, alloyed CdSe/CdS interface. This has been attributed to gQDs exhibiting excellent optical properties such as high excitonic quantum yield (QY), prolonged photostability and inhibition of flourescence intermittency ("blinking"), which is regularly observed in conventional QDs. In gQDs, however, owing to unique fabrication methods and material selection, the Auger process is strongly suppressed resulting in efficient radiative recombination of photogenerated excitons as well as high PL QY of charged excitonic and multiexcitonic species. We perform extensive single gQDs studies that establish the role played by gQD shell thickness and core size in governing their optical properties. It is found that both the core and shell dimensions can be tuned in order to achieve the smallest gQDs with the highest vii Auger suppression resulting in photostable dots with high QYs. Next, we perform a study of multiexcitonic species in gQDs that are encapsulated in an insulating SiO2shell. These silica-coated gQDs exhibit strong PL from charged excitons, biexcitons as well as triexcitons. This observation has led to an accurate description of excitonic and multiexcitonic behavior which is modeled using a statistical scaling approach. As a demonstration of the practical applicability of gQDs, energy transfer of excitons as well as multiexcitons to different substrates is studied. Finally, a back gated silicon nanomembrane FET device is discussed that exhibits a large photocurrent increase when sensitized with QDs.

  18. Line-driven ablation of circumstellar discs – I. Optically thin decretion discs of classical Oe/Be stars

    PubMed Central

    Kee, Nathaniel Dylan; Owocki, Stanley; Sundqvist, J. O.

    2016-01-01

    discussion of future extensions to study line-driven ablation of denser, optically thick, accretion discs of pre-main-sequence massive stars. PMID:27346978

  19. Line-driven ablation of circumstellar discs - I. Optically thin decretion discs of classical Oe/Be stars.

    PubMed

    Kee, Nathaniel Dylan; Owocki, Stanley; Sundqvist, J O

    2016-05-21

    discussion of future extensions to study line-driven ablation of denser, optically thick, accretion discs of pre-main-sequence massive stars.

  20. Nanophotonic interactions between organic excitons and plasmonic metasurfaces (Conference Presentation)

    NASA Astrophysics Data System (ADS)

    O'Carroll, Deirdre M.

    2016-09-01

    Thin-film organic semiconductor materials are emerging as energy-efficient, versatile alternatives to inorganic semiconductors for display and solid-state lighting applications. Additionally, thin-film organic laser and photovoltaic technologies, while not yet competitive with inorganic semiconductor-based analogues, can exhibit small device embodied energies (due to comparatively low temperature and low energy-use fabrication processes) which is of interest for reducing overall device cost. To improve energy conversion efficiency in thin-film organic optoelectronics, light management using nanophotonic structures is necessary. Here, our recent work on improving light trapping and light extraction in organic semiconductor thin films using nanostructured silver plasmonic metasurfaces will be presented [1,2]. Numerous optical phenomena, such as absorption induced scattering, out-of-plane waveguiding and morphology-dependent surface plasmon outcoupling, are identified due to exciton-plasmon coupling between the organic semiconductor and the metasurface. Interactions between localized and propagating surface plasmon polaritons and the excitonic transitions of a variety of organic conjugated polymer materials will be discussed and ways in which these interactions may be optimized for particular optoelectronic applications will be presented. [1] C. E. Petoukhoff, D. M. O'Carroll, Absorption-Induced Scattering and Surface Plasmon Out-Coupling from Absorber-Coated Plasmonic Metasurfaces. Nat. Commun. 6, 7899-1-13 (2015). [2] Z. Shen, D. M. O'Carroll, Nanoporous Silver Thin Films: Multifunctional Platforms for Influencing Chain Morphology and Optical Properties of Conjugated Polymers. Adv. Funct. Mater. 25, 3302-3313 (2015).

  1. Microscopic theories of excitons and their dynamics

    NASA Astrophysics Data System (ADS)

    Berkelbach, Timothy C.

    This thesis describes the development and application of microscopically-defined theories of excitons in a wide range of semiconducting materials. In Part I, I consider the topic of singlet exciton fission, an organic photophysical process which generates two spin-triplet excitons from one photoexcited spin-singlet exciton. I construct a theoretical framework that couples a realistic treatment of the static electronic structure with finite-temperature quantum relaxation techniques. This framework is applied separately, but consistently, to the problems of singlet fission in pentacene dimers, crystalline pentacene, and crystalline hexacene. Through this program, I am able to rationalize observed behaviors and make non-trivial predictions, some of which have been confirmed by experiment. In Part II, I present theoretical developments on the properties of neutral excitons and charged excitons (trions) in atomically thin transition metal dichalcogenides. This work includes an examination of material trends in exciton binding energies via an effective mass approach. I also present an experimental and theoretical collaboration, which links the unconventional disposition of excitons in the Rydberg series to the peculiar screening properties of atomically thin materials. The light-matter coupling in these materials is examined within low-energy models and is shown to give rise to bright and dark exciton states, which can be qualitatively labeled in analogy with the hydrogen series. In Part III, I explore theories of relaxation dynamics in condensed phase environments, with a focus on methodology development. This work is aimed towards biological processes, including resonant energy transfer in chromophore complexes and electron transfer in donor-bridge-acceptor systems. Specifically, I present a collaborative development of a numerically efficient but highly accurate hybrid approach to reduced dynamics, which exploits a partitioning of environmental degrees of freedom into

  2. Interaction of Dirac Fermion excitons and biexciton-exciton cascade in graphene quantum dots

    NASA Astrophysics Data System (ADS)

    Ozfidan, Isil; Korkusinski, Marek; Hawrylak, Pawel

    2015-03-01

    We present a microscopic theory of interacting Dirac quasi-electrons and quasi-holes confined in graphene quantum dots. The single particle states of quantum dots are described using a tight binding model and screened direct, exchange, and scattering Coulomb matrix elements are computed using Slater pz orbitals. The many-body ground and excited states are expanded in a finite number of electron-hole pair excitations from the Hartree-Fock ground state and computed using exact diagonalization techniques. The resulting exciton and bi-exciton spectrum reflects the degeneracy of the top of the valence and bottom of the conduction band characteristic of graphene quantum dots with C3 symmetry. We study the interaction of multi-electron and hole complexes as a function of quantum dot size, shape and strength of Coulomb interactions. We identify two degenerate bright exciton (X) states and a corresponding biexciton (XX) state as XX-X cascade candidates, a source of entangled photon pairs. We next calculate the exciton to bi-exciton transitions detected in transient absorption experiments to extract the strength of exciton-exciton interactions and biexciton binding energies. We further explore the possibility of excitonic instability.

  3. A self-stabilized coherent phonon source driven by optical forces

    PubMed Central

    Navarro-Urrios, D.; Capuj, N. E.; Gomis-Bresco, J.; Alzina, F.; Pitanti, A.; Griol, A.; Martínez, A.; Sotomayor Torres, C. M.

    2015-01-01

    We report a novel injection scheme that allows for “phonon lasing” in a one-dimensional opto-mechanical photonic crystal, in a sideband unresolved regime and with cooperativity values as low as 10−2. It extracts energy from a cw infrared laser source and is based on the triggering of a thermo-optical/free-carrier-dispersion self-pulsing limit-cycle, which anharmonically modulates the radiation pressure force. The large amplitude of the coherent mechanical motion acts as a feedback that stabilizes and entrains the self-pulsing oscillations to simple fractions of the mechanical frequency. A manifold of frequency-entrained regions with two different mechanical modes (at 54 and 122 MHz) are observed as a result of the wide tuneability of the natural frequency of the self-pulsing. The system operates at ambient conditions of pressure and temperature in a silicon platform, which enables its exploitation in sensing, intra-chip metrology or time-keeping applications. PMID:26503448

  4. Thermal radiation from optically driven Kerr (χ{sup (3)}) photonic cavities

    SciTech Connect

    Khandekar, Chinmay; Rodriguez, Alejandro W.; Lin, Zin

    2015-04-13

    We describe thermal radiation from nonlinear (χ{sup (3)}) photonic cavities coupled to external channels and subject to incident monochromatic light. Our work extends related work on nonlinear mechanical oscillators to the problem of thermal radiation, demonstrating that bistability can enhance thermal radiation by orders of magnitude and result in strong lineshape alternations, including “super-narrow spectral peaks” occurring at the onset of kinetic phase transitions. We show that when the cavities are designed to exhibit perfect linear emissivity (rate matching), such thermally activated transitions can be exploited to dramatically tune the output power and radiative properties of the cavity, leading to a kind of Kerr-mediated thermo-optic effect. Finally, we demonstrate that in certain parameter regimes, the output radiation exhibits Stokes and anti-Stokes side peaks whose relative magnitudes can be altered by tuning the internal temperature of the cavity relative to its surroundings, a consequence of strong correlations and interference between the emitted and reflected radiation.

  5. The Complete Redistribution Approximation in Optically Thick Line-Driven Winds

    NASA Astrophysics Data System (ADS)

    Gayley, K. G.; Onifer, A. J.

    2001-05-01

    Wolf-Rayet winds are thought to exhibit large momentum fluxes, which has in part been explained by ionization stratification in the wind. However, it the cause of high mass loss, not high momentum flux, that remains largely a mystery, because standard models fail to achieve sufficient acceleration near the surface where the mass-loss rate is set. We consider a radiative transfer approximation that allows for the dynamics of optically thick Wolf-Rayet winds to be modeled without detailed treatment of the radiation field, called the complete redistribution approximation. In it, it is assumed that thermalization processes cause the photon frequencies to be completely randomized over the course of propagating through the wind, which allows the radiation field to be treated statistically rather than in detail. Thus the approach is similar to the statistical treatment of the line list used in the celebrated CAK approach. The results differ from the effectively gray treatment in that the radiation field is influenced by the line distribution, and the role of gaps in the line distribution is enhanced. The ramifications for the driving of large mass-loss rates is explored.

  6. Tunable High Harmonic Generation driven by a Visible Optical Parametric Amplifier

    NASA Astrophysics Data System (ADS)

    Cirmi, G.; Lai, C.-J.; Huang, S.-W.; Granados, E.; Sell, A.; Moses, J.; Hong, K.-H.; Keathley, P.; Kärtner, F. X.

    2013-03-01

    We studied high-harmonic generation (HHG) in Ar, Ne and He gas jets using a broadly tunable, high-energy optical parametric amplifier (OPA) in the visible wavelength range. We optimized the noncollinear OPA to deliver tunable, femtosecond pulses with 200-500 μJ energy at 1-kHz repetition rate with excellent spatiotemporal properties, suitable for HHG experiments. By tuning the central wavelength of the OPA while keeping energy, duration and beam size constant, we experimentally studied the scaling law of conversion efficiency and cut-off energy with the driver wavelength in argon and helium respectively. Our measurements show a λ-5.9±0.9 wavelength dependence of the conversion efficiency and a λ1.7±0.2 dependence of the HHG cut-off photon energy over the full visible range in agreement with previous experiments of near- and mid-IR wavelengths. By tuning the central wavelength of the driver source and changing the gas, the high order harmonic spectra in the extreme ultraviolet cover the full range of photon energy between ~25 eV and ~100 eV. Due to the high coherence intrinsic in HHG, as well as the broad and continuous tunability in the extreme UV range, a high energy, high repetition rate version of this source might be an ideal seed for free electron lasers.

  7. Optical signatures of electric-field-driven magnetic phase transitions in graphene quantum dots

    NASA Astrophysics Data System (ADS)

    Basak, Tista; Shukla, Alok

    2016-06-01

    Experimental challenges in identifying various types of magnetic ordering in graphene quantum dots (QDs) pose a major hurdle in the application of these nanostructures for spintronic devices. Based upon phase diagrams obtained by employing the π -electron Pariser-Parr-Pople (PPP) model Hamiltonian, we demonstrate that the magnetic states undergo phase transition under the influence of an external electric field. Our calculations of the electroabsorption spectra of these QDs indicate that the spectrum in question carries strong signatures of their magnetic state (FM vs AFM), thus suggesting the possibility of an all-optical characterization of their magnetic nature. Further, the gaps for the up and the down spins are the same in the absence of an external electric field, both for the antiferromagnetic (AFM) and the ferromagnetic (FM) states of QDs. But, once the QDs are exposed to a suitably directed external electric field, gaps for different spins split and exhibit distinct variations with respect to the strength of the field. The nature of variation exhibited by the energy gaps corresponding to the up and down spins is different for the AFM and FM configurations of QDs. This selective manipulation of the spin-polarized gap splitting by an electric field in finite graphene nanostructures can open up new frontiers in the design of graphene-based spintronic devices.

  8. Laser driven intraband optical transitions in two-dimensional quantum dots and quantum rings

    NASA Astrophysics Data System (ADS)

    Barseghyan, M. G.; Kirakosyan, A. A.; Laroze, D.

    2017-01-01

    The intraband optical absorption have been investigated in the presence of hydrogenic donor impurity in GaAs/GaAlAs quantum dot and quantum ring in the intense laser field. The single electron energy spectrum and wave functions have been found using the effective mass approximation and exact diagonalization technique. Different selection rules are obtained for intraband transitions depending on the direction of incident light polarization. Due to the accidental degeneracy of the laser dressed impurity states the crossings of the curves of the threshold energies and the dipole matrix elements on laser field parameter have been observed. The intraband absorption coefficient is calculated for different locations of hydrogenic donor impurity and different values of intense laser field parameter. The obtained results show that the absorption spectrum can exhibit either a blue- or redshift depending on the impurity location, values of the laser field parameter and direction of incident light polarization. The obtained theoretical results indicate a novel opportunity to tune the performance of new devices, based on the quantum dots and quantum rings and to control their specific properties by means of intense laser and hydrogenic donor impurity.

  9. Lindblad theory of dynamical decoherence of quantum-dot excitons

    NASA Astrophysics Data System (ADS)

    Eastham, P. R.; Spracklen, A. O.; Keeling, J.

    2013-05-01

    We use the Bloch-Redfield-Wangsness theory to calculate the effects of acoustic phonons in coherent control experiments where quantum-dot excitons are driven by shaped laser pulses. This theory yields a generalized Lindblad equation for the density operator of the dot, with time-dependent damping and decoherence due to phonon transitions between the instantaneous dressed states. It captures similar physics to the form recently applied to Rabi oscillation experiments [Ramsay , Phys. Rev. Lett.0031-900710.1103/PhysRevLett.104.017402 104, 017402 (2010)] but guarantees positivity of the density operator. At sufficiently low temperatures, it gives results equivalent to those of fully non-Markovian approaches [Lüker , Phys. Rev. B1098-012110.1103/PhysRevB.85.121302 85, 121302 (2012)] but is significantly simpler to simulate. Several applications of this theory are discussed. We apply it to adiabatic rapid passage experiments and show how the pulses can be shaped to maximize the probability of creating a single exciton using a frequency-swept laser pulse. We also use this theory to propose and analyze methods to determine the phonon density of states experimentally, i.e., phonon spectroscopy, by exploring the dependence of the effective damping rates on the driving field.

  10. Exciton interference revealed by energy dependent exciton transfer rate for ring-structured molecular systems

    SciTech Connect

    Yan, Yun-An

    2016-01-14

    The quantum interference is an intrinsic phenomenon in quantum physics for photon and massive quantum particles. In principle, the quantum interference may also occur with quasi-particles, such as the exciton. In this study, we show how the exciton quantum interference can be significant in aggregates through theoretical simulations with hierarchical equations of motion. The systems under investigation are generalized donor-bridge-acceptor model aggregates with the donor consisting of six homogeneous sites assuming the nearest neighbor coupling. For the models with single-path bridge, the exciton transfer time only shows a weak excitation energy dependence. But models with double-path bridge have a new short transfer time scale and the excitation energy dependence of the exciton transfer time assumes clear peak structure which is detectable with today’s nonlinear spectroscopy. This abnormality is attributed to the exciton quantum interference and the condition for a clear observation in experiment is also explored.

  11. Optics and remote sensing of Bahamian carbonate sediment whitings and potential relationship to wind-driven Langmuir circulation

    NASA Astrophysics Data System (ADS)

    Dierssen, H. M.; Zimmerman, R. C.; Burdige, D. J.

    2009-03-01

    Regions of milky white seas or "whitings" periodically occur to the west of Andros Island along the Great Bahama Bank where the bottom sediment consists of fine-grained aragonite mud. We present measurements of inherent optical properties within a sediment whiting patch and discuss the potential for monitoring the frequency, extent, and quantity of suspended matter from ocean colour satellite imagery. Sea spectral reflectance measured in situ and remotely from space revealed highly reflective waters elevated across the visible spectrum (i.e., "whitened") with a peak at 490 nm. Particulate backscattering was an order of magnitude higher than that measured at other stations throughout the region. The whiting also had one of the highest backscattering ratios measured in natural waters (0.05-0.06) consistent with water dominated by aragonite particles with a high index of refraction. Regular periodicity of 40 and 212 s evident in the light attenuation coefficient over the sampling period indicated patches of fluctuating turbidity on spatial scales that could be produced from regular rows of Langmuir cells penetrating the 5-m water column. We suggest that previously described mechanisms for sediment resuspension in whitings, such as tidal bursting and fish activity, are not fully consistent with these data and propose that wind-driven Langmuir cells reaching the full-depth of the water column may represent a plausible mechanism for sediment resuspension and subsequent whiting formation. Optics and remote sensing provide important tools for quantifying the linkages between physical and biogeochemical processes in these dynamic shallow water ecosystems.

  12. Optics and remote sensing of Bahamian carbonate sediment whitings and potential relationship to wind-driven Langmuir circulation

    NASA Astrophysics Data System (ADS)

    Dierssen, H. M.; Zimmerman, R. C.; Burdige, D. J.

    2008-12-01

    Regions of milky white seas or "whitings" periodically occur to the west of Andros Island along the Great Bahama Bank where the bottom sediment consists of fine-grained aragonite mud. We present comprehensive measurements of inherent optical properties within a whiting patch and discuss the potential for monitoring the frequency, extent, and quantity of suspended matter from ocean colour satellite imagery. Sea spectral reflectance measured in situ and remotely from space revealed highly reflective waters elevated across the visible spectrum (i.e., "whitened") with a peak at 490 nm. Particulate backscattering was an order of magnitude higher than that measured at other stations throughout the region. The whiting also had one of the highest backscattering ratios measured in natural waters (0.05-0.06) consistent with water dominated by aragonite particles with a high index of refraction. Regular periodicity of 40 and 212 s evident in the light attenuation coefficient over the sampling period indicated patches of fluctuating turbidity on spatial scales that could be produced from regular rows of Langmuir cells penetrating the 5-m water column. We suggest that previously described mechanisms for sediment resuspension in whitings, such as tidal bursting and fish activity, are not fully consistent with these data and propose that wind-driven Langmuir cells reaching the full-depth of the water column may represent a plausible mechanism for sediment resuspension and subsequent whiting formation. Optics and remote sensing provide important tools for quantifying the linkages between physical and biogeochemical processes in these dynamic shallow water ecosystems.

  13. Ultracold fermions in a one-dimensional bipartite optical lattice: Metal-insulator transitions driven by shaking

    NASA Astrophysics Data System (ADS)

    Di Liberto, M.; Malpetti, D.; Japaridze, G. I.; Morais Smith, C.

    2014-08-01

    We theoretically investigate the behavior of a system of fermionic atoms loaded in a bipartite one-dimensional optical lattice that is under the action of an external time-periodic driving force. By using Floquet theory, an effective model is derived. The bare hopping coefficients are renormalized by zeroth-order Bessel functions of the first kind with different arguments for the nearest-neighbor and next-nearest-neighbor hopping. The insulating behavior characterizing the system at half filling in the absence of driving is dynamically suppressed, and for particular values of the driving parameter the system becomes either a standard metal or an unconventional metal with four Fermi points. The existence of the four-Fermi-point metal relies on the fact that, as a consequence of the shaking procedure, the next-nearest-neighbor hopping coefficients become significant compared to the nearest-neighbor ones. We use the bosonization technique to investigate the effect of on-site Hubbard interactions on the four-Fermi-point metal-insulator phase transition. Attractive interactions are expected to enlarge the regime of parameters where the unconventional metallic phase arises, whereas repulsive interactions reduce it. This metallic phase is known to be a Luther-Emery liquid (spin-gapped metal) for both repulsive and attractive interactions, contrary to the usual Hubbard model, which exhibits a Mott-insulator phase for repulsive interactions. Ultracold fermions in driven one-dimensional bipartite optical lattices provide an interesting platform for the realization of this long-studied four-Fermi-point unconventional metal.

  14. Exciton polarizability in semiconductor nanocrystals.

    PubMed

    Wang, Feng; Shan, Jie; Islam, Mohammad A; Herman, Irving P; Bonn, Mischa; Heinz, Tony F

    2006-11-01

    The response of charge to externally applied electric fields is an important basic property of any material system, as well as one critical for many applications. Here, we examine the behaviour and dynamics of charges fully confined on the nanometre length scale. This is accomplished using CdSe nanocrystals of controlled radius (1-2.5 nm) as prototype quantum systems. Individual electron-hole pairs are created at room temperature within these structures by photoexcitation and are probed by terahertz (THz) electromagnetic pulses. The electronic response is found to be instantaneous even for THz frequencies, in contrast to the behaviour reported in related measurements for larger nanocrystals and nanocrystal assemblies. The measured polarizability of an electron-hole pair (exciton) amounts to approximately 10(4) A(3) and scales approximately as the fourth power of the nanocrystal radius. This size dependence and the instantaneous response reflect the presence of well-separated electronic energy levels induced in the system by strong quantum-confinement effects.

  15. A Survey on Next-Generation Mixed Line Rate (MLR) and Energy-Driven Wavelength-Division Multiplexed (WDM) Optical Networks

    NASA Astrophysics Data System (ADS)

    Iyer, Sridhar

    2015-06-01

    With the ever-increasing traffic demands, infrastructure of the current 10 Gbps optical network needs to be enhanced. Further, since the energy crisis is gaining increasing concerns, new research topics need to be devised and technological solutions for energy conservation need to be investigated. In all-optical mixed line rate (MLR) network, feasibility of a lightpath is determined by the physical layer impairment (PLI) accumulation. Contrary to PLI-aware routing and wavelength assignment (PLIA-RWA) algorithm applicable for a 10 Gbps wavelength-division multiplexed (WDM) network, a new Routing, Wavelength, Modulation format assignment (RWMFA) algorithm is required for the MLR optical network. With the rapid growth of energy consumption in Information and Communication Technologies (ICT), recently, lot of attention is being devoted toward "green" ICT solutions. This article presents a review of different RWMFA (PLIA-RWA) algorithms for MLR networks, and surveys the most relevant research activities aimed at minimizing energy consumption in optical networks. In essence, this article presents a comprehensive and timely survey on a growing field of research, as it covers most aspects of MLR and energy-driven optical networks. Hence, the author aims at providing a comprehensive reference for the growing base of researchers who will work on MLR and energy-driven optical networks in the upcoming years. Finally, the article also identifies several open problems for future research.

  16. Exciton transport and dissociation at organic interfaces

    NASA Astrophysics Data System (ADS)

    Beljonne, David

    2011-03-01

    This paper focuses on modeling studies of exciton transport and dissociation at organic interfaces and includes three parts: 1) Experiments have shown that the values of exciton diffusion length LD in conjugated polymers (CPs) are rather low, in the range of 5-10 nm, apparently regardless of their chemical structure and solid-state packing. In contrast, larger LD values have been reported in molecular materials that are chemically more well-defined than CPs. Here we demonstrate that energetic disorder alone reduces the exciton diffusion length more than one order of magnitude, from values typically encountered in molecules (> 50nm) to values actually measured in CPs (<10nm). 2) A number of organic crystals show anisotropic excitonic couplings, with weak interlayer interactions between molecules that are more strongly coupled within the layers. The resulting energy carriers are intra-layer 2D excitons that diffuse along the interlayer direction. We model this analytically for infinite layers and using quantum-chemical calculations of the electronic couplings for anthracene clusters. We show that the exciton hopping rates and diffusion lengths depend in a subtle manner on the size and shape of the interacting aggregates, temperature and the presence of energetic disorder. 3) The electronic structure at organic/organic interfaces plays a key role, among others, in defining the quantum efficiency of organic-based photovoltaic cells. Here, we perform quantum-chemical and microelectrostatic calculations on molecular aggregates of various sizes and shapes to characterize the interfacial dipole moment at pentacene/C60 heterojunctions. The results show that the interfacial dipole mostly originates in polarization effects due to the asymmetry in the multipolar expansion of the electronic density distribution between the interacting molecules. We will discuss how the quadrupoles on the pentacene molecules produce direct electrostatic interactions with charge carriers and how

  17. Label-free optical characterization methods for detecting amine silanization-driven gold nanoparticle self-assembly.

    PubMed

    Roy, Shibsekhar; Dixit, Chandra K; Woolley, Robert; O'Kennedy, Richard; McDonagh, Colette

    2011-09-06

    Fluorescence lifetime correlation spectroscopy (FLCS) is presented as a single-step label-free detection method for probing the amine silanization-driven spontaneous 3D self-assembly of freestanding gold nanoparticles (GNPs) in solution. Unlike the conventional methods of studying self-assembly, for example, UV-vis spectroscopy and electron microscopy, FLCS utilizes the intrinsic gold fluorescence. The significance of this approach is to amalgamate the measurement of optical and hydrodynamic size properties simultaneously to achieve a more coherent description of the self-assembly pathway. GNP self-assembly has two-stage kinetics. Electrostatic interaction drives the initial amine silanization, and this is followed by siloxane bond formation between hydrolyzed ethoxy groups of GNP-attached APTES, resulting in the formation of micrometer-sized superstructures. The self-assembly has resulted in a 5-fold increase in the fluorescence lifetime (FL), and the FLCS study has shown an 8- to 10-fold increase in the diffusion coefficient using the pure diffusion model. This result is consistent with the transmission electron microscopy (TEM) observation, which shows a few hundred fold increase in the diameter due to assembly formation by the GNPs.

  18. Optical properties of iron oxides

    NASA Astrophysics Data System (ADS)

    Musfeldt, Janice

    2012-02-01

    Magnetoelectric coupling in materials like multiferroics, dilute magnetic semiconductors, and topological insulators has attracted a great deal of attention, although most work has been done in the static limit. Optical spectroscopy offers a way to investigate the dynamics of charge-spin coupling, an area where there has been much less effort. Using these techniques, we discovered that charge fluctuation in LuFe2O4, the prototypical charge ordered multiferroic, has an onset well below the charge ordering transition, supporting the ``order by fluctuation'' mechanism for the development of charge order superstructure. Bragg splitting and large magneto-optical contrast suggest a low temperature monoclinic distortion that can be driven by both temperature and magnetic field. At the same time, dramatic splitting of the LuO2 layer phonon mode is attributed to charge-rich/poor proximity effects, and its temperature dependence reveals the antipolar nature of the W layer pattern. Using optical techniques, we also discovered that α-Fe2O3, a chemically-similar parent compound and one of the world's oldest and most iconic antiferromagnetic materials, appears more red in applied magnetic field than in zero field conditions. This effect is driven by a field-induced reorientation of magnetic order. The oscillator strength lost in the color band is partially transferred to the magnon side band, a process that also reveals a new exciton pattern induced by the modified exchange coupling. Analysis of the exciton pattern exposes C2/c monoclinic symmetry in the high field phase of hematite. Taken together, these findings advance our understanding of iron-based materials under extreme conditions. [4pt] Collaborators include: X. S. Xu, P. Chen, Q. -C. Sun, T. V. Brinzari (Tennessee); S. McGill (NHMFL); J. De Groot, M. Angst, R. P. Hermann (Julich); A. D. Christianson, B. C. Sales, D. Mandrus (ORNL); A. P. Litvinchuk (Houston); J. -W. Kim (Ames); Z. Islam (Argonne); N. Lee, S. -W. Cheong

  19. Temperature Evolution of Excitonic Absorptions in Cd(1-x)Zn(x)Te Materials

    NASA Technical Reports Server (NTRS)

    Quijada, Manuel A.; Henry, Ross

    2007-01-01

    The studies consist of measuring the frequency dependent transmittance (T) and reflectance (R) above and below the optical band-gap in the UV/Visible and infrared frequency ranges for Cd(l-x),Zn(x),Te materials for x=0 and x=0.04. Measurements were also done in the temperature range from 5 to 300 K. The results show that the optical gap near 1.49 eV at 300 K increases to 1.62 eV at 5 K. Finally, we observe sharp absorption peaks near this gap energy at low temperatures. The close proximity of these peaks to the optical transition threshold suggests that they originate from the creation of bound electron-hole pairs or excitons. The decay of these excitonic absorptions may contribute to a photoluminescence and transient background response of these back-illuminated HgCdTe CCD detectors.

  20. Evanescent-Vacuum-Enhanced Photon-Exciton Coupling and Fluorescence Collection

    NASA Astrophysics Data System (ADS)

    Ren, Juanjuan; Gu, Ying; Zhao, Dongxing; Zhang, Fan; Zhang, Tiancai; Gong, Qihuang

    2017-02-01

    An evanescent optical mode existing in various nanophotonic structures always acts as a cavity mode rather than an electromagnetic vacuum in the study of cavity quantum electrodynamics (CQED). Here we show that taking the evanescent mode as an electromagnetic vacuum in which the nanocavity is located is possible through the optical mode design. The proposed evanescent vacuum enables us to enhance both the reversible photon-exciton interaction and fluorescence collection. By embedding the custom-designed plasmon nanocavity into the evanescent vacuum provided by a metallic or dielectric nanowire, the photon-exciton coupling coefficient can achieve 4.2 times that in vacuum due to the exponential decay of the evanescent wave, and spontaneously emitted photons with Rabi splitting can be guided by an evanescent wave with a collection efficiency of 47% at most. Electromagnetic vacuum engineering at subwavelength scale holds promise for controlling the light-matter interaction in quantum optics, CQED, and on-chip quantum information.