Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session U46: Optical Properties of Semiconductors: Excitons and Phonons |
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Sponsoring Units: FIAP Chair: David Reitze, University of Florida Room: Baltimore Convention Center 349 |
Thursday, March 16, 2006 8:00AM - 8:12AM |
U46.00001: Laser induced trapping of excitons in coupled quantum wells A.T. Hammack, M. Griswold, L.V. Butov, A.L. Ivanov, L.E. Smallwood, A.C. Gossard Optical trapping and manipulation of neutral particles plays a major role in single particle studies in physics, chemistry, and biology [1]. An exciting recent outgrowth of the technique has been the experimental implementation of atom Bose- Einstein Condensation [2,3]. In this contribution, we report proposal and demonstration of laser induced trapping for a new system - a cold gas of excitons in coupled quantum wells. We report trapping a cold gas of excitons in laser induced traps and on the formation of a highly degenerate Bose gas of excitons in the trap. [1] A.~Ashkin, {\it IEEE Journal on Selected Items in Quantum Electronics\/} {\bf 6}, 841 (2000). [2] E.~A. Cornell, C.~E. Wieman, {\it Rev. Mod. Phys.\/} {\bf 74}, 875 (2002). [3] W.~Ketterle, {\it Rev. Mod. Phys.\/} {\bf 74}, 1131 (2002). [Preview Abstract] |
Thursday, March 16, 2006 8:12AM - 8:24AM |
U46.00002: Spontaneous First-order Optical Coherence in Cold Exciton Gases in Coupled Quantum Wells Sen Yang, A.T. Hammack, L.V. Butov, A.C. Gossard A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells[1]. A strong enhancement of spontaneous first-order optical coherence was observed at low temperatures below a few Kelvin where the thermal de Broglie wavelength becomes comparable to the interparticle separation and the exciton gas becomes nonclassical. The onset of spontaneous first-order optical coherence was found to be correlated with macroscopic spatial ordering in the exciton system.[1] L.V. Butov, A.C. Gossard, D.S. Chemla, {\it Nature} {\bf 418}, 751 (2002). [Preview Abstract] |
Thursday, March 16, 2006 8:24AM - 8:36AM |
U46.00003: Theory of coherent population trapping and electromagnetically induced transparency in quantum wells and dots Zachary Dutton Recently, there has been important experimental progress in quantum coherent phenomena in quantum-wells and dots, opening up possibilities for observations of quantum optical effects previously observed in atomic systems. In particular, coherent population trapping (CPT), electromagnetically-induced transparency (EIT), and slow light can occur in systems with sufficiently long ground state coherence, optically connected to an excited level, forming a Lambda-like system. We find that a quantum well or ensemble of dots, in Voigt geometry and illuminated by a bi-chromatic circularly polarized laser, can exhibit CPT and EIT. In this scheme, the electron spin provides the long lived ground states and a trion excitation acts as the excited level. By including optical and g-factor inhomogenous broadening, dephasing due to nuclear hyperfine interaction, and coupling to the both trion excitations, we derive a compact set of criteria for observations of these effects. We compare with experiments to date and discuss future prospects. [Preview Abstract] |
Thursday, March 16, 2006 8:36AM - 8:48AM |
U46.00004: Stimulated Emission of Terahertz Radiation from Internal ExcitonTransitions in Cu$_{2}$O B.A. Schmid, R. Huber, Y.R. Shen, R.A. Kaindl, D.S. Chemla Excitons are among the most fundamental optical excitation modes in semiconductors. Resonant infrared pulses have been used to sensitively probe absorptive transitions between hydrogen-like bound pair states [1,2]. We report the first observation of the reverse quantum process: stimulated emission of electromagnetic radiation from intra-excitonic transitions [3]. Broadband terahertz pulses monitor the far-infrared electromagnetic response of Cu$_{2}$O after ultrafast resonant photogeneration of 3$p$ excitons. Stimulated emission from the 3$p$ to the energetically lower 2$s$ bound level occurs at a photon energy of 6.6 meV, with a cross section of $\sim $10$^{-14}$ cm$^{2}$. Simultaneous excitation of both exciton levels, in turn, drives quantum beats which lead to efficient terahertz emission sharply peaked at the difference frequency. Our results demonstrate a new fundamental process of THz quantum optics and highlight analogies and differences between excitonic and atomic systems. [1] R. A. Kaindl et al., Nature \textbf{423}, 734 (2003). [2] M. Kubouchi et al., Phys. Rev. Lett. \textbf{94}, 016403 (2005). [3] R. Huber et al., Phys. Rev. Lett., to appear. [Preview Abstract] |
Thursday, March 16, 2006 8:48AM - 9:00AM |
U46.00005: Ultrafast Raman Studies of Electron Transient Transport in InN Thick Film Grown on GaN Kong-Thon Tsen, D.K. Ferry, Hai Lu, William J. Schaff GaN, AlN, InN and their alloys have long been considered as very promising materials for device applications. Semiconductor alloys such as $In_x Ga_{1-x} N$ have been successfully used in the fabrication of blue-green light emitting diodes and laser diodes. Recently, growth of high quality InN as well as $In_x Ga_{1-x} N$ have been demonstrated. In Particular, progress in the manufacturing of very high quality, single-crystal InN thin films has opened up a new challenging research avenue in the III-nitride semiconductors. InN together with its alloys of GaN and AlN enable the operation of light emitting diodes and diode lasers ranging in spectral wavelength from infrared all the way down to deep ultraviolet. It has also been predicted that InN has the lowest electron effective mass among all the III-nitride semiconductors. As a result, very high electron mobility and very large saturation velocity are expected. In this paper, we report experimental results of electron transient transport on InN thick film grown on GaN. Electron drift velocity as large as $7.5x10^7cm/\sec $ has been found when the sample is excited by an ultrafast laser pulse with pulse width about 600 femtoseconds. Our findings demonstrate that InN has great potential for use in the ultrafast electronic devices. [Preview Abstract] |
Thursday, March 16, 2006 9:00AM - 9:12AM |
U46.00006: Exciton-mediated one phonon resonant Raman scattering from 1-dimensional systems A. N. Vamivakas, A. G. Walsh, Y. Yin, M. S. Unlu, B. B. Goldberg, A. K. Swan The Kramer's-Heisenberg approach is well developed for the theory of one phonon resonant Raman scattering (OPRRS) based on both intermediate states of free electrons and correlated electron-hole pairs in 3, 2 and 0-dimensional (D) systems. But to our knowledge, a theory of OPRRS incorporating excitonic effects has not yet been developed for quantum confined 1D system. In this talk we present a generic expression for the resonant Raman scattering cross section from a 1D system explicitly accounting for excitonic effects. We show how the theory is useful for analyzing the Raman resonance excitation profile lineshapes for of a variety of 1D systems. We apply this formalism to simple model systems to the similarities and differences between the free electron and correlated electron-hole 1D theory and also compare with the 3D excitonic theory. [Preview Abstract] |
Thursday, March 16, 2006 9:12AM - 9:24AM |
U46.00007: Wurtzite Effects on Spin Splitting of GaN Wan-Tsang Wang, M.H. Gau, Ikai Lo, S.F. Tsay, J.K. Tsai, K.L. Hsieh, J.C. Chiang We report the theoretical study of the wurtzite effects on spin-splitting of GaN within a third-neighbor Linear Combination of Atomic Orbitals (LCAO) model. For wurtzite structure, there are two intrinsic wurtzite effects which are band-folding effect and structure inversion asymmetry. The band-folding effect generates two conduction bands ($\Delta _{C1 }$and $\Delta _{C3})$, in which the $p$-wave probability and, consequently, the spin-splitting energy have abrupt changes when $k_{z}$ increases toward the anti-crossing zone. The wurtzite effects, in addition to Rashba and Dresselhaus effect, give significant contributions to the large spin-splitting in GaN/AlN quantum wells. [Preview Abstract] |
Thursday, March 16, 2006 9:24AM - 9:36AM |
U46.00008: Ab initio calculations of excitons in wurtzite-type semiconductors. Niels E. Christensen, Robert Laskowski The optical absorption spectra are calculated with inclusion of electron-hole correlations for GaN, ZnO, and AlN, all in the wurtzite structure. Quasi-particle states are approximated by local-density-functional calculations with gaps corrected by a scissors operator, and the final spectra are obtained by solving the Bethe-Salpeter Equation . The results for ZnO depend sensitively on the energetic positions of the Zn-3d states. These are corrected by means of LDA+U. The excitons originating from the valence-band maximum are labeled A, B, and C, but their symmetry type, and thus dependence on the polarization of the light, are related to the specific values of the spin-orbit and crystal field splittings, SOC and CFS. The SOC is positive in GaN and AlN, negative in ZnO. The CFS is positive in GaN and ZnO, but negative in AlN. The sensitivity of the excitonic states to structural parameters is discussed, and in one case, AlN, we examine the validity of Elliott's model, the effective hydrogen-atom model. [Preview Abstract] |
Thursday, March 16, 2006 9:36AM - 9:48AM |
U46.00009: Pump-Probe studies of Carrier Dynamics in bulk ZnO and ZnO epilayers and Nanorods X. Wang, Y.D. Jho, D.H. Reitze, C. Cook, G.D. Sanders, C.J. Stanton, X. Wei, J.K Yoo, G.-C. Yi ZnO-based devices are potentially useful as short wavelength emitters and in spintronics applications, yet little is known about the ultrafast relaxation properties of ZnO. We have performed time-resolved differential reflectivity (TRDR) measurements of bulk ZnO, ZnO epilayers and nanorods as a function of temperature and excitation wavelength. Bi-exponential decays of the A and B exciton states are observed with fast ($\sim $ps scale) and slower ($\sim $ 50-100 ps scale) components, which depend strongly on excitation wavelength. We find that decay times can be correlated with relaxation channels in the band structure. In addition to their bi-exponential nature, the relaxation times we observe on ZnO epilayers and nanorods are shorter than high quality bulk ZnO, indicating a higher density of defects and impurity states in these samples. [Preview Abstract] |
Thursday, March 16, 2006 9:48AM - 10:00AM |
U46.00010: Rate Equation Model for Carrier and Exciton Dynamics in ZnO C. J. Cook, G. D. Sanders, C. J. Stanton, X. Wang, Y. D. Jho, D. H. Reitze There has been a renewed interest in ZnO materials for possible applications to short wavelength optical devices including blue lasers owing to its wide band-gap (3.37) and large exciton binding energy (approx. 60 meV). Recently, there have been several experimentalstudies of the dynamics of photoexcited carriers in bulk ZnO as well as epitaxial films and nanorods. In this talk, we report on theoretical calculations of the exciton and photoexcited carrier dynamics based on a multi-state, coupled rate equation model. We compare our theoretical results with recent tunable time resolved reflectivity measurements performed at the National High Magnetic Field Laboratory that study the relaxation dynamics when pumping and probing near the A and B excitons. In addition to solving the coupled rate equations, we also discuss the role of diffusion as well as phase space filling (non-linear rate equations) on the experimental results. [Preview Abstract] |
Thursday, March 16, 2006 10:00AM - 10:12AM |
U46.00011: Direct Observation of the Strength of Plasmon-Longitudinal Optical Phonon Interaction in n-type GaAs Hakan Altan, Xuying Xin, David Matten, Robert Alfano The screening of longitudinal optical phonons by plasmons is investigated by time-resolved visible pump-mid infrared probe transmission measurements in a series of light to highly doped n-type GaAs wafers. The reduced relaxation of photogenerated carriers is strongly correlated to the coupling between longitudinal optical phonons and background plasmons as suggested by the variation of the phonon strength over the doping range. Our results show that at low photogeneration ($<$ 10$^{16 }$cm$^{-3})$ the critical doping density at which the strength of the coupling between LO phonons and plasmons decreases significantly is on the order of N$_{c }\sim $ 1x10$^{18}$cm$^{-3}$. The lack of LO phonons that participate in relaxation of carriers due to the hybridization of the longitudinal modes above this doping level, can either result in adverse effects in the spectrum of diode lasers and semiconductor electronic devices or enhance photonic device performance due to longer minority carrier recombination times. [Preview Abstract] |
Thursday, March 16, 2006 10:12AM - 10:24AM |
U46.00012: THz radiation from coherent acoustic phonon waves in strained GaN-based heterostructures Young-Dahl Jho, Jin-Young Sohn, Gary D. Sanders, Christopher J. Stanton, Eunsoon Oh, Dai-Sik Kim We present experimental results and discuss the generation mechanism of newly found THz radiation in GaN/InGaN based light emitting diode (LED) structures. These structures show strong coherent acoustic phonon oscillations under ultra-short optical excitation and we discuss the role these coherent phonons play in the generation of the THz signal. To better understand the role of piezoelectricity on the generation of the acoustic phonons and THz radiation, an external field was applied to compensate the built-in piezoelectric field. The coherent oscillatory behavior of the differential reflectivity spectra was reduced and finally become independent of the increasing applied voltage. However, with reverse bias, the THz emission from these structures was found to increase with increasing reverse voltage and excitation energy, slightly distinct from the trend of the photocurrent. The frequency of the THz emission is related to the transit time of the acoustic phonons between the AlGaN layers. The bias and wavelength dependence of the THz generation suggests that wavefunctions of confined carriers at the AlGaN/GaN and AlGaN/InGaN interfaces, are modulated by a temporally-changing potential shape associated with the piezoelectric field of the lattice and are responsible for the THz radiation. [Preview Abstract] |
Thursday, March 16, 2006 10:24AM - 10:36AM |
U46.00013: Theory of carrier dynamics and coherent phonons in piezoelectric semiconductor heterostructures Gary Sanders, Chris Stanton We model generation and propagation of coherent acoustic phonons in time resolved reflectivity experiments on InGaN/GaN multi quantum wells embedded in a pin diode structure. Carriers are created in the InGaN wells by ultrafast pumping below the GaN band gap. The electronic states in the multiquantum well structure are obtained in an effective mass model and the generation and subsequent relaxation of photogenerated carriers in the well are treated in a Boltzmann equation formalism. Coherent acoustic phonons are generated in the quantum well via a strong piezoelectric electron-phonon interaction with photogenerated carriers. These propagate into the structure at the LA sound speed modifying the optical properties and giving rise to an oscillatory differential reflectivity signal. We also study the THz radiation emitted by the photoexcited carriers and phonons. In addition to studying the multiquantum well structure, we also study chirped superlattices were the well widths increase with distance and investigate the possibility of selectively exciting carriers in a given well to coherently control the response. [Preview Abstract] |
Thursday, March 16, 2006 10:36AM - 10:48AM |
U46.00014: Studies of Coherent Acoustic Phonons in CdMnTe Single Crystals D. Wang, S. Wu, R. Sobolewski, A. Mycielski We have demonstrated generation and detection of coherent acoustic phonons (CAPs) in Cd$_{0.91}$Mn$_{0.09}$Te (CdMnTe) single crystals using a femtosecond pump-probe spectroscopy technique. The Thomsen model based on propagation of a strain pulse in a crystalline lattice accounted very well the observed dependences of the frequency and the dephasing time of our CAP oscillations on the optical probe beam wave-vector. The CAP oscillation frequency was found to be dispersionless with the speed of sound equal to 3579 m/s. The comparison studies, performed using the pump beam with the photon energy well above the CdMnTe energy gap and the sub-gap probe beam demonstrated that in our crystals the measured dephasing time of CAP oscillations was limited by the absorption depth of the probe light rather than the intrinsic decay time of the coherent phonons. The latter value was estimated to be at least in the nanosecond range. Optically-induced electronic stress was determined to be the main generation mechanism of CAPs in CdMnTe. [Preview Abstract] |
Thursday, March 16, 2006 10:48AM - 11:00AM |
U46.00015: Dynamical interfacial-electric-field-induced electro-optics in multilayer semiconductors Y.D. Glinka, J.K. Miller, N.H. Tolk, X. Liu, Y. Sasaki, J.K. Furdyna Multilayer semiconductors with the thickness of layers of a few tens of nanometers are common materials for designing novel multifunctional electronic and opto-electronic devices. Once the materials are subjected to ultrafast laser light, the dynamical interfacial electric fields between adjacent layers is created as a result of charge separation at the interfaces within the carrier thermalization process. This dynamical electric field affects the ultrafast optical properties of the materials additionally to that of the bleaching effect (phase space filling-Pauli blocking). We report the first application of pump--probe technique allowing the interfacial-field-induced both electro-optical refractive-index change and the second harmonic generation to be monitored simultaneously. The pump-probe spectroscopy of GaAs/GaSb/InAs multilayers reveals predominantly the electro-optical nature. The interfacial fields contribute to the variety of electro-optical effects allowing the temporal and spatial resolution in carrier dynamics to be reached by monitoring responses resulted from different order nonlinear polarizations. The absorption bleaching is a secondary effect appearing with much smaller magnitude. [Preview Abstract] |
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