Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V16: Optical Properties of Quantum Well Structures |
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Sponsoring Units: DCMP Chair: Krzysztof Kempa, Boston College Room: LACC 404A |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V16.00001: Excitons and photoluminescence spectra in nanoscale heterostructures AlGaN/GaN/AlGaN E.P. Pokatilov, D.L. Nika, V.M. Fomin, J.T. Devreese For heterostructures AlGaN/GaN/AlGaN with the GaN quantum wells ranging from 4 to 16 monolayers, we use an exciton model, which includes the interaction of an electron and a hole with deformations of the crystal lattice and with built-in electrostatic fields. It is based on a 6-band hole Hamiltonian, as distinct from the common variational approach. Exciton energy spectra and wave functions for the ground state and some excited states are found after numerical diagonalization of the 6-band matrix hole Hamiltonian with an adaptive grid. The developed theoretical approach has allowed us to interpret the position of the photoluminescence bands in good agreement with expe\-riment. For the first time position and intensity of the phonon sidebands are calculated taking into account phonons specific for the wurtzite he\-te\-ro\-structure. We elucidate the transition from the blue shift to the red shift in the photoluminescence spectra with increasing the strength of the built-in electrostatic fields and the quantum-well width. The increase of the exciton radiative lifetime with the quantum- well width is quantitatively explained. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V16.00002: Resonant magnetopolaron effect in a high-electron-density quantum well in a tilted magnetic field. Anticrossing at TO-phonon frequency. S. N. Klimin, J. T. Devreese The cyclotron resonance (CR) spectra are calculated for a high electron density GaAs/AlAs quantum well in a tilted magnetic field taking into account both the electron-phonon and the electron-electron interactions. When the electron density is sufficiently high, the electron-phonon interaction is strongly influenced by screening and by the magnetoplasmon-phonon mixing. As a result of this mixing, a renormalization of the LO- phonon modes occurs, so that the resonant magnetopolaron effect is manifested for CR energies close to the TO-phonon frequency in both perpendicular and tilted configurations of the magnetic field. For a tilted magnetic field, a double splitting of the CR peaks appears due to the resonant magnetopolaron effect and to the anticrossing of CR and plasmon-phonon intersubband modes. The calculated CR peak positions and amplitudes are in good agreement with experiment. It is shown that the experimental CR spectra find an adequate explanation within the polaron concept. [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V16.00003: Two-Electron Linear Intersubband Absorption in a Biased Quantum Well Jian Dai, Mikhail Raikh, Tigran Shahbazyan Linear light absorption of 2D electrons confined within a biased quantum well is studied theoretically. We demonstrate that, for light polarization perpendicular to the 2D plane, in addition to conventional absorption peak at frequency $\hbar\omega \approx \Delta$, where $\Delta$ is the intersubband energy distance, there exists a peak around a {\em double} frequency $\hbar\omega \approx 2\Delta$. This additional peak is entirely due to electron-electron interactions, and corresponds to excitation of {\em two} electrons by {\em one} photon. The magnitude of two-electron absorption is proportional to $U^2$, where $U$ is the applied bias. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V16.00004: Screening of External Electric Field by Photo-Induced Carriers in Multiple Quantum Wells Vadim Puller, Lev Deych, Alexander Lisyansky Multiple quantum well (MQW) structures present a great interest for optoelectronic applications. Their optical characteristics can be controlled by an external electric field. However, ultra-fast applications require an optical control of the spectra. We propose a scheme for ultra-fast optical switching of biased MQW using photo-generated carriers for screening the field in the structure. Using generalized Thomas-Fermi model we obtain the distribution of the electric field in the structure as a function of the applied bias and the excitation conditions. For moderate photo-generated carrier concentrations ( 10$^{15}$-10$^{16}$ cm$^{-3})$ one can distinguish three screening regimes: (i) quantum well screening; (ii) unsaturated screening with participation of mobile electrons excited from the quantum wells; (iii) saturated screening, when the excess charge induced in the structure is not sufficient to screen the applied potential. For high carrier concentrations (10$^{18-}$10$^{19}$cm$^{-3})$ the screening regime (i) is not observed, since the carriers cannot completely condense in quantum wells. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V16.00005: Internal transitions of charged excitons in GaAs QWs with monolayer well-width fluctuations Alexander Dzyubenko, Christian Meining, Vincent Whiteside, Athos Petrou, Bruce McCombe, Joseph Tischler, Allan Bracker, Dan Gammon We report optically detected resonance (ODR) experiments and theoretical studies of interface fluctuation quantum dots (IFQDs) in GaAs/AlGaAs QWs (widths from 2.8 to 14.1 nm) doped in the barriers with donors to allow creation of both excitons and trions. We observe internal transitions of trions and electron cyclotron resonance (CR) in the wider wells. These spectra are similar to those from samples with smooth interfaces except for an additional feature at fields above CR. Based on theoretical predictions, we assign this feature to bound-to-bound triplet transitions, which are strictly forbidden in translationally invariant systems. The ODR signals from the narrow wells show no CR and are interpreted in terms of inhomogeneously broadened internal transitions of charged excitons in the IFQDs. A theoretical model for optical transitions of excitonic complexes in the presence of lateral and magnetic field confinement will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V16.00006: Generation of shift currents in GaAs/AlGaAs quantum wells Mark Bieler, Norman Laman, Henry van Driel, Arthur Smirl Resonant, band-band optical excitation of certain non- centrosymmetric semiconductors produces a change in the electron centre of charge leading to a shift current. Such a current can be described in terms of a nonlinear optical process involving a divergent part of a $\chi^{(2)}$ susceptibility. For zincblende semiconductors this current is nonzero only if a component of the optical electric field exists along two different crystal cubic directions. Here we demonstrate shift currents in (110)- grown GaAs/AlGaAs multiquantum well samples using 60 fs 810 nm pulses and detect them via their emitted terahertz radiation. Due to the reduced symmetry the quantum wells exhibit shift currents for different optical polarizations/crystal orientations than the bulk materials. The shift current varies linearly with laser pulse intensity up to 100 $\textmd{MW cm}^{- 2}$ beyond which high density carrier effects induce a sublinear response. Research is funded by PRO and SERC (Canada) and DARPA (USA). [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V16.00007: Anomalous magneto-plasmonic spectra: evidence for stimulated emission or superradiance? Y.D. Jho, X. Wang, J. Kono, D.H. Reitze, G.D. Sanders, C.J. Stanton, X. Wei, G.S. Solomon Previous studies on electron-hole magneto-plasmas have been limited to relatively low laser intensity and/or low magnetic fields. Here, we extend this regime by probing the emission characteristics of dense magneto-plasmas in high magnetic fields (25 T) and at carrier densities approaching 10$^{13}$/cm$^{2}$. Using a 150 fs, 775 nm Ti:sapphire chirped pulse amplifier and optical parametric amplifier, we have performed intensity and magnetic field dependent magneto-photoluminescence (MPL) measurements on the heavy hole exciton in 8 nm In$_{0.2}$Ga$_{0.8}$As multiple quantum well (QW) samples separated by 15 nm GaAs barriers. Above a threshold intensity, the emission from higher-lying Landau levels (LL's) exhibit anomalous features appearing asymmetrically on the high-energy side of the peaks. These narrow features dominate the spectrum at high excitation power. The line width of the feature is significantly narrower than the lowest LL, implying a different physical origin than simple radiative recombination. In addition, the appearance of the peaks correlates with a threshold magnetic field value of approximately 13 T. An examination of the wavelength dependence of the MPL spectra as well as a line-shape analysis suggest that the inter-LL emission is a stimulated process, arising from the high electron hole densities. [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V16.00008: From Bloch oscillations to plasmon oscillations in biased semiconductor superlattices Lijun Yang, Marc Dignam We have recently shown [1] that, even at moderate excitation densities, many features of the ultrafast interband response of asymmetric semiconductor nanostructures such as Biased Semiconductor Superlattices (BSSLs) cannot be even qualitatively understood via calculations of the response to a finite order in the optical field. We have extended this treatment to the intraband response and present a formalism for calculating the THz radiation emission from BSSLs excited via ultrashort optical pulses. The calculations are done to infinite-order in the optical field, with the inclusion of essential many-body correlations and interactions. This formalism accounts for a variety of phenomena from the low-excitation excitonic Bloch oscillations to the high-excitation plasmon oscillations. As optical intensity is increased, the spectrum of the THz emission from a BSSL first experiences a red-shift due to the asymmetric energy structure of the BSSLs. This red-shift then saturates and a high-frequency plasmon peak emerges as the exciton-exciton interactions begin to dominate. * This work is supported in part by the Natural Sciences and Engineering Research Council of Canada. 1. M. Hawton and M. M. Dignam, Phys. Rev. Lett. 91, 267402 (2003). [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V16.00009: Minigap plasmons in a two-dimensional electron gas modulated by a one-dimensional periodic potential Hiroyuki Sakaki, Manvir Kushwaha We investigate the plasmon excitations in a two-dimensional electron gas (2DEG) subjected to a one-dimensional (1D) weak periodic potential. We derive and discuss the dispersion relations for both intrasubband and intersubband excitations within the framework of Bohm-Pines' random-phase approximation (RPA). For such an anisotropic system with spatially modulated charge density, we observe a splitting of the 2D plasmon dispersion. The splitting is caused by the superlattice effect of the charge-density modulation on the collective excitation spectrum. Several illustrative examples are presented on the computed plasmons excitation energy as a function of the propagation vector as well as the Bloch vector. We also discuss how the tunneling and the potential amplitude influence the minigaps and the plasmon excitations. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V16.00010: Plasma Instabilities in Quantum Well Structures: Line Shape Signature. P. Bakshi, C.G. Du, K. Kempa We have shown [1,2], that a resonant interaction of depolarization shifted emission and absorption modes would lead to a plasma instability in quantum well structures. This is an attractive crossing of the two modes, leading to a merging, with the frequency locked-in over a range of applied biases. There is also a line narrowing, arising from an effective reduction of the collisional damping, due to the nascent plasma instability. When the instability is sufficiently strong to overcome the losses, stimulated generation of plasmons occurs, providing a plasmon laser. This becomes a source of THz radiation. When the two modes merge, the line shape changes from two separate Lorentzians to a modified Lorentzian, with a characteristic oscillatory feature, which can serve as an \textit{additional} diagnostic marker for the plasma instability. The separation between the positions of the maximum and the minimum of this line shape is a measure of the effective damping rate in the precursor stage, and hence an \textit{in-situ} measure of the inter-subband transition rates. [1] P. Bakshi and K. Kempa, Cond. Matter. Theories., Eds. J.W.Clark and P.V. Panat, Nova Science Publishers, NY, vol. 12, pp 399-412, 1997. [2] P. Bakshi and K. Kempa, Physica E 7, 63, (2000). [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V16.00011: Ab Initio Calculations for the Optical Properties of Self-Assembled Nanostructures. Igor Vasiliev Polymeric photonic materials have generated considerable interest in recent years. The dispersion of carbon nanotubes within emissive polymers increases the efficiency of radiative recombination in these materials. The light output can be further enhanced by assembling luminescent molecules on the surface of nanotubes. This study analyzes the interaction of carboxylated nanotubes and molecules of phenosafranin. The geometries, binding energies, and optical absorption spectra of the modeled nanostructures are calculated in the framework of first-principles density-functional and time-dependent density-functional methods. The computed shift in the optical absorption of phenosafranin is consistent with experimental observations and suggests the formation of charge-transfer complexes between phenosafranin and nanotubes. [Preview Abstract] |
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