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Journal of the Optical Society of America B

Optical dephasing and transient grating experiments on 2-D excitons in GaAs single quantum wells

A. Honold, Lothar Schultheis, J. Kühl, C. W. Tu

Published Feb 1, 1987• 0 citations
OpenAlex

Abstract

Optical transitions of 2-D excitons are believed to represent a typical model for inhomogeneous line broadening due to the well width fluctuations of the quantum wells.1 Because of the inhomogeneous broadening and its specified relaxation processes (relaxation of delocalized excitons to localized states), it is difficult to obtain information on the intrinsic relaxation times of delocalized excitons in such multiple-quantum-well structures.

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physica status solidi (a)•Mar 1, 2002•0 citations
Optical transitions of 2-D excitons are believed to represent a typical model for inhomogeneous line broadening due to the well width fluctuations of the quantum wells.1 Because of the inhomogeneous broadening and its specified relaxation processes (relaxation of delocalized excitons to localized states), it is difficult to obtain information on the intrinsic relaxation times of delocalized excitons in such multiple-quantum-well structures.
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This work presents new results of experimental studies of exciton relaxation in GaAs/AlGaAs multiple quantum well materials, using transient nonlinear laser spectroscopy techniques. In the low excitation regime, effective optical Bloch equations similar to the density matrix formalism of a two-level system describe the response. Solutions of these equations appropriate for three-pulse scattering studies are presented, along with a discussion of new experimental techniques used in this work. Non-ideal conditions present in the molecular beam epitaxy growth process introduce monolayer fluctuations in the quantum well thickness, leading to inhomogeneous broadening and exciton localization at sites of minimum energy. Localized excitons are separated in energy from extended, quasi-delocalized states by the mobility edge. Studies of localized exciton relaxation indicate that these excitons relax on a much slower timescale than delocalized excitons. The temperature dependence of the localized exciton relaxation rate shows that above 15K, the excitons are thermally activated to extended states above the mobility edge. Below 15K the excitons relax by phonon assisted migration between localization sites. For temperatures above 5K, and for exciton energies greater than 1 meV below line center, an extra dephasing mechanism characterizes the exciton relaxation. The inhomogeneous broadening of the exciton leads to the emission of stimulated photon echoes in the experimental geometry used in this work. The time-resolved emission demonstrates the stimulated photon echo nature at low excitation densities. At higher excitation densities, the decay dynamics is more complex. The time-resolved emission shows the presence of a stimulated photon echo and a free polarization decay. This unusual result suggests that both a homogeneous and an inhomogeneous exciton distribution comprise the exciton resonance. Exciton relaxation at low excitation densities represents the interaction of the exciton with the lattice. At higher excitation densities, exciton-exciton and exciton-free carrier interactions modify the relaxation rates. The dependence of the exciton relaxation rate on excitation density shows that many-body effects play an important role in the exciton physics. Free carrier interactions are more efficient at increasing the exciton relaxation rate; both types of interactions are more important in quantum well systems than in bulk materials.
Recent experimental and theoretical work suggests that excitons in GaAs/AlGaAs multiple quantum wells may be localized by disorder, or delocalized with Bloch-type wavefunctions1. At low temperature, localized excitons are expected to be characterized by spectral diffusion and by long optical coherence times, while delocalized excitons have short coherence times and exhibit spatial mobility2. The short optical coherence time associated with delocalized excitons results in a linear absorption profile that is primarily homogeneously broadened, while the disorder associated with localized excitons leads to an inhomogeneously broadened profile. In this work, we investigate the energy and phase relaxation of HH1 excitons in GaAs/AlGaAs MQW's at low temperature, and provide evidence that, dependent on the exciton density, both localized and delocalized excitons contribute to the nonlinear optical response.
By degenerate four-wave mixing experiments in a two-beam geometry, we have investigated the initial coherence and dephasing of quasi two-dimensional excitions and biexcitons in GaAs multiple quantum wells. The dephasing has contributions from phonon scattering, interface-roughness scattering and exciton-exciton scattering. Inhomogeneous broadening and generation of coherent wavepackets play a significant role in the coherent exciton dynamics. The incoherent exciton dynamics, diffusion and recombination, is studied by three-beam transient grating experiments. A significant difference in the interface roughness scattering of coherent and incoherent (thermalized) excitons is observed.
In this paper by Wolfgang Langbein and Jørn M. Hvam [1], selected as Editor's Choice in the present issue of phys. stat. sol. (a), the localization of biexcitonic and excitonic states in GaAs quantum wells is investigated by spectrally resolved four-wave mixing (FWM) experiments. The cover picture is a contour plot of the FWM intensity as a function of temporal pulse separation τ12 for cross-linear and co-polarization, scaled with the exciton binding energy R*y. Data are shown for three single and multi quantum well structures with different thicknesses, causing varying homogeneous and inhomogeneous broadening of the excitonic transitions (for details see [1]). The paper is an invited presentation and part of the Proceedings of the International Workshop on “Physics of Light–Matter Coupling in Nitrides (PLMCN-1)” held in Rome, Italy, September 26–29, 2001.
In this paper by Wolfgang Langbein and Jørn M. Hvam [1], selected as Editor's Choice in the present issue of phys. stat. sol. (a), the localization of biexcitonic and excitonic states in GaAs quantum wells is investigated by spectrally resolved four-wave mixing (FWM) experiments. The cover picture is a contour plot of the FWM intensity as a function of temporal pulse separation τ12 for cross-linear and co-polarization, scaled with the exciton binding energy R*y. Data are shown for three single and multi quantum well structures with different thicknesses, causing varying homogeneous and inhomogeneous broadening of the excitonic transitions (for details see [1]). The paper is an invited presentation and part of the Proceedings of the International Workshop on “Physics of Light–Matter Coupling in Nitrides (PLMCN-1)” held in Rome, Italy, September 26–29, 2001.