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NASA STI/Recon Technical Report N

Nonlinear optical studies of resonant systems

Duncan G. Steel

Published Jun 1, 1989• 0 citations
OpenAlex

Abstract

Work has concentrated on the development of high resolution frequency domain nonlinear spectroscopy methods as well as the use of picosecond coherent techniques such as. the stimulated photon echo and time resolved free polarization decay. Using these methods, new understanding has been obtained on the dynamics associated with excitons in GaAs/AlGaAs quantum well structures, particularly as these dynamics are affected by disorder.

Related papers

Stimulated photon echoes and free polarization decay in time-resolved four-wave mixing spectroscopy of HH1 excitons in GaAs/AlGaAs multiple quantum well structures at 5 K

Michael D. Webb, Steven T. Cundiff, Duncan G. Steel

•May 21, 1990•0 citations

Stimulated Photon Echoes and Free Polarization Decay in GaAs Multiple Quantum Well Structures: Evidence for Localized and Delocalized Excitons

Michael D. Webb, Steven T. Cundiff, Duncan G. Steel

Springer series in chemical physics•Jan 1, 1990•0 citations

Picosecond Nonlinear Optical Studies of Exciton Relaxation in Multiple Quantum Well Structures.

Michael D. Webb

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.

•Jan 1, 1990•0 citations

Observation of time-resolved picosecond stimulated photon echoes and free polarization decay in GaAs/AlGaAs multiple quantum wells

M. D. Webb, Steven T. Cundiff, Duncan G. Steel

We report measurements of exciton dynamics in GaAs multiple-quantum-well structures at low temperature using three-pulse picosecond four-wave mixing. The time-resolved measurements of the emission show both a delayed signal (stimulated photon echo) and a prompt signal (free polarization decay) with different decay times. The results suggest that, in addition to the inhomogeneously broadened resonance, a homogeneously broadened resonance contributes to the optical response of the exciton.

Physical Review Letters•Feb 18, 1991•116 citations

Coherent nonlinear laser spectroscopy of excitons in GaAs/AlGaAs multiple quantum wells

Duncan G. Steel, Hailin Wang, Michael D. Webb +2 more

Conference on Lasers and Electro-Optics•May 12, 1991•0 citations

Stimulated Photon Echoes and Free Polarization Decay in GaAs Multiple Quantum Well Structures: Evidence for Localized and Delocalized Excitons

M. D. Webb, Steven T. Cundiff, Duncan G. Steel

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.

International Conference on Ultrafast Phenomena•Jan 1, 1990•0 citations
Work has concentrated on the development of high resolution frequency domain nonlinear spectroscopy methods as well as the use of picosecond coherent techniques such as. the stimulated photon echo and time resolved free polarization decay. Using these methods, new understanding has been obtained on the dynamics associated with excitons in GaAs/AlGaAs quantum well structures, particularly as these dynamics are affected by disorder.
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.