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Quantitative measurements of many-body exciton dynamics in GaAs quantum-well structures

TitleQuantitative measurements of many-body exciton dynamics in GaAs quantum-well structures
Publication TypeThesis
Year of Publication2010
AuthorsSmith, RP
Abstract

Heterostructures in semiconductors, such as quantum wells, give rise to new optoelec- tronic properties compared to bulk materials by confining the motion of carriers. We study quasiparticles in gallium arsenide (GaAs) quantum wells known as excitons. The absorp- tion spectrum of these excitons exhibits changes at elevated carrier-excitation levels because of many-body interactions between particles. While the spectral absorption changes from these interactions have been studied, a connection between measurements and a microscopic theory has been lacking.

The quantitative spectrally resolved transient absorption measurements described in this thesis are combined with a microscopic theory. This combination relates observed spectral changes in the probe after ultrashort optical excitation to a unique mixture of electron hole plasma, exciton, and polarization effects. Through theory-experiment comparison, we deduce the actual carrier-density levels, exciton populations, and other correlations in the system. We find that Coulomb scattering of polarization with free-carrier densities dominates the excitation-induced shifting and broadening of the exciton resonance and that exciton populations do not significantly contribute to the excitation-induced effects. We observe a strong transient gain and attribute this feature to a coherent transfer of energy between the pump and probe.

Since the excitation-induced effects are strong, theory predicts that the system should display light-statistic-dependent excitation-induced effects. To demonstrate that light statistics of optical fields have a meaningful effect on the many-body dynamics, we develop a measurement technique involving optical pulse shaping. Our measurements are consistent with theoretical predictions that light with incoherent statistics gives rise to weaker excitation induced effects than does a coherent source that creates the same density of electron-hole pairs. Preliminary results suggest that the observed effects are related to the increased temporal duration of the exciting light source. We explore the effects of different pulse shapes to elucidate the primary underlying effects that give rise to the observed weaker excitation-induced effects.