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physica status solidi (b)

Analysis of Excitonic Absorption Changes Induced by Incoherent Exciton and Electron-Hole Pair Populations

T. Meier, S. W. Koch

Published Sep 1, 2000• 5 citations
Publisher

Abstract

The density-dependent absorption in the presence of incoherent occupations of excitons and electron–hole pairs is analyzed including many-body correlations represented by exciton to two-exciton transitions. At low temperatures the exciton resonance is bleached and shifts to higher energies with increasing density. For higher temperature, the blue shift disappears and pure bleaching is obtained.

DOI
10.1002/1521-3951(200009)221:1<211::aid-pssb211>3.0.co;2-l

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The density-dependent absorption in the presence of incoherent occupations of excitons and electron–hole pairs is analyzed including many-body correlations represented by exciton to two-exciton transitions. At low temperatures the exciton resonance is bleached and shifts to higher energies with increasing density. For higher temperature, the blue shift disappears and pure bleaching is obtained.
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The intensity dependence of pump-induced excitonic absorption changes is studied in high-quality In0.04Ga0.96As{\mathrm{In}}_{0.04}{\mathrm{Ga}}_{0.96}\mathrm{As}In0.04​Ga0.96​As quantum wells for different detunings and polarization configurations. For resonant excitation with increasing pump intensity a saturation of both the exciton bleaching and the induced absorption due to two-exciton resonances is observed, whereas for pumping below the exciton a characteristic polarization dependence of higher-order contributions to the optical Stark effect is found. Microscopic calculations including Coulomb correlations show that the experimental observations can be described by the coherent dynamics of single- and two-exciton resonances up to fifth order in the optical field.
Summary form only given. Since the original discovery of the excitonic optical Stark effect, substantial progress has been made in the understanding of this ultrafast nonlinear optical effect. Recently, a red shift of the excitonic absorption line has been reported using low intensity excitation detuned 4.5 meV below the 1s-heavy-hole (hh) resonance and cross-circularly-polarized pulses. This was attributed to higher-order Coulomb correlations, interpretable as bound and unbound two-exciton states and memory effects. For co-circularly polarized pulses the well-known blue shift was found. Here we present a related effect occurring in the higher-intensity regime. Standard pump-probe experiments were performed on high-quality 8.5 nm-thick In/sub 0.04/Ga/sub 0.96/As multiple quantum well (MQW) samples.
Excitonic resonances in semiconductors are very sensitive to applied optical fields. We present here a comparison between the effects of resonant and non resonant optical excitation, creating respectively real or virtual excitons. A blue shift of the exciton line and a reduction of the oscillator strength are simultaneously observed in multiple-quantum-well-structures for the two following cases : presence of a population of real excitons in very thin wells (two-dimensional systems), or presence of an intense offresonance non absorbed optical field (virtual excitons) independently of the well thickness. The latter configuration (optical Stark effect) has been studied as a function of pump intensity, determining in particular how the saturation of the blue shift is related to the exciton bleaching.