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Journal of Modern Optics

Time-dependent Light Absorption by Impurity Centres

Boris P. Antonyuk, S.F. Musichenko, Peter Bussemer

Published Nov 1, 1993• 0 citations
Publisher

Abstract

Light absorption by impurity centres with generation of charge-transfer excitons is considered. The static electric field produced by already existing excitons shifts the excitation energy of non-excited centres and, hence, the probability of their excitation. This dependence of the absorption cross-section of a centre on the spatial distribution of excitons results in the nonlinear behaviour of the system under strong pumping. Computer simulation of exciton generation and annihilation shows that stationary powerful light transmitted through a sample has a time dependent power.

DOI
10.1080/09500349314552281

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Light absorption by impurity centres with generation of charge-transfer excitons is considered. The static electric field produced by already existing excitons shifts the excitation energy of non-excited centres and, hence, the probability of their excitation. This dependence of the absorption cross-section of a centre on the spatial distribution of excitons results in the nonlinear behaviour of the system under strong pumping. Computer simulation of exciton generation and annihilation shows that stationary powerful light transmitted through a sample has a time dependent power.
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I report on the effect of a moderate excess electron population on nonlinearities in modulation-doped CdTe quantum wells. I show that the electron population does not qualitatively affect the nature of correlations between excitons. In this respect, I bring strong evidence of the existence of unbound and bound (stable) two-exciton states in the presence of electrons and charged excitons (trions). In time-resolved pump and probe experiment, they lead to the observation of electromagnetically induced transparency and optical Stark shift of the exciton resonance. Rabi flopping of excitons within a sea of electrons is also clearly evidenced through ac Stark splitting and gain without inversion. The quantum coherence is more robust to electron induced dephasing than what would have been expected. I demonstrate third and higher-order exciton correlations in the presence of electrons, which manifest, for increasing exciton densities, through the blue-shift of counter-polarized exciton resonance, the red-shift of the biexciton resonance and the exciton to biexciton crossover. I also evidence correlated behavior of excitons and trions under excitation which manifests itself by crossed trion-exciton effects. I observe a wealth of phenomena encompassing bleaching, crossed bleaching, induced-absorption and energy shifts of the resonances. Significant differences are found between the nonlinear optical effects induced by an exciton and a trion population. Electron scattering with electron and exciton is shown to strongly broaden the high energy tail of exciton and trion resonance lineshapes. Variations of electron density result in a clear modification of both resonance lineshapes. The dynamics of the formation of trions is also studied. I propose a two-channel mechanism for their formation; they are formed through bi- and tri-molecular processes. This implies that both negatively and positively charged excitons coexist in a quantum well, even in the absence of excess carriers. The model is applied to a time-resolved photoluminescence experiment performed on a very high quality InGaAs quantum well sample, in which the photoluminescence contributions at the energy of the trion, exciton and at the band edge can be clearly separated and traced over a broad range of times and densities. The unresolved discrepancy between the theoretical and experimental radiative decay time of the exciton in a doped semiconductor is explained.
The propagation of a laser pulse through a GaAs-GaAlAs MQW is investigated in the frame of classical optics. Optical spectra are calculated in the vicinity of the 1 s -exciton resonance of GaAs taking into account interference effects and nonlinear absorption caused by many-exciton effects at high excitation. The density of excited e-h pairs and intensities of transmitted and reflected beam are determined in dependence on the intensity of stationary incident laser beam by simultaneous solution of the problem of exciton diffusion and propagation of the electromagnetic wave. The observed strong nonlinearities refer to an optical bistable behaviour.
The behaviour of the excitonic system stimulated by an external microscopic electric field is analyzed. The mathematical analysis is applied using the Green function of the type [Formula: see text]. This opens the possibility of strictly applying Wick's theorem for the decoupling. The pairing of operators is performed for the same, as well as for different, moments of time. Using the Green function calculated in this way, we derive the dielectric constant and the refraction and absorption indices. It is shown that the presense of external stimulation shifts the principal absorption and refraction peaks towards higher frequencies, making them, at the same time, less prominent. Thus, the conclusion is that external stimulation cannot be used to construct better absorbers of the light energy although external stimulation is able to increase absorption as well as the refraction at frequencies higher than the resonant one.