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European Quantum Electronics Conference (EQEC)

Relationship between spin relaxation and excitonic nonlinearity in an organic-inorganic hybrid quantum-well material

M. Toda, K. Umeda, 千恵子 矢島, Y. Arai, Hideyuki Kunugita, Kazuhiro Ema

Published Jan 18, 2006• 0 citations
Publisher

Abstract

This study analyzed the influence of the spin relaxation on the excitonic nonlinearity using a 7-level model, and found that it is important to clarify whether coherence is conserved in spin relaxation process or not. If the coherence is perfectly conserved, the nonlinearity will not have size dependence. On the other hand, if the coherence is not conserved, the nonlinearity has coherent size dependence. In order to clarify this point three-pulse four-wave mixing with several polarization configurations at the exciton resonance was performed.

DOI
10.1109/eqec.2005.1567270

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This study analyzed the influence of the spin relaxation on the excitonic nonlinearity using a 7-level model, and found that it is important to clarify whether coherence is conserved in spin relaxation process or not. If the coherence is perfectly conserved, the nonlinearity will not have size dependence. On the other hand, if the coherence is not conserved, the nonlinearity has coherent size dependence. In order to clarify this point three-pulse four-wave mixing with several polarization configurations at the exciton resonance was performed.
The paper presents a description of two-pair correlations in an electron-hole system in laser-excited direct-gap semiconductors in terms of the exciton-boson formalism. The dependence of the states of quasi-particles on their angular momentum orientation is properly taken into account following group theory rules. It is shown for the example of heavy hole excitons in GaAs quantum wells that Coulomb two-exciton correlations include effective repulsion between excitons with nonzero total momenta and effective attraction between excitons with zero total momenta. They generate exciton and biexciton nonlinearities, respectively, in the exciton-biexciton system. The Heisenberg equations of motion derived are applied to four-wave mixing in the coherent mode. Two components of Coulomb two-exciton correlations were found to generate coherent four-wave mixing either separately, one at the cocircular and the other at the perpendicular-linear polarization of applied pulses, or jointly at parallel-linear polarization. General equations are obtained for the amplitude of four-wave mixing signals with time resolution in these polarization configurations, which, in the limit of ultranarrow pulses, reduce to analytic functions of time and excitation and sample parameters. These equations are used to analyze signal characteristics in every polarization configuration. The results give a correct picture of biexciton effects and the polarization dependence of coherent four-wave mixing in GaAs quantum wells.
A systematic analysis of excitonic and biexcitonic contributions to four-wave mixing in dependence on the polarization state and on the intensity of the exciting beams is presented for ZnSe/Zn1—xMgxSySe1—iiy quantum wells. The relative signals of excitons and biexcitons for different polarization configurations are discussed in terms of a microscopic density matrix model revealing excellent correspondence to the complex experimental data. In configurations which do not allow for a third-order contribution of the biexciton of zero angular momentum higher-order correlations are shown to become important already at densities as low as 1010 excitons cm—2/pulse.
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By use of an optical coherent-control technique we demonstrate and analyze the control of transitions involved in the generation of four-wave-mixing signals in a semiconductor quantum well for the two different four-wave-mixing directions and for different sequences of the excitation pulses. Results are presented for frequency- as well as for time-resolved signals. A doubling of the coherent switching frequency is found which occurs if the laser pulses performing the control process contribute quadratically to the wave-mixing signal. A direct comparison between experiment and microscopic theory reveals how the coherent-control process acts on the coherent polarization of the exciton-biexciton system in all different configurations. An additional interpretation of the experimental results based on a simplified few-level model is able to provide an intuitive understanding of the relevant processes which govern the coherent control of the excitonic and biexcitonic wave-mixing polarizations.