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AIP conference proceedings

Exciton Associated Photorefractive Effect in ZnMgSe/ZnSe Quantum Wells

Hans Peter Wagner

Published Jan 1, 2005• 0 citations
Publisher

Abstract

We observe a phase coherent all‐optical photorefractive effect in a ZnSe single quantum well in four‐wave‐mixing configuration using ultrashort pulses. The microscopic origin of the observed effect is attributed to the coherent formation of an electron grating by interfering excitons.
DOI
10.1063/1.1994564

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We investigate the spectral and thermal dependence of an exciton resonant all-optical and phase coherent photorefractive effect in a ZnSe single quantum well by four-wave mixing using 90fs90\phantom{\rule{0.3em}{0ex}}\mathrm{fs}90fs pulses. The observed phase coherence is attributed to an exciton-induced formation of an electron grating within the quantum well. The diffracted signal exhibits an exponential decay that is equal to the exciton dephasing rate and it shows a pronounced suppression at the temporal overlap of incident pulses. With decreasing photon energy the diffracted signal traces reveal faster decay and the exciton transition is shifted towards lower energies indicating an increasing electron concentration within the quantum well. In temperature dependent measurements we observe a filling of the signal dip at pulse overlap and a transition from the photorefractive effect to a ${\ensuremath{\chi}}^{(3)}$ four-wave-mixing process below 35K35\phantom{\rule{0.3em}{0ex}}\mathrm{K}35K. Additional experiments using a cw pump laser are consistent with the proposed mechanism of the observed photorefractive effect. The phase coherent photorefractive signal is reproduced by a phenomenological model that is based on the optical Bloch equations for a five-level system.
We report on the decoherence of excitons in ZnSe single quantum wells by intensity dependent four‐wave‐mixing (FWM) experiments using 30 fs pulses. The FWM traces at the 11h exciton transition reveal marked quantum beats and a fast decay at pulse overlap that is attributed to a destructive interference of the polarization wavepacket and to an effective diffraction of the exciton polarization at an electron‐hole‐pair density grating. Three‐pulse FWM experiments are performed to study the formation of excitons from this electron‐hole‐pair grating.
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