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Physical Review B

Spectral and thermal dependence of phase coherent photorefractivity in ZnSe quantum wells

Hans Peter Wagner, S. Tripathy, Pradeep Bajracharya, H.-P. Tranitz

Published Feb 17, 2006• 5 citations
Publisher

Abstract

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 $90\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$ 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 $35\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. 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.

DOI
10.1103/physrevb.73.085318

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We observe an efficient phase coherent photorefractive effect in ZnSe single quantum wells for ultrashort light pulses resonant to the excitonic transition. The effect is attributed to the formation of an electron grating in the quantum well induced by the interference of coherent excitons that preserve phase and polarization of the incident light fields. All characteristic features of the diffracted signal are explained and reproduced by numerical calculations that are based on the optical Bloch equations for a three-level system.
We investigate the influence of electron density and trion formation on the phase-coherent photorefractive effect in ZnSe single quantum wells by laser energy and temperature dependent degenerate four-wave-mixing experiments using 90-fs pulses. Two different structures with the same quantum well width and confinement energy but different barriers adjacent to the GaAs substrate are studied in order to compare the formation of a photorefractive electron density grating at specific excitation conditions. At temperatures below 35 K and laser excitation energy close to the exciton energy the formation of trions significantly suppresses the generation of an electron density grating. At lower excitation energies increasing space-charge fields reduce the trion binding energy which leads to an enhanced thermal ionization of trions resulting in a strong phase-coherent photorefractive effect. Due to the thermal dissociation of trions at temperatures exceeding 45 K a significant photorefractive effect exists even at exciton resonant excitation. The experimentally observed signal traces obtained at different excitation conditions are in good agreement with model calculations that are based on the optical Bloch equations, including inhomogeneous broadening at strong space-charge fields.
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