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AIP conference proceedings

Phase Coherent Photorefractive Effect in ZnSe Quantum Wells Using Ultrashort Pulses

S. Tripathy, Pradeep Bajracharya, Hans Peter Wagner

Published Jan 1, 2007• 0 citations
Publisher

Abstract

We report on an exciton resonant phase coherent photorefractive effect in ZnMgSe/ZnSe single quantum wells using 30 fs light pulses. The experiments are performed in a four‐wave‐mixing configuration using two or three pulses. The PCP effect is attributed to the formation of an electron grating within the QW that is caused by the interference of excitons in the ZnMgSe barrier.

DOI
10.1063/1.2730457

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We report on an exciton resonant phase coherent photorefractive effect in ZnMgSe/ZnSe single quantum wells using 30 fs light pulses. The experiments are performed in a four‐wave‐mixing configuration using two or three pulses. The PCP effect is attributed to the formation of an electron grating within the QW that is caused by the interference of excitons in the ZnMgSe barrier.
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 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.
Spectrally resolved and time-integrated degenerate four-wave mixing (FWM) experiments with ultra-short pulses have been used to investigate the relaxation times and coherent interactions of excitons in the Boltzmann and in the quantum kinetic regime in ZnSe single quantum wells.Three-pulse FWM experiments using 100 fs pulses in a 10 nm thick ZnSe/ZnMgSe single quantum well reveal nearly equal zero-density dephasing rates for coherences between + σ and - σ heavy-and/or light-hole exciton states as compared to the dephasing rate of the interband coherence between these states and the ground state.The measurements also demonstrate that exciton-exciton scattering destroys the phase coherence between heavy-and light-hole excitons about three times more effectively than the coherence between + σ and + σ heavy-hole excitons.The different coherent nonlinear signals obtained in a three-beam FWM experiment are explained and reproduced by numerical calculations of the optical Bloch equations for a ten-level model including excitation induced dephasing (EID).Two-beam FWM experiments using 30 fs pulses on a 10 nm ZnSe quantum well reveal marked quantum beats and a pronounced non-exponential decay for delay times smaller ~500 fs.The pronounced peak at pulse overlap is attributed to rapid destructive interference of the created polarization and to spin dependent quantum kinetic effects caused by the transient formation of an electron-hole (eh) pair grating.The three-beam FWM trace shows a bi-exponential decay which is attributed to a nonlinear polarization that is caused by the interaction of polarization 3 k with both an exciton grating and an eh-pair grating.Model calculations using the OBEs of a two level system that includes ∑ -+
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.