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Slow and Fast Light (SL)

Tunable optical delay with carrier induced exciton dephasing in semiconductor quantum wells

Susanta K. Sarkar, Yan Guo, Hailin Wang

Published Jan 1, 2006• 0 citations
Publisher

Abstract

We report the experimental demonstration of tunable optical delay using carrier induced exciton dephasing in a GaAs quantum well. Fractional delays exceeding 200% have been obtained for an 8 ps optical pulse.
DOI
10.1364/sl.2006.mc1

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We report the experimental realization of a tunable optical delay by exploiting unique incoherent nonlinear optical processes in semiconductors. The tunable optical delay takes advantage of the strong Coulomb interactions between excitons and free carriers and uses optical injection of free carriers to broaden and bleach an exciton absorption resonance. Fractional delay exceeding 200% has been obtained for an 8 ps optical pulse propagating near the heavy-hole excitonic transition in a GaAs quantum well structure. Tunable optical delay based on optical injection of free carriers avoids strong absorption of the pump beam and is also robust against variations in the frequency of the pump beam.
The polarization dependence of the ultrafast nonlinear response of localized excitons in GaAs/AlGaAs quantum wells has been investigated by spectrally resolved, transient four-wave mixing. The role of the excitation-induced dephasing in the delay dependence of the signal is discussed in detail. In particular, we show experimentally that the excitation-induced dephasing gives rise to a photon echo in inhomogeneous systems. We develop a simple analytical model for the delay dependence of the signal, which enables us to deduce the dephasing time from the four-wave mixing decay.
We study the coherent response of an optically dense exciton system in GaAs quantum wells (QWs). For this, we resonantly excite 1s1s1s heavy-hole excitons in the QWs at 8 K with subpicosecond laser pulses. The excitonic polarization is probed using time integrated (TI) degenerate four wave mixing (DFWM) measurements in self-diffraction geometry. The average power density (I1){(I}_{1})(I1​) of the leading laser pulse is systematically increased to large values, keeping that of the delayed pulse small and fixed. The dephasing rate of the excitonic coherent polarization (determined by the rate of decrease of the TI-DFWM signal with delay between the exciting pulses) attains saturation at large I1{I}_{1}I1​ after an initial rise at small I1.{I}_{1}.I1​. In comparison, the coherent polarization (as given by the DFWM signal at a fixed delay) increases initially with I1,{I}_{1},I1​, reaches a peak and then decreases at large I1.{I}_{1}.I1​.
The pulse distortion in GaAs quantum wells was studied by means of a light-gating technique. In the experiment, we used the incoherent picosecond pulse to observe accurately the changes of the coherent spike and its surrounding envelope. The experimental results clearly showed that the pulse is not anomalously delayed but is strongly distorted around the exciton resonance. We also compared the results with a computational simulation based on the absorption saturation mechanism. The simulated results agreed well with experimental results. The simulation showed that the recovery time of the exciton saturation is 90 ps around the tail part of the exciton absorption. We also observed a reduction of the coherent spike around the exciton resonance. Theoretical calculations indicate that the coherent-propagation effect in the linear absorber causes the reduction of the coherent spike.