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Physical Review Letters

Femtosecond ac Stark effect in semiconductor quantum wells: Extreme low- and high-intensity limits

Wayne H. Knox, D. S. Chemla, David A. B. Miller, J. B. Stark, Stefan Schmitt-Rink

Published Mar 6, 1989• 163 citations
Publisher

Abstract

We show that in quasi-2D GaAs quantum wells, below-resonance low-intensity excitation produces a pure shift of the excitons, contrary to ac Stark shifts in atomic systems. At high pump intensities two-photon 3D-real-carrier generation competes with the virtual 2D-exciton effects. We also show that dc fields applied perpendicular to the layers reduce the excitonic ac Stark effect.

DOI
10.1103/physrevlett.62.1189

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We show that in quasi-2D GaAs quantum wells, below-resonance low-intensity excitation produces a pure shift of the excitons, contrary to ac Stark shifts in atomic systems. At high pump intensities two-photon 3D-real-carrier generation competes with the virtual 2D-exciton effects. We also show that dc fields applied perpendicular to the layers reduce the excitonic ac Stark effect.
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Summary form only given. In the excitonic ac Stark shift in quantum wells the excitonic absorption line is blueshifted by shining on the sample an intense laser pulse, whose frequency is lower than the exciton transition frequency. The effect is bigger when the laser spectrum approaches the resonance with the excitonic absorption. But if the laser and excitonic absorption spectra sensibly overlap, a great amount of electronic population is excited and consequently the Coulomb nonlinearities due to the interaction between excited carries or excitons dramatically increase. One prominent effect under these conditions is the excitation- induced dephasing which can destroy the coherence of the electronic system and therefore the coupling between excitons and light, which is inherently coherent. Some evidence for the resonant Stark effect was already reported, but the direct observation of the spectral splitting was still missing. This observation demonstrates that the coherent exciton light coupling in semiconductors can be strong enough to survive the fast dephasing, and that therefore an efficient ultrafast manipulation of the exciton absorption in the coherent transient is possible. In order to magnify the ac Stark effect and its visibility a special sample was designed: a single In/sub 0.04/Ga/sub 0.96/As quantum well is grown on the top of an AlAs/AlGaAs distributed Bragg reflector; two /spl lambda//2 spacers separate the quantum well from the mirror and from the air. In this configuration the light field incident on the sample and the one reflected by the mirror constructively interfere at the quantum well position, giving a high effective absorption.
Recent works in the area of ultrafast nonlinearities due to virtual carriers in semiconductor heterostructures have generated significant interest among researchers. It has been shown that excitons in QW's behave very much like conventional two-level atoms, to the extent that exciton-exciton interaction can be neglected. Therefore, fast coherent phenomena usually associated with two-level system may occur in excitonic QW transitions. Indeed, dynamic (AC) Stark effect in QW's have been demonstrated. When semiconductor QW's were pumped below the absorption edge there were a strong "blue" shift of the exciton resonance, accompanied by a strong bleaching.
We describe the first reported experimental observation of an extremely fast shift of the n = 1 exciton transition energy in GaAs quantum-well heterostructures. The shift is produced by optical pumping below the band gap and is not associated with a carrier or exciton population. We interpret the shift in terms of an optical Stark effect. We present a model for the Stark effect on the ground-state exciton in quantum wells and find good agreement between the predictions of the model and our experimental results.
The coherent optical response of high quality microcavities containing quantum wells is investigated. The dynamics of the ac Stark effect is investigated by degenerate pump–probe experiments, by exciting the quantum well excitons at various intensities and detunings. The evolution from a simple dynamical excitonic Stark shift to a splitting is clearly observed, even when the excitation is degenerate with the continuum states. The experimental results agree well with the predictions of a theoretical model based on the semiconductor Maxwell-Bloch equations.