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International Conference on Ultrafast Phenomena

Ultrafast gain in semiconductors

J.-P. Likforman, M. Joffre, Daniele Hulin

Published Jan 1, 1996• 0 citations
Publisher

Abstract

Excitonic resonances in direct-gap semiconductors have proved to exhibit similar optical non-linearities as in atoms. One example is the optical Stark effect [1] which occurs when the sample is excited by a strong pump field whose frequency is detuned from the optical transition. The optical Stark effect has been described as a coupling between the electromagnetic field and the resonance with no real absorption. It can also be seen as the creation of a virtual population in the excited state. A transient gain, quadratic in pump intensity, is associated to this effect as already observed in atomic systems [2]. This hyper-Raman process involves two pump photons and occurs at frequency 2ω p – ω0, where ω p and ω0 are the pump and atomic transition frequencies, respectively (see inset of Fig. 1).

DOI
10.1364/up.1996.thc.4

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Excitonic resonances in direct-gap semiconductors have proved to exhibit similar optical non-linearities as in atoms. One example is the optical Stark effect [1] which occurs when the sample is excited by a strong pump field whose frequency is detuned from the optical transition. The optical Stark effect has been described as a coupling between the electromagnetic field and the resonance with no real absorption. It can also be seen as the creation of a virtual population in the excited state. A transient gain, quadratic in pump intensity, is associated to this effect as already observed in atomic systems [2]. This hyper-Raman process involves two pump photons and occurs at frequency 2ω p – ω0, where ω p and ω0 are the pump and atomic transition frequencies, respectively (see inset of Fig. 1).
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.
Excitonic resonances in semiconductors are very sensitive to applied optical fields. We present here a comparison between the effects of resonant and non resonant optical excitation, creating respectively real or virtual excitons. A blue shift of the exciton line and a reduction of the oscillator strength are simultaneously observed in multiple-quantum-well-structures for the two following cases : presence of a population of real excitons in very thin wells (two-dimensional systems), or presence of an intense offresonance non absorbed optical field (virtual excitons) independently of the well thickness. The latter configuration (optical Stark effect) has been studied as a function of pump intensity, determining in particular how the saturation of the blue shift is related to the exciton bleaching.
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.
The optical Stark effect of excitons in semiconductors, where the exciton resonance is blue shifted by an off-resonance light beam, has been actively studied. Here we report on the observation of a very fast recovery of the exciton bleaching for an off-resonance excitation condition in a quantum well sample at room temperature. The good agreement between the experimental results and our numerical calculations of the time-resolved absorption spectra suggests a coherent transient population buildup and decay occurring on the time scale of the pump pulse.
Irradiation of a medium with non-resonant light leads to a shift of the energy levels in the system. This optical, or dynamical Stark effect has been studied for many years in atomic gases. It is only recently, however, that it could be detected for excitonic transitions in solids. This development has stirred considerable interest, because the optical Stark effect has inherent features which makes it well suited for applications such as in ultrafast all-optical data processing. Since the pump light beam falls in a transparency region of the medium, it experiences little loss, in contrast to optical non-linearities based on a saturation mechanism. Also, because virtual transitions are involved, the light-induced shift of exciton levels follows closely the pump pulse duration, making it possible to design ultrafast all-optical logic gates with subpicosecond switch-on and -off times.
The coherent response of a semiconductor three-state system to one or two intense light pulses is investigated experimentally on a 100 fs time scale. Three experiments constitute this dissertation: observation of excitonic Rabi oscillations, measurement of two-exciton coupled Stark shifting, and an attempt to observe dark states. Basic concepts of time-resolved ultrafast semiconductor spectroscopy are explained, followed by an analysis of semiconductor two- and three-state systems. Pure two- and three-state dynamics are derived from first principles, followed by the development of the appropriate semiconductor Bloch equations (SBE). Two-color pump-probe, two-color pump-pump, and pump-pump-probe techniques are explained in the context of three-state semiconductor experiments. The experimental setup is explained in detail. Resonant two-color pump-probe measurements resulted in the first observation of multiple excitonic Rabi oscillations. The common conduction band shared by the light-hole and heavy-hole excitons of an InGaAs multiple quantum well allowed us to measure hh-exciton density (Rabi) oscillations by probing lh-exciton absorption. By studying the intensity dependence of the Rabi frequency, we showed the important role of many-body effects in renormalizing the dipole energy. The shared conduction band also causes the lh-exciton resonance to Stark shift when the hh exciton is Stark shifted. We measured a transient Stark shifting of both resonances due to virtual hh-exciton transitions. We observed that the ratio of the hh-exciton shift to lh-exciton shift was 2:1 at large pump-exciton detuning, as predicted from a simple three-state dressed exciton picture. For smaller detunings we saw an increase in the ratio and a redshift of the non-dipole coupled exciton state. Both of these observations are consistent with the most recent theories and experiments on excitonic Stark shifting. For strong near-resonant pumping of both lh- and hh-exciton transitions, an intervalence-band Raman-type coherence follows from the SBE that results in a transparent eigenstate (dark state) when both pumps are equally detuned from resonance. The existence of this coherence is well-known in atomic-optical systems, but has been elusive in semiconductors. Our inconclusive experimental result is presented along with an evaluation of experimental shortcomings. In brief, the expected change is absorption was too small to see.