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Nonlinear Optical Properties of Materials

Oscillator Strength, Dephasing and Stark Shift of Free and Three-Dimension Confined Excitons

M. Dagenais

Published Jan 1, 1988• 0 citations
Publisher

Abstract

The concept of giant oscillator strength for both free and bound excitons is elucidated. It is shown that, the oscillator strength of these transitions is enhanced over the free exciton oscillator strength per molecule of the crystal by the ratio of the volume of the system divided by the volume of the crystal unit cell. An expression for the free exciton oscillator strength is obtained that can be used to describe the AC Stark shift of an excitonic resonance and also its saturation behavior.

DOI
10.1364/nlopm.1988.tua4

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The concept of giant oscillator strength for both free and bound excitons is elucidated. It is shown that, the oscillator strength of these transitions is enhanced over the free exciton oscillator strength per molecule of the crystal by the ratio of the volume of the system divided by the volume of the crystal unit cell. An expression for the free exciton oscillator strength is obtained that can be used to describe the AC Stark shift of an excitonic resonance and also its saturation behavior.
The theory of the coherent pairing of excitons in the presence of a resonant electromagnetic field is generalized to describe the biexciton spectra of molecular crystals, which contain several molecules (atoms) in the unit cell. The excitation spectrum for crystals like anthracene having two molecules per unit cell is discussed in detail. It is found that, at high exciton densities and under resonance of near-resonance conditions and at temperatures below some value T c , the formation of four biexciton states is feasible provided that the dielectric gaps induced by the electromagnetic field are less than those of the corresponding biexciton states. Expressions for the absorption coefficient describing optical transitions to the biexciton states are derived.
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.
When a direct-gap semiconductor is irradiated in the transparency region, the exciton line blue shifts. This so-called optical Stark effect of the exciton is due to a coupling between the exciton and all biexcitonic states (bound or unbound). More precisely we show that the exciton shift results only from interactions between excitons (Coulomb interaction and Pauli exclusion). At large detuning, Pauli exclusion dominates, and the excitonic shift is the same as the dressed-atom blue shift. In material having a bound biexcitonic molecule, the excitonic blue shift, observed at large detuning, becomes a red shift at small detuning close to the exciton-biexciton resonance.
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.