C. Sieh

Influence of carrier correlations on the excitonic optical response including disorder and microcavity effects

A model study of the characteristic signatures of carrier-correlation effects on the excitonic differential absorption spectra in semiconductors is presented. Using an effectively one-dimensional tight-binding system Coulomb-induced carrier-correlations up to third-order in the optical field are treated without additional approximations. To illustrate the influence of the different many-body contributions excitonic differential absorption spectra are computed for various polarizations and pump-probe time delays. The simultaneous influence of energetic disorder and correlations on the differential absorption spectra is discussed. Also presented are numerical results for the case when the semiconductor system is placed inside a microcavity in the strong-coupling regime. It is shown that the correlations induce characteristic signatures in the normal mode spectra.

Theory of coherent effects in semiconductors

A microscopic theory is applied to discuss coherent excitation effects in semiconductors. The theory is evaluated to analyze ultrafast absorption changes around the exciton resonance in semiconductor quantum-well structures where the relative strength of the different many-body contributions can be manipulated by proper selection of pump and probe polarizations. For two-band systems and oppositely circular polarization it is shown that Coulombic correlation dynamics dominates the optical response. As a consequence, the optical Stark effect in this configuration corresponds to a red shift of the exciton resonance in striking contrast to the usual atomic-like blue shift. The predictions are confirmed by experiments using high-quality InGaAs quantum-well systems.

Excitation induced shift and broadening of the exciton resonance

The influence of coherent photoexcited excitons on the absorption spectra and the Four Wave Mixing response of ZnSe/ZnMgSSe quantum wells is investigated for various polarization configurations. For purely resonant excitation at the heavy-hole-exciton a blue shift and an increasing homogeneous line width of the exciton absorption peak is found. The clear and stable blue shift shows a linear dependence on excitation fluence both in experiment and theory with different slopes for the different pump and probe polarization configurations. The equivalence of pump and probe, and Four Wave Mixing results is shown experimentally.

Higher order correlations and semiconductor optical nonlinearities

A microscopic theory for optical semiconductor nonlinearities systematically including Coulomb correlation effects is able to explain AC Stark shifts measured with all possible pump/probe circular polarizations and gives evidence for a 3-level-atom-like intervalence band coherence.

The excitonic Stark effect: absorption splitting and the influence of the light-hole exciton

Summary form only given. Since the original discovery of the excitonic optical Stark effect, substantial progress has been made in the understanding of this ultrafast nonlinear optical effect. Recently, a red shift of the excitonic absorption line has been reported using low intensity excitation detuned 4.5 meV below the 1s-heavy-hole (hh) resonance and cross-circularly-polarized pulses. This was attributed to higher-order Coulomb correlations, interpretable as bound and unbound two-exciton states and memory effects. For co-circularly polarized pulses the well-known blue shift was found. Here we present a related effect occurring in the higher-intensity regime. Standard pump-probe experiments were performed on high-quality 8.5 nm-thick In/sub 0.04/Ga/sub 0.96/As multiple quantum well (MQW) samples.

Influence of carrier-correlations on the optical Stark effect of semiconductors

Summary form only given. For a proper understanding of nonlinear optical experiments in semiconductors even the low-density limit Coulomb-induced many body correlations beyond the Hartree-Fock decoupling scheme need to be considered. In the coherent X(3) limit these correlations manifest themselves in the contributions of bound and unbound two-exciton states to the optical response. The understanding of nonlinear excitonic effects in the Stark effect of semiconductors including carrier-correlations is still of fundamental interest.