We present a detailed experimental and theoretical analysis of the optical orientation of electron spins in GaAs/AlAs quantum wells. Using time and polarization resolved photoluminescence excitation spectroscopy, the initial degree of electron-spin polarization is measured as a function of excitation energy for a sequence of quantum wells with well widths between 63 and 198 Å. The experimental results are compared with an accurate theory of excitonic absorption taking fully into account electron-hole Coulomb correlations and heavy-hole-light-hole coupling. We find in wide quantum wells that the measured initial degree of polarization of the luminescence follows closely the spin polarization of the optically excited electrons calculated as a function of energy. This implies that the orientation of the electron spins is essentially preserved when the electrons relax from the optically excited high-energy states to quasithermal equilibrium of their momenta. Due to initial spin relaxation, the measured polarization in narrow quantum wells is reduced by a constant factor that does not depend on the excitation energy.

We theoretically predict a strong influence of stimulated exciton-exciton scattering on semiconductor luminescence. The stimulated scattering causes circularly polarized instead of unpolarized emission at the biexciton emission line in a degenerate gas of partly spin polarized excitons. The biexciton polarization effect increases with increasing exciton densities and decreasing temperatures and approaches almost unity in the ultimate case of Bose-Einstein condensation. Time- and polarization-resolved luminescence measurements evidence the biexciton polarization effect both in ZnSe and GaAs quantum wells.

We study the optical orientation of electron spins in GaAs/AlGaAs quantum wells for excitation in the growth direction and for in-plane excitation. Time-and polarization-resolved photoluminescence excitation measurements show, for resonant excitation of the heavy-hole conduction band transition, a negligible degree of electron spin polarization for in-plane excitation and nearly 100% for excitation in the growth direction. For resonant excitation of the light-hole conduction band transition, the excited electron spin polarization has the same (opposite) direction for in-plane excitation (in the growth direction) as for excitation into the continuum. The experimental results are well explained by an accurate multiband theory of excitonic absorption taking fully into account electron-hole Coulomb correlations and heavy-hole light-hole coupling.

We observe signatures of stimulated bosonic scattering of excitons, a precursor of Bose-EinsteinCondensation (BEC), in the photoluminescence of semiconductor quantum wells. The optical decay of a spinless molecule of two excitons (biexciton) into an exciton and a photon with opposite angular momenta is subject to bosonic enhancement in the presence of other excitons. In a spin polarized gas of excitons the bosonic enhancement breaks the symmetry of two equivalent decay channels leading to circularly polarized luminescence of the biexciton with the sign opposite to the excitonic luminescence. Comparison of experiment and many body theory proves stimulated scattering of excitons, but excludes the presence of a fully condensed BEC-like state.In the last few decades intensive theoretical and experimental work has been devoted to the understanding and observation of a macroscopic ground state of correlated electrons and holes in semiconductor structures. Analogous to Bose-Einstein-Condensation (BEC) of atoms, a dilute gas of bound electron-hole pairs -so called excitons -is predicted to condense into a common ground state [1,2,3]. Observation of condensation had been reported in CuO 2 , where excitons with extraordinary long radiative lifetime exhibit signatures of BEC [4]. More recently, the observation of a macroscopically ordered state in a GaAs based heterostructures was interpreted in terms of exciton condensation [5]. However, all reports on possible exciton condensation are controversially discussed and alternative interpretations of the experimental data have been given [6,7]. Clear criteria that are able to give unambiguous evidence for bosonic effects and ultimately BEC are therefore highly desirable. A precursor of excitonic BEC is stimulated scattering of excitons into other exciton states. The scattering rate is proportional to a bosonic enhancement factor (1 + n), where n is the number of excitons in the final state. In this letter we present experimental evidence for stimulated bosonic scattering in the exciton system of optically excited semiconductor quantum wells (QWs). The optical decay of a spinless biexciton (molecule of two excitons) into an exciton and a photon with opposite spin is subject to bosonic enhancement. The presence of spin polarized excitons causes the biexciton to decay preferentially into an exciton with the same spin. The resulting imbalance of circularly polarized photons in the final state leaves as signature a finite degree of circular polarization at the biexciton photoluminescence (PL) line. We report on clear experimental evidence for the described effect in semiconductor QWs. A full quantum mechanical many body model yields a criterion for bosonic effects which by comparison with our data proofs stimulated scattering but excludes the presence of a BEC-like state.We investigate stimulated bosonic scattering in the photoluminescene of a high quality 10 nm thick ZnSe QW embedded in 500 nm ZnS 0.07 Se 0.93 barriers grown by molecular beam epitaxy (MBE) on GaAs substrate [8]. T...