Heavy-hole and light-hole oscillations in the coherent emission from quantum wells: Evidence for exciton-exciton correlations

We demonstrate the presence of strong exciton-exciton correlations in GaAs- AlGaAs multiple quantum wells by using the polarization selection rules for four-wave mixing and a test that was originally designed to distinguish be tween quantum beats and polarization interference. We show that, when the f our-wave-mixing signal is produced by two pulses that have the same circula r polarization, dynamic beating behavior is observed at the heavy-hole-ligh t-hole frequency that provides evidence of Coulomb-induced correlations tha t go beyond those expected from the semiconductor Bloch equations in the Ha rtree-Fock approximation and beyond the local field corrections used in sim ple phenomenological models. Moreover, this behavior cannot be explained by invoking biexcitonic effects. We demonstrate that this dynamic excitonic c oupling can be explained in terms of a competition between quantum-beat-lik e and polarization-interference-like behavior, and we show that such behavi or can be used to place quantitative limits on the coupling strength. In ad dition, we use a simple phenomenological model to show that excitation-indu ced-dephasing can produce such correlations.