Anomalous selection rules and heavy-light hole beats: Stress effects in GaAs

Time-resolved measurements of the coherent emission due to excitons in GaAs show a polarization-dependent phase shift of the heavy-light hole exciton beating observed in the linear optical response and deviations from the usu al polarization selection rules. The anomalies are ascribed to the presence of strain in the plane perpendicular to that of the light propagation. The data are in qualitative agreement with theoretical calculations based on t he strain-orbit Hamiltonian and a simple harmonic model for the excitonic r esonances.