RESONANT FEMTOSECOND EMISSION FROM QUANTUM-WELL EXCITONS - THE ROLE OF RAYLEIGH-SCATTERING AND LUMINESCENCE

We study the ultrafast properties of secondary radiation of semiconduc tor quantum wells under resonant excitation. We show that the exciton density dependence allows one to identify the origin of secondary radi ation. At high exciton densities, the emission is due to incoherent lu minescence with a rise time determined by exciton-exciton scattering. For low densities, when the distance between excitons is much larger t han their diameter, the temporal shape is independent of density and r ises quadratically, in excellent agreement with recent theories for re sonant Rayleigh scattering.