We report on the observation of coherent secondary emission from resonantly excited bound and unbound two-exciton states in quantum wells. The emission is intrinsic, in contrast to the well-known Rayleigh scattering from excitons that requires disorder. We measure the spectral amplitude and the phase (and, hence, the femtosecond dynamics) of this coherent emission using ultrafast spectral interferometry, and obtain direct information on the contributions to the nonlinear susceptibility from exciton-exciton correlations beyond the mean-field approximation.

• Received 26 March 1999

DOI:https://doi.org/10.1103/PhysRevB.60.15585