Coherent secondary emission from resonantly excited two-exciton states: Anintrinsic phenomenon

We report on the observation of coherent secondary emission from resonantly excited bound and unbound two-exciton states in quantum wells. The emissio n is intrinsic, in contrast to the well-known Rayleigh scattering from exci tons that requires disorder. We measure the spectral amplitude and the phas e (and, hence, the femtosecond dynamics) of this coherent emission using ul trafast spectral interferometry, and obtain direct information on the contr ibutions to the nonlinear susceptibility from exciton-exciton correlations beyond the mean-field approximation. [S0163-1829(99)-2].