Spin-dependent exciton-exciton interaction in ZnSe quantum wells

The loss of phase coherence of optically oriented excitons in ZnSe quantum wells is determined from the decay of the four-wave mixing signal in the presence of a background exciton density generated by a prepulse. The homogeneous linewidth of the exciton resonance strongly depends on the optical orientation, the density, and the spin-relaxation dynamics of the background excitons. Modelling of the data reveals that exciton-exciton interaction including fermion exchange dominates while the exciton-exciton exchange process and dipole-dipole scattering are negligible. Qualitative agreement between theory and experiment is achieved when exciton spin relaxation to dark states is considered.