The role of spin orientation and relaxation in exciton-exciton scattering

The homogeneous linewidth of optically oriented excitons in ZnSe quantum wells is deduced from four-wave mixing experiments. A background exciton density is generated by a prepulse to induce exciton-exciton scattering processes. We find a faster dephasing for excitons in pure spin populations compared to ones in mixed spin populations. This enhanced scattering vanishes with time due to spin relaxation of the background excitons. Exciton-exciton interaction including fermion exchange is found to be the dominant scattering mechanism while the exciton-exciton exchange process and dipole-dipole scattering are negligible. We model the exciton population dynamics and the related homogeneous linewidth accounting for exciton spin flip and relaxation to dark states. Femtosecond pump-probe experiments show a consistent behaviour for the blueshift of the exciton resonance in pure and mixed spin populations.