Relaxation of nonthermal hh and lh excitons in ZnSe quantum wells

The strong exciton-LO phonon coupling in ZnSe QWs gives a direct access to the relaxation dynamics of nonthermal, free heavy-hole and light-hole excitons. Narrow hot-exciton distributions can be generated by LO-phonon assisted exciton formation. The thermalization of these excitons is monitored by cw or time-resolved phonon-sideband spectroscopy. A thermal exciton distribution evolves on a 100 ps timescale at low exciton densities when only scattering with acoustic phonons is possible. Exciton-exciton scattering accelerates the relaxation dynamics at high densities. Filling of the free heavy-hole exciton states and thermal population of the light-hole exciton states is observed. Measurements of the polarization degree after excitation with a sigma(+)-polarized picosecond pulse show that light-hole excitons with zero in-plane momentum change sign of their spin with each emission of an LO phonon.