Dynamics of exciton-polariton recombination in CdS

The dynamics of exciton-polariton recombination in CdS are investigated by measuring time-resolved luminescence spectra between 1.5 and 80 K. Decay times of free and bound excitons in $\overrightarrow{\mathrm{E}}\perp \overrightarrow{\mathrm{c}}$ and $\overrightarrow{\mathrm{E}}\parallel \overrightarrow{\mathrm{c}}$ polarization are determined for various types of crystals using low-excitation densities. The experiments prove the existence of a bottleneck for the relaxation and establish its location. Transverse excitons above the "knee" of the dispersion curve are found to be in thermal equilibrium with the lattice at temperatures higher than 30 K. At low lattice temperatures, excitons above the "knee" are in partial quasiequilibrium shortly after pulse excitation. The effective temperature of this exciton distribution exceeds that of the lattice and decreases rapidly within the observed exciton lifetime of 2.5-3 nsec.