We investigate with a Boltzmann approach the spin relaxation kinetics of microcavity polaritons after an excitation pulse with near-resonant polarized light and calculate the polarization of the emitted light around and above the threshold for stimulated emission. Considering only the optically active excitons with an angular momentum $m=\pm 1$, we calculate the corresponding $2\times 2$ single-particle density matrix. Our kinetic treatment takes the polariton-acoustic phonon as well as the polariton-polariton scattering as the dominant relaxation processes into account. Both processes are spin conserving. Particularly for excitation with circular light, we find in isotropic crystals above threshold a nearly complete circular polarization degree which lasts (typically $40–60\mathrm{ps}$) much longer than the exciting $3\mathrm{ps}$ pulses due to the dominance of the stimulated spin-conserving scattering processes over the spontaneous spin-flip processes. These and other results also for linearly polarized pump light are in very good agreement with corresponding experiments on GaAs microcavities. In addition, we present time- and wave-number-dependent results which too are in qualitative agreement with the available angle- and polarization-resolved luminescence measurements.

• Received 21 February 2007

DOI:https://doi.org/10.1103/PhysRevB.77.075320