Spin relaxation in polarized interacting exciton gas in quantum wells

Fast initial decays of both the luminescence intensity and the circular luminescence polarization, under resonant excitation of high exciton densities (typically above 2×${10}^{10}$ ${\mathrm{cm}}^{\mathrm{-}2}$), are reported. These fast decays, which are not observed in a dense excitonic system with well-defined angular momentum ${\mathrm{J}}_{\mathrm{z}}$=1 (or ${\mathrm{J}}_{\mathrm{z}}$=-1), are simultaneously initiated by the increase of the ellipticity of the photogenerating picosecond laser beam. We show that all the experimental observations support the driving role of the exciton-exciton exchange interaction in the spin-relaxation mechanism at high density. The theory of the mechanism is developed, leading to the simulation of luminescence and polarization dynamics for varied photogeneration conditions (intensity and polarization of the laser beam, temperature of the exciton gas). The theory provides an excellent interpretation of all the very specific features of the experimental data. The dephasing mechanism in polarized interacting exciton gas is identified.