On the Exciton Luminescence at Low Temperatures: Importance of the Polariton Viewpoint

Abstract

The intensity ratio of the LO-phonon sideband to the zero-phonon line and the width of the zero-phonon line in the free-exciton luminescence spectrum can never be understood, especially at low temperatures, by the usual theory with the assumption of the thermal equilibrium for excitons. The theory assuming exciton thermal-equilibrium gives too small values for these quantities compared with the observed ones. These large discrepancies are removed by considering the exciton luminescence from the polariton viewpoint. In this viewpoint, polaritons accumulate around the bottleneck in polariton decay lower a little than the lowest exciton energy E 0. Polaritons at the bottleneck determine the zero-phonon line, while polaritons maintaining approximately the thermal-equilibrium distribution in the energy region above E 0 determine the LO-phonon sideband. The temperature and crystal-thickness dependences of the luminescence spectrum and the quenching of the luminescence by impurities are also discussed in terms of the polariton bottleneck.