Temporal coherence of spatially indirect excitons across Bose-Einstein condensation: the role of free carriers

We study the time coherence of the photoluminescence radiated by spatially indirect excitons confined in a 10 mu m electrostatic trap. Above a critical temperature of about 1 K, we show that the photoluminescence is homogeneously broadened in the dilute regime, with a spectral width around 500 mu eV that weakly varies with the exciton density. By contrast, the spectral width reduces by two-fold below the critical temperature and for experimental parameters at which excitons undergo a gray Bose-Einstein condensation. We find evidence showing that the photoluminescence temporal coherence is limited by interactions between excitons and a low concentration of residual excess charges, leading to a minimum photoluminescence spectral width of around 300 mu eV in the condensed regime.