Résumé
Microcavity polaritons attract nowadays a lot of interest from the scientific community as they allow for the realization of low-threshold coherent light emitters that operate up to 300 K. It is usually admitted that, at low-temperature and negative cavity detunings, the mechanism limiting the polariton lasing threshold is the relaxation of polaritons towards the bottom of the lower-polariton branch, which is mediated by scattering with acoustic phonons. However, it has been theoretically predicted that in multi quantum well (QW) microcavities, dark exciton and biexciton states should play an important role in the overall relaxation mechanisms of cavity polaritons. Here, we address this issue through a time-resolved photoluminescence (TR-PL) study of the relaxation dynamics at 10 K of microcavity polaritons in a 3!-cavity containing 67 Al0.2Ga0.8N/GaN QWs. The vacuum Rabi splitting of our microcavity is 60 meV and cavity detunings (!) ranging between -100 and 0 meV are investigated. At 10 K, the microcavity PL at small angle is dominated by a transition at 3.63 eV attributed to lower polaritons with a small in-plane wave vector. Two additional non-dispersive lines at 3.673 and 3.693 eV are also observed. From excitation/power-dependent experiments, we attribute them to the recombination of cavity biexcitons and localized dark excitons, respectively. We show by TR-PL that the lower polariton PL at zero-angle exhibits at all times the same PL decay as biexcitons. From this dynamical behavior, together with the !-dependence of the biexciton emission energy, we can describe the interplay between polaritons and biexcitons. In particular, we demonstrate that in nitride-based multi QW microcavities, the mechanism limiting the relaxation of polaritons towards the center of the Brillouin zone is the radiative dissociation of cavity biexcitons into lower polaritons with high in-plane wave vector.