Résumé
Since their prediction in 1996, non-equilibrium polariton condensates have attracted a lot of interest, as they should allow for the realization of ultralow threshold coherent light-emitters [1]. However, the steep dispersion and the short radiative lifetime of lower polaritons (LP) within the light cone hinder their relaxation toward the center of the Brillouin zone, limiting the polariton lasing threshold. Experimental studies carried out on GaAs and CdTe-based microcavities however revealed that one available channel to bypass efficiently the relaxation bottleneck was the relaxation of LPs from the excitonic reservoir directly to the ground state of the LP branch (LPB) through the emission of one LO-phonon [2]. Here, we study by non-resonant angle- and time-resolved photoluminescence experiments the LP relaxation dynamics in a GaN-based multi-quantum well microcavity, for which polariton condensation has been reported between 10 and 340 K [3]. Regarding the strong Fröhlich interaction in GaN, we particularly investigate the role of LO-phonon on the overall LP relaxation mechanism. We first show that LPs efficiently bind into cavity biexcitons and we demonstrate that the latter complexes are in thermal equilibrium with the excitonic reservoir. We then not only demonstrate the existence of a LO-phonon enhanced relaxation channel from the excitonic reservoir toward the ground state, but also the direct feeding of the LPB by the radiative dissociation of cavity biexcitons mediated by one LO-phonon. In other words, when the energy of the bottom of the LPB corresponds to that of the first LO-phonon replica of the cavity biexciton, we observe an enhanced scattering of polaritons toward the k// = 0 state, as well as a decrease in the condensation threshold. This peculiar observation indicates that the LO-phonon assisted dissociation of excitonic molecules constitutes an additional and efficient relaxation channel driving the condensation threshold of polaritons [4]. A. Imamoglŭ et al., Phys. Rev. A 53, 4250 (1996). see for instance F. Boeuf et al., Phys. Rev. B 62, R2279 (2000). J. Levrat et al., Phys. Rev. B 81, 125305 (2010). P. Corfdir et al., submitted to Phys. Rev. B (2012).