Résumé:

ZnO is a wide bandgap semiconductor with strong excitonic properties, in particular a large oscillator strength and a large exciton binding energy. It therefore raises a strong interest for the demonstration of polariton lasing up to room temperature with microcavities in the strong exciton-photon coupling regime. The strong coupling regime has recently been demonstrated for planar ZnO microcavities fabricated by various approaches. In this work, the investigated cavities consist in a ZnO active layer grown by molecular beam epitaxy embedded between an AlGaN/AlN distributed Bragg reflector (DBR) and a SiO2/SiN DBR. Coherent non-linear emission under non-resonant optical pumping is demonstrated in two different regimes as a function of the exciton-cavity detuning. At large negative detuning, from T=80 K to 300 K, the cavity switches at threshold from the strong coupling to the weak coupling regime. A gain-related transition, which appears while still observing polariton branches and, thus, with stable excitons, is observed below 240K. This shows that exciton scattering processes, typical of II-VI semiconductors, are involved in the gain process, and that we realized an exciton laser [Guillet et al., APL 98, 211105 (2011)]. At small negative detuning, and at T=120 K, the cavity persists in the strong coupling regime at threshold with negligible blue shift. Angle-resolved µPL experiments demonstrate the spectral narrowing at threshold, as well as a competition between multiple lasing polariton modes. Those results are compared with macroPL experiments. The far-from-equilibrium behavior of the system is typical of a polariton laser in a cavity presenting photonic disorder [To be published].