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, we report the improvement of the quality factor of ZnO microcavities, and the corresponding evolution of lasing mechanisms. For a quality factor Q=450, we have shown that the cavity switches from an exciton laser at large negative detuning, to a polariton laser at smaller negative detuning. The exciton laser appears while still observing polariton branches and, thus, with stable excitons, and is observed below 240K. It emphasizes the role of exciton scattering processes, typical of II-VI semiconductors, that are involved in the gain process [Guillet et al., APL 98, 211105 (2011)]. At smaller 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. The far-from-equilibrium behavior of the system is typical of a polariton laser in a cavity presenting photonic disorder [Guillet et al., APL 99, 161104 (2011)]. In a more recent cavity with a quality factor larger than 2000 and a Rabi splitting of 200 meV, polariton lasing is obtained systematically, over a much wider range of exciton-photon detuning and of temperature up to 300K. The large variation of the cavity thickness allows to analyse polariton lasing processes in a monomode (one polariton mode) and in a multi-mode regime (two and more competing polariton modes). The complete phase diagram of the ZnO polariton laser is drawn, showing that the lasing threshold is only 6 times larger at 300 K than at 8 K. Strongly excitonic (92% exciton fraction) as well as strongly photonic condensates (88% photon fraction) are realized, that pave the way to promising advances of the polariton physics in ZnO microcavities.