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 polariton condensation over a broad range of temperatures, up to room temperature, and of exciton-photon composition for the polariton condensate. This is the outcome of the improvement beyond Q=2000 of the microcavity quality factor, one of the key parameters for polariton condensation.Our fully-hybrid bulk ZnO microcavity combines a high quality active region, made up of bulk ZnO from a thinned substrate, and a cavity with a high quality factor, enabled by two dielectric mirrors. The large variation of the cavity thickness allows to analyze polariton condensation processes in thin () as well as thick (2 to 3 ) cavities. The Rabi splitting approaches 250 meV. Polariton condensation is obtained systematically [1], over a wide range of exciton-photon detuning and of temperature up to 300K. The complete phase diagram of the ZnO polariton laser is drawn (cf figure 1a), showing that its threshold is only 6 times larger at 300 K than at 8 K. It is in a good qualitative agreement with the simulations of exciton and polariton relaxation in a kinetic model (figure 1b). Strongly excitonic (96% exciton fraction) as well as strongly photonic condensates (83% photon fraction) are realized, that pave the way to promising advances of the polariton physics in ZnO microcavities. This tunability represents an important progress compared to our previous demonstration of a ZnO polariton laser in a Q=450 microcavity [2], as well as to other recent reports [3]. It also confers a strong advantage to ZnO microcavities compared to GaN [4], since thanks to their very large Rabi splitting the optimal detuning for condensation is positive, instead of negative, at moderate temperatures, and close to zero at room temperature. One important consequence is that more excitonic condensates (i.e. with stronger interactions, and therefore nonlinearities) can be generated and investigated.