Résumé
Semiconductor-based microcavities appear as a prolific system for studying light-matter interaction between a spatially-confined photonic mode and an excitonic resonance. The quasiparticles arising from this coupling (microcavity-polaritons) have enabled in the last years the observation of new lasing regimes as well as polariton Bose-Einstein condensates, vortices and lately solitons. In this panorama ZnO appears as an alternative material to more mature ones, such as GaAs or CdTe, with larger oscillator strengths and enhanced exciton stability. These two properties render ZnO very interesting for studies and applications where large particle densities and/or high temperatures are required. However, the fabrication of ZnO-based microcavities is still challenging and it often requires the use of either nitrides or dielectric materials for the DBRs. Indeed, polariton lasing was demonstrated for the first time in a ZnO-based microcavity only in 2011 [1]. In this work we report on the optical study of a fully-hybrid ZnO-based microcavity in which we combine a high quality active region made up of bulk ZnO and a high cavity quality factor, thanks to the use of two dielectric DBRs. With the cavity Q-factor measured to be more than 1500, polariton lasing is clearly observed from low to room temperature, characterized by a strong linewidth reduction, a small blueshift compared to the Rabi splitting, and an increased emission intensity (three orders of magnitude increase). Furthermore, the wedged-shape of the ZnO active region allows accessing a large range of detunings between the exciton and cavity modes. Under these conditions, the polariton lasing regime has been systematically studied as a function of temperature and detuning, from low to room-temperature. The detailed phase diagram demonstrates the important role played by LO-phonons in the dynamics of the polariton relaxation in ZnO [2], evidenced by a local threshold minimum as a function of detuning. The different relaxation regimes, i.e. kinetic Vs thermodynamic, are further investigated by analyzing a thick cavity region where several lower polartion branches (LPBs), with very different excitonic/photonic fractions are observed. Condensation is observed to take place on the optimum branch as determined by the actual detuning and excitation power. These observations are promising for realizing future multi-mode tunable lasers and room temperature optical switches. References [1] T. Guillet et al., Polariton lasing in a hybrid bulk ZnO microcavity, Appl. Phys. Lett. 99 161104, 2011. [2] L. Orosz et al., LO-phonon-assited polariton lasing in a ZnO-based microcaivty, Phys. Rev. B, 85, 121201(R), 2012.