Room-temperature condensation in whispering gallery microresonators assisted by longitudinal optical phonons

We report condensation of hexagonal whispering gallery modes (WGM) at room temperature in ZnO microwires that embody nearly perfect polygonal whispering gallery microresonators. The condensate regime is achieved in the UV spectral range only at energies below the first longitudinal optical (LO) phonon replica of the free ZnO A-exciton transition and at non-zero wave vectors. We demonstrate that the multimodality of the WGM system and the high population of free excitons and phonons with various momenta strongly enhance the probability of an interaction of quasiparticles of the cavity exciton-photon system with LO phonons. We further examine the far-field mode pattern of lasing WGM and demonstrate their spatial coherence.