Ballistic propagation of exciton–polariton condensates in a ZnO-based microcavity

We report on the observation of macroscopically coherent states of exciton–polaritons in a ZnO-based bulk planar microcavity up to 250 K. Excitation power-dependent photoluminescence investigations show a clear threshold behaviour and corresponding spectral narrowing of the emission for negative detunings, revealing clear signatures of a Bose–Einstein condensate. For positive detunings, no condensation occurred but the emission from an electron–hole plasma was detected. Above threshold interscattering phenomena of condensate polaritons between roughly equidistant energy levels have been observed. As a special feature we found ballistic propagation of the condensate in the pump-induced potential landscape, making these ZnO-based microcavities promising candidates for applications based on polariton transport. This effect is caused by strong repulsive interactions in our system, leading to an immense blueshift of the condensate emission and hence to pronounced dynamic effects.