Temperature-dependent exciton polariton photoluminescence in ZnO films

The polarized photoluminescence (PL) in a (0001) oriented ZnO epitaxial film is studied in the temperature range 4.5-250 K. We report the evidence of exciton polariton emission between 50 and 130 K. In this range the PL of mixed polariton modes is detected in the geometry of an extraordinary beam, realized by the use of a large-aperture microobjective collecting light from the cleaved sample edge at different angles with respect to the c axis. The elevated temperatures facilitate the polariton emission due to the thermal population of both A and B exciton branches and the enhanced polariton scattering into the photonlike mixed modes.