Effect of a microstructure on the formation of self-assembled laser cavities in polycrystalline ZnO

The optical properties of polycrystalline ZnO have been studied to elucidate the occurrence of random laser action. The spatially-resolved refractive index has been mapped out by using the scanning electron energy loss spectroscopy across the grain boundary. It is observed that the refractive index decreases gradually when the probe beam is approaching to the grain boundary. A thin reflective layer of ∼10 nm is found to form in the vicinity of the grain boundary, which assists the optical scattering. The photon scattering factor of the reflective layer has been determined and is shown to correlate well with the results of the coherent backscattering method. Together with the cathodoluminescence studies, it is suggested that the overall structure, which includes the grain and grain boundary, determines the laser action in ZnO.