Emission properties under high-intensity excitation in ZnO epilayers

We present room temperature stimulated emission investigations on high-quality ZnO epilayers grown by molecular beam epitaxy. Under near-resonant high-intensity excitation, cavity-mode-like sharp peaks emerge in the emission spectra. The presence of threshold pumping intensity, and above which the superlinear increasing in the emission intensity with pumping indicate the fingerprint of laser action. However, structural characterizations did not reveal significant amount of plane defects in the ZnO epilayer, which could serve as accidentally formed cavities. This raises the controversy comparing with the previous interpretations with the concepts such as naturally formed optical cavity and Anderson localization effect.