Two-photon-pumped stimulated emission from ZnO single crystal

We report on two-photon-pumped stimulated emission (SE) from ZnO single crystal at low temperature. Under nanosecond pulse excitation, two-photon absorption induced SE with a threshold of 2.8 MW/cm(2) is observed, which is ascribed to the inelastic exciton-exciton scattering. The mechanism of the SE is further confirmed by temperature dependent photoluminescence spectra. However, it is interesting to find that under one photon pumping with the same power density, no SE is observed from the sample. Such unusual phenomenon is explained by the nonradiative surface recombination induced carriers depletion under single photon excitation. (C) 2011 American Institute of Physics. [doi:10.1063/1.3628329]