Ultraviolet lasing with low excitation intensity in deep-level emission free ZnO films

We have prepared high-quality zinc oxide (ZnO) films by using laser ablation and magnetron sputtering. The deep-level emission free spectra have been obtained for ZnO films on both sapphire and fused silica. It was found that the stimulated emission and band-edge emission were sensitive to the quality of the ZnO films. Strong band-edge emission can be achieved and the stimulated emission can be excited at low threshold excitation density of 7 W/cm2 in single crystal ZnO films. By using special devised optical geometry, we have proved that the traditional resonant cavity is not imperative to form the lasing and that in-plane closed loop paths via multiple scattering between crystalline grains seems more suitable to account for the lasing mechanism.