We have observed ultraviolet laser emission from ZnO nanocrystal thin films at room temperature. ZnO films were epitaxially grown on sapphire substrates by laser molecular beam epitaxy. Closely packed and hexagonally shaped nanocrystals were formed in a spiral growth mode presumably due to a large lattice mismatch (similar to 18% between ZnO and sapphire. Photoluminescence of these films clearly showed a sharp emission due to free excitons at room temperature. Above a threshold intensity as small as 24 kW cm(-2) of pumping laser pulses (355 nm, 15 ps), we observed stimulated emission at 3.2 eV, increasing with a power of eight as increasing pumping intensity. This emission is shown to be due to an exciton-exciton collision process. Excitons are confined in these nanocrystals to show the giant oscillator strength effect and resulting in an excitonic stimulated emission even at room temperature.
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