  Zinc Oxide thin films were grown on c-sapphire substrates using pulsed laser
deposition. Pump power dependence of surface emission spectra, acquired using a
quadrupled 266 nm laser, revealed room temperature stimulated emission
(threshold of 900 kW/cm2). Time dependent spectral analysis plus gain
measurements of single-shot, side-emission, spectra pumped with a nitrogen
laser revealed random lasing indicative of the presence of self-forming laser
cavities. It is suggested that random lasing in an epitaxial system rather than
a 3-dimensional configuration of disordered scattering elements, was due to
waveguiding in the film. Waveguiding causes light to be amplified within
randomly-formed closed-loops acting as lasing cavities.
