Abstract
We report on an experimental study of macroscopic energy transfer in ZnO bulk crystals. We observe the ultraviolet photoluminescence (PL) emission from the near band edge (3.4eV at room temperature), a green emission band from a deep center transition, and an orange emission band. Unusually, the orange PL is not only visible at the excitation spot and at crystal edges, but we also observe a slow spatial expansion of the light emission in the orange band up to a distance of several millimeters. In contrast to that, the green and ultraviolet emission originate only from the excitation spot or are scattered from the crystal edges. We investigate the temporal dynamics of the orange PL as a function of the distance from the laser spot, by turning on and off the laser excitation. We describe the results with a two-dimensional diffusion model and discuss some possible mechanisms that may cause this energy transfer, such as Förster/Dexter transfer or a thermally induced hopping process.
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