Strong yellow emission of high-conductivity bulk ZnO single crystals irradiated with high-power gyrotron beam

We report the strong yellow emission of bulk ZnO single crystals irradiated with the high-power gyrotron beam. Hydrothermally grown bulk crystals with high conductivity are irradiated at room temperature with up to 60-W output of a sub-terahertz gyrotron wave source. During gyrotron irradiation, the high-conductivity crystals exhibit intense emissions with a peak of around 2 eV (600 nm) and a longer-wavelength tail. The sample temperatures were also elevated from room temperature to above 1000 K by irradiation. However, when heated up to 1250 K using a heater without irradiation, the ZnO crystals do not exhibit similar visible emissions. We then use the generalized Planck's radiation in non-equilibrium states as an explanation of our experimental observations. The emission peak intensity can be enhanced by the gyrotron-induced non-equilibrium states, and the emission peak position can be related to the Urbach energy. With high intensities in the visible wavelengths, the emissions of the irradiated crystals can be readily observed with our bare eyes or with inexpensive digital cameras. As the spatial distribution of the yellow emission reflects the gyrotron beam pattern, the bulk ZnO single crystals can then be utilized for the quick diagnosis of gyrotron beam patterns and positions.