Simultaneous formation of visible and ultraviolet random lasings in ZnO films

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We have demonstrated visible and ultraviolet random laser actions simultaneously from highly disordered ZnO polycrystalline thin films under optical excitation. It is found that the realization of ZnO grain boundaries by thermal annealing can provide sufficient optical gain (i.e., related to deep-defect-level radiative recombination) and coherent feedback to achieve random laser action at visible wavelength. Furthermore, the coexistence of visible and ultraviolet random lasings inside the highly disordered ZnO films is due to the size difference of the random cavities at different wavelengths.