UV photoluminescence (PL) from GaN thin films was observed by multiphoton excitation. The two-photon PL excitation spectrum near the band gap agrees with the theoretical two-photon-absorption spectrum. The pump-intensity dependence and the PL excitation spectrum in the infrared indicate the existence of midgap defect states around 1 eV above the valence band. This is confirmed by the PL excitation spectrum obtained with a two-color, two-photon excitation process.
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Kim et al. (1997) studied this question.