Resonantly enhanced femtosecond second-harmonic generation and nonlinear luminescence in GaN film grown on sapphire

At room temperature, by using a tunable broadband femtosecond laser as excitation source we observed second-harmonic generation (SHG) and nonlinear photoluminescence (NPL) in GaN film grown on sapphire simultaneously or individually. In addition to the observation of the resonance effect of the nonlinear response when the SHG is tuned to coincide with the near-band-edge emission, we carefully measured dependence of the SHG and NPL signals on polarization of the excitation light. The results reveal that the reabsorption of the SHG photons with energies higher than the fundamental gap of GaN significantly contributes to generation of the efficient NPL signal.