Optical nonlinearities of gallium nitride

Luminescence, induced absorption and degenerate four wave mixing experiments are performed on GaN epilayers grown on a sapphire substrate by MOCVD. We measure the nonlinear behaviour of the luminescence spectra near the excitonic resonance, by using an excitation at 4.026 eV from an excimer laser. At low intensities of excitation, spectra show a saturation of the I2 line due to the finite donor density in the sample. Higher intensities of excitation induce collision process between photo-created particles. Using a dye laser as a pump beam, we measure the induced variation of absorption of a probe beam as a function of the intensity and of the wavelength of the excitation. With increasing intensities of the pump beam, curves show a red-shift of the absorption edge and of the excitonic resonance. Pulsed degenerate four-wave mixing experiments were performed using the third harmonics of a picosecond Nd-YAG laser at 3.492 eV. A characteristic time of 16 ps has been measured, which is independent of the temperature, of the fringe spacing and of the intensity of the pump beams.