Femtosecond degenerate four-wave mixing of GaN on sapphire: Measurement of intrinsic exciton dephasing time

We describe time-integrated and spectrally resolved degenerate four-wave mixing (DFWM) on hexagonal GaN on sapphire for different laser energies and intensities. Our measured DFWM signal decays exhibit no contributions originating from polariton propagation effects in the 1- to 3-mu m-thick bulk samples or from exciton/free-carrier scattering. At low temperatures and low excitation intensities the dephasing time of the A exciton is as long as 3 ps and is most likely due to scattering by defects. At excitation densities above N-ex=5x10(15) cm(-3) exciton-exciton scattering becomes the dominant dephasing mechanism and a fifth-order diffraction signal is observed above N-ex=3x10(16) cm(-3).