Picosecond four-wave-mixing in GaN epilayers at 532 nm

Pulsed probe degenerate four wave mixing experiments were performed on GaN epilayers using 13 ps pulses at 532 nm. Intensity and time response of the scattering efficiency was studied. Intensity dependence of the observed signal suggests carrier generation by both single and two photon effects. The absolute scattering efficiency was measured and related to pump-induced nonlinear index change. The nonlinear refractive coefficient found was 1 x 10(-3) cm(2)/GW which is greater than an order of magnitude larger than the expected value. Time response of the signal was found to be dictated by carrier lifetimes. Double-exponential decays to trap levels with lifetimes of 100 ps and 1.1 ns are suggested as the dominant recombination processes. (C) 1996 American Institute of Physics.