Degenerate four-wave mixing experiments on GaN in the quasistationary regime

In order to determine the third order of the nonlinear susceptibility chi((3)) of GaN, we perform degenerate four-wave mixing measurements in a two-beam configuration at low temperature on a GaN epilayer on sapphire substrate. We measure the excitation spectrum of chi((3)) close to the exciton resonances. Besides the chi((3)) contribution, higher orders of the nonlinear susceptibility show already up around 10 kW cm(-2), leading to a saturation of the signal. Results are complemented by a study of the influence of a band-to-band excitation ((h) over bar omega(P) = 4.02 eV) on the nonlinear susceptibility. (C) 1999 American Institute of Physics. [S0003-6951(99)02810-7].