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-be am configuration at low temperature on a GaN epilayer on sapphire substrate . We measure the excitation spectrum of chi((3)) close to the exciton reson ances. Besides the chi((3)) contribution, higher orders of the nonlinear su sceptibility show already up around 10 kW cm(-2), leading to a saturation o f the signal. Results are complemented by a study of the influence of a ban d-to-band excitation ((h) over bar omega(P) = 4.02 eV) on the nonlinear sus ceptibility. (C) 1999 American Institute of Physics. [S0003-6951(99)02810-7 ].