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In order to determine the third order of the nonlinear susceptibility χ(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 χ(3) close to the exciton resonances. Besides the χ(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 (ℏωP=4.02 eV) on the nonlinear susceptibility.

We describe an experiment to be carried out by advanced students as part of their laboratory practice. Its aims are (i) to measure simultaneously the frequency dependence of both the refractive index n and the extinction coefficient of an atomic gas interacting with a resonant laser beam and (ii) to examine that n and satisfy the Kramers-Kronig relations. The medium to be probed consists of a dilute 85Rb vapour exposed to collisions with a buffer gas. The D2 resonance line is thereby broadened to such an extent that its hyperfine components merge into single Lorentzian dispersion and absorption profiles, respectively. The main features of the apparatus are a tunable laser diode, a Mach-Zehnder interferometer at nearly zero path difference, an array of photodiodes to measure the interference pattern as a function of laser frequency and Rb density in one of the interferometer arms, an absorption monitor, computer-controlled electronics to read out and analyse the data, and a video camera for visual control and adjustment of the interference pattern.