Résumé
The samples studied are thin GaN epilayers grown by metalorganic vapor phase epitaxy (MOVPE). At liquid helium temperature, the linear luminescence, transmission and reflection spectra of the films has been recorded, as well as the nonlinear emission spectra under high UV excitation, By using the variable strip length method, we have measured the gain coefficient as a function of the excitation intensity and the temperature. Pump and probe experiments have been performed in the femtosecond regime, showing crossed two-photon absorption without long lasting response. This allows to envisage the use of GaN for the characterization femtosecond pulses in the blue spectral range. (C) 1997 Elsevier Science S.A.