We measure the nonlinear behavior of the luminescence spectra near the excitonic resonance, by using an excitation at 4.026 eV from an excimer laser. At low intensities of excitation, spectra show a saturation of the I2 line due to the finite donor density in the sample. Higher intensities of excitation induce collision processes between photo-created quasi-particles.

Using a dye laser as a pump beam, we measure the induced variation of absorption of a probe beam as a function of the intensity and of the wavelength of the excitation. With increasing intensities of the pump beam, transmission spectra show a red-shift of the absorption edge and of the excitonic resonance.

Pulsed degenerate four-wave mixing experiments were performed using the third harmonic of a picosecond Nd-YAG laser at 3.492 eV. A characteristic time constant of 16 ps has been measured, which is independent of the temperature, spacing of the interference fringe and of the intensity of the pump beams.

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