We present the study of the ultrafast carrier dynamics in bulk GaAs at 6 K, by time-resolved optical reflectivity in a wide range of excitation densities. We observe an exciton-phonon interaction at low excitation densities, in terms of a mono-exponential relaxation. Further increase of excitation density above a critical value nc, stimulates a bi-exponential relaxation in which the fast one is added owing to an exciton-exciton scattering channel. Our results reveal nc∼2×1024photons/m3, about an order of magnitude higher than the Mott density previously determined by photoluminescence and terahertz spectroscopy measurements and theoretical calculations. Keywords: Exciton, Semiconductor, Ultrafast pump-probe spectroscopy
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