Optical two-photon absorption in GaAs measured by optical-pump terahertz-probe spectroscopy

Time-resolved terahertz experiments in a novel setup were used for a direct observation of the competition between the single- and two-photon absorption in highly excited GaAs. The experiments were carried out near 800 nm where the single photon absorption usually dominates. The crystal surface was excited by femtosecond laser pulses with fluences up to 4.2 mJ/cm(2), for which the single-photon absorption saturates and the two-photon absorption becomes the leading absorption mechanism. The two-photon absorption coefficient beta, its anisotropy and the depth profile of the photoexcited carriers were determined.