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 $800\mathrm{nm}$ where the single photon absorption usually dominates. The crystal surface was excited by femtosecond laser pulses with fluences up to $4.2\mathrm{mJ}∕{\mathrm{cm}}^2$, 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.