Measurement of nonlinear absorption coefficients in GaAs, InP and Si by an optical pump THz probe technique

An optical pump terahertz (THz) probe method for measuring carrier mobility and multiphoton absorption coefficients in semiconductors is demonstrated. A THz probe pulse is used to detect the transient photoconductivity generated by an optical pump pulse. The change in transmission coefficient at THz frequencies due to a pump pulse with photon energy greater than the band gap energy is used to determine the sum of electron and hole mobilities. The weak nonlinear absorption of a pump pulse with photon energy less than the band gap energy produces an approximately uniform free carrier distribution. The THz transmission coefficient vs. pump fluence, and the mobility, are used in a bulk photoconductivity model to determine the multiphoton absorption coefficients. For GaAs, InP and Si we find two photon absorption coefficients at 1305 nm of 42.5 ± 11, 70 ± 18 and 3.3 ± 0.9 cm/GW, respectively. For GaAs and InP we determine three photon absorption coefficients at 2144 nm of 0.19 ± 0.07 and 0.22 ± 0.08 cm³/GW².