Two-photon absorption of femtosecond laser pulses in GaS crystals

Experimental results of the influence of high-power density (up to I-0=400 MW/cm(2)), ultrashort laser pulses with duration of 42 fs (FWHM, E = 1.98 eV) on the transient transmission change of layered GaS crystals at room temperature are reported. The strong absorption at Delta t=0 (zero time delay between pump and probe pulse) is identified as being due to two-photon direct interband transitions. The estimated value of the two-photon absorption coefficient is beta(dir) = (0.6 +/- 0.2) x 10(-3) cm/MW. (C) 1998 Elsevier Science B.V.