Optical absorption in GaSe under high-density ultrashort laser pulses

Experimental results of the influence of high-density ultra short laser pulses of 60 fs duration (E = 1.98 eV) on the transient transmission change of thick (0.86, 1.92, 2.32 mm) GaSe crystals are reported. Two peculiarities of the differential transmission change - a significant absorption at Deltat = 0 (zero time delay between pump and probe pulses) and an induced absorption at Deltat = 1.5 ps (pump pulse precedes probe pulse) are discussed. The strong absorption at Deltat = 0 is identified as due to two-photon absorption. The estimated value of the two-photon absorption coefficient at low densities of excitation (less than similar to3 mJ/cm(2)) is beta(1) = 3 x 10-(10) cm/W. On increasing the excitation density from similar to3 to similar to16 mJ/cm(2) the two-photon absorption coefficient decreases to beta(2) approximate to 1 x 10-(10) cm/W. The induced absorption at Deltat = 1.5 ps is assigned to the non-equilibrium free carriers absorption. The estimated value of the absorption cross-section for the free carriers is sigma similar to (4-6) x 10(-19) cm(2). (C) 2004 Published by Elsevier B.V.