Optical absorption in GaSe under high-density ultrashort laser pulses

Abstract 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 Δ t  = 0 (zero time delay between pump and probe pulses) and an induced absorption at Δ t  = 1.5 ps (pump pulse precedes probe pulse) are discussed. The strong absorption at Δ t  = 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 ∼3 mJ/cm 2 ) is β 1  ≈ 3 × 10 −10 cm/W. On increasing the excitation density from ∼3 to ∼16 mJ/cm 2 the two-photon absorption coefficient decreases to β 2  ≈ 1 × 10 −10 cm/W. The induced absorption at Δ t  = 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 σ  ∼ (4–6) × 10 −19 cm 2 .