Infrared two-photon photo-induced absorption in Y-Ba-Cu-O films

Photo-induced structural transformations in oriented Y-Ba-Cu-O films deposi ted on ZnSe single crystals were studied using two-photon absorption (TPA). The ZnSe single crystals were cut parallel to the [110] crystallographic p lane. The reversible changes of third-order nonlinear optical susceptibilit ies,: particularly of two-photon absorption, were induced in the Cu-O in-pl ane and out-of-plane structural fragments after the first cycle of infrared photoexposure. The whole scheme of the photo-induced and thermo-induced tr ansformations in the oriented Y-Ba-Cu-O films can be explained as created a nd annihilated defects during illumination. The density of vibration states calculated using quantum-chemical solid-state methods is in good agreement with experimental results. We show also the essential influence of the lim e delay between the probe and pump beams on two-photon absorption in Y-Ba-C u-O films.