Interband two-photon absorption in strontium barium niobate crystals

Interband two-photon absorption in photorefractive strontium — barium niobate crystals Sr_xBa_1-xNb₂O₆ (x = 0.61, 0.75), both pure and doped with Ni and La ions, is studied under excitation by trains of 523.5-nm picosecond laser pulses. The two-photon absorption coefficients are found to vary from 0.17 to 0.31 cm GW^-1 depending on the azimuth of the linear polarisation of the pump radiation and on the chemical composition of samples. The dependences of the transmission of crystals on the pump radiation intensity show a hysteresis related to induced one-photon absorption from excited levels.