Nonlinear absorption in crystalline niobates with stimulated defects of structure

Processes of formation of radiation-induced defects in crystals of lithium niobate and barium-sodium niobate are investigated by spectrophotometric methods. Colour centers, being responsible for the increase of the linear absorption in gamma-irradiated crystals, are identified as the F-centers. The effect of colour centers on the nonlinear absorption in LiNbO3 and Ba2NaNb5O15 crystals is studied. It is shown that the dependence of the nonlinear absorption in crystals on the colour-centers concentration has the nonmonotonic character. This nonmonotonic character of the nonlinear absorption is based on the band theory of solids and is explained by superposition of the two-photon and two-stepped absorption processes