Influence of intrinsic defects on light-induced changes in the refractive index of lithium niobate crystals

The results of a detailed investigation into the variation of the composition on the photorefractive effect in nominally pure LiNbO3 crystals at continuous-wave illumination are reported. The investigation clearly indicates an increase in the photorefraction amplitude Delta n(s) (at saturation) with light intensity J for all crystals studied. The main result is a correlation between partial derivative(Delta n(s))/partial derivative J and the density N of intrinsic defects such as Nb-Li. This effect is independent of the accidental iron contamination. Therefore at low J, Delta n(s) decreases with an increase in N, whereas at high J the opposite dependence is observed. The threshold intensity value of such a transformation correlates with the optical quality of the crystal. It has been established that these photorefraction changes are connected with the light induced absorption of the secondary centers, caused by the existence of the metastable small polarons at room temperature during illumination.