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 comp osition on the photorefractive effect in nominally pure LiNbO3 crystal s at continuous-wave illumination are reported. The investigation clea rly indicates an increase in the photorefraction amplitude Delta n(s) (at saturation) with light intensity J for all crystals studied. The m ain result is a correlation between partial derivative(Delta n(s))/par tial derivative J and the density N of intrinsic defects such as Nb-Li . This effect is independent of the accidental iron contamination. The refore at low J, Delta n(s) decreases with an increase in N, whereas a t 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 change s are connected with the light induced absorption of the secondary cen ters, caused by the existence of the metastable small polarons at room temperature during illumination.