Photorefractive properties of congruent and stoichiometric lithium niobate at high light intensities

In lithium niobate crystals the focused beam of an argon-ion laser induces birefringence changes. We investigate the intensity dependences of the saturation value of these birefringence changes, the photoconductivity, and the photorefractive sensitivity in nominally pure crystals with different lithium contents and in an iron-doped sample. All experimental results can be described assuming Fe2+/3+ centers—of low concentration in the nominally pure samples—and, in addition, intrinsic NbLi polarons as a second photoactive center.