Strongly sublinear growth of the photorefractive effect for increasing pump intensities in doped lithium-niobate crystals

The pump intensity dependence of the birefringence change due to the photorefractive effect in Hf-doped and Mg-doped congruent lithium niobate LN crystals is reported. The birefringence change is found to be very weakly dependent on the light intensity. This is in strong contrast with the behavior of undoped congruent LN, and much more similar to what happens in stoichiometric LN. The data also indicate that the minimum HfO2 concentration necessary to strongly reduce photorefractivity is lower than the value of 4 mol % indicated by previous experiments.