Elasto-optic and roto-optic contribution to the photorefractive effect

In a photorefractive experiment the space-charge grating is built-up along a certain axis and it modulates the refractive indices via the linear electro-optic (EO) effect. In many of the works in the field there is an on-going dilemma which EO coefficient and also dielectric constants to use in order to model the photorefractive effect.[1] The relevance of the question lies in the fact that the unclamped (stress-free) and the clamped (strain-free) values typically differ by more than a factor of two. Experiments in KNbO3[2] and BaTiO3[3] tend to indicate that the unclamped values are more appropriate. In this contribution we show that neither the clamped nor the unclamped linear EO coefficients are to be used without caution but the appropriate effective coefficients must be calculated for every particular crystal symmetry and sample orientation.