Clamped-unclamped electro-optic coefficient dilemma in photorefractive phenomena

In a usual photorefractive experiment neither the unclamped nor the clamped linear electro-optic coefficients should be used for calculating the refractive-index change. We show that a crystal deformation in a form of a static plane wave is produced by the space-charge electric-field grating. The total refractive-index change is then calculated by adding together the strain-free electro-optic contribution and the elasto-optic contribution, properly taking into account also the rotation of the axes of the optical indicatrix that is produced by shear deformations. This roto-optic contribution to the photorefractive effect has been overlooked until now.