Abstract

The calculation of photorefractive-index changes has been complicated by uncertainty over whether to use the clamped (strain-free) or undamped (stress-free) electro-optic coefficients. The piezoelectrically induced deformation of the crystal lattice by the spacecharge field contributes to the refractive- index changes, through the elasto-optic effect, and this contribution varies as a function of the grating orientation and crystal symmetry. Recently Gunter and Zgonik laid much of this controversy to rest.¹ They showed that the inhomogeneous space-charge field produces lattice distortions different from those produced by uniform fields, and they also showed how to calculate correctly the associated refractive-index changes. Their model also predicts that the inhomogeneous spacecharge field induces strain components different from those induced by a uniform field. However, this model assumes an infinite photorefractive crystal. X-ray topographic imaging of photorefractive gratings in barium titanate has shown that the lattice distortions near the surface of a crystal can be quite different from those in the bulk.^2,3 In particular, images of gratings in barium titanate with the grating vector parallel to the c-axis show strain components normal to the c-axis in the region near the crystal surface, which are not predicted by the bulk model.

© 1995 Optical Society of America