There is a linear electro-optic effect called Pockels Effect The brief is that refractive index changes due to electric field. If there is an anisotropy (like birefringence) and electric field is in perpendicular to optical axis then refractive index changes for different polarization of ordinary rays. It gets eigendirections 45 degrees to electric field. Why so? Why it gets min 45 degrees but not exactly towards direction of E?
Pavel, I think an excellent and concise description of the Pockels effect is provided here: http://www.rp-photonics.com/pockels_effect.html. If you need some more information, please provide an illustration to your question, it will help to understand it.