X-ray birefringence in highly Anisotropic Materials

Birefringence is the dependence of a material's refractive index on the direction of linear polarization. It induces a phase shift between two perpendicular polarization directions and thus couples linear and circular polarization states. Birefringence in X-ray absorption is as common as linear dichroism but is rarely discussed in the literature. We outline a mathematical framework for describing experiments on birefringence and illustrate the importance of the phenomenon with three examples.