Refractive Index Ellipsoid for Circular Birefringence Materials When light travels through an anisotropic medium, its refraction may depend on both its polarization and propagation. Such materials may be classified in terms of their (Linear) Birefringence as uniaxial or biaxial and the refractive index ellipsoid is used to describe them. Is there any analogous description, model and/or classification for materials exhibiting Circular Birefringence?
 A: For a circular birefringence, the refractive index is complex quantity and its modulus and phase represents the absorption and phase shift respectively caused by the material.
The analogous description/model/classification for materials Circular Birefringence lies in the Faraday effect that occurs when linearly polarized light is passed through a material with circular birefringence in the presence of a static magnetic field. This causes the plane of polarization of the light to rotate by a certain angle that determines the circular birefringence of that materialand is quantified by the product of the length of the Faraday rotator, the magnetic field strength, and the Verdet constant of the material.
In terms of classification, unlike linear birefringence, which can be classified as uniaxial or biaxial, circular birefringence is not classified based on the properties of the medium itself. Rather, it depends on the magnetic field applied to the medium. Materials that can exhibit circular birefringence can be natural, such as certain crystals and biological materials, or artificial, such as metamaterials.
Moreover, for the description of Circular birefringence, The Verdet Constant is used instead of the refractive index ellipsoid, it is a dimensionless constant of the medium that describes the rotation of the plane of polarization per unit length in the presence of an applied magnetic field.
