ABSTRACT
The diagonal elements are called principal permittivities and are real numbers, due to absence of absorption. However, perturbative methods allow extending it to low absorption materials in view of analyzing the phenomenon of dichroism and pleiochroism. Uniaxial and biaxial materials are of great interest in the field of Optics. However, modem technology involves the use of anisotropic materials with strong losses, as it occurs in optical information storage. The problem of diffraction by a grating made of an anisotropic lossy medium was tackled as soon as mid 1980s in view of optical information storage. An extension of the classical differential method was used, so that the numerical treatment was only successful with low modulated gratings.
