On the symmetry properties of dielectric tensor in incommensurately modulated insulators

General symmetry properties of the dielectric permittivity tensor which det ermines nonlocal optical response of crystals with incommensurately modulat ed phases are analyzed. The relations resulted from the Onsager's principle and the condition of transparency of the optical medium are derived. Speci fic features of a macroscopic averaging procedure for the dielectric permit tivity of incommensurate crystals are in brief discussed. It is revealed a possibility for existing small non-Hermitian part in the dielectric tensor related to inhomogeneity of the medium on a mesoscopic scale and originated from the modulation-induced spatial dispersion.