Features of the optical response of dielectric crystals with incommensurate phases

The constitutive relation describing the optical response of dielectric crystals in incommensurately modulated phases is derived for the plane-wave range of modulation. The symmetry properties of modulation corrections to the permittivity tensor of such crystals are analyzed with allowance for spatial dispersion. These properties are dictated by the Onsager principle for a nonabsorbing medium and by the mesoscopic periodicity of the medium. It is shown that the basic results are consistent with the general conventional approach in crystal optics of spatially inhomogeneous media.