Diffractionless light beams in linear and nonlinear optics

The parameters of anisotropy and gyrotropy area are shown to exist in biaxial gyrotropic crystals, where local sites of wave vectors surface in the vicinity of optical axis directions have negative or zero curvature. In these directions effects of focusing and diffractionless propagation of light beams occur. At Gaussian beams example a number of focusing and diffractionless propagation schemes are presented. The description of the new type of nondiffracting light beams (spatial solitons) in photorefractive crystals in the case of phase shifted diffraction gratings is given.