Anomalies in the spectrum of electromagnetic eigenwaves in anisotropic plates

A radical transformation of the spectrum of electromagnetic eigenwaves in anisotropic plates of optically biaxial and uniaxial crystals is investigated for small deviations of the direction of wave propagation from crystallographic orientations. It is demonstrated that, in the case when the sagittal plane and plate faces are the symmetry planes of the permittivity tensor ?, there exist four independent families of bulk eigenwaves, each characterized by an infinite set of dispersion curves. Dispersion curves of the independent families intersect one another to form a network structure. Under the action of perturbations, the sagittal plane and the plate faces cease to be the symmetry planes. Moreover, these perturbations lead to interactions between waves belonging to different families, the generation of mixed polarization modes, and divergence of the dispersion curves, i.e., the disappearance of intersection points. In the vicinity of these points, the polarization of wave fields changes drastically even with insignificant variations in the frequency.