The objective of this work is to investigate how the anisotropy of the interaction between dipole-active vibrations of a crystal lattice and infrared electromagnetic waves is manifested in the spontaneous parametric light scattering spectra of polaritons (Raman scattering by small angles). The case where scattering occurs by extraordinarily polarized polaritons—quasiparticles formed as a result of the coupling of the wave polarized in the symmetry plane of a biaxial crystal simultaneously with two phonons possessing orthogonal dipolar moments—is studied. A series of spectra of equilibrium fluctuations of the electromagnetic (infrared) field, each of which represents an intensity distribution in frequency-wave number coordinates for a fixed direction of the wave vector, are constructed on the basis of a scattering model that takes account of the tensor character of the permittivity and the quadratic and cubic susceptibilities of the crystal. Analysis of the computed spectra identified the basic laws and dependences which are determined by the anisotropy of the electromagnetic susceptibilities of various orders and made it possible to explain previous experimental results which cannot be interpreted on the basis of the generally accepted model of transversely polarized polaritons. A method is proposed for determining the contributions of the dipole-active vibrations of the crystal lattice to the permittivity and the quadratic and cubic susceptibilities, as well as the absorption of the material from the spectra of the extraordinarily polarized polaritons.
