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The influence of the relative group velocity finiteness of the scattering polariton on the polariton Raman scattering (RS) intensities is discussed. It is proved that this effect is not very essential only in the region of polariton frequencies wp near the frequency wf of the corresponding polar mechanical phonon because here the relative group velocity tP of the polariton is small. But the influence of the group velocity tp finiteness appears t o be essential in the case of sufficiently distant frequencies wP from of. This influence results not only in essential quantitative changes but also in qualitative changes, in particular, of the angular dependence of the polariton RS intensity. It is manifested particularily in the vicinity of the "critical" scattering angles where the neglection of this effect distorts absolutely the character of the above-mentioned dependence. The Ep-finiteness influence is illustrated by numerical calculations for a number of real crystals (ZnO, ZnSe, Gap, CdS, LiIO,). Experimental investigations are made of the RS-intensity angular dependence in the region of the upper polariton branch in LiIO, crystals in the vicinity of the critical scattering angle. The results confirm the conclusion of the theory about the essential influence of the effect mentioned above on the polariton R S intensity.

The fluctuation-dissipation theory of spontaneous Raman scattering (RS) by surface polaritons (SP) is carried out. The appearance of scattering emission is treated as the result of mixing of the pump emission and the fluctuating surface electromagnetic field. A calculation is made of the surface electromagnetic fluctuations near the interface between the two dissipative media and in the absorbing layer surrounded by transparent dielectrics. Expressions are found and analysed describing the frequency-angular distribution of the scattering emission intensity in the layer, taking into account the interference of SP localized near different surfaces (and near the interface of two absorbing media). The RS intensities are expressed by the phenomenological parameters of the media. It is also shown that the RS intensity decreases essentially with increasing scattering angle 0 (when the scattering frequency approaches the frequency of the surface phonon) due to the increasing localieation of SP. Various essential physical features of the phenomenon are analysed in detail: scattering line shape and its angular dependence; the B'ermi resonance of SP and its display in the RS; mixing of SP of different media and RS by appearing mixed modes; RS by SP connected genetically with the linear medium which borders with the transparent nonlinear medium; evolution of RS in a layer by decreasing its thickness, and others.

The theory of the gain factor of the stimulated Raman scattering by polaritons in biaxial crystals is carried out in the fixed stationary pump field approximation. By sufficient length of the polariton wave vector the polariton scattering turns into the scattering by polar optical phonons which may be regarded as quasilongitudinal electromagnetic field quanta. Typical features of scattering by anisotropic polar phonons whose energy depends on the wave vector direction are investigated. Mixing of different symmetry mechanical vibrations is analysed in anisotropic crystals. The Fermi resonance of polar mechanical phonons is studied in detail. Numerical calculations made for some particular cases agree with available experimental data.

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