Parametric and polariton scattering in noncentrosymmetric crystals

The work is dedicated to studying of the somewhat nontrivial effect of parametric light scattering (also known as parametric luminescence), which could be treated as an elastic scattering of photons in noncentrosymmetric media at vacuum fluctuations. The essence of the effect is the observation of a conic scattered light spectrum, the frequency of which is connected by means of simple dependence with the angle of scattering due to conservation laws for energy ( _p = _s + _i ) and momentum (k _p=k _s+k _i). The photon which propagates in a noncentrosymmetric medium is a mixed electromagnetic-mechanical exitation called the polariton.The theory of parametric light scattering in noncentrosymmetric crystals is given. Experimentally the effect was studied by means of two methods (photoelectric and photographic ones) in single-axial crystals of ADP, KDP, LiNbO₃ and biaxial crystal of a-HIO₃. The measurements fulfilled permitted us to find the eigenvalues of the refractive indexes of the crystals in the region of deep absorption of infrared radiation, where study of these characteristics is strongly hampered or even impossible.In the -HIO₃ crystal spectrum strange-looking polariton lines (without mutual pushing away when crossing, wide and diffuse, with sharp decreasing of the intensity with increasing to the scattering angle beginning after some value of it) were observed. The theory of the effect was constructed, computer simulation in the frame of which had shown that such resonant lines could exist, if one took into account the absorption in the medium.