<title>Polariton spectroscopy: a method of investigating spectral and spatial properties of nonlinear optical materials</title>

Abstract

Rapid development of new materials for nonlinear optics initiates research in the area of testing these materials and measuring their optical properties, such as refractive index, absorption, and nonlinear susceptibilities. As a rule, it is necessary to measure both the spectral dependence (dispersion) of these parameters and their spatial distribution. Speaking about the dispersion dependences, one should take into account the resonant behaviour of the optical properties in the vicinity of electron and lattice resonances; spatial distribution can be connected with inhomogeneities, growth layers, domains and so on. Studying of spectral and spatial distributions of optical properties is especially complicated in the IR region of spectrum, which is, however, very important for the materials used as parametric up- and down- converters and frequency doublers. Polariton spectroscopy is rather effective for these purposes. The spectra of light scattering by polaritons are in fact the up-converted to the visible range spectra of the infrared equilibrium fluctuations in the matter, and the scattering intensity is determined by the quadratic susceptibility. Therefore the spectra contain the information about the dispersion of complex dielectric function and complex quadratic susceptibility in IR. In case when these parameters are spatially modulated, the character of modulation can be studied. The spectra also allow to determine the dispersion of the cubic susceptibility imaginary part. The experimental setup for polariton spectroscopy - a "polariton scattering spectrograph" - consists of an Ar laser generating at 488 nm which is used as a pump, the nonlinear crystal, two Gian prizms, an optical system for analysing the angular spectum and a spectrograph for analysing the frequency spectrum. The resulting two­ dimensional frequency-angular spectra can be measured and processed at the exit of the spectrograph. It should be stressed that though the effect of light scattering by polaritons is possible only in non-centrosymmetric media, in order to investigate centrosymmetric materials one can use the effect of hyper-Raman scattering by po­ laritons. In the first part of the work we show how the dispersion of different-order complex susceptibilities can be measured using a polariton scattering spectrum, and also how such a spectrum can give information about the periodic modulation of the optical properties. The second part of the paper describes the experimental results demonstrating the abilities of the method which were obtained in o: -iodic acid, lithium niobate and barium-sodium niobate crystals.