Nonlinear dispersion of light in some ferroelectrics having soft modes

Full text: In these work the results of the analysis of spectra of dispersion of light on strongly damping soft modes are given. The comparative characteristics of spectra are given in cases, when the soft mode has resonant and relaxation character. It is analyzed in details those cases, when the distinction between resonant and relaxation modes is erased. The spectra of nonlinear dispersion on polaritons are considered. It is notice, that there are a many works devoted to the analysis of spectra of Raman scattering on soft modes in ferroelectrics without the account of polariton effects. It is known, that on the basis of the concept of a soft mode lays the assumption, that the phase transitions in some ferroelectrics can be result of instability of one normal oscillatory modes in a lattice, which frequency in neglect of damping approaches to zero at the temperatures of a sample near to temperature of phase transition. As ferroelectric state of solid state is characterized by macroscopic spontaneous polarization, the soft oscillatory modes (phonons) should be polar, i.e. active in the infrared spectrum and, hence, should be transformed in polaritons at small sizes of wave vectors. As the frequency of the soft modes approaches to zero, actually always the condition is realized near the temperatures of phase transition, at which the damping becomes comparable and even more then the frequency of the soft mode. In this case the wide spectrum with a maximum on zero frequency is observed in the spectra of dispersion. It complicates to obtain in the information about parameters of soft modes from spectra of Raman scattering by optical phonons. The advantage of Raman scattering by polaritons is the result of photon-phonon character of polaritons: with the reduction of the wave vector of polaritons their damping decreases faster, than their frequency. It results to appearance of peaks on Raman scattering of light by polaritons, although soft mode is overdamped