Polariton spectroscopy as a method of studying the effects of lattice anharmonicity in ferroelectrics

M.V. Chekhova, T.V. Laptinskaya and A.N. Penin

Dept. of Quantum Radiophysics, Moscow State University, Moscow, 119899, Russia

The method of polariton scattering spectroscopy is based on the effect of the spontaneous decay of a pump photon (w₁, k₁) into a signal photon (w_s, k_s) and a polariton (w_p, k_p) in a nonlinear crystal without the centre of symmetry. The signal waves relate to the visible range, and the polariton waves - to the infrared (IR) range. One analyzes the frequency-angular spectrum of the signal wave scattered at small angles. This spectrum is in one-to-one correspondence with the spectrum of the IR fluctuations of the electromagnetic field I(k_p, w_p). The characteristic values of k_p correspond to the centre of Brillouin zone. If the polariton frequency is close to a lattice vibration frequency, then the observed intensity distribution is the result of the interference between the contributions of this vibration into the linear and quadratic susceptibilities with the background values of these susceptibilities. The higher-order vibrations, as a rule, make small contributions into the dispersion dependences of linear and nonlinear susceptibilities and are difficult to study by means of traditional Raman and IR methods. The special methods of polariton spectroscopy is 2-3 orders more sensitive. They enable to observe vibrations with contributions into the dielectric function 10^-4-10^-7 or Raman activities of order 10^-10-10^-15 cm^-2dyne^-1.

An effective version of polariton spectroscopy - K-spectroscopy consists in the measurement of angular line shapes at a set of fixed frequencies. A simple calculation yields in this case the complex values of linear and quadratic susceptibilities as well as the imaginary part of cubic susceptibility. Then, the contribution of the second-order spectrum can be extracted.

All these parameters have been measured for an HIO₃ crystal in the range containing a few second-order excitations, and also a first-order vibration of the OH-group at 1160 cm^-1. The results allow some conjectures be made regarding the nature of the second-order excitations.