Contribution of spontaneous polarization and its fluctuations to refraction of light in ferroelectrics

Abstract

The expressions for the spontaneous polar contribution δn_i^s to the principal values of the refractive index due to the quadratic electro-optic effect in ferroelectrics have been considered within the phenomenological approach taking into account the polarization fluctuations. A method has been proposed for calculating the magnitude and temperature dependence of the root-mean-square fluctuations of the polarization (short-range local polar order) P_sh = 〈P_fl²〉^1/2 below the ferroelectric transition temperature T_c from temperature changes in the spontaneous polar contribution δn_i^s (T) if the average spontaneous polarization P_s = 〈P〉 characterizing the long-range order is determined from independent measurements (for example, from dielectric hysteresis loops). For the case of isotropic fluctuations, the proposed method has made it possible to calculate P_sh and P_s only from refractometric measurements. It has been shown that, upon interferometric measurements, the method developed in this work allows calculating P_sh and P_s directly from the measured temperature and electric-field changes in the relative optical path (the specific optical retardation) of the light.