Two-oscillator description of optical properties of oxygen-octahedra ferroelectrics

Abstract

Refractive index,birefringence, and linear electro‐optic (EO) effect of BO6‐type oxides have been analyzed using a two‐term Sellmeier dispersion relation. Two oscillators correspond to two main transitions; the transition to dε conduction band with a transition energy of 4–6 eV and that to the dγ band with an energy of 9–11 eV. It is found that the contribution of the latter dγ transition to the refractive index in the visible to the infrared region is larger than that of the former, and that the dispersion energy E i disp for the dγ transition is approximately equal to that for the average single oscillator. The birefringence and the linear EO effect are explainable by the polarization‐induced changes in the position and the strength of the dε transition. In the ferroelectric phase, the change in the oscillator position is found to be proportional to the square of the spontaneous polarization P s 2 . On the other hand, the change in the oscillator strength consists of the terms proportional to P s 2 and P s 4 . The second term leads directly to two observed features: the quadratic EO coefficient depends on Ps ; and a slight difference exists between the coefficients derived from the spontaneous birefringence and the linear EO effect.