QUADRATIC ELECTROOPTIC EFFECT AND 2ND-ORDER STRAIN DERIVATIVES OF ELECTRONIC SUSCEPTIBILITY

Electro-optic effects are related to second order strain derivatives of electronic susceptibility. The method is applied to halite-type alkali halides and KDP crystals. Employing the bond polarizability approach, the values of strain and volume derivatives are evaluated from experimental quadratic electro-optic coefficients. In alkali halides the model based on the decomposition of optical susceptibility into contributions of intraionic and interionic excitations is taken into consideration. For alkali halides, results obtained within the Clausius-Mosotti theory and the Penn model are also presented. Since in noncentrosymmetric KDP crystals the influence of the linear electro-optic effect can lead to large apparent quadratic electro-optic effects, the conditions of measurement are discussed. The earlier experimental results are critically analysed, and to estimate in terms of the bond polarizability model, the second order strain derivative of electronic susceptibility of KDP, we employed our experimental values of the quadratic coefficient g1111 - g1122.