Nonlinear optic coefficients that have been calculated through the resonant region in the far-infrared are looked at. It is possible to make such calculations by using a systematic expansion based on the Landau-Devonshire expression for the free energy together with the Landau-Khalatnikov dynamical equations. In this paper the coefficients considered have been calculated for an isotropic paraelectric phase. Two third order examples are presented: third harmonic generation and the d.c. Kerr effect. Experimental and practical implications include those for far-infrared sources and Terrahertz wave technology.
Webb, J. F., Chew, K-H., Osman, J., & Tilley, D. R. (2001). The nature of some nonlinear optic coefficients in ferroelectrics. Ferroelectrics, 251(1-4), 117-121. https://doi.org/10.1080/00150190108008508