Two wave mixing gain and electrical polarization were compared when a BaTiO3 crystal was placed in an alternating electric field. The electric field was used to switch the direction of polarization in the crystal. Each time the polarization was switched the direction of two wave gain changed. A relationship between the time dependent polarization and the optical gain was obtained.
D. M. Gookin,
G. W. Gross,
"Optical Switching In Barium Titanate (BaTiO3)", Proc. SPIE 0567, Advances in Materials for Active Optics, (4 March 1986); doi: 10.1117/12.949826; https://doi.org/10.1117/12.949826