Microwave Nonlinear Susceptibilities Due to Electronic and Ionic Anharmonicities in Acentric Crystals

A microwave nonlinear susceptibility analogous to the optical and electro-optical non-linear susceptibilities is investigated in a number of acentric ferroelectric and semiconductor crystals. Contributions from ionic nonlinearities result in enormously larger nonlinear coefficients in some materials than any known for the optical or electro-optical effects. An interpretation is presented in terms of a relationship between the non-linear coefficients and the electronic and ionic linear susceptibilities.