Nonlinear optics of ferroelectrics

The nonlinear optic coefficients of ferroelectrics in the far-infrared reso nance region can be determined by means of a systematic expansion based on the Landau-Devonshire free-energy expansion together with the Landau-Khalat nikov dynamical equations. The formalism is reviewed and the general struct ure of the chi((2)) and chi((3)) coefficients is discussed. Examples of the frequency and temperature dependence of selected coefficients are presente d. We conclude with brief discussions of possibilities for experimental mea surement of the nonlinear-optic coefficients and of possible applications.