Nonlinear Optical Properties of Liquids

Nonlinear optical polarization quadratic in the optical electric fields is shown to occur in optically active liquids and to lead to sum- and difference-frequency generation; second-harmonic generation is forbidden. The nonlinearity is described by components of the second-order polarizability tensor χᵢⱼₖ antisymmetric in j and k; the form of the antisymmetric part of χᵢⱼₖ is given for all the crystal classes and textures and for isotropic media. The magnitude of nonlinear polarizability of liquids is estimated from second-order perturbation theory and calculated to be readily detectable in many optically active liquids. The mechanism of the nonlinearity is illustrated by a simple single-electron molecular model.