Optical second-harmonic generation by electrically polarized isotropic media

In relation to recent experiments, a quantitative discussion of the microscopic mechanisms accounting for second-harmonic generation (SHG) and harmonic mixing in isotropic media immersed in a dc electric field is given, comprising the temperature-independent effect of nonlinear electronic polarizability and the temperature-dependent effect of reorientation of permanent electric dipoles. It is shown that in the case of a very strong dc electric field, when all the microsystems undergo complete alignment in the direction of the field vector, these two mechanisms attain saturation, raising considerably the intensity of SHG in the case of prolate microsystems and lowering it in the case of oblate ones. Electric saturation effects can take place in strongly dipolar molecular substances but are especially intense in solutions of macromolecules and colloid particles. The discussion covers, moreover, the symmetry relations between the nonzero elements of the nonlinear susceptibility tensor of the medium in weak as well as in strong electric fields, and the application of these relations in determining the values of elements of the tensors of second- and third-order polarizabilities of the individual microsystems. A preliminary interpretation of the available experimental data is proposed, and new procedures of SHG measurements are suggested.