Effects of permanent dipole moments in high-order optical nonlinearity

High-order optical nonlinearity in molecular systems is of considerable interest, due to the dynamical information it affords, the flexibility in geometry it offers, and due to its involvement in the generation of even harmonics in isotropic media. Here, attention is focused on several processes whose optical response is dominated by two electronic states and which involve the coupling of up to six different optical modes. It is shown that transformation of the electric-dipole interaction provides a convenient means for ascertaining the role of intramolecular charge transfer on the high-order nonlinear optics of such systems. The transformation offers calculational expediency and a direct route to expressions for nonlinear molecular polarizabilities. In particular, the method identifies directly terms dependent on various powers of the displacement vector for intramolecular charge transfer.