An algorithm for the nonlinear optical susceptibilities of dipolar molecules, and an application to third harmonic generation

It is well known that a simple transformation of the electric dipole intera ction provides a convenient means for ascertaining the effects of permanent dipoles on the optical behaviour of systems with a response dominated by t wo energy levels. By establishing the general validity of the procedure for parametric processes such as harmonic generation, it is shown how the deta iled structure of the optical susceptibilities associated with arbitrary fo rms of optical nonlinearity can be ascertained by an algorithmic method, ba sed on a novel interpretation of the relevant quantum electrodynamical Feyn man diagrams. Application of the algorithm to second and third harmonic gen eration illustrates its usefulness and simplicity, whilst also providing ne w results and revealing features, related to the role of permanent dipoles, which have not hitherto been apparent.