A simplified three-level model describing the molecular third-order nonlinear optical susceptibility



A simplified three-level model for γ, the molecular third-order nonlinear optical susceptibility, is presented and discussed. The perturbation theory-based approach suggests that there are three primary avenues to optimizing molecular four-wave mixing susceptibilities and that with each of these is associated a particular class of molecular electronic structures. The three electronic structure classes consist of (1) conjugated donor–acceptor dipolar molecules with a large secondorder susceptibility, β; (2) even-member conjugated chains such as -enes, -ynes, and ene–ynes with large two-photon dominated susceptibilities; and (3) “charged” odd-member conjugated chaines with large linear absorption dominated third-order susceptibilities such as squaryliums (perhaps, more generally, the polymethine dyes). Classes (1) and (2) have been known and investigated in the past, while recent results of ours suggest the existence of the third and perhaps final class.