A four-level "essential states" model of third order optical nonlinearity in π-conjugated polymers

Theoretical description of optical nonlinearity in π-conjugated polymers must necessarily include both the on-site and the intersite electron correlations. We show that only three dominant nonlinear optical channels, involving four "essential states", determine the bulk of the optical nonlinearity. In ideal isolated strands, third harmonic generation experiments should find two three-photon resonances, and not merely one, as has been generally assumed. Two-photon resonances other than the one due to the essential two-phonon excited state will have vanishingly weak intensities due to cancellations between nonlinear optical channels with opposite signs.