Excitons and Optical Nonlinearities in Conjugated Polymers

Within the random phase approximation, single-exciton states in the 1D Peierlas-extended Hubbard model are investigated. Two low-lying bound-exciton states (lB_a and 2A_g) are found for a moderate interaction. The excitons are of an intermediate size. The electron correlation strongly affects energles of the bound-exciton states and leads to the change of the order between lB_a and 2A_g. The lB_a and 2A_g excitons are almost, degenerate for some interaction parameters. The calculated third-harmonic generation clearly shows a three-photon resonance at 0.6 eV and a two-photon resonance at 0.9 eV observed experimentally in polyacetylene.