Static and dynamic optical nonlinearities in conjugated polymers: Third-harmonic generation and the dc Kerr effect in polyacetylene, polyparaphenylene vinylene, and polythienylene vinylene

The static and dynamic second hyperpolarizability tensors of oligomers of polyacetylene, polyparaphenylene vinylene, and polythienylene vinylene are evaluated by means of the valence effective Hamiltonian (VEH) method. The sum-over-states (SOS) approach is exploited, by constructing excited states from both single- and double-electron excitations. The ab initio coupled perturbed Hartree-Fock method is also applied in the case of small representative molecules such as benzene, thiophene, styrene, and thienylethylene in order to test the validity of the VEH-SOS approach. The VEH-SOS results are found to be comparable with the ab initio results. The length dependence of the static hyperpolarizability is analyzed and found to scale with the optical gap evolution of the different oligomers. The dynamic properties are presented in terms of third-harmonic generation and the dc Kerr effect.