Nonlinear optical susceptibility of conjugated polymers: d.c. Kerr effect

With the help of Keldysh diagram technique we rigorously calculate the nonlinear optical (NLO) susceptibility of a conjugated polymer in one electron two band continuum linearized model with constant dimerization for the d.c. Kerr effect (Electroabsorption, EA). The explicit analytical formula obtained for χEA(3)(ω) is accurate in the entire frequency range in the lowest order in the quasiparticle relaxation constant. Its comparison to the results on the quadratic electro-optic nonlinear susceptibility in PTV reveals qualitative agreement between theory and experiment.