Static and dynamic third-harmonic generation in long polyacetylene and polyparaphenylene vinylene chains

We have calculated the full spectrum of the third-order optical susceptibility (3)-3;,) in long oligomers of polyacetylene and polyparaphenylene vinylene, in the framework of a simple one-electron tight-binding approach. All doubly excited states are explicitly included in a sum-over-states formulation. The damping factors associated with resonances with excited states are modulated as a function of excited-state energy. Both the length dependence and the dynamic behavior of (3)-3;,) are found to be in excellent agreement with experiment. Our results thus show that, contrary to the situation in very short polyenes, a one-electron theory can provide an accurate description of the nonlinear optical response in conjugated polymers.