Essential-states mechanism of optical nonlinearity in π-conjugated polymers

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Abstract
We present a microscopic mechanism of optical nonlinearity in conjugated polymers. It is shown that the bulk of the nonlinearity is determined by only two well-defined channels, even though an infinite number of channels are possible in principle. We are able to explain the complete frequency dependence of the third-harmonic generation in both trans-polyacetylene and polydiacetylene within the same model without invoking weak Coulomb interactions between electrons or interchain interactions.