Observation of three resonances in the third harmonic generation spectrum of conjugated polymers: Evidence for the four-level essential states model

Much of the theoretical effort to understand the mechanism of optical nonlinearity in π-conjugated polymers has been focused towards explaining the early observation of only two resonances in the third harmonic generation spectrum of trans-polyacetylene. A recent theory of third order optical nonlinearity predicts three, instead of two, resonances, with their intensities decreasing with increasing frequencies. This is a fundamental difference between the band and the exciton pictures of π-conjugated polymers. Very recent experimental results obtained with cis-polyacetylene, as well as earlier experiments on two blue forms of polydiacetylenes, are in complete agreement with this prediction. An isolated single strand of cis-polyacetylene should exhibit exciton behavior. We briefly discuss the latest experimental results for trans-polyacetylene.