Bipolaron-induced third-order optical nonlinearity in conjugated conducting polymers

A mechanism, due to the presence of bipolarons, for large resonant chi((3)) in doped cis-polyacetylene and other conjugated conducting polymers is presented. Under optical pumping, the electronic occupations of the two intragap energy levels of the bipolarons will vary, resulting in a shift in their energies, which in turn causes a dramatic change in the absorption spectrum. Even for moderate doping concentrations of bipolarons, chi((3)) is found to be as high as 10(-8) esu.