Third harmonic generation spectrum of a degenerate ground state conjugated polymer. Direct evidence of simultaneous two- and three-photon resonance

We report the third harmonic generation (THG) spectrum of poly(dioctyldipropargylmalonate) (PDDPM), covering pump energies from 0.6 to 1.4 eV. Like trans-polyacetylene, PDDPM has a bond-alternated structure and a twofold degenerate ground state. The phase of the nonlinear optical susceptibility, χ(3)(3ω), undergoes a 2π shift, demonstrating that two nearly simultaneous resonances (at 0.7 and 0.9 eV) are traversed, as expected for degenerate ground state polymers where the two-photon mid-gap state formed by neutral soliton-antisoliton pairs plays a dominant role in χ(3) processes.