3RD-HARMONIC GENERATION SPECTRUM OF A DEGENERATE GROUND-STATE CONJUGATED POLYMER - DIRECT EVIDENCE OF SIMULTANEOUS 2-PHOTON AND 3-PHOTON RESONANCE

We report the third harmonic generation (THG) spectrum of poly(dioctyl dipropargylmalonate) (PDDPM), covering pump energies from 0.6 to 1.4 e V. Like trans-polyacetylene, PDDPM has a bond-alternated structure and a twofold degenerate ground state. The phase of the nonlinear optical susceptibility, chi((3))(3 omega), undergoes a 2 pi shift, demonstrat ing that two nearly simultaneous resonances (at 0.7 and 0.9 eV) are tr aversed, as expected for degenerate ground state polymers where the tw o-photon mid-gap state formed by neutral soliton-antisoliton pairs pla ys a dominant role in chi((3)) processes.