CONJUGATED POLYMERS WITH DEGENERATE GROUND-STATE - THE ROUTE TO HIGH-PERFORMANCE 3RD-ORDER NONLINEAR OPTICAL-RESPONSE

Third harmonic generation (THG) measurements on cis- and trans-polyacetylene demonstrate a symmetry specific mechanism favoring the degenerate ground state: the third-order susceptibility, chi(3)(-3omega;omega,omega,omega), is an order of magnitude larger in trans-(CH)x than in cis-(CH)x over the entire spectral range, even comparing the respective three-photon resonance maxima. Reduced disorder in high-quality oriented trans-(CH)x increases chi(3)(3omega) by more than an order of magnitude to values greater-than-or-equal-to 10(-8)-10(-7) esu for HBARomega within the energy pp. The THG data are consistent with theory which predicts a dominant contribution to chi(3)(3omega) from neutral soliton pair intermediate states with A(g) symmetry.