FREQUENCY-DEPENDENCE OF 3RD-HARMONIC GENERATION IN CIS-POLYACETYLENE AND TRANSPOLYACETYLENE - IMPORTANCE OF THE DEGENERATE GROUND-STATE TO NONLINEAR OPTICAL-RESPONSE

Third-harmonic generation (THG) measurements on cis- and trans-polyacetylene over the pump frequency range from 0.55 through 1.25 eV show that chi(3)(3omega; omega, omega, omega) for trans-(CH)x is an order of magnitude larger than that for the cis-isomer over the entire spectral range, even comparing the respective 3omega resonance maxima. The THG data demonstrate a symmetry specific mechanism favoring the degenerate ground state system, consistent with the existence of an important contribution from virtual soliton pairs enabled by nonlinear zero-point fluctuations. Because the soliton pair creation energy at finite separation is less than the energy gap, chi(3) can be simultaneously resonant at 3omega and at 2omega, and, therefore, significantly enhanced in polymers with a degenerate ground state.