Spectrum of chi (3)(-3 omega; omega, omega, omega ) in polyacetylene: An application of free-electron laser in nonlinear optical spectroscopy.

We have measured the full spectrum of the third-order optical susceptibiltiy, ${\ensuremath{\chi}}^{(3)}$(\ensuremath{\omega}), in polyacetylene using an infrared free-electron laser. The magnitude of ${\ensuremath{\chi}}^{(3)}$(\ensuremath{\omega}) reaches \ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}8}$ esu at 0.6 eV, the largest value of an electronic ${\ensuremath{\chi}}^{(3)}$(\ensuremath{\omega}) inside the gap of a semiconductor. There are two peaks at 0.6 and 0.89 eV in the spectrum of ${\ensuremath{\chi}}^{(3)}$(\ensuremath{\omega}) that are assigned to three- and two-photon resonance enhancements, respectively, implying states of opposite symmetry lie near the 1D band gap at ${E}_{g}$\ensuremath{\sim}1.8 eV.