Spectrum ofχ(3)(-3ω;ω,ω,ω) in polyacetylene: An application of free-electron laser in nonlinear optical spectroscopy

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