Spectrum of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>χ</mml:mi></mml:mrow><mml:mrow><mml:mn/><mml:mo>(</mml:mo><mml:mn>3</mml:mn><mml:mo>)</mml:mo><mml:mn/></mml:mrow></mml:msup></mml:mrow></mml:math>(-3<i>ω;ω</i>,<i>ω</i>,<i>ω</i>) 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.