Anisotropy of the third-order nonlinear-optical susceptibility in a degenerate-ground-state conjugated polymer: trans-(CH)x.

Abstract

We present the results of a series of third-harmonic-generation (THG) measurements on the conjugated polymer polyacetylene which probe the magnitude and the origin of the third-order nonlinear-optical susceptibility of this material. By performing reflection THG measurements relative to a silicon standard, we have unambiguously determined the magnitude of the third-order susceptibility associated with tripling the fundamental of the Nd:YAG laser to be ${\ensuremath{\chi}}_{?}^{(3)}$(3\ensuremath{\omega};\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega}/B) =(4\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ esu, where ${\ensuremath{\chi}}_{?}^{(3)}$ refers to that component of the third-order susceptibility tensor with all indices parallel to the chain direction. By measuring anisotropic THG in oriented films, we have shown that this component dominates. The magntiude and anisotropy are directly compared with results obtained from single crystals of polydiacetylene-(toluene-sulfonate) measured in the same apparatus. Finally, we have measured THG in both cis-rich and trans isomers of the same sample. We found that the measured response of the cis-rich samples scales with the residual trans content of the sample; ${\ensuremath{\chi}}_{?}^{(3)}$ of the trans isomer is 15--20 times larger than that of the cis isomer. This symmetry-specific aspect of ${\ensuremath{\chi}}^{(3)}$ implies a mechanism which is sensitive to the existence of a degenerate ground state, as in trans-(CH${)}_{\mathrm{x}}$; it is consistent with the virtual generation of nonlinear solitons as a mechanism for the large measured third-order nonlinear-optical susceptibility of polyacetylene.