Third-order optical susceptibilities and their temperature dependence in polysilane and polygermane materials.

The first measurements of the third-order nonlinear susceptibilities of a novel class of polymers, polysilane and polygermane with planar zigzag backbones, are reported. Third-harmonic generation was performed using 50-800-nm-thick polymer films on 1-mm-thick fused silica substrates. Thermally induced phase changes between the planar trans structure and other backbone disorder structures were found to result in marked and reversible changes in the nonlinearity. A significant enhancement of the nonlinear susceptibility was observed to result from the optimal sigma bonded conjugation in the low-temperature all-trans structure.