Two-photon resonant third-harmonic generation in polysilanes

Third-order nonlinear-optical susceptibility spectra ‖${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(-3ω;ω,ω,ω)‖ have been obtained for poly(dihexylsilane) (PDHS) and poly(dibutylsilane) (PDBS) films by third-harmonic generation (THG) measurements for a fundamental photon energy region from 1.90 to 2.15 eV, which covers the two-photon absorption (TPA) for PDHS but not for PDBS. The ‖${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$‖ spectrum for PDHS shows a prominent peak at about 2.08 eV with a maximum value of about 2.3×${10}^{\mathrm{-}12}$ esu, while there is no enhancement effect in PDBS in the same fundamental photon energy region. These results provide direct evidence for the two-photon-resonant THG process in polysilanes due to the one-photon-forbidden ${}_{}{}^1{}_{}{}^{}$ ${\mathit{A}}_{\mathit{g}}$ exciton detected by TPA and electroabsorption measurements.