Third-order nonlinear response of conjugated polymer coated carbon nanotubes

Carbon nanotubes coated with an nonlinear optical (NLO) polymer, poly [3-hexylthiophene-2,5-diyl]- [3-methoxyl-4-hydroxyl-benzylidenequino-methane](PHTHMOBQ/MWNTs), were synthesized via in-situ polymerization. The micrograph of PHTHMOBQ/MWNTs has the coaxial tubular structure, which indicated that the polymer was restricted and wrapped on the surface of carbon nanotubes while propagating during polymerization. PHTHMOBQ /MWNTs showed fluorescence quenching, which involved the inter-molecular photo-induced charge transfer process. Third-order nonlinear susceptibility χ₍₃₎ and second-order hyperpolarizability γ of PHTHMOBQ/MWNTs were measured using degenerate four wave mixing at 532 nm under nanosecond pulse excitation. The two pump beams were vertically polarized and the probe beam was horizontally polarized to avoid thermal effect. The third-order NLO responses were enhanced gradually when nanotube content increased, which was attributed to inter-molecular photo-induced electron transfer between PHTHMOBQ and MWNTs.