Third-order non-linear optical susceptibility measurements of thin films of 3-methylthiophene and methylmethacrylate copolymers

Abstract

The thir-order non-linear optical susceptibility χ (3) of thin films of 3-methylthiophene and methylmethacrylate copolymers were measured using the degenerate four-wave mixing (DFWM) and the third-harmonic generation (THG) techniques. A red copolymer has a large resonant χ (3)(−ω; ω, ω, −ω) on the order of 8.7 × 10 −11 e.s.u., 12 times as large as that of a yellow copolymer determined using the DFWM at 0.602 μm. The THG-measured χ (3)(−3ω; ω, ω, ω) values were in the range of 5.0 × 10 −13 e.s.u. for a yellow copolymer and 1.3 × 10 −11 e.s.u. for a red copolymer at 1.860 μm. These results show that the optical transparency and third-order optical non-linearity can be tailored in random copolymers of 3-methylthiophene and methylmethacrylate. The large resonant optical non-linearities of the red copolymer are attributed to the highly delocalized π-electron system of 3-methylthiophene segments whereas the increased non-conjugated methylmethacrylate segments significantly reduce the magnitude of the optical non-linearity in yellow copolymer. The optical and non-linear optical properties of solution-processable copolymers are discussed in the light of the present results.