The spectra of the third-order optical nonlinearity, $|{\chi }^{\mathrm{}\left(3\right)\mathrm{}}(\omega ;\omega ,\omega ,-\omega )|,$ around excitonic resonance in a spin-coated film and a Langmuir-Blodgett (LB) film of polythiophene derivative, poly(3-[2-((S)-2-methylbutoxy)ethyl]thiophene), were measured using the degenerate four-wave-mixing method. The obtained value of $|{\chi }^{\mathrm{}\left(3\right)\mathrm{}}(\omega ;\omega ,\omega ,-\omega )|$ at the excitonic resonance was $\sim {10}^{-7}\mathrm{esu},$ which was the largest value ever obtained in conjugated polymers. From the absorption spectra and $|{\chi }^{\mathrm{}\left(3\right)\mathrm{}}|$ spectra, the average effective conjugation lengths of the spin-coated film and the LB film were estimated to be approximately 20- and 36-thiophene rings, respectively.

• Received 14 July 1998

DOI:https://doi.org/10.1103/PhysRevB.58.R13430