Wavelength dependence of the third-order nonlinear optical properties of a polythiophene/selenophene derivative film

The third-order nonlinear optical properties of a hybrid polymer film containing substituted thiophene rings copolymerized with unsubstituted selenophene were measured using the Z-scan technique. Large, resonantly enhanced third-order nonlinearities were found at wavelengths on the low-energy side of the absorption band. In contrast to the results on a similar copolymer film incorporating only thiophene, a strong two-photon absorption appeared in the near infrared. The imaginary part of the third-order susceptibility can be adequately explained as saturation of an exciton band, while the origin of the large negative real part is unknown.