Femtosecond Z-scan and degenerate four-wave mixing measurements of real and imaginary parts of the third-order nonlinearity of soluble conjugated polymers

We investigated the third-order nonlinear optical properties of several soluble π-conjugated polymers with a goal of reliable determination of the parameters that characterize the nonlinearity, such as the real and imaginary parts of the nonlinear refractive index $n_2$ and the one-photon loss and two-photon loss merit factors $W$ and $T,$ respectively. The measurements were performed at 800 nm with 100-fs pulses from an amplified Ti:sapphire system. We present results of investigations of several 2,5-substituted poly($p$-phenylene vinylenes), including poly[2-methoxy-5-(2^′-ethyl-hexyloxy)-$p$-phenylenevinylene] and a soluble ladder poly ($p$-phenylene) polymer. In particular, Z-scan measurements of polymer solutions were found to be useful for the determination of the complex nonlinear refractive index. The results could be verified by time-dependent degenerate four-wave mixing studies performed on thin films of the polymers. We found generally good agreement between the values of $n_2$ determined from Z scan and those from degenerate four-wave mixing. The results indicate that the nonlinear refractive index of the order of ${10}^{-12} {\mathrm{cm}}^2/\mathrm{W}$ can be readily obtained for various conjugated polymers. However, the two-photon merit factors $T$ larger than unity are commonly encountered within the two-photon absorption ranges of these compounds.