Two-photon absorption spectra of luminescent conducting polymers measured over a wide spectral range

We report the two-photon absorption (TPA) spectra of poly(2,5-dibutoxy-p-phenylene acetylene) (PPADBO), poly(2,5-dioctyloxy-p-phenylene vinylene) (PPV-DOO), and poly(3-hexylthiophene) (PAT6) in the spectral range extending from 576 nm to 846 nm. Using the Z-scan technique on the polymers in solution, we measured a strong two-photon allowed transition in ail three materials which we attribute to the mAg essential state. In the case of PPA-DBO and PPV-DOO, TPA peaks were coincident with dispersion in the nonlinear refractive indices as detected by reduced aperture Z scan. In all three polymers this peak occurs at approximately 1.3 the bandgap energy (Eg^). The excitonic nature of the excited electronic states in PPA-DBO is indicated by the lack of a TPA band at or near the 1 Bu exciton position. Saturation was observed in the nonlinear index of refraction near spectral peaks, as well as an apparent reverse Kramers-Kronig effect.