INTERPRETING THE DISPERSION OF CHI(3)(-3-OMEGA, OMEGA, OMEGA, OMEGA) OF POLYTHIOPHENES

The dispersion of chi(3)( -3omega; omega, omega, omega) for poly(3-hexylthiophene) (P3HT) and five other polythiophenes which possess a mixed aromatic-quinoid backbone structure has been characterized using third-harmonic generation (THG). The data were interpreted using perturbation theory and the essential states mechanism. The nonlinearity of the P3HT is well described by the two-level model, while the mixed backbone systems need three levels to reproduce properly the measured dispersion. It is demonstrated that the three-level model leads to an interference minimum in the dispersion of the nonlinearity. The chi(3)( - omega; omega, - omega, omega) is predicted from the parameters derived by fitting the THG data, and the resulting figure of merit is discussed.