Two-photon absorption measured in the presence of strong one-photon saturation in cumulene-containing polymer

The Z-scan technique is used to extract the real and imaginary third order nonlinear susceptibilities, Rex(3) and Imx(3), respectively. A series of Z scans were conducted at 590 nm (near one-photon resonance) on a cumulene-containing polymer, poly(p-phenylene-l,4-diphenyl-l,2,3-butatriene) or PPC3, prepared in solution. At very high peak focal point intensities two-photon absorption is seen superimposed on a one-photon saturation signature in open aperture Z scans. A negative real third-order nonlinear susceptibility is also observed in closed aperture Z scans. We describe a procedure to extract the desired third-order nonlinear susceptibilities by conducting Z scans at various peak focal point intensities and then fitting the results by adjusting Rex(3) and saturation intensity. Molecular second hyperpolarizability, is calculated to be (-1.4+2.2i)xl029 esu which is almost 40 times larger than that measured in a monomer equivalent, and scaling nonlinearly with chain length. Z scans were also conducted at 780 nm (below one-photon resonance) on PPC3 and also gives a molecular second hyperpolarizability that is greater than that seen in the monomer. The higher value of molecular second hyperpolarizability measured near one-photon resonance is attributed to the existence of a real state at the first transition in a two-photon process. The extent of the exciton wave function obtained from the saturation intensity, is 60 A , larger than a single repeat unit.