MULTIPHOTON NONLINEAR-INTERACTIONS IN CONJUGATED ORGANIC POLYMERS

The optical nonlinearity at a wavelength of 1.064 μm has been studied in a range of soluble conjugated organic polymers using the technique of degenerate forward four-wave mixing. This technique enables determination of the third-order nonlinear susceptibility |χ(3)| of the material. In all cases, determination of the response time of the material nonlinearity is limited by the experimental resolution (≈ 70 ps). Measurements of |χ(3)| as a function of polymer solution concentration were performed and contributions from both real and imaginary components were observed and estimated. The origin of the imaginary contribution in these 'off-resonant' conditions was investigated by intensity-dependent measurements of the four-wave mixing signal. In solutions of poly-3 and -4(butoxycarbonylmethylurethane diacetylene), (BCMU) in both chloroform and chlorobenzene, a dependence characteristic of a fifth-order nonlinear interaction is observed. Furthermore, in chloroform solutions of poly(3-butylthiophene) a seventh-order nonlinearity is observed. These high-order nonlinearities are associated with two-photon and three-photon resonant enhancement of the four-photon process and are characterized by fifth- and seventh-order susceptibilities, χ(5) and χ(7) respectively. The presence of imaginary contributions to the optical nonlinearity in all materials investigated implies that this nonlinearity is dominated in all cases by the nearest resonance. This conclusions renders redundant such simplistic models as the free-electron model. © 1990.