The polydiacetylenes are known for their large and fast third order optical nonlinearities, mainly due to the one dimensional delocalization of their π-electron system. Since they are available in several easily processable forms (Solutions,gels...) they can be considered as prototype materials for nonlinear optics. These nonlinearities are usually characterized by a third order nonlinear susceptibility (x(3)) but higher order effects take place when the pump intensity is increased. These higher order effects are usually observed under one photon resonant conditions, where they correspond to a saturation of the optical nonlinearity with pump intensity. Here we report on investigations of high order nonlinearities with a pump wavelength in the transparancy domain of the polydiacetylenes. Under our experimental conditions the nonlinear responses are dominated by higher order effects for pump intensities of the order of 1 GW/cm and, on the contrary to the one photon resonant situation, they correspond to an increase of the optical nonlinearity.
J. M. Nunzi,
"Strong Field Nonlinear Optical Properties Of Polydiacetylenes", Proc. SPIE 1127, Nonlinear Optical Materials II, (22 December 1989); doi: 10.1117/12.961417; https://doi.org/10.1117/12.961417