Irradiance dependence of the resonant nonlinearities in an organic material

We present experimental data for the irradiance dependence of the optical n onlinearity of an organic material at different wavelengths across resonanc e; The material studied was a suspension of vanadyl-phthalocyanine nanocrys tals in cyclohexane. The refractive and absorptive contributions to the non linearity were resolved by use of the z-scan technique with a tunable picos econd laser source. The observed dependence of the nonlinearity with irradi ance is explained by excited-state absorption, and a three-level model is u sed to describe the nonlinear response. A fit to the experimental data is m ade, allowing the extraction of molecular parameters that are of interest f or a full characterization of the nonlinearity. (C) 2000 Optical Society of America [S0740-3224(00)01808-7] OCIS codes: 190.4710, 160.4330, 070.4340.