Irradiance dependence of the resonant nonlinearities in an organic material

We present experimental data for the irradiance dependence of the optical nonlinearity of an organic material at different wavelengths across resonance. The material studied was a suspension of vanadyl-phthalocyanine nanocrystals in cyclohexane. The refractive and absorptive contributions to the nonlinearity were resolved by use of the z-scan technique with a tunable picosecond laser source. The observed dependence of the nonlinearity with irradiance is explained by excited-state absorption, and a three-level model is used to describe the nonlinear response. A fit to the experimental data is made, allowing the extraction of molecular parameters that are of interest for a full characterization of the nonlinearity.