We report our results on the femtosecond nonlinear optical studies of alkyl-phthalocyanine nanoparticles dispersed in water and dissolved in chloroform. Nonlinear refractive and absorptive properties were investigated using the closed and open aperture Z-scan techniques. The nonlinear optical coefficients obtained for nanoparticles suspended in water are compared with those dissolved in chloroform, which resulted in simple phthalocyanine solutions. Our studies clearly demonstrate the nonlinear

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... efractive index of nanoparticles to be positive, while that of the nanoparticles in chloroform was negative. Our results and analysis point out a high nonlinearity in the ultrafast domain for both the nanoparticles and solutions indicating their potential in optical signal processing and limiting applications.