Anomalous Nonlinear Absorption Behavior in an Unsymmetrical Phthalocyanine Studied Near 800 nm using Femtosecond and Picosecond Pulses

The nonlinear optical properties of an unsymmetrical Zinc phthalocyanine were studied using the Z-scan technique in femtosecond and picosecond time domains. Open aperture Z-scan studies with <110 femtosecond pulses revealed an intriguing behavior exhibiting switching phenomena (from saturable absorption to reverse saturable absorption and back to saturable absorption) as a function of concentration and the peak intensities used. Similar behavior was observed with <2 picosecond pulse excitation. The observed results are qualitatively explained using an effective three-level model vis-à-vis a recent report on nonlinear absorption in a chloro-aluminum phthalocyanine where analogous performance was reported due to the solute migration. Such behavior is explicated as due to the consequence of combined effects of solvent contribution, large peak intensities, concentration, etc. The nonlinear absorption coefficients were established from the fits to the data.