Intensity Dependent Third Order Optical Nonlinearity of Zinc-Tetraphenyl Porphyrin Ultrathin film in Nano-Second Regime

The third order nonlinear optical properties of 5,10,15,20 Tetraphenyl 21H,23H-porphine zinc (II) (ZnTPP) thin films have been investigated by using single beam Z-scan technique with 6 ns laser pulse operating at 532 nm. The ultrathin films of ZnTPP with the thickness of the order of 120 nm were fabricated by means thermal vacuum coating technique. The UV-Visible spectrum depicts signature absorption curve identifying the enhancement in the symmetry of the molecule from D-2h symmetry to D-4h, due to the presence of central Zn metal ion. The ZnTPP thin film is exhibiting reverse saturation absorption (RSA) and strong self-defocusing effect. The effective two photon nonlinear absorption coefficients (beta(eff)) and nonlinear refractive index (n(2)) obtained at different input on-axis peak intensities (I-0) are of the order of 10(-6) to 10(-5) m/W and 10(-13) to 10(-11) m(2)/W. The decrease in beta(eff) with the increase in intensity indicates that the observed RSA is due to the excited state absorption (ESA) from T-1 triplet state to higher triplet state (T-n).