Enhanced third order optical nonlinearity in ultrathin amorphous film of tetraphenyl-porphyrin in picosecond regime

The third order optical nonlinearity of ultrathin 5, 10, 15, 20-Tetraphenyl-21H, 23H-porphine (TPP) films fabricated on an ultrasonically cleaned glass-substrate, by high vacuum thermal evaporation method is investigated. The powder X-ray diffraction (XRD) pattern demonstrates the amorphous nature of the fabricated thin film. The atomic force microscopy (AFM) and the field emission scanning electron microscope (FESEM) images show that the surface morphology of thin film composes of randomly oriented particles with the mean surface roughness of 17.73 nm. The structure of TPP thin film portrays a characteristic UV-Visible spectrum due to pi-pi* transitions in the porphyrin molecule. The photoluminescence spectroscopic study reveals that the TPP exhibits excellent fluorescence emission from S-1 singlet excited state. The third order optical nonlinearity is studied using single beam Z-scan technique at 532 nm with picosecond pulsed (Delta tau = 30 ps) laser. The TPP thin film exhibits saturation absorption property, with the effective two-photon absorption coefficient (beta(eff)) of the order of 10(-6) m/W. The observed nonlinear saturation absorption behavior is largely influenced by one-photon absorption due to the filling effect of the surface states and the localized defect states in the thin film. The closed aperture Z-scan study highlights the self-defocusing nature of the TPP thin film with negative nonlinear refractive index (n(2)) of the order of 10(-13) m(2)/W. The enhanced n(2) value is attributed to the highly polarizable structure of free base TPP molecule and modified electronic band structure due to the strong intermolecular interactions observed in the condensed state.