Influence of structural and morphological features of zinc (II)-tetraphenylporphyrin thin film on its third order optical nonlinearity at pico and nano second regimes

Implications of laser pulse width on the third-order nonlinear optical properties of Zinc(II) 5, 10, 15, 20-Tetraphenyl-21H, 23H-Porphine (ZnTPP) thin films is investigated. Thin films of ZnTPP are developed on glass substrate by means of thermal vapor deposition technique. X-ray diffraction analysis illustrates polycrystalline nature of these films. Atomic force microscopy and field emission scanning electron microscope images confirm the homogeneous film surface with mean surface roughness value of 8.45 nm. UV–Visible absorption spectrum depicts signature peaks attributable to π-π* orbital transitions. Photoluminescence spectrum illustrates fluorescence emission in the wavelength range of 590 nm–650 nm from S1 state. Nonlinear optical investigations are carried out using single-beam Z-scan technique. ZnTPP thin-film at pico-second regime depicts saturable nonlinear absorption nature, attributable to the collective effect of singlet excited state absorption and filling effect of localized defect states. Whereas, at nanosecond timescale the film exhibits reverse saturable nonlinear absorption, sensitive to triplet excited state absorption. Closed aperture Z-scan measurements at pico and nano second regime highlight self-defocusing optical nonlinearity. Enhanced nonlinear optical coefficient (βeff and n2) values are strongly influenced by the heavy zinc metal ion and modified excited state dynamics due to the strong intermolecular interactions through J-aggregation.