Phase-dependent ultrafast third-order optical nonlinearities in metallophthalocyanine thin films

We present a comprehensive study on the impact of phase transformations of metallophthalocyanine thin films on their third-order nonlinear optical (NLO) properties. The metallophthalocyanine thin films are prepared by thermally evaporating the commercially available Copper(II) 2,9,16,23-Tetra-tert-butyl-29H, 31H-phthalocyanine (CuPc) and Zinc(II) 2,9,16,23-Tetra-tert-butyl-29H, 31H-phthalocyanine (ZnPc) powder on glass substrate. Thermal annealing causes a phase transformation which has a distinct signature in powder X-ray diffraction and UV-Vis-NIR spectroscopy. The NLO characteristics which include nonlinear refractive index n2, as well as nonlinear absorption coefficient (beta(eff)), were measured by using a single beam Z-scan technique. An ultrashort pulsed fiber laser emitting femtosecond pulses (Delta(tau) approximate to 250 fs) at 1064 nm central wavelength is used as a source for the Z-scan experiment. The beta(eff) values in as prepared thin films were ascertained to be smaller as compared to the annealed one due to the smaller value of saturation intensity (I-s) which, in turn, is a consequence of ground-state bleaching in the thermally unstable amorphous state of the molecule. Interestingly, the nonlinear refractive indices bear opposite sign for CuPc and ZnPc. The variations in the third-order nonlinearity in CuPc and ZnPc are discussed in terms of molecular packing and geometries of metallophthalocyanine molecules. Published by AIP Publishing.