Large third-order optical nonlinearities of ultrathin films containing octacarboxylic copper phthalocyanine

An ultrathin composite film with alternating layers containing anionic octacarboxylic copper phthalocyanine [CuPc(COONa)8] and cationic polyethylenimine (PEI) has been prepared by the electrostatic layer-by-layer self-assembly technique. The alternating multilayer assemblies of CuPc(COONa)8/PEI were characterized by UV–Vis spectroscopy, showing the regularly consecutive adsorption process. The film surface was smooth and homogeneous as indicated by atomic force microscopy. The closely packed nanoparticles, with the average diameter of 36 nm and the mean interface roughness of 2.984 nm, can be easily seen on the film surface uniformly. Further structure investigation was carried out by using small angle X-ray diffraction. Third-order nonlinear properties of the film and CuPc(COONa)8 aqueous solution were studied by Z-scan measurements at 532 nm with 8 ns pulses. The ultrathin film depicted strong nonlinear reverse saturable absorption and excellent self-defocusing performance. The second-order molecular hyperpolarizability γ value of the film was 68 times larger than that of solution sample.