Nonlinear optical characteristics of vanadyl phthalocyanine thin film grown by the molecular beam epitaxial method

We have studied several aspects of nonlinear optical behaviors of unidirectionally oriented vanadyl phthalocyanine (VOPc) epitaxial film. The characteristic nature of the epitaxial film has been found to be the rotational anisotropy in third-harmonic intensity in the film surface plane and demonstrated to be associated with the symmetry elements of the film. The essential features are explained based on the symmetries of C-4v for VOPc/KBr epitaxial film and C-infinity V for VOPc/silica polycrystalline film. The epitaxial film is distinguished from the polycrystalline one using in-plane angle-dependence THG measurement. It is found that the rotational anisotropy in the epitaxial film is strongly wavelength dependent, We attribute the spectral dependence to resonant coupling of the fundamental and harmonic fields with electronic states of the pi-electron system of the epitaxial film. We have also discussed the orientation of VOPc molecules on KBr substrate and proposed an accommodation model of the VOPc lattice on the KBr (001) surface based on third harmonic generation measurement and computer simulation.