Thickness dependence of morphology and third-order nonlinear optical characteristics of metal-phthalocyanine thin films prepared with different thicknesses on KBr substrate

The third-order nonlinearity of nonlinear optical materials is of great importance in achieving all-optical switching. However, the relationship between the morphology and third-order nonlinear optical characteristics of metal-phthalocyanine thin film is unclear. In this paper, the thickness dependence of the morphologies and the nonlinear optical properties of titanylphthalocyanine (TiOPc) and vanadyl-phthalocyanine (VOPc) thin films prepared on a KBr substrate are investigated by using UV/Vis spectra, X-ray diffraction (XRD), and third-order harmonics (TH) and second-order harmonics (SH) detected using the Maker fringe method. TiOPc and VOPc thin films having different thicknesses were prepared on a KBr substrate using a molecular beam epitaxy (MBE) method. From the Vis/UV spectra of VOPc thin films fabricated on KBr substrate, the VOPc thin film epitaxially grows until the thickness of 90nm and VOPc thin films that are thicker than 90 nm change to a pseudomorphic layer. In the incident angle dependence of TH intensity, the maximum TH intensity of the pseudomorphic layers is not proportional to the square of film thickness. This indicates that defects inside the pseudomorphic layers are closely related to the maximum TH intensity not being proportional to the square of film thickness. These defects may be due to the misfit between the VOPc and TiOPc molecules and the KBr substrate. On the other hand, within the thickness of the TiOPc thin films fabricated on KBr substrate, the TiOPc thin films mainly grow with epitaxy. However, the maximum TH intensity measured with the TiOPc thin film is lower than that of the VOPc thin film. This is related to the molecular packing density of VOPc thin film being larger than that of TiOPc thin film.