Second-harmonic generation in centrosymmetric molecular films: Analysis under anisotropic conditions

In order to interpret relatively strong second-harmonic generation (SHG) in centrosymmetric molecular films such as ${\mathrm{C}}_{60}$ and copper phthalocyanine (CuPc), higher-order contributions to bulk SHG have been considered. The contributions examined were of electric quadrupole, magnetic dipole, electric quadrupole coupling, and magnetic dipole coupling mechanisms, in addition to an electric dipole one in ${\mathit{K}}_{\mathit{h}}$, ${\mathit{D}}_{\mathrm{\infty }\mathit{h}}$ and ${\mathit{C}}_{\mathrm{\infty }\mathit{v}}$ point-group symmetries. Analytical expressions of the SHG intensities were obtained under a three-layer model. Using these expressions, we analyzed the experimental results on the thickness dependence of SHG in CuPc films evaporated on a glass substrate in various polarization combinations, and demonstrated that the dominant origin of SHG is attributed to the magnetic dipole coupling mechanism.