Resonant enhancement of second-harmonic generation of electric quadrupole origin in phthalocyanine films

Second-harmonic-generation (SHG) resonance spectra were measured in thin films of centrosymmetric copper phthalocyanine and metal-free phthalocyanine. A sharp peak was observed at the high-energy edge of the $Q$-absorption band using 1100-nm fundamental light, and was attributed to the resonant enhancement of the electric-quadrupole transition based on an orientation gas model. The SHG resonance spectra provide us with a useful tool for studying electric-dipole-forbidden states.