Second harmonic generation from copper-tetratert-butyl-phthalocyanine Langmuir–Blodgett film/metal interface: Electric quadrupole or electric field induced second harmonic generation effect?

Abstract

It has been found that a high electric field is created at the interface between copper-tetratert-butyl-phthalocyanine (CuttbPc) Langmuir–Blodgett (LB) film/metal due to the exchanged charges. The symmetry of the CuttbPc molecule is broken by this space charge induced electric field (SCIEF), and optical second harmonic (SH) signal is generated from the CuttbPc LB film/metal interface. A theoretical expression of the reflected SH signal intensity is derived using a four-layer model with the SCIEF being treated as a mean field. The origins of the SHG are considered as the quadrupole effect and the electric field induced SHG (EFISHG) due to the SCIEF. On the basis of experimental results and the theoretical analysis, the nonlinear susceptibilities of these two effects are calculated. It is found that at the wavelength of incident fundamental light around 1100 nm the electric quadrupole effect makes a main contribution to the SHG from the interface, while at the wavelength around 1200 nm it is attributed to the EFISHG. It is expected that the SCIEF could be determined by SHG.