Investigation of the electrostatic phenomena at pentacene/metal interface by second-harmonic generation

Optical second-harmonic generation (SHG) in a vacuum evaporated pentacene thin film deposited on an aluminum (Al) surface was investigated on the basis of the electrostatic phenomena occurring at the interface. The SH process in centrosymmetric molecules is activated by the static electric field, i.e., the surface potential. Since electric field-induced SHG is classified as a third-order nonlinear optical process, its resonant condition differs from the one originating in the electric quadrupole. In the SHG spectrum of the pentacene/Al structure, two prominent peaks were observed at approximately 560 nm and 660 nm, whereas only one peak located at approximately 560 nm was observed in pentacene/glass structure. These results are discussed on the basis of the surface potential formed in the pentacene layer deposited on Al due to the displacement of electrons from metal to pentacene. The effects of photoirradiation on the SH spectrum were investigated to confirm the presence of the electric field-induced SHG.