Phase of the electric field localized at surface-immobilized gold nanospheres determined by second-harmonic interferometry

Gold nanospheres immobilized above a gold surface, with a nanometer gap between, comprise a promising plasmonic system. Although many works have studied the magnitude of the enhanced electric fields that are localized at the plasmonic nanostructure, little is known about their phase. To probe the phase of the electric field localized adjacent to the plasmonic nanostructure, we undertook interferometry for second-harmonic (SH) light from hemicyanine monolayers on the surface-immobilized gold nanospheres (SIGNs), and from a hemicyanine monolayer in the nanogap. The SH phase from the hemicyanine monolayer on the SIGNs was found to be almost opposite to that from the hemicyanine monolayer in the nanogap. This result is supported by a theoretical calculation of the localized electric fields based on higher order multipoles.