title: Optical Dark-Field and Electron Energy Loss Imaging and Spectroscopy of Symmetry-Forbidden Modes in Loaded Nanogap Antennas.
abstract: We have produced large numbers of hybrid metal-semiconductor nanogap antennas using a scalable electrochemical approach and systematically characterized the spectral and spatial character of their plasmonic modes with optical dark-field scattering, electron energy loss spectroscopy with principal component analysis, and full wave simulations. The coordination of these techniques reveal that these nanostructures support degenerate transverse modes which split due to substrate interactions, a longitudinal mode which scales with antenna length, and a symmetry-forbidden gap-localized transverse mode. This gap-localized transverse mode arises from mode splitting of transverse resonances supported on both antenna arms and is confined to the gap load enabling (i) delivery of substantial energy to the gap material and (ii) the possibility of tuning the antenna resonance via active modulation of the gap material's optical properties. The resonant position of this symmetry-forbidden mode is sensitive to gap size, dielectric strength of the gap material, and is highly suppressed in air-gapped structures which may explain its absence from the literature to date. Understanding the complex modal structure supported on hybrid nanosystems is necessary to enable the multifunctional components many seek.
authors.affiliation: †Materials Division, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, United States.
authors.forename: Todd
authors.initials: T
authors.lastname: Brintlinger
authors.affiliation: ‡Materials Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States.
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Herzing
authors.affiliation: §Chemistry Division, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, United States.
authors.forename: James P
authors.initials: JP
authors.lastname: Long
authors.affiliation: ∥Optical Sciences Division, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, United States.
authors.forename: Igor
authors.initials: I
authors.lastname: Vurgaftman
authors.affiliation: †Materials Division, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, United States.
authors.forename: Rhonda
authors.initials: R
authors.lastname: Stroud
authors.affiliation: §Chemistry Division, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, DC 20375, United States.
authors.forename: B S
authors.initials: BS
authors.lastname: Simpkins
keywords: dark-field spectroscopy; electron energy loss spectroscopy; hybrid nanoparticles; localized surface plasmon; nanogap antenna; plasmon hybridization