title: Extremely confined gap surface-plasmon modes excited by electrons.
abstract: High-spatial and energy resolution electron energy-loss spectroscopy (EELS) can be used for detailed characterization of localized and propagating surface-plasmon excitations in metal nanostructures, giving insight into fundamental physical phenomena and various plasmonic effects. Here, applying EELS to ultra-sharp convex grooves in gold, we directly probe extremely confined gap surface-plasmon (GSP) modes excited by swift electrons in nanometre-wide gaps. We reveal the resonance behaviour associated with the excitation of the antisymmetric GSP mode for extremely small gap widths, down to ~5 nm. We argue that excitation of this mode, featuring very strong absorption, has a crucial role in experimental realizations of non-resonant light absorption by ultra-sharp convex grooves with fabrication-induced asymmetry. The occurrence of the antisymmetric GSP mode along with the fundamental GSP mode exploited in plasmonic waveguides with extreme light confinement is a very important factor that should be taken into account in the design of nanoplasmonic circuits and devices.
authors.affiliation: 1] Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark [2] Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark [3].
authors.forename: Søren
authors.initials: S
authors.lastname: Raza
authors.affiliation: 1] Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark [2] Center for Nanostructured Graphene (CNG), Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark [3].
authors.forename: Nicolas
authors.initials: N
authors.lastname: Stenger
authors.affiliation: 1] Department of Technology and Innovation, University of Southern Denmark, DK-5230 Odense M, Denmark [2].
authors.forename: Anders
authors.initials: A
authors.lastname: Pors
authors.affiliation: Department of Physics and Nanotechnology, Aalborg University, DK-9220 Aalborg Øst, Denmark.
authors.forename: Tobias
authors.initials: T
authors.lastname: Holmgaard
authors.affiliation: Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark.
authors.forename: Shima
authors.initials: S
authors.lastname: Kadkhodazadeh
authors.affiliation: Center for Electron Nanoscopy, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark.
authors.forename: Jakob B
authors.initials: JB
authors.lastname: Wagner
authors.affiliation: Department of Physics and Nanotechnology, Aalborg University, DK-9220 Aalborg Øst, Denmark.
authors.forename: Kjeld
authors.initials: K
authors.lastname: Pedersen
authors.affiliation: 1] Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark [2] Center for Nanostructured Graphene (CNG), Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark.
authors.forename: Martijn
authors.initials: M
authors.lastname: Wubs
authors.affiliation: Department of Technology and Innovation, University of Southern Denmark, DK-5230 Odense M, Denmark.
authors.forename: Sergey I
authors.initials: SI
authors.lastname: Bozhevolnyi
authors.affiliation: 1] Department of Photonics Engineering, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark [2] Center for Nanostructured Graphene (CNG), Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark.
authors.forename: N Asger
authors.initials: NA
authors.lastname: Mortensen