Experimental Verification of \u3cem\u3en\u3c/em\u3e = 0 Structures for Visible Light

We fabricate and characterize a metal-dielectric nanostructure with an effective refractive index n=0 in the visible spectral range. Light is excited in the material at deep subwavelength resolution by a 30-keV electron beam. From the measured spatially and angle-resolved emission patterns, a vanishing phase advance, corresponding to an effective ϵ=0 and n=0, is directly observed at the cutoff frequency. The wavelength at which this condition is observed can be tuned over the entire visible or near-infrared spectral range by varying the waveguide width. This n=0 plasmonic nanostructure may serve as a new building block in nanoscale optical integrated circuits and to control spontaneous emission as experimentally demonstrated by the strongly enhanced radiative optical density of states over the entire n=0 structure

Plasmonic excitation and manipulation with an electron beam

When an electron beam passes through or near a metal structure, it will excite surface plasmons, providing a unique way to access surface plasmon behavior at the nanoscale. An electron beam focused to nanometer dimensions thus functions as a point source that is able to probe the local plasmonic mode structure at deep-subwavelength resolution. In this article, we show how well-controlled coupling between an electron beam and surface plasmons, combined with a far-field detection system, allows characterization and manipulation of plasmons on a variety of plasmonic devices. By mapping the spatial profile of inelastic scattering to resonant modes, the dispersion and losses of surface plasmons are resolved. The technique further allows probing of the confinement of plasmons within cavities and measuring the angular emission profile of nanoantennas. The coupling of electrons to surface plasmons allows the use of the electron beam as a dipole emitter that can be positioned at will. The beam position thereby can select between modes with different symmetries. This effect can be used to exert forces on plasmonic structures on the nanometer length scale with great control. © 2012 Materials Research Society.This work is part of the research program of the “Stichting voor Fundamenteel Onderzoek der Materie (FOM),” which is financially supported by the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO).” P.E.B. acknowledges financial support from the Basic Energy Sciences Division of the US Department of Energy, Award #DE-SC0005132. J.A. acknowledges financial support from the project FIS2010-19609-C02-01 of the Spanish Ministery of Science and A.R.C. acknowledges Consejo Nacional de Ciencia y Tecnología of Mexico and Benemérita Universidad Autónoma de Puebla.Peer Reviewe

Broadband Purcell enhancement in plasmonic ring cavities

We present a theoretical study of a surface plasmon ring resonator geometry that shows strong spontaneous emission control due to an extremely small optical cavity mode volume. V grooves made in the surface of a metal confine whispering gallery surface plasmon polariton modes at the bottom of the groove, with the confinement determined by groove depth and width. Resonances in ringlike cavities defined by grooves patterned into circular shape, determined using the boundary-element method, are in agreement with calculations based on dispersion for linear V grooves. Cavity quality factors are relatively insensitive to cavity geometry (Q=10-50) while mode volume is very sensitive to small differences in the cavity shape. The smallest mode volume V=0.00073 λ03 is found for a ring with a 10-nm wide, 100-nm deep groove with straight sidewalls and a diameter of 180 nm. Purcell factors well above 2000 are found in the energy range from E=1.0-1.8eV depending on cavity geometry. For a given cavity geometry the Purcell enhancement is observed over a broad spectral range (50-100 meV), enabling application of these cavities beyond the typical low-temperature cavity quantum electrodynamics experiments. © 2010 The American Physical Society.Peer Reviewe

Modal decomposition of surface - Plasmon whispering gallery resonators

We resolve the resonant whispering gallery modes of plasmonic subwavelength ring cavities defined by circular grooves patterned into a gold surface. An interesting interplay is observed between subwavelength confinement and guiding along the groove. Full spatial and spectroscopic information is directly obtained using cathodoluminescence, including details of the nanoscale intensity distribution (spatial resolution 11 ±( 8 nm). Excellent agreement between measurements and rigorous electromagnetic theory is obtained, thus allowing us to assess the symmetry, ordering, degree of confinement, and near-field enhancement of the modes with unprecedented detail. © 2009 American Chemical Society.This work is part of the research program “Microscopy and Modification of Nanostructures with Focused Electron and Ion Beams: (MMN) of the “Stichting voor Fundamenteel Onderzoek der Materie” (FOM), which is financially supported by the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek” (NWO). The MMN program is cofinanced by FEI Company. F.J.G.A. acknowledges support from the Spanish MCeI (MAT2007- 66050 and NanoLight.es) and the EU-FP6 (NMP4-2006- 016881 “SPANS”).Peer Reviewe

Local density of states, spectrum, and far-field interference of surface plasmon polaritons probed by cathodoluminescence

The surface plasmon polariton (SPP) field intensity in the vicinity of gratings patterned in an otherwise planar gold surface is spatially resolved using cathodoluminescence (CL). A detailed theoretical analysis is presented that successfully explains the measured CL signal based upon interference of transition radiation directly generated by electron impact and SPPs launched by the electron and outcoupled by the grating. The measured spectral dependence of the SPP yield per incoming electron is in excellent agreement with rigorous electromagnetic calculations. The CL emission is shown to be similar to that of a dipole oriented perpendicular to the surface and situated at the point of electron impact, which allows us to establish a solid connection between the CL signal and the photonic local density of states associated to the SPPs. © 2009 The American Physical Society.Peer Reviewe

Plasmonic Modes of Annular Nanoresonators Imaged by Spectrally Resolved Cathodoluminescence

We report the observation of plasmonic modes of annular resonators in nanofabricated Ag and Au surfaces that are imaged by spectrally resolved cathodoluminescence. A highly focused 30 keV electron beam is used to excite localized surface plasmons that couple to collective resonant modes of the nanoresonators. We demonstrate unprecedented resolution of plasmonic mode excitation and by combining these observations with full-field simulations find that cathodoluminescence in plasmonic nanostructures is most efficiently excited at positions corresponding to antinodes in the modal electric field intensity

Plasmonic modes of annular nanoresonators imaged by spectrally resolved cathodoluminescence

We report the observation of plasmonic modes of annular resonators in nanofabricated Ag and Au surfaces that are imaged by spectrally resolved cathodoluminescence. A highly focused 30 keV electron beam is used to excite localized surface plasmons that couple to collective resonant modes of the nanoresonators. We demonstrate unprecedented resolution of plasmonic mode excitation and by combining these observations with full-field simulations find that cathodoluminescence in plasmonic nanostructures is most efficiently excited at positions corresponding to antinodes in the modal electric field intensity. © 2007 American Chemical Society.This work has benefited from numerous discussions with colleagues, including T. van Wijngaarden, J. Dionne, R. Walters, J. Biteen, D. Hofmann, E. Verhagen, and R. de Waele. We acknowledge financial support from the Air Force Office of Scientific Research under MURI Grant FA9550-04-1-0434.Peer Reviewe

Plasmonic Modes of Annular Nanoresonators Imaged by Spectrally Resolved Cathodoluminescence NANO LETTERS

We report the observation of plasmonic modes of annular resonators in nanofabricated Ag and Au surfaces that are imaged by spectrally resolved cathodoluminescence. A highly focused 30 keV electron beam is used to excite localized surface plasmons that couple to collective resonant modes of the nanoresonators. We demonstrate unprecedented resolution of plasmonic mode excitation and by combining these observations with full-field simulations find that cathodoluminescence in plasmonic nanostructures is most efficiently excited at positions corresponding to antinodes in the modal electric field intensity. Excitation and localization of surface plasmon polariton (SPP) modes in metallodielectric structures is currently a topic of intensive research motivated by the ability to achieve truly nanophotonic materials and devices with tunable optical dispersion. 1 In particular, nanoresonators are essential building blocks of future subwavelength-scale photonic systems as both active 2 and passive 3 device components. Nanostructures consisting of annular grooves and gratings in metal films exhibit exciting properties such as photon-to-plasmo