A metallic nanostructure consisting of a disk inside a thin ring supports superradiant and very narrow subradiant modes. Symmetry breaking in this structure enables a coupling between plasmon modes of differing multipolar order, resulting in a tunable Fano resonance. The LSPR sensitivities of the subradiant and Fano resonances are predicted to be among the largest yet for individual nanostructures.

We observe the appearance of Fano resonances in the optical response of plasmonic nanocavities due to the coherent coupling between their superradiant and subradiant plasmon modes. Two reduced-symmetry nanostructures probed via confocal spectroscopy, a dolmen-style slab arrangement and a ring/disk dimer, clearly exhibit the strong polarization and geometry dependence expected for this behavior at the individual nanostructure level, confirmed by full-field electrodynamic analysis of each structure. In each case, multiple Fano resonances occur as structure size is increased.

On the basis of conformal transformation, a general strategy is proposed to design plasmonic nanostructures capable of an efficient harvesting of light over a broadband spectrum. The surface plasmon modes propagate toward the singularity of these structures where the group velocity vanishes and energy accumulates. A considerable field enhancement and confinement is thus expected. Radiation losses are also investigated when the structure dimension becomes comparable to the wavelength.

Plasmonic nanocavities consisting of the concentric arrangement of a disk and a ring sustain both subradiant and superradiant dipolar plasmon modes with large associated field enhancements and high refractive index sensitivities. In structures with broken symmetry, additionally a highly tunable Fano interference feature appears, which can be explained with a simple analytical harmonic oscillator model. The spectral tunability of these resonances from the visible to the mid-infrared is investigated, highlighting a potential for applications in surface enhanced spectroscopies.

Subradiant and superradiant plasmon modes in concentric ring/disk nanocavities are experimentally observed. The subradiance is obtained through an overall reduction of the total dipole moment of the hybridized mode due to antisymmetric coupling of the dipole moments of the parent plasmons. Multiple Fano resonances appear within the superradiant continuum when structural symmetry is broken via a nanometric displacement of the disk, due to coupling with higher order ring modes. Both subradiant modes and Fano resonances exhibit substantial reductions in line width compared to the parent plasmon resonances, opening up possibilities in optical and near IR sensing via plasmon line shape design.

Unidirectional side scattering of light by a singleelement plasmonic nanoantenna is demonstrated using full-field simulations and back focal plane measurements. We show that the phase and amplitude matching that occurs at the Fano interference between two localized surface plasmon modes in a V-shaped nanoparticle lies at the origin of this effect. A detailed analysis of the V-antenna modeled as a system of two coherent point-dipole sources elucidates the mechanisms that give rise to a tunable experimental directivity as large as 15 dB. The understanding of Fano-based directional scattering opens a way to develop new directional optical antennas for subwavelength color routing and selfreferenced directional sensing. In addition, the directionality of these nanoantennas can increase the detection efficiency of fluorescence and surface enhanced Raman scattering.KEYWORDS: Nanoantenna, surface plasmon resonance, directionality, Fano resonance, side scattering T he interaction of light with metal nanoparticles is largely governed by resonant oscillations of the free electrons at the metal-dielectric interface. These so-called localized surface plasmon resonances (LSPR) can reach frequencies in the visible spectrum, have large extinction cross sections, are very sensitive to the surrounding medium, and lead to deepsubwavelength electromagnetic field confinement and enhancement. Plasmonic resonators, therefore, bring optics into the nanoscale and have already found applications in disease diagnostics and treatment, photovoltaics, and optical communications. 1−7One of the most determinative characteristics of a plasmonic resonator is its shape. It is well-known that the shape determinesto a large extentthe LSPR spectral positions. 8Specific resonator designs, consisting of a single or multiple particles, also allow to control the LSPR quality-factor by scattering loss engineering based on plasmon hybridization, 9 sub-and superradiance, and Fano interference.10−13 Additionally, similar to classical antennas, a proper plasmonic antenna design will impact its directionalitythat is, the ability to direct scattered radiation in a particular direction. Achieving high directivities in combination with a high degree of flexibility for the direction is elementary to devise efficient subwavelength plasmonic transmitters, receivers, and sensors.To obtain directional scattering, constructive and destructive interferences of multiple coherent radiation sources with carefully designed spatial separation and phase differences are required. Directional scattering of a plane wave along its propagation direction has recently been observed in core−shell nanoparticles, 14 as well as in nonmetallic silicon nanospheres. 15The obtained large forward-to-backward scattering ratios were shown to result from interfering dipoles and quadrupoles where retardation of the incident light over the particle volume activates the higher order mode and induces the required phase differences. 16 Higher order modes in a tilted plasmonic nanocup c...

Polarization-controlled excitation of plasmonic modes in nanometric Au particle-on-film gaps is investigated experimentally using single-particle dark-field spectroscopy. Two distinct geometries are explored: nanospheres on top of and inserted in a thin gold film. Numerical simulations reveal that the three resonances arising in the scattering spectra measured for particles on top of a film originate from highly confined gap modes at the interface. These modes feature different azimuthal characteristics, which are consistent with recent theoretical transformation optics studies. On the other hand, the scattering maxima of embedded particles are linked to dipolar modes having different orientations and damping rates. Finally, the radiation properties of the particle-film gap modes are studied through the mapping of the scattered power within different solid angle ranges.

Defects are known to affect nanoscale phase transitions, but their specific role in the metal-to-insulator transition in VO(2) has remained elusive. By combining plasmon resonance nanospectroscopy with density functional calculations, we correlate decreased phase-transition energy with oxygen vacancies created by strain at grain boundaries. By measuring the degree of metallization in the lithographically defined VO(2) nanoparticles, we find that hysteresis width narrows with increasing size, thus illustrating the potential for domain boundary engineering in phase-changing nanostructures.