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.

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.

The detection of small changes in the wavelength position of localized surface plasmon resonances in metal nanostructures has been used successfully in applications such as label-free detection of biomarkers. Practical implementations, however, often suffer from the large spectral width of the plasmon resonances induced by large radiative damping in the metal nanocavities. By means of a tailored design and using a reproducible nanofabrication process, high quality planar gold plasmonic nanocavities are fabricated with strongly reduced radiative damping. Moreover, additional substrate etching results in a large enhancement of the sensing volume and a subsequent increase of the sensitivity. Coherent coupling of bright and dark plasmon modes in a nanocross and nanobar is used to generate high quality factor subradiant Fano resonances. Experimental sensitivities for these modes exceeding 1000 nm/RIU with a Figure of Merit reaching 5 are demonstrated in microfluidic ensemble spectroscopy.

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...

Nanoscale self‐folding of electron‐beam lithography patterned templates is used to create 3D devices for optics and biosensing.

An optical antenna forms the subwavelength bridge between free space optical radiation and localized electromagnetic energy. Its localized electromagnetic modes strongly depend on its geometry and material composition. Here, we present the design and experimental realization of a novel V-shaped all-dielectric antenna based on high-index amorphous silicon with a strong magnetic dipole resonance in the visible range. As a result, it exhibits extraordinary bidirectional scattering into diametrically opposite directions. The scattering direction is effectively controlled by the incident wavelength, rendering the antenna a passive bidirectional wavelength router. A detailed multipole decomposition analysis reveals that the excitation and abrupt phase change of an out-of-plane polarized magnetic dipole and an in-plane electric quadrupole are essential for the directivity switching. Previously, noble metals have been extensively exploited for plasmonic directional nanoantenna design. However, these inevitably suffer from high intrinsic ohmic losses and a relatively weak magnetic response to the incident light. Compared to a similar gold plasmonic nanoantenna design, we show that the silicon-based antennas demonstrate stronger magnetic scattering with minimal absorption losses. Our results indicate that all-dielectric antennas will open exciting possibilities for efficient manipulation of light-matter interactions.

In nanoplasmonic sensing, the bulk refractive index sensitivity is often used as a metric for performance evaluation. However, for biosensing applications, which involve molecular binding events, only the refractive index in a confined region close to the metal surface is altered. The correlation between the bulk and the surface sensitivity strongly depends on the nanostructure geometry, especially in strongly coupled systems. In this paper, we thoroughly investigate the surface sensing performance of diffractively coupled plasmonic crystals using the atomic layer deposition of conformal Al 2 O 3 layers with well-defined thickness and refractive index. It is demonstrated that the surface sensing capacity cannot be fully described by the bulk sensitivity. It not only shows opposite dependence on the coupling strength compared to the bulk sensitivity, but also the bulk sensitivity cannot reflect the fact that the surface sensitivity could be different in different thickness ranges on the metal surface. The reason rests on the different decay lengths of the plasmonic crystal arrays with different coupling strengths and can be well explained by the second order surface sensitivity that has recently been proposed. Furthermore, we provide a quantitative method to evaluate the surface sensing performance of specific target analyte. This method is generic and can be applied to other nanoplasmonic systems and a broad range of biomolecules with various sizes.

We present a new electromagnetic phenomenon-the asymmetric second-harmonic generation from planar chiral structures. The effect consists in distinguishing the handedness of a chiral material by rotating the sample in an experiment involving solely linearly polarized light. This phenomenon originates in the surface plasmon resonance of chiral gold nanostructures, where homodyne interference of anisotropic and chiral electric and/or magnetic multipoles appears to play an important role.