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...

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Specially designed plasmonic antennas can, by far-field interference of different antenna elements or a combination of multipolar antenna modes, scatter light unidirectionally, allowing for directional light control at the nanoscale. One of the most basic and compact geometries for such antennas is a nanorod with broken rotational symmetry, in the shape of the letter V. In this article, we show that these V-antennas unidirectionally scatter the emission of a local dipole source in a direction opposite the undirectional side scattering of a plane wave. Moreover, we observe high directivity, up to 6 dB, only for certain well-defined positions of the emitter relative to the antenna. By employing a rigorous eigenmode expansion analysis of the V-antenna, we fully elucidate the fundamental origin of its directional behavior. All findings are experimentally verified by measuring the radiation patterns of a scattered plane wave and the emission pattern of fluorescently doped PMMA positioned in different regions around the antenna. The fundamental interference effects revealed in the eigenmode expansion can serve as guidelines in the understanding and further development of nanoscale directional scatterers.

Heralded single surface plasmon polaritons are excited using photons generated via spontaneous parametric down conversion. The mean excitation rates, intensity correlations, and Fock state populations are studied. The observed dependence of the second-order coherence in our experiment is consistent with a linear uncorrelated Markovian environment in the quantum regime. Our results provide important information about the effect of loss for assessing the potential of plasmonic waveguides for future nanophotonic circuitry in the quantum regime.

We study in real time the optical response of individual plasmonic nanoparticles on a mirror, utilized as electrodes in an electrochemical cell when a voltage is applied. In this geometry, Au nanoparticles are separated from a bulk Au film by an ultrathin molecular spacer. The nanoscale plasmonic hotspot underneath the nanoparticles locally reveals the modified charge on the Au surface and changes in the polarizability of the molecular spacer. Dark-field and Raman spectroscopy performed on the same nanoparticle show our ability to exploit isolated plasmonic junctions to track the dynamics of nanoelectrochemistry. Enhancements in Raman emission and blue-shifts at a negative potential show the ability to shift electrons within the gap molecules.