Unidirectional Side Scattering of Light by a Single-Element Nanoantenna

Abstract

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...

Directional emission from a single plasmonic scatterer

“…This can be achieved by introducing an asymmetry in the nanostructure itself 26,28 . Also in this case, it has been shown that it is better to have larger structures with more polarizable material in order to promote the higher-order multipoles that facilitate a strong directional response.…”

“…However, these array antennas are complex to design and fabricate, and have a relatively large spatial footprint. Strong directionality can also be achieved using just a single element antenna if the interference between electric dipole and higher-order multipole components can be controlled [21][22][23][24][25][26][27][28] . However, efficient excitation of higher-order magnetic and electric multipoles requires strong field gradients on the scale of the nanoparticle, which is difficult to obtain using plane wave excitation.…”

Controlling surface-plasmon-polaritons launching with hot spot cylindrical waves in a metallic slit structure

“…[7][8][9][10][11] In order to realize such potential, it's crucial to launch the SPPs effectively and flexibly from the freespace light. For the widely used nanostructures, such as nanoslit, [12][13][14][15][16][17][18] nanogroove, [19,20] and nanoantenna, [21,22] all of them have sharp metallic corners where the charges can accumulate there. As a result, the localized electromagnetic fields become extremely strong at the sharp corners, which are usually termed as hot spots.…”

Accurate Feeding of Nanoantenna by Singular Optics for Nanoscale Translational and Rotational Displacement Sensing

“…The total far-field scattering pattern of the two nanorods can be calculated using the far-field expressions for the radiation of two dipoles. The right to left power ratio can be calculated as [32,40,41] …”