In recent years, nanotechnology and nanoscience have been expanding their toolbox dramatically. Metallic nanoantennas – also known as plasmonic resonators – can be considered as one of these novel tools providing an effective route to couple photons in and out of nanoscale volumes. The higher the level of control over the way a nanoantenna interacts with light, the more effective this tool becomes and the further its applications will reach. Essential to this end is a detailed knowledge of such an antenna’s scattering characteristics. Especially in nanophotonics applications where every photon counts, one immediately benefits from directed photon routing for efficient photon collection.
Recent studies have demonstrated that proper antenna designs can result in scattering patterns strongly deviating from the trivial dipole distribution, allowing one to route light in specific directions. [1–4] For instance, left-to-right directionality or unidirectional side scattering (i.e. preferential scattering in a single direction perpendicular to the incident wave) with only a single nanoparticle geometry, namely a V-antenna, was demonstrated by our group. [3, 4] Nevertheless, selectively steering photons at the nanoscale remains a fundamental challenge and most works are, unfortunately, lacking a clear rigorous analysis of the antenna behavior. This constrains the further development of more complex radiative functionality, such as for instance bi-directional scattering where photons of different energy are routed into opposite directions.
Here, first, we elucidate the basic principles underlying directional plasmonic nanoantennas. By applying an eigenmode decomposition of full-field 3D simulations (Method of Moments and Finite Difference Time Domain) we are able to rigorously determine the underlying mode interferences that give rise to the experimentally observed directional behaviour.  An important result from this analysis is the conclusion that directional behaviour is strongly dependent on the type of antenna feed that is applied, an effect mostly ignored so far. For instance, plane wave excitation is directed in opposite direction from the radiation of a point dipole source.
Next, the obtained design principles are applied to construct a bi-directional metallic nanoantenna with a single feed-gap. This means that, for instance, a mix of quantum emitters can be placed in the antenna gap and that the multi-colored emission can be de-multiplexed by beaming different spectral bands in opposite directions. Both the bi-directional scattering of a plane wave and bi-directional emission of local dipole emitters are experimentally verified by means of back-focal-plane microscopy. We demonstrate directional tunability by varying the antenna structural parameters, allowing further optimisation and spectral control.
The obtained insight in how directional emission and scattering are generated and how different modes come together to form far-field properties of a nanoantenna device is indispensable to create new nanoscale optical devices for sub-wavelength color routing and self-referenced directional sensing. Moreover, we believe the same concepts are applicable to dielectric nanoantennas.
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 T. Shegai, et al. Nat. Comm. (2011) 2(481)
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 D. Vercruysse, et al. ACS Nano (2014) 8(8), 8232−8241
Niels Verellen, Dries Vercruysse, Xuezhi Zheng, Stef Boeckx, Jiaqi Li, Guy A. E. Vandenbosch, Liesbet Lagae, and Pol Van Dorpe, "Designing complex radiative behaviour of metallic nanoantennas
(Conference Presentation)," Proc. SPIE 9884, Nanophotonics VI, 98841O (Presented at SPIE Photonics Europe: April 07, 2016; Published: 26 July 2016); https://doi.org/10.1117/12.2227911.5042345296001.
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