Directional fluorescence emission by individual V-antennas explained by mode expansion

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.