Dual-Band Unidirectional Emission in a Multilayered Metal-Dielectric Nanoantenna.

Controlling the emission efficiency, direction, and polarization of optical sources with nanoantennas is of crucial importance in many nanophotonic applications. In this article, we design a subwavelength multilayer metal-dielectric nanoantenna consisting of three identical gold strips that are separated by two dielectric spacers. It is shown that a local dipole source can efficiently excite several hybridized plasmonic modes in the nanoantenna, including one electric dipole (ED) and two magnetic dipole (MD) resonances. The coherent interplay between the ED and MDs leads to unidirectional emissions in opposite directions at different wavelengths. The relative phase difference between these resonant modes determines the exact emission direction. Additionally, with a proper spacer thickness and filling medium, it is possible to control the spectral positions of the forward and backward unidirectional emissions and to exchange the wavelengths for two unidirectional emissions. An analytical dipole model is established, which yields comparable results to those from the full-wave simulation. Furthermore, we show that the wavelength of the peak forward-to-backward unidirectionality is essentially determined by the MD and is approximately predictable by the plasmonic wave dispersion in the corresponding two-dimensional multilayer structure. Our results may be useful to design dual-band unidirectional optical nanoantennas.