Unidirectional Light Scattering With High $f/b$ at Optical Frequencies Based on Coupled Nanoantennas

We propose an optically coupled nanoantenna which contains an asymmetric dimer of core-dual shells nanoparticles, and the individual nanoparticle consisting of two identical metal strips that separated by a dielectric spacer known as the metal-dielectric-metal (MDM) structure. According to the dipole-dipole theory and the Mie theory, we theoretically and numerically demonstrate that this design can provide unidirectional forward scattering along the incident axis of light with high f /b radio (known as the ratio of scattering spectra to forward and backward direction) and narrow beam-width owing to interference between electric dipole and magnetic high-order modes. We also find that all properties of this nanoantenna that the performance of it is related with the gap separation of the asymmetric dimer. Comparing with the asymmetric dimer of pure metal nanoparticles and the symmetric dimer of MDM nanoparticles, the proposed nanoantenna has higher directivity and low loss at the same time, due to effectively coupled electric dipole mode and magnetic multipole modes. The ultra-directional nanoantenna may shed new light to overcome power losses and low directivity of coupled nanoantennas, which can find various applications in bio-sensing and metamaterials.