Ultra-unidirectional Emission with Enhanced Spectral Splitting Based on Plasmonic Nano-pillars and its Metasurface

Here, we proposed a plasmonic nanoantenna based on exquisitely designed double nano-pillars. Firstly, the enhanced splitting behavior in the scattering spectrum with Δ λ = 142 nm is observed by asymmetric-breaking of the nano-pillars. Then, when turn to investigate the far-field in moment space, we found that there is also splitting behavior of unidirectional emission in moment space accompanied by the spectral splitting. Multipolar expansion and Green’s function method are used to analyze the splitting behavior and construct the far-field angular emission patterns. The results show that the spectral splitting comes from the electric dipole mode splitting and the multipolar interference is the origin of the unidirectional emission. Moreover, the single nanoantenna unit is used to form a two-dimensional metasurface. By combining radiation modes of the nanoantenna unit and diffraction modes supported by the metasurface, we got an ultra-unidirectional far-field emission lobe with divergence angle Δ θ = 17.2°. Our study on the combination of spectral splitting behavior and ultra-unidirectional emission in one structure will be potentially applied in wavelength routing and other multifunctional optoelectronic devices.