Toroidal-Assisted Generalized Huygens' Sources for Highly Transmissive Plasmonic Metasurfaces

Metasurface operating in the transmission scheme has shown a promising scenario for flat optics applications. Nevertheless, the inherently low working efficiency of transmissive plasmonic metasurfaces at optical frequencies severely hinders them from future technology development. This work reports on a hybrid plasmonic meta-atom (HPMA) with a simple fabrication and cost-effective single-lithographic process featuring a toroidal-assisted generalized Huygens' source with a state-of-the-art circular polarization conversion efficiency beyond 50%. The HPMA representsa new upper limit for transmission efficiency in the near-infrared. The high transmission is realized via balanced multipoles of different orders including toroidal dipole that satisfies the generalized Kerker condition. The introduction of toroidal dipole provides an additional degree of freedom to tailor the wave interference and radiation symmetry rather than the use of a conventional electric and magnetic multipolar coupling. In addition, two high-performance metasurfaces by combining the HPMAs with the geometric phase method are highlighted. The highly-transmissive beam deflection metasurface and plasmonic metalens respectively yield anomalous refraction with 38.2% optical efficiency and 46.56% focusing efficiency, both experimentally showing a record transmission level. The findings may open new ways to design highly-efficient plasmonic metasurfaces and to take one step forward to facilitate nearly optimal and practical nanophotonic devices.