Core-Shell Structured Dielectric-Metal Circular Nanodisk Antenna: Gap   Plasmon Assisted Magnetic Toroid-like Cavity Modes
  Plasmonic nanoantennas, the properties of which are essentially determined by their resonance modes, are of interest both fundamentally and for various applications. Antennas with various shapes, geometries and compositions have been demonstrated, each possessing unique properties and potential applications. Here, we propose the use of a sidewall coating as an additional degree of freedom to manipulate plasmonic gap cavity modes in strongly coupled metallic nanodisks. It is demonstrated that for a dielectric middle layer with a thickness of a few tens of nanometers and a sidewall plasmonic coating of more than ten nanometers, the usual optical magnetic resonance modes are eliminated, and only magnetic toroid-like modes are sustainable in the infrared and visible regime. All of these deep-subwavelength modes can be interpreted as an interference effect from the gap surface plasmon polaritons. Our results will be useful in nanoantenna design, high-Q cavity sensing, structured light-beam generation, and photon emission engineering. 
Qiang Zhang, Jun Jun Xiao, Xiao Ming Zhang, Dezhuan Han, and Lei Gao
physics.optics