Strong Purcell effect for magnetic dipole emission with spoof plasmonic spiral structure

The ability to manipulate the interaction between light and optical emitters is essential for enhancing the capability of optical devices. Multifarious metallic and all-dielectric structures have been proposed frequently to enhance the emission of electromagnetic dipoles through the Purcell effect, in which its performances depend on two confinement mechanisms: temporal confinement (photon cavity period) and spatial confinement (localized light in an enclosed space), which can be described by the quality factor and mode volume, respectively. Here, we demonstrate that a hollow spoof plasmonic spiral structure in deep-subwavelength scale, which is constituted by periodically inserting spiral-shaped metallic arms into a hollow silicon cylinder, can drastically enhance emission of magnetic dipoles. Particularly, ultrahigh quality factor and ultrasmall mode volume of the magnetic resonance can be realized by further increasing the spiral degree of metallic arms. The results indicate that the quality factor of magnetic dipole mode in the structure can be enhanced to 2600 (silicon ∼ 5.5 for same scale) for spiral degree 4 π , and the Purcell factor can be enhanced to 5 × 10 6 (silicon ∼ 5.1 ) for a magnetic dipole emission. These results may provide a new avenue for designing optical cavities and enhancing magnetic dipole emission in low frequency.