Ultrastrong Purcell enhancement of magnetic dipole emission based on quasi-BIC.

Here, we propose a hollow quasi-periodic spoof plasmonic structure to realize the ultrastrong Purcell enhancement of magnetic dipole emission in terahertz frequency range based on quasi-bound states in the continuum. By independently tuning the substructures, we can severally tune the responses of the electric dipole and quadripolar modes supported by groove-substructure, and the magnetic dipolar mode supported in slit-substructure. In particular, we also observe a whispering gallery mode supported by the composite structure, which will move with changing structural parameters. When the magnetic dipole mode close to the whispering gallery mode, the interference between them will emerge to generates a quasi-bound state in the continuum with ultra-high quality factor. Then, based on the quasi-periodic structure, we put a magnetic dipole into the hollow structure center and realize the ultrastrong Purcell enhancement of magnetic dipole emission, which can be more than 105. The designed structure can be applied to a variety of magneto-optical devices to greatly enhance Purcell effect in terahertz region. Manipulating the spontaneous emission rate of magnetic dipole with artificial magnetic resonators have been widely investigated to provide the road toward novel magnetic light-matter interactions. However, the emission strength of magnetic dipole still an open challenge due to the lack of high-quality factor cavity with magnetic dipole resonance. Here, we propose a hollow quasi-periodic spoof plasmonic structure to realize the ultrastrong Purcell enhancement of magnetic dipole emission in terahertz frequency range based on quasi-BIC as shown in (a). By independently tuning the substructures, we can severally tune the responses of the electric dipole and quadripolar modes supported by groove-substructure, and the magnetic dipolar mode supported in slit-substructure. Particularly, when the magnetic dipole close to the electric quadrupole, the interference between the magnetic dipolar mode as a bright one and electric quadrupolar modes as a dark one in the quasi-structure can generate a sharp Fano resonance with high quality factor. When we put a magnetic dipole into the structure center and realize the ultrastrong Purcell enhancement of magnetic dipole emission, which can be more than 105 as Figure (b). The designed structure can be applied to a variety of magneto-optical devices to greatly enhance magnetic Purcell effect in terahertz region. [ABSTRACT FROM AUTHOR]