Deep-Subwavelength and High-Q Trapped Mode Induced by Symmetry-Broken in Toroidal Plasmonic Resonator

We propose a toroidal plasmonic resonator composed of double-layer concentric inward and outward spoke patterns, which tightly compacts all Mie resonance modes while reduces the radiation loss. Introducing a slit perturbation to the top spoke to break the in-plane symmetry, a trapped mode is excited and the confinement is double folded. The trapped mode behaves as a mixture of the electric and magnetic plasmon dipoles, realizing a quality factor ( Q-factor) of 104.8 within the diameter of 1/20 wavelength. On-chip excitation of the trapped mode is analyzed. Experimental results agree well with numerical simulations. The microstrip compatible structure, outstanding compactness, and high Q-factor of the proposed plasmonic resonator will find significant applications in fundamental cavity electrodynamics and on-chip microwave sensing.