Absorption Improvement of the Anapole Metastructure for Sensing Applications

In this paper, a metastructure absorber based on the non-radiating anapole mode is firstly proposed and theoretically demonstrated. The combination of four split rings leads to a classical resonant anapole behavior upon the normally illuminating of the
${y}$
-polarized wave, which is originated from the destructive interference of the toroidal and electric dipoles. The results show that a point electric field hotspot with up to 20 times enhancement is strictly confined at the geometric center of the proposed structure in the near field and the absorption in the far-field reaches 50.18% at 1.25 THz. To further effectively enhance the absorption efficiency, an improved version is further presented by inserting two vertical asymmetric split rings on both sides of the first design, in which the strengthened magnetic dipole moments reconfigure the mode coupling of multipolar excitations and greatly enhance the absorption to 98.51% at 1.2 THz, while the confinement of the electric field amplifies to nearly 200 times. In addition, the sensing performance of this improved design for measuring the relative permittivity of the background surrounding from 1 to 1.5 has also been explored with the sensitivity, maximum FOM, minimum detection limit and minimum resolution of 341 GHz/RIU, 11.37, 0.00438 and 0.02481. Such an implementation of the absorption enhancement mechanism based on anapole mode enriches the theoretical framework and provides a new platform for multipole electrodynamics in the fields of ultra-sensitive sensing, label-free detection, optical switch, optical modulation, and so on.