Absorptivity enhancement of higher-order electric sextupole plasmonic modes by the outer-square inner-ring coupled resonators

Plasmonic resonances in metal-dielectric-metal structures have shown strong electric and magnetic fields enhancement in the sub-wavelength region, which can significantly boost the performance of plasmon-based metamaterial absorbers. Square resonators (SR) and ring resonators (RR) are both the most common blocks for designing metamaterial perfect absorbers (MPAs) by exciting fundamental electric dipole plasmonic modes. Actually, they can also excite the higher-order electric sextupole plasmonic modes but the absorptivity is very low, which are not available for the design of MPAs. In this paper, the near-field-coupling idea is introduced to enhance the absorptivity of higher-order electric sextupole modes. We propose the outer-square inner-ring coupled resonators (OSIRCR) made of the Ag and ZnS. When the coupling distance has decreased from 70 to 10 nm, the electric field intensity has increased 10.8 times from 55 to 594, which leads to a 5.4 times increase in absorptivity of higher-order electric sextupole modes of the outer-square from 17.6% to 95.3%. In addition to the higher-order electric sextupole modes, the OSIRCR can also excite the fundamental electric dipole modes of the outer-square and the inner-ring. Then a six-band polarization-independent MPA in the infrared range is designed with an average absorptivity of 91.5% utilizing two different sized OSIRCRs.