Numerical investigation of tunable and switchable refractive index-based mid-infrared metamaterial absorber

In this paper, we numerically investigate a tunable and switchable metamaterial absorber operating in the mid-infrared region. The metamaterial structure comprises an array of metal–dielectric concentric nanodisks and a metal square patch on a substrate coated with thin dielectric and metal layers. The two dielectric regions induce two absorption bands, which can be independently tuned by changing the refractive index and geometrical parameters of the dielectric layers. In an optimised geometry, a dual-band to a single-band switchable frequency response is achieved by interchanging the refractive index of the dielectric layers. The origin of the resonant absorptions is traced to the simultaneous excitation of electric and magnetic fields in the dielectric regions. Due to architectural symmetry, the investigated structure is polarisation-insensitive and exhibits high absorption over a wide range of angles.