Near-Perfect Broadband Infrared Metamaterial Absorber Utilizing Nickel

We propose a thin, compact, broadband, polarization and angle insensitive metamaterial absorber based on a tungsten reflector, silicon spacer and a top pattern composed of a double square-like ring resonator utilizing nickel(Ni). In such a structure, a high absorption (above 80 %) bandwidth ~ 4.8 {\mu}m from 3.52 up to 8.32 {\mu}m corresponding to the relative bandwidth ~81% can be achieved with deeply subwavelength unit cell dimensions. Here the physical origin of the broadband absorption is associated with low Q-factor dipole modes of the top pattern inner and outer sides functioning as rectangular nanoantennas. Owing to the structural symmetry, the absorber shows a good incidence angle tolerance in the relatively wide range for both TE and TM polarizations. The effective parameters of the Ni-based absorber were retrieved using the constitutive effective medium theory and the absorption characteristics of the effective medium and metamaterial were compared.