A broadband and polarization-insensitive high impedance surface (HIS) metamaterial absorber (MA) based on octagonal ring-shaped resistive patches is presented. The absorber is investigated theoretically, experimentally and by simulation. The simulated results indicate that this structure obtains 10.28 GHz-wide absorption from 3.65 to 13.93 GHz with absorptivity larger than 90% at the normal incidence. Experimental results are in accordance with those of the simulation results. The electromagnetic (EM) field distributions and the plots of surface power loss density have been illustrated to analyze the absorption mechanism of the structure. Further simulations of the absorptivity of the proposed absorber with different surface resistances and substrate thicknesses indicate that there exist optimal values for the design. The polarization-insensitive feature and the properties under oblique incidence are also investigated. Finally, the interference theory is introduced to analyze and interpret the broadband absorption mechanism at both normal and oblique incidence. The calculated absorption rates of the proposed absorber coincide well with the simulated results. Therefore, the simulated and experimental results verify the validity of the theoretically analytical method for this type of broadband absorber.Index Terms-High impedance surface (HIS), metamaterial absorber (MA), broadband, microwave, interference theory.

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We present a broadband and polarization-insensitive metamaterial absorber (MA) composed of a dielectric substrate sandwiched with double-circle rings (DCRs) and welded with lumped resistances and continuous metal film. The structure is designed with thickness of 3 mm and investigated by simulation, fabrication, and experiment. The results show that the composite MA loaded with lumped resistances has wider absorptivity compared with a structure with only DCRs. The simulation results indicate that the proposed absorber achieves 7.60-GHz-wide absorption from 8.87 GHz to 16.47 GHz with absorptivity greater than 90%, in excellent agreement with experimental results. Further simulations indicate that there exist optimal values for the lumped resistances for which the absorptivity is the highest and the bandwidth widest. Additionally, the proposed MA is polarization insensitive at normal incidence. Simulation results for wide angles of incidence of both transverse electric and transverse magnetic waves are also investigated.

Terahertz perfect metamaterial absorbers (PMA) based on higher order resonances have exhibited important applications in detecting, sensing, and imaging. However, most of them suffer from polarization and incident-angle dependence due to their asymmetrical structure. Here, we numerically investigate a kind of ultrathin triple-band PMA with polarization stability and wide angle of incidence. Numerical results reveal that three resonance peaks with nearly 100% absorptivity are obtained at 1.605, 4.425, and 4.946 THz under different polarization angles. It is interestingly found that the absorptance of two higher order resonances is inversely increased when the size of the middle dielectric spacer is smaller. The three resonance peaks in transverse electric and transverse magnetic modes nearly maintain fixed for oblique incidence up to 60°. Importantly, another four-band and dual-band perfect absorptions are easily achieved by a complementary method without doing iterative numerical simulation. The complementary metamaterial based absorber reaches more than 98% absorptance at four resonance frequencies. The metamaterial absorber based on complementary resonator also obtains two absorption peaks at 2.65 and 5.84 THz.

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