Reverse design of metamaterial absorbers based on an equivalent circuit

We present a reverse design method useful for designing and analyzing metamaterial absorbers; we demonstrate its power by designing both a narrowband absorber and a wideband absorber. The method determines the structure of the absorber using an equivalent-circuit model. The narrowband metamaterial absorber structures were based on the equivalent-circuit model, and the narrowband metamaterial absorber designed using the method has an absorption fraction greater than 90% in a bandwidth of 500 nm centered at about 1450 nm. In order to extend the absorption bandwidth for the absorber, the narrowband absorber structure is adjusted based on the equivalent-circuit model, and the broadband metamaterial absorber structure is investigated. The numerical results show that the absorption bandwidth is substantially increased; the absorbance is greater than 90% for a band nearly reaching the limits of our experiment, from about 400 nm (near-ultraviolet) to about 2800 nm (deep infrared). The absorption spectrum of the wideband absorber is more sensitive to the angle of incident polarization due to the asymmetric structure, but the whole band shows polarization independence. For a large angle of 60° (TM polarization) oblique incidence, the average absorption of the broadband metamaterial absorber reaches 81%. The physical mechanism of the wideband high absorption is analyzed, which is mainly caused by Fabry-Perot resonance, surface plasmon resonance, local surface plasmon resonance, and the hybrid coupling among them. Our proposed design with high-broadband absorption has significant potential for thermoelectric and thermal emitters, solar thermal energy harvesting, and invisible device applications.