Unified analyses and optimization for achieving perfect absorption of layered absorbers with ultrathin films

Radiation absorbers are vital components in energy conversion and detection devices such as solar cells, thermal absorbers, photodetectors and biological sensors. Recently, layered absorbers with ultrathin films have attracted increasing attentions considering the good tradeoff between high absorption ability and low cost of simple structures. Although some theories have been proposed to understand absorption enhancement by introducing ultrathin films, few of them explicitly show effects of thicknesses and materials for perfect absorption. In this manuscript, the perfect absorption in layered absorbers is attributed to admittance matching by adjusting the loss factor of artificial magnetic conductors. Realistically, unified theories based on admittance analyses are developed to achieve perfect absorption in these absorbers. Admittance matching conditions related to thicknesses and materials for multilayer layered absorbers are deduced to achieve perfect absorption. Thus optimal film materials or thicknesses of films can be obtained directly. Moreover, admittance loci diagrams are employed to clearly illustrate influences of both material properties and structural parameters on radiative behaviors. Therefore, unified analyses are not only used as explanations for absorption mechanism, but also used as practical tools for the optimization of layered absorbers with ultrathin films.