  Metallic rugate structures are theoretically investigated for achieving
perfect absorption in the visible and near-infrared regions. Our model builds
on nanoporous metal films whose porosity (volume fraction of voids) follows a
sine-wave along the film thickness. By setting the initial phase of porosity at
the top surface as 0, perfect absorption is obtained. The impacts of various
structural parameters on the characteristic absorption behaviors are studied.
Furthermore, multiple peaks or bands with high-absorption can be achieved by
integrating several periodicities in one structure. The rugate absorbers show
perfect or near-perfect absorption for TE and TM polarizations and large
incident angles.
