Metallic rugate structures for perfect absorbers in visible and   near-infrared regions
  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. 
Shiwei Shu and Yang Yang Li
physics.optics