Unity absorbance layers - optimal design criteria
  We present the necessary and sufficient conditions for the unity absorbance of thin planar layers. With a simple structure comprising a double-layer grid, it is shown that zero transmission and reflection is feasible only if the second sheet is a perfectly conducting wall. The operational frequencies can be varied by tuning the grid impedance. Explicit conditions are given at the large wavelength limit; these underline the fundamental of ultra-thin perfect antennae and absorbers. 
K. Chalapat, G. S. Paraoanu, Z. Du, J. Tervo, I. Nefedov, S. Tretyakov
cond-mat.mes-hall physics.optics