Void plasmons and total absorption of light in nanoporous metallic films

We have calculated light absorption spectra of planar metal structures containing a two-dimensional (2D) lattice of spherical nanocavities. Two types of porous metal structures are considered: (i) unsupported planar metal films containing a lattice of voids and (ii) planar metal films with a lattice of voids supported by the bulk metal substrate. It is shown that nearly total absorption of light occurs at the plasma resonance of the void lattice in the visible when the intervoid spacing and the void deepening into the bulk metal substrate are thinner than the skin depth, which ensures optimal coupling of void plasmons to external light. It is remarkable that even a single 2D lattice of nanovoids beneath the planar metal surface can produce the total light absorption. This giant light absorption is accompanied by strong local-field enhancement at the plasma resonance. The absorption and local-field properties of these types of nanoporous metal structures can be effectively tuned by nanoengineering the spherical pores.