  Power dissipation in electromagnetic absorbers is a quadratic function of the
incident fields. To characterize an absorber, one needs to deal with the
coupling that may occur between different excitations. Energy Absorption
Interferometry (EAI) is a technique that highlights the independent degrees of
freedom through which a structure can absorb energy: the natural absorption
modes of the structure. The coupling between these modes vanishes. In this
paper, we use the EAI formalism to analyse different kinds of plasmonic
periodic absorbers while rigorously accounting for the coupling: resonant
golden patches on a grounded dielectric slab, parallel free-standing silver
wires and a silver slab of finite thickness. The EAI formalism is used to
identify the physical processes that mediate absorption in the near and far
field. First, we demonstrate that the angular absorption, which is classically
used to characterize periodic absorbers in the far field and which neglects the
coupling between different plane waves, is only valid under stringent
conditions (subwavelength periodicity, far field excitation and negligible
coupling between the two possible polarizations). Using EAI, we show how the
dominant absorption channels can be identified through the signature of the
absorption modes of the structure, while rigorously accounting for the
coupling. We then exploit these channels to improve absorption. We show that
long-range processes can be exploited to enhance the spatial selectivity, while
short-range processes can be exploited to improve absorptivity over wide angles
of incidence. Lastly, we show that simply adding scatterers with the proper
periodicity on top of the absorber, the absorption can be increased by more
than one order of magnitude.
