  Complete absorption of electromagnetic waves is paramount in today's
applications, ranging from photovoltaics to cross-talk prevention into
sensitive devices. In this context, we use a genetic algorithm (GA) strategy to
optimize absorption properties of periodic arrays of truncated square-based
pyramids made of alternating stacks of metal/dielectric layers. We target
ultra-broadband quasi-perfect absorption of normally incident electromagnetic
radiations in the visible and near-infrared ranges (wavelength comprised
between 420 and 1600 nm). We compare the results one can obtain by considering
one, two or three stacks of either Ni, Ti, Al, Cr, Ag, Cu, Au or W for the
metal, and poly(methyl methacrylate) (PMMA) for the dielectric. More than 10^17
configurations of geometrical parameters are explored and reduced to a few
optimal ones. This extensive study shows that Ni/PMMA, Ti/PMMA, Cr/PMMA and
W/PMMA provide high-quality solutions with an integrated absorptance higher
than 99% over the considered wavelength range, when considering realistic
implementation of these ultra-broadband perfect electromagnetic absorbers.
Robustness of optimal solutions with respect to geometrical parameters is
investigated and local absorption maps are provided. Moreover, we confirm that
these optimal solutions maintain quasi-perfect broadband absorption properties
over a broad angular range when changing the inclination of the incident
radiation.
  The study also reveals that noble metals (Au, Ag, Cu) do not provide the
highest performance for the present application.
