  We suggest to exploit dielectric-metal core-shell nanostructures for
efficient resonant and yet broadband absorption of infrared radiation with
deeply subwavelength configurations. Realizing that nanostructures would
efficiently absorb radiation only when their dielectric properties match those
of the environment and making use of the effective medium approach, we develop
the design strategy using core-shell nanostructures with very thin shells made
of poor metals, i.e., metals having real and imaginary parts of their
dielectric permittivities of the same order of magnitude. Analyzing in detail
spherical and cylindrical core-shell nanostructures, we demonstrate that the
resonant infrared absorption can be not only very efficient, i.e., with the
absorption cross sections exceeding geometrical ones, but also broadband with
the spectral width being of the order of the resonant wavelength. We obtain
simple analytical expressions for the absorption resonances in spherical and
cylindrical configurations that allow one to quickly identify the configuration
parameters ensuring strong infrared absorption in a given spectral range.
Relations to effective medium parameters obtained by the internal
homogenization are established and discussed. We believe that our results can
be used as practical guidelines for realization of efficient broadband infrared
absorbers of subwavelength sizes desirable in diverse applications.
