  Spoof plasmons mimic noble metal plasmons. The equivalent of the plasma
frequency is an energy scale imposed by the geometry of the metal grating upon
which they propagate. In this paper we show that the dispersion of spoof
plasmons in the THz can be controlled placing a doped graphene sheet on the
proximity of a metallic grating, adding more versatility to this type of
system. We develop a semi-analytical model, based on a perfect-metal
diffraction grating. This model allows to reproduce well FDTD calculations for
the same problem but with much less computer time. We discuss the optical
properties of the system covering a spectral range spanning the interval from
the THz to the mid-IR. It is shown that the system can be used as both a
perfect absorber and a sensing device. For illustrating the latter property we
have chosen different alcohols as analytes. The frequency at which perfect
absorption appears can be controlled by the geometric parameters of the grating
and by the value of the Fermi energy in graphene. The theoretical results
predicted throughout this work can be verified experimentally in the future.
