  In this paper we show that graphene surface plasmons can be excited when an
electromagnetic wave packet impinges on a single metal slit covered with
graphene. The excitation of the plasmons localized over the slit is revealed by
characteristic peaks in the absorption spectrum. It is shown that the position
of the peaks can be tuned either by the graphene doping level or by the
dielectric function of the material filling the slit. The whole system forms
the basis for a plasmonic sensor when the slit is filled with an analyte.
