  We study in this work the so-called "instantaneous time mirrors" in the
context of surface plasmons. The latter are associated with high frequency
waves at the surface of a conducting sheet. Instantaneous time mirrors were
introduced by M. Fink et al, with the idea that singular perturbations in the
time variable in a wave-type equation create a time-reversed focusing wave. We
consider the time-dependent three-dimensional Maxwell's equations, coupled to
Drude's model for the description of the surface current. The time mirror is
modeled by a sudden, strong, change in the Drude weight of the electrons on the
sheet. Our goal is to characterize the time-reversed wave, in particular to
quantify the quality of refocusing. We establish that the latter depends on the
distance of the source to the sheet, and on some physical parameters such as
the relaxation time of the electrons. We also show that, in addition to the
plasmonic wave, the time mirror generates a free propagating wave that offers,
contrary to the surface wave, some resolution in the direction orthogonal to
the sheet. Blurring effects due to non-instantaneous mirrors are finally
investigated.
