  In this paper we study the role of surface plasmon modes in the Casimir
effect. The Casimir energy can be written as a sum over the modes of a real
cavity and one may identify two sorts of modes, two evanescent surface plasmon
modes and propagative modes. As one of the surface plasmon modes becomes
propagative for some choice of parameters we adopt an adiabatic mode definition
where we follow this mode into the propagative sector and count it together
with the surface plasmon contribution, calling this contribution "plasmonic".
We evaluate analytically the contribution of the plasmonic modes to the Casimir
energy. Surprisingly we find that this becomes repulsive for intermediate and
large mirror separations. The contribution of surface plasmons to the Casimir
energy plays a fundamental role not only at short but also at large distances.
This suggests possibilities to taylor the Casimir force via a manipulation of
the surface plasmons properties.
