  The group velocity of a plasmonic guided mode can be written as the ratio of
the flux of the Poynting to the integral of the energy density along the
profile of the mode. This theorem, linking the way energy propagates in metals
to the properties of guided modes and Bloch modes in a multilayer, provides a
unique physical insight in plasmonics. It allows to better understand the link
between the negative permittivity of metals and the wide diversity of exotic
phenomenon that occur in plasmonics -- like the slowing down of guided modes,
the high wavevector and the negative refraction.
