  We consider a p-polarized surface electromagnetic wave (a classical surface
polariton) at the interface between the vacuum and a metal or left-handed
medium. We show that the evanescent electromagnetic waves forming the surface
polariton inevitably possess a backward spin energy flow, which, together with
a superluminal orbital energy flow, form the total Poynting vector. This spin
energy flow generates a well-defined (but not quantized) spin angular momentum
of surface polaritons which is orthogonal to the propagation direction. The
spin of evanescent waves arises from the imaginary longitudinal component of
the electric field which makes the polarization effectively elliptical in the
propagation plane. We also examine the connection between the spin and
chirality of evanescent modes.
