  We study surface plasmons localized on interfaces between topologically
trivial and topologically non-trivial time reversal invariant materials in
three dimensions. For the interface between a metal and a topological insulator
the magnetic polarization of the surface plasmon is rotated out of the plane of
the interface; this effect should be experimentally observable by exciting the
surface plasmon with polarized light. More interestingly, we argue that the
same effect also is realized on the interface between vacuum and a doped
topological insulator with non-vanishing bulk carrier density.
