  Polarization, spin, and helicity are important properties of electromagnetic
waves. It is commonly believed that helicity is invariant under the Lorentz
transformations. This is indeed so for plane waves and their localized
superpositions. However, this is not the case for evanescent waves, which are
well-defined only in a half-space, and are characterized by complex wave
vectors. Here we describe transformations of evanescent electromagnetic waves
and their polarization/spin/helicity properties under the Lorentz boosts along
the three spatial directions.
