  In the recently discovered Weyl semimetals, the Fermi surface may feature
disjoint, open segments -- the so-called Fermi arcs -- associated with
topological states bound to exposed crystal surfaces. Here we show that the
collective dynamics of electrons near such surfaces sharply departs from that
of a conventional three-dimensional metal. In magnetic systems with broken time
reversal symmetry, the resulting Fermi arc plasmons (FAPs) are chiral, with
dispersion relations featuring open, hyperbolic constant frequency contours. As
a result, a large range of surface plasmon wave vectors can be supported at a
given frequency, with corresponding group velocity vectors directed along a few
specific collimated directions. Fermi arc plasmons can be probed using near
field photonics techniques, which may be used to launch highly directional,
focused surface plasmon beams. The unusual characteristics of FAPs arise from
the interplay of bulk and surface Fermi arc carrier dynamics, and give a window
into the unusual Fermiology of Weyl semimetals.
