  The electromagnetic response of three-dimensional topological insulators can
be described by an effective axion action with a quantized axion field. Once
both time-reversal and inversion symmetries are broken, for example, in
antiferromagnetic topological insulators, the axion field becomes dynamical
along with magnetic fluctuations. The dynamical axion field, when coupled to
electromagnetic fields, can lead to rich physical phenomena. Here, based on the
modified Maxwell's equations, we reveal the existence of an exotic type of
polariton excitation, termed a surface axion polariton, which propagates in the
interface between a dynamical axion insulator and a dielectric. When doping the
dynamical axion insulator to be metallic, the coexistence of axion-photon
coupling and plasmon-photon coupling will further generate a mixed surface
axion plasmon polariton. We also propose a Kretschmann-Raether configuration to
experimentally detect the surface axion polariton. Our result provides an
alternative way to study the axion electrodynamics in condensed matter physics.
