  It has been suggested that certain antiferromagnetic topological insulators
contain axion quasiparticles (AQs), and that such materials could be used to
detect axion dark matter (DM). The AQ is a longitudinal antiferromagnetic spin
fluctuation coupled to the electromagnetic Chern-Simons term, which, in the
presence of an applied magnetic field, leads to mass mixing between the AQ and
the electric field. The electromagnetic boundary conditions and transmission
and reflection coefficients are computed. A model for including losses into
this system is presented, and the resulting linewidth is computed. It is shown
how transmission spectroscopy can be used to measure the resonant frequencies
and damping coefficients of the material, and demonstrate conclusively the
existence of the AQ. The dispersion relation and boundary conditions permit
resonant conversion of axion DM into THz photons in a material volume that is
independent of the resonant frequency, which is tuneable via an applied
magnetic field. A parameter study for axion DM detection is performed,
computing boost amplitudes and bandwidths using realistic material properties
including loss. The proposal could allow for detection of axion DM in the mass
range between 1 and 10 meV using current and near future technology.
