  The lack of both time-reversal and spatial inversion symmetry in polar
magnets is a prerequisite for the occurrence of optical magnetoelectric effects
such as nonreciprocal directional dichroism with the potential for the
realization of optical diodes. In particular, antiferromagnetic materials with
magnetic excitations in the THz range such as Fe2Mo3O8 are promising candidates
for next-generation spintronic applications. In a combined experimental and
theoretical effort we investigated the THz excitations of the polar honeycomb
antiferromagnet Fe2Mo3O8 in external magnetic fields and their nonreciprocal
directional dichroism, together with the temperature dependence of the
electronic transitions in the mid- and near-infrared frequency range. Using an
advanced single-ion approach for the Fe ions, we are able to describe optical
excitations from the THz to the near-infrared frequency range quantitatively
and successfully model the observed nonreciprocal directional dichroism in the
THz regime.
