  Non-reciprocal directional dichroism, also called the optical-diode effect,
is an appealing functional property inherent to the large class of
non-centrosymmetric magnets. However, the in-situ electric control of this
phenomenon is challenging as it requires a set of conditions to be fulfilled:
Special symmetries of the magnetic ground state, spin-excitations with
comparable magnetic- and electric-dipole activity and switchable electric
polarization. We demonstrate the isothermal electric switch between domains of
Ba$_2$CoGe$_2$O$_7$ possessing opposite magnetoelectric susceptibilities.
Combining THz spectroscopy and multiboson spin-wave analysis, we show that
unbalancing the population of antiferromagnetic domains generates the
non-reciprocal light absorption of spin excitations.
