  Non-reciprocal directional dichroism assigns an optical diode-like property
to non-centrosymmetric magnets, making them appealing for low-dissipation
optical devices. However, the direct electric control of this phenomenon at
constant temperatures is scarce. In Ba$_2$CoGe$_2$O$_7$, we demonstrate the
isothermal electric switch between domains possessing opposite magnetoelectric
susceptibilities. Combining THz spectroscopy and multiboson spin-wave analysis,
we show that unbalancing the domain population realizes the non-reciprocal
light absorption of spin excitations.
