The variation of absorption coefficient with the direction of light propagation in a solid is called nonreciprocal directional dichroism (NDD). In polar magnets with polarization $\mathit{P}$ and magnetization $\mathit{M}$, NDD depends on the direction of $\mathit{P}\times \mathit{M}$; its magnitude is expected to be controlled via a change in $\mathit{P}\times \mathit{M}$ by simultaneously applying electric and magnetic fields. Here, we show that the ultrafast control of NDD can be achieved using a strong terahertz pulse in an antipolar antiferromagnet, ${\text{Bi}}_2{\text{CuO}}_4$, in which no macroscopic polarization and magnetization appear. By measuring the dependence of the absorption changes on the incident directions of the terahertz pump and visible probe pulses, as well as on the phase of magnetic domains, the primary component of the NDD changes induced by the terahertz pulse is demonstrated to be proportional to the product of its orthogonal electric field $E_{\mathrm{THz}}$ and magnetic field $B_{\mathrm{THz}}$. This is ascribed to the magnetoelectric effect modulation by the electromagnetic fields.

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• Strong electromagnetic field effects
• Ultrafast phenomena
• Multiferroics
• Terahertz techniques
• Transient absorption spectroscopy
• Ultrafast pump-probe spectroscopy