Ultrafast probes of giant dynamical magnetoelectricity in the layered multiferroic NiI₂

The recent discovery of type-II multiferroicity in the van der Waals helimagnet NiI₂ down to the single-layer limit raises the prospect of unprecedented magnetoelectric couplings in quasi-2D layered materials. Here, by combining ultrafast second harmonic generation and reflective Kerr rotation microscopy probes, we disentangle the dynamics of intertwined dipolar and magnetic orders in NiI₂. Our analysis focuses on chiral terahertz-frequency electromagnons, using careful dependences of the optical driving field to reveal the mechanism behind the coherent excitation of these modes. Moreover, by extracting their corresponding transient polarization and magnetization modulations, we recover a record-breaking dynamical magnetoelectric coupling driven by direct interactions between electronic spin and orbital degrees of freedom. Finally, we examine the underlying incoherent material response originating from photoexcited carriers and demonstrate their impact on the system's magnetoelectric order.