Quantum Matter Seminar

Complex magnetic orders can intertwine spin and charge degrees of freedom in ways that endow their collective modes with novel properties. A striking example occurs in chiral multiferroics, where non-collinear spin textures break inversion symmetry and generate an electric polarization. Their low-energy excitations – electromagnons – carry magnetic- and electric-dipole character, providing a direct window into the dynamical coupling between magnetic and polar order.

In this talk, I will discuss how we visualize and coherently manipulate these dynamics in the van der Waals multiferroic NiI2. Using time-resolved nonlinear and magneto-optical probes, we track the electric-polarization and magnetization components of coherently driven electromagnons within individual multiferroic domains. Their oscillations evolve in quadrature, revealing a giant dynamical magnetoelectric response and a natural optical activity at terahertz frequencies that strongly exceeds that of conventional helimagnets.

I will then show that the handedness of this collective motion can be encoded in an electronic excitation. By identifying opposite spiral-chirality domains and resonantly driving their electromagnons with tailored terahertz pulses, we induce coherent modulations of an exciton coupled to the magnetic background. As the terahertz polarization is rotated, the electromagnon phase winds in opposite directions in domains of opposite chirality. This behavior originates from interference between the electric- and magnetic-dipole excitation channels of the electromagnon, allowing an exciton to act as a phase-sensitive optical sensor of magnetic handedness.

Together, these results establish a route for resolving the amplitude, phase, and chirality of coupled magnetoelectric motion on its intrinsic timescale. More broadly, they illustrate how electronic excitations can be used to read out – and ultimately help control – the symmetry encoded in collective dynamics of complex quantum materials.