Coherent terahertz magnon–phonon three-wave mixing in a layered antiferromagnet

The coherent nonlinear dynamics between collective excitations, such as magnons and phonons, drive emergent phenomena in quantum materials, yet their direct observation remains limited. Here, using double-terahertz-pump optical-probe spectroscopy, we report the direct observation of coherent magnon–phonon three-wave mixing in the layered antiferromagnetic insulator FePS3. We resolve both second- and third-order nonlinear responses of antiferromagnetic magnons and identify a suite of nonlinear couplings in two-dimensional (2D) coherent spectra, including definitive sum- and difference-frequency generation between magnons and phonons. These results lay the groundwork for exploiting coherent nonlinearities to entangle magnetic and vibrational excitations, opening avenues for quantum control and hybrid quantum technologies in the terahertz regime. Higher order non-linear involving both magnons and phonons are key to the entanglement of the lattice and magnetic degrees of freedom. Here, Li, Wu, Lin, and coauthors succeed in observing three wave mixing in the van der Waals antiferromagnet, FePS3, offering a direct route to the creation of hybrid non-classical magnon phonon states at terahertz frequencies.