Proximity-mediated magnon-exciton coupling at a van der Waals heterointerface

Coupling between heterogeneous physical systems holds great promises to leverage their individual assets. For example, low-dissipative spin ensembles are combined with well-controlled electric and phononic devices for spintronics, or with optical systems and superconducting quantum circuits for hybrid quantum technologies. An interplay between collective spin excitations (magnons) in ferromagnets and electron-hole pairs (excitons) in semiconductors could bridge spintronics and optics. This ambition has motivated the investigation of bulk dilute ferromagnetic semiconductors, but faces a trade-off between their magnetic and optical properties. Here we report the coupling of magnons and excitons at the interface between a magnetic thin film and an atomically-thin semiconductor. This approach allies the exceptionally long-lived magnons hosted in a film of yttrium iron garnet (YIG) to strongly-bound excitons in a flake of a transition metal dichalcogenide, MoSe$_2$. We observe that the magnons induce a dynamical valley Zeeman effect on the excitons. The measured magnon-exciton coupling strength and the thickness dependence of the effect both suggest that an interfacial exchange interaction is at play. Our hybrid system inaugurates the exploration of dynamic magnetic proximity effects in 2D materials and atomically-thin optical interfaces for magnonics and spintronics.