Spin-correlated exciton-polaritons in van der Waals magnets

The recent discovery of optically active excitons in magnetic van der Waals crystals offers extraordinary opportunities to study collective phenomena in quantum materials via light-matter interactions. One of the prime candidates in this endeavor is nickel phosphorus trisulfide (NiPS₃), a van der Waals antiferromagnet with highly correlated magnetic and electronic degrees of freedom. By coupling optical fields to its excitonic excitations, we demonstrate a previously unobserved class of polaritons with unique properties tied to excitons, photons and spins [1]. A detailed spectroscopic analysis of these newly formed quasiparticles in conjunction with our microscopic theory shows that magnetically coupled excitations can have an origin and interactions that are very different from those of excitons in conventional band semiconductors. Application of this approach to CrSBr, a van der Waals magnetic semiconductor with exceptionally strong excitons, provides a path towards the design and control of magneto-optical responses via cavity quantum electrodynamics.