  Controlling the properties of quantum materials with light is of fundamental
and technological importance. While high-power lasers may achieve this goal,
more practical strategies aim to exploit the strong coupling of light and
matter in optical cavities, which has recently been shown to affect elemental
physical phenomena, like superconductivity, phase transitions, and topological
protection. Here we report the capacity of strong light-matter coupling to
modify and control the magneto-optical properties of magnets. Tuning the
hybridization of magnetic excitons and cavity photons allows us to realize
distinct optical signatures of external magnetic fields and magnons in the
archetypal van der Waals magnetic semiconductor CrSBr. These results highlight
novel directions for cavity-controlled magneto-optics and the manipulation of
quantum material properties by strong light-matter coupling.
