Excitonic negative refraction mediated by magnetic orders.

Excitons are prevalent in many bosonic quantum phenomena in semiconductors. During their optical transitions, excitons not only emit or absorb photons, but also determine light propagation behaviours within the host medium. While absorption and emission processes have found numerous practical applications, chiefly in light-emitting diodes and solar cells, excitons' capability of manipulating light propagation remains largely underexplored. Here we observe negative refraction-light bending in the opposite direction to conventional refraction-in an excitonic van der Waals magnet, namely chromium sulphide bromide (CrSBr). We also develop an excitonic hyperlens on an integrated nanophotonic chip, whose functionality is mediated by the magnetic orders of CrSBr. Specifically, the observed negative refraction and hyperlens effects emerge when CrSBr is magnetically ordered, driven by a magnetic enhancement of excitonic resonances. This work establishes excitons in van der Waals magnets as a versatile platform for controlling anomalous light propagation at the nanoscale.