Magnetic Order Unlocks Optical Access to Dark Excitons in CrSBr

Abstract: Hybrid quasiparticles that intertwine magnetic and electronic degrees of freedom underpin emerging strategies for manipulating and transducing quantum information in solids. A key missing element has been the ability to optically access dark excitons - optically forbidden but functionally crucial states that shape energy flow, coherence, and spin dynamics in quantum materials. Here we show that exciton-magnon coupling provides an optical gateway to dark excitons in the antiferromagnetic van der Waals semiconductor CrSBr. Broadband femtosecond reflectivity reveals a dark exciton at 1.46 eV that is entirely absent in static optical spectra but becomes visible through its coherent hybridization with a GHz magnon. High-photon-energy excitation further allows active control of the hybrid dispersion, enabling strong renormalization and selective enhancement of exciton-magnon interactions. These results establish a general mechanism by which magnetic order renders dark excitons optically addressable, opening a pathway toward engineered hybrid spin-exciton platforms for microwave-to-optical quantum transduction.