Exciton-coupled coherent magnons in a 2D semiconductor

The recent discoveries of two-dimensional (2D) magnets^1-6 and their stacking into van der Waals structures^7-11 have expanded the horizon of 2D phenomena. One exciting application is to exploit coherent magnons¹² as energy-efficient information carriers in spintronics and magnonics^13,14 or as interconnects in hybrid quantum systems^15-17. A particular opportunity arises when a 2D magnet is also a semiconductor, as reported recently for CrSBr (refs. ^18-20) and NiPS₃ (refs. ^21-23) that feature both tightly bound excitons with a large oscillator strength and potentially long-lived coherent magnons owing to the bandgap and spatial confinement. Although magnons and excitons are energetically mismatched by orders of magnitude, their coupling can lead to efficient optical access to spin information. Here we report strong magnon-exciton coupling in the 2D A-type antiferromagnetic semiconductor CrSBr. Coherent magnons launched by above-gap excitation modulate the exciton energies. Time-resolved exciton sensing reveals magnons that can coherently travel beyond seven micrometres, with a coherence time of above five nanoseconds. We observe these exciton-coupled coherent magnons in both even and odd numbers of layers, with and without compensated magnetization, down to the bilayer limit. Given the versatility of van der Waals heterostructures, these coherent 2D magnons may be a basis for optically accessible spintronics, magnonics and quantum interconnects.