  Two-dimensional (2D) magnetic semiconductors feature both tightly-bound
excitons with large oscillator strength and potentially long-lived coherent
magnons due to the presence of bandgap and spatial confinement. While 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 van der Waals (vdW)
antiferromagnetic (AFM) semiconductor CrSBr. Coherent magnons launched by
above-gap excitation modulate the interlayer hybridization, which leads to
dynamic modulation of excitonic energies. Time-resolved exciton sensing reveals
magnons that can coherently travel beyond 7 micrometer, with coherence time
above 5 ns. We observe this exciton-coupled coherent magnons in both even and
odd number of layers, with and without compensated magnetization, down to the
bilayer limit. Given the versatility of vdW heterostructures, these coherent 2D
magnons may be basis for optically accessible magnonics and quantum
interconnects.
