Strong coupling between optical and magnetic excitations could enable contactless, spatially resolved, or ultrafast interrogation and control of magnetism in two-dimensional (2D) materials and devices. The layered 2D A-type antiferromagnet ${\mathrm{CrPS}}_4$ stands out among van der Waals (vdW) magnets for its rich optical fine structure, but its spectroscopy is not yet understood and has so far been interpreted without consideration of magnetic exchange. Here, we show that this fine structure comes primarily from exchange-mediated coupling between on-site optical “spin-flip” transitions of $\mathrm{C}{\mathrm{r}}^{3+}$ and low-energy spin transitions involving the surrounding lattice. Well-resolved magnon sidebands to optical ${}^4\mathrm{A}_2\leftrightarrow {}^2\mathrm{E}$ transitions are observed in photoluminescence (PL) and PL excitation spectra, as well as a pronounced PL sideband associated with short-range exchange splitting. Energy migration is probed using $\mathrm{Y}{\mathrm{b}}^{3+}$ dopants as traps, revealing subpicosecond intersite excitation hopping. Formation of dispersive Frenkel excitons of coupled on-site d-d transitions because of intersite exchange is discussed. In addition to impacting how optical fine structure is interpreted in this and potentially other vdW magnets, these findings may have ramifications for future applications of layered 2D magnets by suggesting potential routes to drive mode-specific spin-wave excitations using light.

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• Crystal-field excitations
• Doping effects
• Luminescence
• Magnetism
• Magnons
• Antiferromagnets
• Layered crystals
• van der Waals systems