Despite its tiny net magnetization, the antiferromagnetic (AFM) van der Waals material ${\mathrm{Co}}_{1/3}{\mathrm{NbS}}_2$ exhibits a large transverse Hall conductivity ${\sigma }_{xy}$ even at zero applied magnetic field, which arises, as recently shown, from the topological nature of its noncoplanar “tetrahedral” AFM order. This triple-$\mathbf{q}$ magnetic order can be regarded as the short-length-scale limit of a magnetic skyrmion lattice and has an intrinsic spin chirality. Here, we show, using optical wavelengths spanning the ultraviolet to infrared (400–1000 nm), that magnetic circular dichroism provides an incisive optical probe of the topological AFM order in ${\mathrm{Co}}_{1/3}{\mathrm{NbS}}_2$. Measurements as a continuous function of photon energy are directly compared with first-principles calculations, revealing the influence of the underlying quantum geometry on optical conductivity. Leveraging the power and flexibility of optical methods, we use scanning magnetic circular dichroism microscopy to directly image chiral AFM domains and demonstrate writing of chiral AFM domains.

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• Magnetic domains
• Topological Hall effect
• Antiferromagnets
• Noncollinear magnets
• Triangular lattice
• Magneto-optical Kerr effect