An object is considered chiral if its mirror image cannot be brought to coincide with itself by any sequence of simple rotations and translations1. Chirality on a microscopic scale—in molecules2, 3, clusters4, crystals5 and metamaterials6, 7—can be detected by differences in the optical response of a substance to right- and left-handed circularly polarized light2, 3. Such ‘optical activity’ is generally considered to be a consequence of the specific distribution of electronic charge within chiral materials. Here, we demonstrate that a similar response can also arise as a result of spin excitations in a magnetic material. Besides this spin-mediated optical activity (SOA), we observe notable differences in the response of Ba2CoGe2O7—a square-lattice antiferromagnet that undergoes a magnetic-field driven transition to a chiral form—to terahertz radiation travelling parallel or antiparallel to an applied magnetic field. At certain frequencies the strength of this magneto-chiral effect8, 9, 10 is almost complete, with the difference between parallel and antiparallel absorption of the material approaching 100%. We attribute these phenomena to the magnetoelectric 11, 12 nature of spin excitations as they interact with the electric and magnetic components of light.
At a glance
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