Chirality of matter shows up via spin excitations

An object is considered chiral if its mirror image cannot be brought to coincide with itself by any sequence of simple rotations and translations ¹ . Chirality on a microscopic scale - in molecules ^2,3 , clusters ⁴ , crystals ⁵ and metamaterials ^6,7 - can be detected by differences in the optical response of a substance to right- and left-handed circularly polarized light ^2,3 . 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 Ba ₂ CoGe ₂ O ₇ - 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 effect is almost complete, with the difference between parallel and antiparallel absorption of the material approaching 100%. We attribute these phenomena to the magnetoelectric nature of spin excitations as they interact with the electric and magnetic components of light.