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 squarelattice 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 8-10 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.Natural circular dichroism (NCD) and gyrotropy are observed respectively as the change in the ellipticity and the rotation of the polarization of light transmitted through chiral media. Because the sign of these quantities depends on the handedness of the material, NCD is a common probe of chirality over a broad spectrum of the electromagnetic radiation, as reviewed in Fig. 1, when applied to the chiroptical study of proteins. Ultraviolet NCD spectroscopy of peptide-bond excitations is a well-established method for the determination of the secondary structure of proteins 3,13 . Recent extensions of NCD spectroscopy to the infrared and X-ray regions have shed light on new signatures of chirality of matter expressed by molecular vibrations 2,5,14 and core electron excitations 15 . Extending this context, the handedness of magnetic matter should also be detected in the NCD spectra of spin excitations, typically at gigahertz to terahertz frequencies, although these have seldom been investigated.When chirality is accompanied by magnetism, an intriguing optical cross effect, the magneto-chiral dichroism 8,9 (MChD), emerges besides the conventional magnetically induced circular dichroism (MCD). MChD is a directional dichroism and is measured as the absorption difference for unpolarized (or linearly polarized) light propagating parallel and antiparallel to the magnetization of the media. MChD is generally recognized as a weak effect and has been found for visible light in metallic complexes 10 , molecular magnets 16 , inorganic crystals 17 and cholesteric liquid crystals 18 . However, a
