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

The need to develop new methods for the high-sensitivity diagnosis of malaria has initiated a global activity in medical and interdisciplinary sciences. Most of the diverse variety of emerging techniques are based on research-grade instruments, sophisticated reagent-based assays or rely on expertise. Here, we suggest an alternative optical methodology with an easy-to-use and cost-effective instrumentation based on unique properties of malaria pigment reported previously and determined quantitatively in the present study. Malaria pigment, also called hemozoin, is an insoluble microcrystalline form of heme. These crystallites show remarkable magnetic and optical anisotropy distinctly from any other components of blood. As a consequence, they can simultaneously act as magnetically driven micro-rotors and spinning polarizers in suspensions. These properties can gain importance not only in malaria diagnosis and therapies, where hemozoin is considered as drug target or immune modulator, but also in the magnetic manipulation of cells and tissues on the microscopic scale.

The coupling between spins and electric dipoles governs magnetoelectric phenomena in multiferroics. The dynamical magnetoelectric effect, which is an inherent attribute of the spin excitations in multiferroics, drastically changes the optical properties of these compounds compared with conventional materials where light-matter interaction is expressed only by the dielectric permittivity or magnetic permeability. Here we show via polarized terahertz spectroscopy studies on multiferroic Ca 2 CoSi 2 O 7 , Sr 2 CoSi 2 O 7 and Ba 2 CoGe 2 O 7 that such magnetoeletric spin excitations exhibit quadrochroism, that is, they have different colours for all the four combinations of the two propagation directions (forward or backward) and the two orthogonal polarizations of a light beam. We demonstrate that one-way transparency can be realized for spin-wave excitations with sufficiently strong optical magnetoelectric effect. Furthermore, the transparent and absorbing directions of light propagation can be reversed by external magnetic fields. This magnetically controlled optical-diode function of magnetoelectric multiferroics may open a new horizon in photonics.