Directional dichroism is the effect when materials absorb light beams travelling in opposite direction differently. This has been found to exist in the THz frequencies in multiferroic materials with magnetic and charge order. In these materials the spin waves are coupled to the oscillations of electric polarization. The spin waves acquire electric dipole activity due to optical magnetoelectric coupling and, therefore, interact with the electric (E) component of the THz radiation in addition to magnetic (B) component. This effect is similar to dc magnetoelectric effect where static magnetization M and electric polarization P are coupled.
One way transparency would present itself if there would be a large difference in absorbtion between counterpropagating light beams. Some multiferroics, e.g. Ba2CoGe2O7, Sr2CoSi2O7, exhibit some degree of this effect. Since this effect is sensitive to the direction of external magnetic field a directional light switch can be created. However, the needed magnetic and charge order in materials with strong directional dichroism happens at cryogenic temperatures rendering their practical applications impractical.
We studied spin excitations in Sr2CoSi2O7 at THz frequencies between 3 and 100 K and in magnetic fields up to 17 T. Magnetic field was applied in the a or b direction. Directional dichroism occurs because of the magneto-chiral dichroism where the crystal lattice and the magnetic structure form a chiral lattice. Almost one way transparency is seen in some spin wave modes. What is more, the directional dichroism increases above the Neel temperature (7K) in high magnetic fields. This is unusual in multiferroics, but is explained by the ME coupling on a single spin site.