Colossal Directional Dichroism in a Multiferroic CuB$_{2}$O$_{4}$

In multiferroic materials, electric and magnetic responses to light can interfere with each other, resulting in novel optical phenomena. One typical example is directional dichroism, where the absorption coefficient changes with the reversal of the propagating direction of light. It has been reported that CuB$_{2}$O$_{4}$ shows giant directional dichroism at 1.40 eV, which corresponds to the intratomic from d$_{\mathrm{x2-y2}}$ to d$_{\mathrm{xy}}$ transition of Cu$^{2+}$ hole. The optical absorption coefficient changes by a factor of three at low magnetic fields. In this study, we investigated magnetic field dependence of directional dichroism up to 52 T. We have observed colossal directional dichroism where the ratio of the optical absorption coefficient is as large as 1000{\%} at 52 T. The observed extraordinary large directional dichroism is explained by a modification of the electric and magnetic dipole transition moments due to the canting of the spin direction of Cu$^{2+}$ hole by the application of a high magnetic field.