Nonreciprocal Refraction of Light in a Magnetoelectric Material.

In magnetoelectric materials, where the time-reversal and space-inversion symmetries are simultaneously broken, optical properties can differ between the opposite propagation directions of light. We report on an experimental observation of nonreciprocal trajectory of a light ray in magnetoelectric material CuB_{2}O_{4}. The light is refracted in different ways between the opposite propagation directions of light. We find a nonreciprocal refraction at the interface between a matter with macroscopic toroidal moment and vacuum. The resultant nonreciprocal deflection of the light is 0.005 deg, which is quantitatively explained using Fermat's principle.