Nonreciprocal optical responses of multiferroics

Multiferroics exhibit the strong magnetoelectric coupling, giving rise to the many intriguing phenomena. One such example is the optical response characterized by the nonreciprocity, in which the electric and magnetic dipole transitions interfere through the dynamical magnetoelectric coupling. Here, we focus on the optical response of the electromagnon, which is the magnon endowed with the electric activity, with strong dynamical magnetoelectric coupling. The spin orders which break the space-inversion symmetry induce the many nonreciprocal optical effects, which are especially enhanced on the electromagnon resonance, in the terahertz region. For example, the magnetically-induced chirality causes the large natural optical activity and magnetochiral effect. The magnetically-induced polarity, on the other hand, causes the directional dichroism and gyrotropic birefringence. We demonstrate these four distinct nonreciprocal optical effects on the spin-spiral magnets by using the time-domain terahertz polarimetry.