Asymmetry of atomic coordinates in crystalline lattices and molecules can produce the chirality, giving rise to the natural optical activity. The spin spiral also induces the chirality regardless of the symmetry of underlying chemical lattice and the sign of spin-induced chirality can be controlled by the electric field. We demonstrated the electric field control of the natural optical activity on the electromagnon resonance in terahertz region.

Weyl semimetals have the pairs of Weyl points near the Fermi level due to the topological properties of their electronic band structure. The ferromagnetic Weyl semimetals potentially exhibits the magneto-optical effects stemming from these topological electronic structures. We observed the giant magneto-optical effects enhanced on the optical transitions among these topological bands in terahertz and far-infrared region. This large magneto-optical responses of Weyl semimetal apply to the optical devices for low-energy photonics.

Magnetic skyrmion is a quantum particle in condensed matter, which is composed of the swirling spins. The motion of electron on the skyrmions is twisted by the emergent magnetic field, resulting in the topological Hall effect. We observed the resonance of topological and anomalous Hall effects on the 3D skyrmion lattice by using the time-domain terahertz polarimetry.

Gyrotropic birefringence is the novel optical rotation phenomena having the nonreciprocity. However, the observation of gyrotropic birefringence is still rare and their optical properties remain elusive. We demonstrated the resonantly enhanced gyrotropic birefringence on the electromagnon in multiferroics with spin-driven ferroelectricity. It is clearly revealed that the gyrotropic birefringence can be viewed as the nonreciprocal rotation of the optical axis.