Observation of nonreciprocal directional dichroism via electromagnon resonance in a chiral-lattice helimagnet B a 3 NbF e 3 S i 2 O 14

Nonreciprocal directional dichroism (NDD) is investigated on a magnetic resonance with both electric and magnetic dipole activities, i.e., electromagnon, for a chiral-lattice helimagnet $\mathrm{B}{\mathrm{a}}_{3}\mathrm{NbF}{\mathrm{e}}_{3}\mathrm{S}{\mathrm{i}}_{2}{\mathrm{O}}_{14}$ by terahertz spectroscopy under the magnetic field. In the Voigt geometry, the electromagnon resonance causes the NDD in the helimagnetic phase in accord with the spin-driven ferroelectric polarization, whose polarity is uniquely selected owing to the structural chirality. In the Faraday geometry, all possible optical effects, including natural optical activity, magnetic optical activity and magnetochiral effect showing NDD, are separately evaluated by terahertz polarimetry technique. The NDD is manifested in the electromagnon resonance in the helimagnetic phase, while a formation of helimagnetic spin structure enhances the natural optical activity inherent to the structural chirality.