Terahertz broadband one-way transparency with spontaneous magnon decay

Nonreciprocity is a phenomenon in which broken spatial inversion symmetry appears on a macroscopic scale and is an essential issue in condensed matter physics. Directional dichroism is the nonreciprocal phenomenon of light; the light transmittance varies depending on the direction of light propagation. It arises from interference between electromagnetic fields of light, resulting in the nonreciprocal transmittance determined by the resonance light absorption. For the maximum interference, it shows one-way transparency, and achieving this is an outstanding experimental challenge. Concerning energy dispersion, the resonance generally occurs only at a specific energy with a narrow bandwidth. When the excited state of the magnon spontaneously decays into lower energy states, the absorption linewidth gets broader. Here, using electron spin resonance, we identify the specific absorption mode in a multiferroic material Sr₂CoSi₂O₇ with the spontaneous magnon decay accompanied by the maximum interference, achieving the broadband one-way transparency.