Floquet Theory and Ultrafast Control of Magnetism

The development of laser science and technology have stimulated the study of condensed matter physics, especially, dynamical or non-equilibrium nature in solids. The laser technique in terahertz (THz) regime, whose photon energy is comparable to those of typical collective modes in solids such as magnetic excitations, phonons, etc., has remarkably proceeded in the last decade. Theoretical tools for non-equilibrium states have also progressed. Thanks to these backgrounds, magneto-optics, especially, the study of controlling magnetism with laser, now enters a new stage. For such controls, Floquet engineering is a key concept, which means the method of controlling static properties of targets with high-frequency external fields like laser. I review the theoretical foundation of Floquet engineering and its application to magnetic insulators. Basic magnetic quantities such as magnetization, spin chirality, and spin current are shown to be controlled with intense THz laser or wave.