Axion optical induction of antiferromagnetic order

Using circularly-polarized light to control quantum matter is a highly intriguing topic in physics, chemistry and biology. Previous studies have demonstrated helicity-dependent optical control of spatial chirality and magnetization M. In our work, we report the surprising observation of helicity-dependent optical control of fully-compensated antiferromagnetic (AFM) order in 2D even-layered MnBi2Te4, a topological Axion insulator with neither chirality nor M. By shining circularly-polarized light while cooling across the Néel temperature, surprisingly, we found that light helicity and wavelength can directly control the AFM order parameter. We further demonstrate the optical creation of AFM domain walls by double induction beams and the direct switching of AFM domains by ultrafast pulses. To understand this optical control, we study a novel type of circular dichroism (CD) proportional to the AFM order, which only shows up in reflection but is absent in transmission. We show that the optical control and CD both arise from the optical Axion electrodynamics, which can be visualized as a Berry curvature real space dipole.