Nonlinear optical domain imaging in magnetic Weyl semimetal CeAlSi

Topological magnets recently emerged as a new class of topological quantum materials. The interaction between topologically protected electronic states and long-range magnetic order not only gives rise to fundamentally new phenomena, but may also provide novel pathways for the electrical manipulation of magnetization with immediate implications for magnetoelectric devices. Exploring the intricate coupling between topology and magnetism, however, hinges critically on the ability to visualize and control the magnetic state. Here, we use optical second-harmonic spectroscopy and imaging to spatially resolve the complex multi-domain state in the magnetic Weyl semimetal CeAlSi. Second-harmonic generation (SHG) is a highly symmetry sensitive nonlinear optical process that couples linearly to the magnetic order parameter. Using SHG spectroscopy we identify components that are selectively sensitive either to the noncentrosymmetric crystal structure or to the magnetic order. Interference between the individual components allows us to uniquely distinguish all four magnetic domain states. Our results enable the further investigation of the intricate interplay between topology and magnetism.