Revealing Antiferromagnetic Fluctuations Through Resonant Nonlinear Optical Coupling in a Magnetic Topological Insulator

Optical second harmonic generation (SHG) is a sensitive probe of crystal symmetries and is most often used to identify inversion symmetry breaking. Rotational anisotropy measurements of SHG reveal detailed information about symmetry and can detect subtle changes in bonding, charge order, or magnetism. Here we show that near-resonant magnetic dipole SHG is sensitive to magnetic ordering in the antiferromagnet and candidate axion insulator EuSn₂(As,P)₂, despite the presence of both inversion and time-reversal symmetry which prevent new SHG processes from turning on. This unexpected sensitivity arises from the resonant electronic transition underlying the SHG process, revealing a previously unrecognized mechanism by which magnetic order may become visible to nonlinear optical probes. Using this mechanism, we track magnetic fluctuations as a function of temperature and directly image the antiferromagnetic domain structure. These results demonstrate the power of nonlinear optical spectroscopy as a probe of quantum coherence and magnetic fluctuations in correlated materials.