Observation of Giant Optical Linear Dichroism and Pseudo Critical Slowing Down in van der Waals Zigzag Antiferromagnets

Two-dimensional antiferromagnetism (AFM) has been a topic of great interest due to its intertwined relationship with various correlated phenomena like unconventional superconductivity and its potential applicability in novel spintronic devices. In contrast to its ferromagnetic counterpart, direct optical probing and study of AFM order is challenging due to the absence of net magnetization and the resulting weak magneto-optical coupling. Here, we report an optical study of an atomically thin zigzag antiferromagnet, FePS₃. We observed a large optical linear dichroism associated with the zigzag AFM order. By performing time-resolved polarimetry measurements, we further examined the critical fluctuations of the AFM order parameter, which exhibit a prominent pseudo-critical slowing down behavior near the Néel temperature. Our findings point to a new optical approach to identify zigzag AFM order and study its non-equilibrium dynamics in strongly correlated systems.