Cavity-Enhanced Circular Dichroism in a van der WaalsAntiferromagnet

Broken symmetry plays a pivotal role in determining the macroscopic electrical, optical, magnetic, and topological properties of materials. Circular dichroism (CD) has been widely employed to probe broken symmetry in various systems, from small molecules to bulk crystals, but designing CD responses on demand remains a challenge, especially for antiferromagnetic materials. Here we develop a cavity-enhanced CD technique to sensitively probe the magnetic order and broken symmetry in the van der Waals antiferromagnet FePS₃. By introducing interfacial inversion asymmetry in cavity-coupled FePS₃ crystals, we demonstrate that the induced CD is strongly coupled with the zigzag antiferromagnetic order of FePS₃ and can be tuned both spectrally and in magnitude by varying the cavity length (FePS₃ thickness). Our findings open new avenues for using cavity-modulated CD as a sensitive diagnostic probe to detect weak broken symmetries, particularly at hidden interfaces, and in systems exhibiting hidden spin polarization or strong correlations.