Cavity-Enhanced Linear Dichroism in a van der Waals Antiferromagnet

The coupling of electronic states with crystal lattice underlies a wealth of condensed matter phenomena. Coupling of electronic charge with lattice vibrations or distortions underlies superconductivity, charge density wave as well as metal-insulator transitions. In contrast, coupling between electronic charge and spins has been scarcely investigated, particularly in the optical frequency domain. The recent discovery of anti-ferromagnetic (AFM) insulators of the form MPX3 (M = Fe, Mn, Ni; X= S, Se) has opened new avenues in studying spin-charge correlations via optical means. Here we report such an interaction in AFM FePS₃ crystals which induces strong in-plane anisotropy, resulting in linear dichroism (LD). We identify that this LD is characteristic of the AFM spin ordering in the crystal coincident with the Neel transition temperature (~118 K). Further, we show that the LD is tunable both spectrally and in magnitude up to near-unity (~98.4%) levels when coupled to an optical cavity. Finally, through transfer-matrix simulations, we also demonstrate the dispersion of the LD in the visible-near infrared range as a function of the cavity length and the FePS₃ thickness. We observe a close agreement between experiments and simulations and prove that the LD can be enhanced from 5% (in bulk) to 98.4% (in ~200nm thick samples) by virtue of cavity coupling and engineering the dielectric stack. Our findings open a new route to probing spin-charge coupling in magnetic materials and have implications in the design of novel birefringent nanophotonic elements.