Cavity-enhanced linear dichroism in a van der Waals antiferromagnet

Optical birefringence is a fundamental optical property of crystals widely used for filtering and beam splitting of photons. Birefringent crystals concurrently possess the property of linear dichroism (LD), which allows asymmetric propagation or attenuation of light with two different polarizations. This property of LD has been widely studied from small molecules to polymers and crystals but has rarely been engineered on demand. Here we use the newly discovered spin-charge coupling in the van der Waals antiferromagnetic insulator FePS 3 to induce large in-plane optical anisotropy and consequently LD. We report that the LD in this antiferromagnetic insulator is tunable both spectrally and in terms of its magnitude as a function of the cavity coupling. We demonstrate near-unity LD in the visible–near-infrared range in cavity-coupled FePS 3 crystals and derive its dispersion as a function of the cavity length and FePS 3 thickness. Our results hold wide implications for the use of cavity-tuned LD as a diagnostic probe for strongly correlated quantum materials and offer new opportunities for miniaturized, on-chip beamsplitters and tunable filters.