  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) that 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 per will. Here, we use
the newly discovered spin-charge coupling in van der Waals antiferromagnetic
(AFM) insulator FePS3 to induce large in-plane optical anisotropy and
consequently LD. We report that the LD in this AFM insulator is tunable both
spectrally and magnitude-wise as a function of cavity coupling. We demonstrate
near-unity LD in the visible-near infrared range in cavity-coupled FePS3
crystals and derive its dispersion as a function of cavity length and FePS3
thickness. Our results hold wide implications for use of cavity tuned LD as a
diagnostic probe for strongly correlated quantum materials as well as opens new
opportunities for miniaturized, on-chip beam-splitters and tunable filters.
