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 FePS₃ 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 FePS₃ crystals and derive its dispersion as a function of the cavity length and FePS₃ 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.

光学双折射是晶体的基本光学特性，广泛用于光子的过滤和分束。双折射晶体同时具有线性二向色性 (LD) 的特性，它允许具有两种不同偏振的光的不对称传播或衰减。LD 的这一特性已被广泛研究，从小分子到聚合物和晶体，但很少按需设计。在这里，我们使用范德华反铁磁绝缘体 FePS _{3中新发现的自旋电荷耦合}诱导大的面内光学各向异性并因此产生LD。我们报告说，这种反铁磁绝缘体中的 LD 在光谱和作为腔耦合函数的幅度方面都是可调谐的。_{我们展示了腔耦合 FePS 3}晶体中可见-近红外范围内的近统一 LD，并得出其色散是腔长和 FePS ₃厚度的函数。我们的结果对于使用腔调谐 LD 作为强相关量子材料的诊断探针具有广泛的意义，并为小型化片上分束器和可调谐滤波器提供了新的机会。