Observation of maximum circular dichroism in twisted hyperbolic van der Waals homostructures.

Achieving maximum circular dichroism (CD) at nanoscale stands as an ultimate goal in chiral photonics, but it remains challenging for conventional chiral materials and anisotropic crystals. While two-dimensional van der Waals materials and their twisted assemblies have enabled atomic-scale control of chiral light-matter interactions, the attainable CD magnitudes are still limited due to the lack of versatile design and fabrication strategies. Here we demonstrate that the giant CD in twisted hyperbolic van der Waals homostructures originates from microscopic linear dichroism-linear birefringence (LD-LB) interference, rather than intrinsic geometric chirality. By establishing a rigorous interlayer LD-LB coupling framework, we predict maximum CD in twisted MoOCl2 homostructures at merely 220 nm, while the measured CD reaches a near-unity magnitude of 0.9, without resorting to intricate nanofabrications. Furthermore, by modulating the interlayer twist angle through a peel-and-restack process, we achieve reconfigurable CD engineering from -0.9 to 0. 9. Based on the nanoscopic platform, we present the first stick-and-use circular polarizer capable of generating high-purity chiral emission from light-emitting devices, which opens up new avenues for chiral photonics and optoelectronics.