Giant Optical Anisotropy in a Natural van der Waals Hyperbolic Crystal for Visible Light Low-Loss Polarization Control.

Optically anisotropic bidimensional crystals offer a promising path toward compact, lithography-free polarization control in integrated photonic devices. However, most materials exhibit only modest optical anisotropy, requiring long propagation lengths to effectively modify the polarization state of light, hindering miniaturization and integration. While some materials achieve strong polarization extinction via directional absorption, this often comes at the cost of high optical losses, limiting their practical use. Here, we investigate the van der Waals crystal MoOCl that exhibits broadband in-plane hyperbolicity spanning the visible to near-infrared spectrum, driven by a Drude-like response. Thin MoOCl (∼100-200 nm) flakes achieve high reflectivity (>80%) along the metallic axis and strong transmission (>50%) along the orthogonal dielectric axis, enabling polarization extinction with minimal loss. From polarization-resolved transmission and reflection measurements, we extract an in-plane dielectric permittivity anisotropy exceeding |Δ(ε)| > 10 for wavelengths above 600 nm, among the highest reported to date. We further demonstrate the integration of a MoOCl flake directly onto a connected optical fiber to realize a broadband, ultrathin polarizer. These results establish MoOCl as a compelling platform for low-loss, miniaturized polarization control in next-generation photonic systems.