Unveiling α‐MoO ₃ as a High‐Index, Low‐Loss Anisotropic Material: Bridging the Gap From Mid‐Infrared to Ultraviolet Nanophotonics

ABSTRACT Modern nanophotonics demands light manipulation at a subwavelength scale, a capability historically constrained by the isotropic dielectric response of conventional optical semiconductors. Here, we establish α‐MoO 3 as a high‐index, low‐loss anisotropic material that bridges the gap between mid‐infrared polaritonics and short‐wavelength nanophotonics. Integrating spectroscopic ellipsometry, first‐principles calculations, and near‐field optical microscopy, we determine its broadband 250–1700 nm anisotropic optical constants and combine them with a phonon model in mid‐infrared to 30 µm. While the mid‐infrared phonon polariton properties of α‐MoO 3 are well established in the literature, the present work provides the first experimental characterization of its complete anisotropic dielectric tensor in the ultraviolet‐to‐near‐infrared regime, conceptually bridging these two spectral domains in which this material finds application. The pronounced out‐of‐plane birefringence 0.35 suppresses evanescent crosstalk beyond competing van der Waals (vdW) waveguide materials, and the high in‐plane anisotropy enables zero‐order waveplates at wavelengths as short as 325 nm. As a practical demonstration, we prepared nanowaveguide which exhibits wavelength‐selective light guidance at visible range. Together, these results reposition α‐MoO 3 from a mid‐infrared specific material to a versatile wide‐bandgap platform bridging mid‐infrared polaritonics and ultraviolet‐visible nanophotonics.