Unveiling α-MoO₃ as a High-Index, Low-Loss Anisotropic Material: Bridging the Gap From Mid-Infrared to Ultraviolet...

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-MoO₃ 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-MoO₃ 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-MoO₃ from a mid-infrared specific material to a versatile wide-bandgap platform bridging mid-infrared polaritonics and ultraviolet-visible nanophotonics.