Observation of canalized phonon polaritons in a single <i>α</i>-MoO₃ flake

The canalization effect of phonon polaritons (PhPs) shows highly directional and diffraction-less propagation characteristics in van der Waals (vdW) materials, offering new opportunities to mold the light flow at nanoscale for near-field energy, information, and thermal management. Previously, canalized PhPs have only been experimentally realized in the hexagonal boron nitride metasurface, heterostructures of twisted α -phase molybdenum trioxide ( α -MoO 3 ) crystal flakes, or the hybridized system. However, these systems typically have complex structures, and require strict operational conditions, such as fine structural parameters, a specific photonic magic angle, or a doping level of graphene, for realizing polariton canalization with a modest performance. Here, we demonstrate the high-quality PhPs canalization in a single natural α -MoO 3 crystal flake. The canalized PhPs exhibit the highly directional and diffraction-free propagation features associated with lateral confinement ratios up to λ 0 /80 (where λ 0 is the free-space wavelength of the incident laser). We believe this work is important to effectively manipulate PhPs in natural vdW materials, with potential applications in nanoimaging, directional energy transfer, and enhanced nonlinearity at the deep subwavelength scale.