Configurable phonon polaritons in twisted α-MoO3

Moiré engineering is being intensively investigated as a method to tune the electronic, magnetic and optical properties of twisted van der Waals materials. Advances in moiré engineering stem from the formation of peculiar moiré superlattices at small, specific twist angles. Here we report configurable nanoscale light–matter waves—phonon polaritons—by twisting stacked α-phase molybdenum trioxide (α-MoO3) slabs over a broad range of twist angles from 0° to 90°. Our combined experimental and theoretical results reveal a variety of polariton wavefront geometries and topological transitions as a function of the twist angle. In contrast to the origin of the modified electronic band structure in moiré superlattices, the polariton twisting configuration is attributed to the electromagnetic interaction of highly anisotropic hyperbolic polaritons in stacked α-MoO3 slabs. These results indicate twisted α-MoO3 to be a promising platform for nanophotonic devices with tunable functionalities. Infrared nanoimaging of phonon polaritons in twisted α-phase molybdenum trioxide bilayers reveals tunable wavefront geometries and topological transitions over a broad range of twist angles, offering a configurable platform for nanophotonic applications.