In-Plane Anisotropic Plasmons in Van Der Waals Thin Films of WTe2

Anisotropic plasmonic surface supports elliptic, hyperbolic and even flattened polaritons, which is quite interesting for the diffractionless and highly collimated propagation of infrared light at the nanoscale. However, direct real-space near-field observation of anisotropic plasmons as well as frequency dependent topological transitions in natural materials have not been realized. In this paper, we theoretically investigate real-space anisotropic plasmons in WTe2 thin films by using a phenomenological cavity model, anisotropic near-field plasmonic images with specific interference patterns and isofrequency curves in momentum space have been demonstrated. Due to the frequency selective forbidden of plasmons along b axis, a topological transition from the elliptic to the hyperbolic regime is manifested. Moreover, the plasmons as well as topological transition present significant electrostatic-gating tunability. Our studies provide new insights into WTe2 based plasmonic components for the manipulation of plasmon propagation, which capable of tailoring anisotropic two-dimensional light confinement in the far-infrared regime and can be applied to investigate other anisotropic materials.