Tunable plasmonic topological transitions in Mo-doped WTe2 van der Waals films

Naturally existing in-plane hyperbolic polaritons and the associated topological transitions, which avoid the nano-structuring to achieve hyperbolicity, can outperform their counterparts in artificial metasurfaces. Such polaritons based on plasmons are rare and only experimentally revealed recently in WTe2 van der Waals thin films, though plasmons form the mainstream in nanophotonics. Different from phonon polaritons, hyperbolic plasmons originate from the interplay of free carrier Drude response and interband transitions, which promise good intrinsic tunability. However, tunable in-plane hyperbolic plasmon and its topological transition to elliptical regime in a natural material have not been realized. Here we demonstrate the tuning of the plasmonic topological transition through Mo doping. The topological transition is tuned in a wide range, with frequencies from 429 cm-1 (23 microns) for pure WTe2 to 270 cm-1 (37 microns) at the 50% Mo-doping level. Surprisingly, the localized plasmon in skew ribbons shows unusual polarization dependence, accurately manifesting its topology, which renders a reliable means to track the topology with far-field techniques. Our results open an avenue for reconfigurable photonic devices capable of plasmon polariton steering, such as canaling, focusing and routing, and pave a way for low-symmetry plasmonic nanophotonics based on anisotropic natural materials.