Twist-Angle and Thickness-Ratio Tuning of Plasmon Polaritons in Twisted Bilayer van der Waals Films

Stacking bilayer structures is an efficient way to tunethe topologyof polaritons in in-plane anisotropic films, e.g., by leveraging thetwist angle (TA). However, the effect of another geometric parameter,the film thickness ratio (TR), on manipulating the plasmon topologyin bilayers is elusive. Here, we fabricate bilayer structures of WTe2 films, which naturally host in-plane hyperbolic plasmonsin the terahertz range. Plasmon topology is successfully modifiedby changing the TR and TA synergistically, manifested by the extinctionspectra of unpatterned films and the polarization dependence of theplasmon intensity measured in skew ribbon arrays. Such TR- and TA-tunabletopological transitions can be well explained based on the effectivesheet optical conductivity by adding up those of the two films. Ourstudy demonstrates TR as another degree of freedom for the manipulationof plasmonic topology in nanophotonics, exhibiting promising applicationsin biosensing, heat transfer, and the enhancement of spontaneous emission.