Surface plasmons induce topological transition in graphene/MoO₃ heterostructures

Polaritons in hyperbolic van der Waals materials – where principal axes have permittivities of opposite signs – have helped develop new nanotechnologies, study fundamental nano-optics, and probe complex response functions. The isofrequency contours of hyperbolic polaritons may undergo topological transitions from open hyperbolas to closed ellipse-like curves, prompting a discontinuous change in physical properties. Electronically-tunable topological transitions are especially desirable for future integrated technologies but have yet to be demonstrated. In this work, we present a doping-induced topological transition effected by plasmon-phonon hybridization in a graphene/MoO₃ heterostructure. Scanning near-field optical microscopy was used to image hybrid polaritons in graphene/MoO₃. We demonstrate the topological transition and characterize the hybrid modes, studying their evolution from surface waves to bulk waveguide modes and the dependence of properties of hybrid polaritons on plasmon-phonon coupling strength. Graphene/MoO₃ is an exciting platform for exploring dynamical topological transitions and directional plasmon-phonon coupling in nanophotonics.