▎ 摘　　要

Hyperbolic phonon polaritons - mixed states of photons and anisotropic lattice vibrations - offer appealing properties for nanophotonic applications. Here, the authors show that the plasmon-phonon hybridization upon electronic doping in graphene/alpha-MoO3 heterostructures can induce topological transitions of the polariton wavefront. Polaritons in hyperbolic van der Waals materials-where principal axes have permittivities of opposite signs-are light-matter modes with unique properties and promising applications. Isofrequency contours of hyperbolic polaritons may undergo topological transitions from open hyperbolas to closed ellipse-like curves, prompting an abrupt 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 graphene/alpha-MoO3 heterostructures. Scanning near-field optical microscopy was used to image hybrid polaritons in graphene/alpha-MoO3. We demonstrate the topological transition and characterize hybrid modes, which can be tuned from surface waves to bulk waveguide modes, traversing an exceptional point arising from the anisotropic plasmon-phonon coupling. Graphene/alpha-MoO3 heterostructures offer the possibility to explore dynamical topological transitions and directional coupling that could inspire new nanophotonic and quantum devices.