Surface plasmons induce topological transition in graphene/α-MoO3 heterostructures

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.

Automated summary

The authors demonstrate a topological transition of polariton wavefront induced by plasmon-phonon hybridization in graphene/alpha-MoO3 heterostructures, which offers the possibility to explore the topological properties of hyperbolic phonon polaritons. The transition can be achieved by tuning the hybrid modes from surface waves to bulk waveguide modes.