Enhanced near-field coupling and tunable topological transitions in hyperbolic van der Waals metasurfaces for optical nanomanipulation.

Hyperbolic metasurfaces based on van der Waals (vdW) materials support propagation of extremely anisotropic polaritons towards nanoscale light compression and manipulation, and thus have great potential in the applications of planar hyperlenses, nanolasing, quantum optics, and ultrasensitive infrared spectroscopy. Two-dimensional hexagonal boron nitride ( h -BN) subwavelength gratings as vdW metasurfaces can manipulate the propagation of hyperbolic polaritons at the level of single atomic layers, possessing a higher degree of field confinement and lower losses than conventional media. However, active manipulation of hyperbolic polaritonic waves in h -BN midinfrared metasurfaces remains elusive. Herein, we provide an effective strategy for tunable topological transitions in mid-infrared hyperbolic vdW metasurfaces (HMSs) via enhanced plasmon-phonon polaritons coupling. They are composed of in-plane heterostructures of thin-layer h -BN and monolayer graphene strips (iHBNG) as meta -atoms. The graphene-plasmon-enhanced near-field coupling enables a large tunability of light fields by tailoring the chemical potentials of graphene without frequency shift, which involves topological transitions of polaritonic modes, unidirectional polariton propagation, and local-density-of-state enhancement. Simulated visual near-field distributions of iHBNG metasurfaces reveal the unique transformations of hyperbolic polariton propagations, distinguished from that of individual h -BN and graphene metasurfaces. Our findings provide a platform of optical nanomanipulation towards emerging on-chip polaritonic devices.