Asymmetric steering of phonon polaritons based on bilayer metagratings

Polaritons in van der Waals (vdWs) materials enable light-matter interactions at the nanoscale. Polaritonic manipulation is of significance in fundamental physics and various promising nanophotonic applications. Here we study the asymmetric steering of phonon polaritons (PhPs) in hexagonal boron nitride (hBN) based on numerical simulations. Empowered by metagratings, the deflection angle of PhPs can be controlled. Furthermore, by employing the combination of metagrating and uniform grating, asymmetric steering of PhPs not only can be achieved but also can be switched to symmetric steering by tuning the period of grating. More intriguingly, an asymmetric dual-functional polaritonic metalens with the function of convergence/divergence for forward/backward incident PhPs is demonstrated. Our work provides insights into the manipulation of polaritons in vdWs materials and a promising strategy for developing nanophotonic devices.