Configurable topological phonon polaritons in twisted hBN metasurfaces.

Phonon polaritons are hybrid excitations that originate from coupling of photons with optical phonons in polar crystals. Hexagonal boron nitride (hBN) is a representative phonon polariton material in mid-infrared that exhibits long lifetimes and ultraslow propagation. However, due to in-plane isotropic permittivities, the dispersion engineering and highly canalized ray-like propagation along the in-plane surface required in photonic and optoelectronic applications cannot be realized in a bare hBN structure. In this paper, we theoretically investigate phonon polaritons in twisted hBN metasurfaces. Due to interactions between different propagating polaritons in the top and bottom metasurfaces, configurable polaritons can be hybridized. Importantly, the hybridized polariton dispersion can be changed from the hyperbolic type to elliptical type via tuning the twisting angle. The demonstrated steerable dispersion evolution and highly canalized propagating polaritons hold promise for nano-optical applications such as in-plane hyperlensing, waveguiding, and focusing.