Advanced Optical Materials 2302804

Hyperbolic phonon polaritons in optically uniaxial hexagonal boron nitride (hBN) have shown considerable potential for manipulating light on the deep-subwavelength scale, but the inherent in-plane isotropy of hBN hinders their further application in planar polariton meta-optics. Engineering these polaritons to achieve ultraconfined in-plane anisotropic propagation is of scientific significance for facilitating light–matter interactions toward an extreme scale. Here, the polariton features are systematically investigated in a vertically stacked van der Waals heterostructure consisting of hBN and tellurium (Te), a highly birefringent van der Waals crystal in the mid-infrared region. Polaritons in this heterostructure exhibit Te-thickness-dependent anisotropic dispersion, and the configurable in-plane anisotropy and enhanced field confinement are experimentally demonstrated using scanning near-field optical microscopy. It is concluded that these findings contribute to the fundamental understanding of anisotropic polaritons in hBN van der Waals heterostructures, providing a practical avenue for effectively engineering anisotropic PhPs in an intrinsically isotropic van der Waals material.