Dispersion Engineering of In‐Plane Anisotropic Phonon Polaritons in hBN/Te van der Waals Heterostructures

Abstract 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.