Phonon engineering enables hyperbolic asymptotic line polaritons

Advances in polaritonic materials, where coupling between light and matter creates hybrid states, have enhanced our ability to control light propagation at nano and atomic scales. Conventional polariton modulation techniques, particularly topological modulation, are limited by the stringent momentum-matching requirement between light and the material's coupling mode. Here we propose a phonon-engineering strategy that utilizes anisotropic phononic materials in α-MoO₃ to transform circular surface polaritons into hyperbolic asymptotic line polaritons (HALPs) in high-symmetry AlN semiconductors. This approach circumvents the strict requirement for momentum matching via phonon-induced anisotropic Lorentz-type dielectric oscillations. Our system shows broadband modulation of HALP in AlN (~55 cm^-1), achieving an approximate 90° tuning range for the isofrequency contour's open angle. This enables precise phase control for diffraction-free zero-phase propagation. Notably, precise control of atomic isotopes and crystal structure allows further modulation of HALP propagation directions. Our strategy can be generalized to other systems to achieve hyperbolic polaritons in high-symmetry materials.