Topological polaritons in natural uniaxial materials.

Surface polaritons in natural materials offer unique opportunities for tailoring light-matter interactions at the nanoscale. While various classes of surface polaritons have recently been unveiled via near-field imaging techniques, their connection to the underlying topological properties of bulk polaritonic bands has remained unexplored. Here, we unveil polaritonic topological nodal rings in unstructured natural materials, arising from the anisotropic coupling between photons and material excitations and supporting topologically nontrivial polaritonic bands. Through topological band theory, we identify drumhead-like surface polariton modes associated with such nodal rings, directly visualize them using scattering-type scanning near-field optical microscopy, and engineer their near-field features by tuning the orientation of the material optical axis relative to the interface. Furthermore, we demonstrate transverse spin-momentum locking, associated with these topological features, enabling control of the propagation direction through the handedness of circularly polarized excitations. Our findings establish an unexplored link between topological semimetals and polaritonic photonics, opening a new pathway for the exploration and application of topological polaritons in natural materials without requiring sophisticated nanofabrication.