Long-range hyperbolic polaritons on a non-hyperbolic crystal surface

Hybridized matter-photon excitations in hyperbolic crystals-anisotropic materials characterized by permittivity tensor components with opposite sign-have attracted substantial attention owing to their strong light-matter interactions in the form of hyperbolic polaritons^1-3. However, these phenomena have been restricted to hyperbolic crystals, whose optical responses are confined to fixed spectral regions and lack tunability, thereby limiting their broader applicability^4,5. Here we demonstrate the emergence of hyperbolic surface phonon polaritons in a non-hyperbolic yttrium vanadate (YVO₄) crystal. Using real-space nanoimaging combined with theoretical analyses, we visualize hyperbolic wavefronts of surface phonon polaritons on YVO₄ crystal surfaces within its non-hyperbolic frequency range, where the permittivity tensor components of the material have the same negative sign. Furthermore, by varying the temperature from room temperature to cryogenic levels, we realize in situ manipulation of polariton dispersions, enabling a topological transition from hyperbolic to canalization and eventually to the elliptic regime. This temperature-controlled dispersion engineering not only provides precise control over polariton topology but also modulates their wavelength and group velocity, showing remarkable sensitivity alongside low-loss, long-range propagation. These findings extend the realm of hyperbolic nano-optics by removing the reliance on hyperbolic crystals, unlocking opportunities for applications in negative refraction^6-10, superlensing^11,12, polaritonic chemistry¹³, integrated photonics^14-16 and beyond.