Far-Infrared Hyperbolic Phonon-Polaritons in Zirconium Disulfide

Group-IVB transition-metal dichalcogenides (TMDs) have recently emerged as a promising material platform for extreme light confinement, with Hf-based compounds demonstrating confinement factors exceeding two orders of magnitude in the far-infrared. As a complementary Zr-based member of this material family, zirconium disulfide ( ${\mathrm{ZrS}}_2$ ) combines a comparably broad first Reststrahlen band with semiconducting electronic character, providing a wide spectral window for phonon-dominated far-infrared hyperbolic polaritonics. Here, we report the first experimental demonstration of far-infrared hyperbolic phonon polaritons in the group-IVB TMD ${\mathrm{ZrS}}_2$ using a resonator-assisted far-field spectroscopy platform. An unpatterned ${\mathrm{ZrS}}_2$ flake integrated with a metallic ribbon array forms a phonon polariton resonator, enabling efficient far-field excitation of phonon polaritons while suppressing extrinsic scattering losses. This high coupling efficiency enables far-field observation of multiple polaritonic resonances beyond the fundamental branch. The large normalized light-matter coupling strength of ${\mathrm{ZrS}}_2$ enables ultrahigh in-plane momenta, with effective refractive indices as high as 223. Despite this extreme confinement, linewidth analysis indicates that the measured damping is primarily governed by intrinsic propagation loss, corresponding to a sub-picosecond polariton lifetime. These results establish ${\mathrm{ZrS}}_2$ as a van der Waals hyperbolic material platform for ultraconfined far-infrared phonon polaritons and highlight the potential of group-IVB TMDs for compact far-infrared nanophotonic and thermal photonic applications.