Far‐Infrared Hyperbolic Phonon–Polaritons in Zirconium Disulfide

ABSTRACT

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 () 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 using a resonator‐assisted far‐field spectroscopy platform. An unpatterned 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 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 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.