High quality-factor terahertz phonon-polaritons in layered lead iodide.

While hyperbolic phonon-polaritons in van der Waals materials such as h-BN and α-MoO3 have driven major advances in mid-infrared (IR) nanophotonics, further progress at longer THz wavelengths has been hampered due to material limitations and experimental challenges. Here, we report the discovery of long-lived hyperbolic phonon-polaritons in the deep THz range in layered PbI2. Using room-temperature scattering-type scanning near-field optical microscopy, we achieved real-space imaging and broadband spectral analysis of PbI2 2D crystals transferred onto different substrates with high near-field amplitude contrast and good agreement with theoretical models. Our measurements revealed an experimental figure-of-merit related to the propagating efficiency of the polaritons above 15-on par with state-of-the-art mid-IR benchmarks-and extreme field confinement of 264 for a 144 nm-thick flake, which can exceed 300 in slightly thinner samples. These findings demonstrate that PbI2 combines strong anisotropy, low losses, and extreme mode confinement, making it a compelling candidate for deep-THz nanophotonic applications.