Abstract
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.
Data availability
Relevant data supporting the key findings of this study are available within the article and the Supplementary Information file. All raw data generated during the current study are available from the corresponding authors upon request.
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Acknowledgements
The authors thank O. Lancry (HORIBA) for the acquisition of the Raman spectra, H. Hassani (attocube systems GmbH) for the assistance with the s-SNOM TDS experiments, T. Reck, J. Hesler (VDI Inc.) for their precious help concerning the Schottky diode detection, and C. Boyaval (IEMN) for performing SEM imaging. This work benefited from the financial support of the Agence Nationale de la Recherche projects QUICKTERA (ANR-22-CE09-0018, J.-F.L. and S.E.), HYPSTER (ANR-20-CE42-0016, M.F., B.W., R.P., T.H., C.N.S., J.-F.L., and S.E.), NANOFUTUR (ANR-21-ESRE-0012, M.F.), and TRAPIST (ANR-21-CE24-0011, M.W., C.N.S., and J.-F.L.). T.H. and R.P. thank the ERC-2022-COG grant TUSCany #101089040 for the support. R.O.F. and J.-F.L. acknowledge FAPESP (processes 2022/14245-4 and 2024/09159-7). F.H.F. and R.O.F. acknowledge the FAPESP post-doc project process 2023/09839-5. R.O.F. acknowledges FAPESP Young Investigator process 2019/14019-7 and CNPq grants 309946/2021-2, 300197/2025-0. The authors also thank CPER P4S and CPER Wavetech@hdf and the RENATECH network.
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J.-F.L. proposed and synthesized the material. C.N.S., R.O.F., and J.-F.L. conducted the s-SNOM experiments at IEMN. R.O.F., J.-F.L., and A.C. carried out the s-SNOM TDS measurements at Attocube. C.N.S., M.V., and F.H.F. provided the dispersion curves. F.H.F. performed the real-space polariton wave simulations. C.N.S. and F.H.F. extracted key polariton parameters from the experiments. T.H., S.E., and R.P. conducted the far-field TDS measurements and analysis. B.W. and M.F. developed the long s-SNOM tips. C.N.S., R.O.F., and J.-F.L. co-wrote the manuscript. R.O.F. and J.-F.L. supervised the project. All authors contributed to the discussion of results and the review of the manuscript.
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B.W. and M.F. are cofounders of Vmicro SAS, a company producing AFM and s-SNOM probes. Special probes designed and fabricated by the company were used in this study. This does not compromise the objectivity, integrity, and value of this publication. The remaining authors declare no competing interests.
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Santos, C.N., Feres, F.H., Hannotte, T. et al. High quality-factor terahertz phonon-polaritons in layered lead iodide. Nat Commun (2026). https://doi.org/10.1038/s41467-026-69027-6
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DOI: https://doi.org/10.1038/s41467-026-69027-6
