Abstract
Hyperbolic media enable unique optical phenomena including hyperlensing, negative refraction, enhanced photonic density of states (PDOS), and highly confined polaritons. While most hyperbolic media are artificially engineered metamaterials, certain natural materials with extreme anisotropy can exhibit hyperbolic dispersion. Here, based on experimental evidence and theoretical fitting estimates to the experimental data, we suggest the presence of natural hyperbolic dispersion in hexagonal boron nitride (hBN) in the deep-ultraviolet (DUV) regime, induced by strong, anisotropic exciton resonances. Using all-optical imaging spectroscopic ellipsometry (ISE), we characterize the complex dielectric function along in-plane and out-of-plane directions down to 190 nm (6.53 eV), revealing a potential type-II hyperbolic window in the DUV regime. We predict that hyperbolicity supports hyperbolic exciton polaritons (HEP) with high directionality and slow group velocity, as confirmed by numerical calculations. Our findings suggest hBN as a platform for nanophotonic applications in the technologically significant DUV spectral range.
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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
D. J., B.C., and J.L. acknowledge support from the Office of Naval Research Young Investigator Award, Metamaterials Program (N00014-23−1-2037). This work was carried out in part at the Singh Center for Nanotechnology, which is supported by the NSF National Nanotechnology Coordinated Infrastructure Program under Grant NNCI-2025608. J.K. acknowledges the support from the Institute for Basic Science (IBS), Korea under Project Code IBS-R014-A1 and the National Research Foundation of Korea grants (NRF-2023R1A2C2007998). N. E. acknowledges partial support from the US Air Force Office of Scientific Research (AFOSR) Multidisciplinary University Research Initiatives (MURI) program with grant # FA9550-21−1-0312 and AFOSR grant # FA9550-23-1-0307.
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D.J. supervised and acquired funding for the project. D.J. and B.C. conceived and designed the experiment. B.C. performed the ellipsometry measurements and conducted the data fitting. B.C. performed the optical simulations and calculations in discussion with D.J., N.E., and J.L. W.C. fit preliminary ellipsometry data. K.Y. fabricated samples. S. J. and H.C. performed the reflectance measurement under the supervision of J.K., B.C., and D.J. wrote the manuscript with inputs from all authors. All authors discussed the results and revised the manuscript.
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Choi, B., Lynch, J., Chen, W. et al. Natural hyperbolicity of hexagonal boron nitride in the deep ultraviolet. Nat Commun (2026). https://doi.org/10.1038/s41467-026-69536-4
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DOI: https://doi.org/10.1038/s41467-026-69536-4
