Natural hyperbolicity of hexagonal boron nitride in the deep ultraviolet.

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.