Natural Hyperbolicity of Hexagonal Boron Nitride in the Deep Ultraviolet

TL;DR

This paper reports the first observation of natural hyperbolic dispersion in hexagonal boron nitride within the deep-ultraviolet range, enabled by anisotropic exciton resonances, opening new avenues for nanophotonics.

Contribution

It demonstrates natural hyperbolic behavior in hBN in the DUV, characterized by spectroscopic ellipsometry, revealing a hyperbolic window and supporting hyperbolic exciton polaritons.

Findings

Identification of a type-II hyperbolic window in hBN in the DUV range

Observation of hyperbolic exciton polaritons with high directionality

Establishment of hBN as a platform for DUV nanophotonics

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, we report the first observation of natural hyperbolic dispersion in hexagonal boron nitride (hBN) in the deep-ultraviolet (DUV) regime, induced by strong, anisotropic exciton resonances. Using 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 type-II hyperbolic window in the DUV regime. This hyperbolicity supports hyperbolic exciton polaritons (HEP) with high directionality and slow group velocity. Our findings establish hBN as a promising platform for nanophotonic applications in the technologically significant DUV spectral range.