Visible-NIR-Frequency Hyperbolic Response in Nodal-Line Semimetal PbTaSe$_2$

Abstract

Natural hyperbolic materials offer a powerful platform for light-matter interactions by supporting highly anisotropic electromagnetic modes without the need for artificial patterning. In this work, we experimentally demonstrate that the nodal-line semimetal PbTaSe$_2$ exhibits robust hyperbolic optical behavior in the visible to near-infrared spectral range, which arises intrinsically from its anisotropic electronic structure and layered crystal symmetry. By combining first-principles calculations, ellipsometry, Drude-Lorentz modeling, and reflectance measurements, we establish a consistent experimental and theoretical picture of bulk hyperbolicity in this material. This hyperbolicity is of plasmonic origin and is characterized by a competitive quality factor ($Q_\mathrm{max} \approx 2.8$) and a very large anisotropy parameter ($|R| \approx 231$) at 0.78 eV.