Hyperbolic Plasmon Polaritons Propagating in Ultrapure Metal PdCoO₂

Plasmons, the collective motion of free carriers in conducting materials, hold significant potential for applications in quantum microelectronics and quantum optics. Among these phenomena, hyperbolic polaritons, which emerge in layered anisotropic metals due to the contrasting dielectric permittivity between in-plane and out-of-plane directions, facilitate unique optical effects such as negative reflection, diffraction-free propagation, and enhanced photonic density. Layered metallic materials like MgB₂, YBa₂Cu₃O₇, and Sr₂RuO₄ exhibit hyperbolic plasmons across specific frequency ranges due to their anisotropic dielectric tensor elements. Metallic delafossite oxides, including PdCoO₂, PdRhO₂, PdCrO₂, and PtCoO₂, display exceptional anisotropic transport properties, with ab-plane electrical conductivities rivaling those of noble metals like silver and gold. These properties make delafossite oxides prime candidates for low-loss hyperbolic plasmon polaritons.

In this study, we demonstrate that PdCoO₂ (PCO), a metallic delafossite oxide, is an effective hyperbolic medium with minimal plasmonic losses over a broad frequency range extending from the mid-infrared to near-infrared. PCO exhibits the highest ratio of real to imaginary optical conductivity among layered metallic materials, signifying enhanced plasmonic response and reduced dissipation. Using electron energy loss spectroscopy (EELS) combined with monochromated scanning transmission electron microscopy (STEM), we directly observe Fabry-Perot resonances in nanogaps formed from PCO single crystals. Numerical modal analysis confirms that these polaritons are hybridized surface hyperbolic plasmon polaritons. This work highlights the potential of PdCoO₂ and other metallic delafossite oxides as ultrapure hyperbolic plasmonic media for advanced applications in quantum microelectronics, photonics, and optoelectronics.