Observation of Hyperbolic Plasmon Polaritons in PdCoO ₂ using Monochromated Electron Energy Loss Spectroscopy

Hyperbolic plasmon polaritons, which propagate in materials with a hyperboloid-shaped isofrequency surface, exhibit unique optical phenomena such as negative reflection, diffraction-free propagation, and an enhanced photonic density of states. The metallic delafossite oxides have been considered promising low-loss hyperbolic plasmonic media due to their exceptionally high conductivity and anisotropic transport characteristics. However, the direct observation of hyperbolic polaritons in these materials using nanoscale spectroscopy techniques has not been achieved.

In this study, we employ monochromated electron energy loss spectroscopy (EELS) within scanning transmission electron microscopy (STEM) to observe Fabry-Perot resonances in finite-length nanogaps fabricated on metallic delafossite oxide PdCoO ₂ single crystals. Through orientation-dependent STEM-EELS experiments, we confirm the hyperbolic nature of the plasmon polaritons. Numerical modal analysis further reveals that the detected photonic modes are hybridized hyperbolic surface plasmon polaritons. This work uncovers a new optical property of metallic delafossites and demonstrates the potential of monochromated STEM-EELS as a powerful tool to explore the characteristics of plasmon polaritons.