Hyperbolic Plasmon Polaritons in the Delafossite PdCoO2

Hyperbolic plasmon polaritons are uncommon light-matter modes in materials at the extreme of anisotropy - where metallic and dielectric characters coexist along different crystallographic directions. Using Kramers-Kronig analysis and finite-difference time-domain simulations, we have identified a new quasi-two-dimensional material, palladium cobaltate (PdCoO₂), that is expected to support low-loss hyperbolic plasmon polaritons at infrared frequencies. Simulations are consistent with the polariton dispersion calculated from the complex reflection coefficient. The large optical anisotropy can be recognized by the order of magnitude difference between the in-plane and out-of-plane plasma frequencies. Scanning near-field optical microscopy can be used to study these light-matter modes. However, because hyperbolic polaritons travel inside the material and reflect off the surfaces, observing them requires a thin layer (50-150 nm) of PdCoO₂ with smooth surfaces. This presents a challenge to an experimental demonstration of hyperbolic modes in PdCoO₂ since the material does not exfoliate readily from flux-grown single crystals and forms twin domains in films grown by molecular beam epitaxy. Efforts to isolate flat, thin flakes of PdCoO₂ for near-field measurements will be presented.