Dual optical hyperbolicity of PdCoO2 and PdCrO2 delafossite single crystals

Hyperbolic materials exhibit a very peculiar optical anisotropywith simultaneously different signs of the dielectric tensor components.This anisotropy allows the propagation of exotic surface-wave excitationslike hyperbolic phonons and plasmon polaritons. While hyperbolic materialshold promise for applications in subwavelength photonics and enhancedlight-matter interactions, their natural occurrence is limited to a few materials,often accompanied by significant dielectric losses and limited hyperbolicspectral bandwidth. Focusing on PdCoO2 and PdCrO2 delafossite transition-metal oxides, in this paper unique dual hyperbolic regimes are demonstrated::one localized around a phonon absorption in the mid-infrared spectral region,and the other extending into the visible range. Both hyperbolic regimesshow exceptional properties including low dissipation and high hyperbolicquality factors. These results pave the way for innovative applications ofdelafossite layered metals in subwavelength photonics, imaging, and sensing.