Hyperbolic materials, featured by the hyperbolic dispersion relations of waves, enable highly directional electromagnetic modes and extreme light confinement beyond the diffraction limit. However, current realizations of hyperbolic materials rely on complex, lossy metamaterials or narrow-band phononic crystals. In this study, we propose copper oxides ${\mathrm{Ba}}_2{\mathrm{CuO}}_x$ as natural hyperbolic materials whose hyperbolic response can be tuned by altering the oxygen stoichiometry. First-principles calculations reveal that ${\mathrm{Ba}}_2{\mathrm{CuO}}_3$, which features one-dimensional (1D) Cu-O chains, behaves as an ideal quasi-1D electron gas and exhibits a broadband, low-loss type-I hyperbolic window (0.13–2.57 eV) with an exceptional quality factor ($Q$ > 50). By contrast, its fully oxidized counterpart ${\mathrm{Ba}}_2{\mathrm{CuO}}_4$, which consists of two-dimensional ${\mathrm{CuO}}_2$ planes, displays broadband type-II hyperbolicity in the range of 0.51–3.43 eV and Lorentz-driven ultraviolet type-I hyperbolicity from 4.46–5.22 eV. We also investigate the directional surface plasmon polaritons arising from the different types of equifrequency contours. These findings offer a promising platform for reconfigurable, low-loss hyperbolic photonics without the need of nanofabrication.

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• Electronic structure
• First-principles calculations
• Plasmons
• Surface plasmon polariton