Hyperbolic dispersion and low-frequency plasmons in electrides

Natural hyperbolic materials have attracted significant interest in the field of photonics due to their unique optical properties. Based on the initial successful explorations on layered crystalline materials, hyperbolic dispersion was associated with extreme structural anisotropy, despite the rarity of natural materials exhibiting this property. Here, we show that non-cubic electrides are generally promising natural hyperbolic materials owing to charge localization in interstitial sites. This includes elemental and binary electrides, as well as some two-dimensional materials that show prominent in-plane hyperbolic dispersion. They exhibit low plasma frequencies and a broad hyperbolic window spanning the infrared to the ultraviolet. In semiconductor electrides, anisotropic interband transitions provide an additional mechanism for hyperbolic behaviour. These findings remove the previously held prerequisite of structural anisotropy for natural hyperbolic materials and open up new opportunities, which might change the current strategy for searching and design photonic materials. Natural hyperbolic materials have attracted interest in photonics and until now were sought mainly in crystals with strong structural anisotropy. Here the authors propose that many noncubic electrides, owing to charge localization in interstitial sites, can support hyperbolic dispersion without depending on pronounced structural anisotropy.