Hyperbolic Dispersion and Low-Frequency Plasmons in Electrides

TL;DR

This paper demonstrates that non-cubic electrides are natural hyperbolic materials with broad infrared to ultraviolet hyperbolic dispersion, challenging the previous belief that structural anisotropy is necessary for such properties.

Contribution

It reveals that charge localization in electrides enables hyperbolic dispersion without structural anisotropy, expanding the class of natural hyperbolic materials.

Findings

Electrides exhibit low plasma frequencies and broad hyperbolic windows.

In-plane hyperbolic dispersion is observed in some two-dimensional electrides.

Anisotropic interband transitions contribute to hyperbolic behavior in semiconductor electrides.

Abstract

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.