The Two‐Dimensional Electrides XONa (X=Mg, Ca) as Novel Natural Hyperbolic Materials

Abstract

In two‐dimensional electrides, anionic electrons are spatially confined in interlayer regions with high density, comparable to metals, and they are highly mobile, just as free electrons, resembling hyperbolic metamaterials with metal‐dielectric multilayered structures. In this work, two‐dimensional electride materials MgONa and CaONa are proposed as good natural hyperbolic materials. By using the first‐principles calculations based on density functional theory (DFT), the electronic structures, stabilities, and optical properties of two‐dimensional electride materials XONa (X=Mg, Ca) are investigated. Our results show that they are stable in 1‐monolayer (1‐ML) structures as well as in bulk states. They exhibit hyperbolic dispersions from visible to near infrared spectral range with high qualities up to about 700, which is two orders‐of‐magnitude larger than the preceding bulk hyperbolic materials. Numerical results reveal that they exhibit negative refraction with low losses. Their high‐quality hyperbolic responses over a wide spectral range pave the way of broad photonic applications as natural hyperbolic materials.

Photonic Topological Transitions and Epsilon-Near-Zero Surface Plasmons in Type-II Dirac Semimetal NiTe$_2$

“…All three surface plasmon peaks arise from the anisotropic dielectric function of NiTe 2 , which in turn originates from the structural anisotropy and the resulting anisotropy in the electronic dispersion. Corresponding to highmomentum surface plasmon polaritons [10], the condition for the existence of a surface plasmon is ξ(ω) = −1. This is exactly satisfied in the ideal case where the imaginary parts of dielectric functions are negligible for ε ⊥ (ω) = 1/ε (ω).…”

“…The second flat region (2.22 < hω < 2.88 eV) arises from the superposition of the resonant surface plasmon at 2.22 eV and the peak in the loss function related to the unusual ENZ response (Reε (ω) = 0) of NiTe 2 . Note that surface plasmons are high-momentum evanescent waves (viz., k ≫ ω/c) [9,10] and, generally, they are difficult to be excited in the standard metamaterials, since the effective medium theory does not hold when the EM momentum approaches the inverse size of the metamaterial inclusions. Thus, the desired metamaterial response can be completely washed out by the spatial scales associated with the metamaterial inclusions.…”

“…On the other hand, some natural materials have been reported to exhibit EM 'meta'-responses overcoming some limits of artificial composite metamaterials. Natural van der Waals crystals, stacked structures of atomically thin layers, hold great promise as an alternative platform to the anisotropic and even hyperbolic metamaterials [9][10][11][12][13][14] exhibiting peculiar optical properties such as negative refraction, strong enhancement of spontaneous emission, and spatial filtering, among others [15][16][17][18][19][20].…”

High‐Harmonic Generation from 2D Monolayer Electrides

“…[47] Among them, 2D electrides have the advantage of low-loss hyperbolic responses. [48][49][50][51] The optical responses of monolayer alkaline earth subnitrides are characterized by free electrons along the in-plane directions and bound electrons along the outof-plane direction, which is quite different from the cases of dielectric and semiconducting monolayers such as transition metal dichalcogenides [12,13] and h-BN. [14,15] The purpose of this work is to study the HHG behavior in monolayer electrides for both out-of-plane and in-plane pumping in comparison with other monolayer materials, mainly focusing on monolayer magnesium subnitride Mg 2 N. The major distinctive feature can be observed for out-of-plane pumping.…”

Polarization-dependent mode coupling in hyperbolic nanospheres

“…Hyperbolic metamaterials are presently an active research field, with investigated topics encompassing spontaneous emission [17], waveguides [18], subdiffraction focusing and imaging [19], and others. This type of dispersion can be found naturally existing van der Waals (vdW) materials [20] such as hexagonal boron nitride (hBN) [21], WTe 2 [22] and current research is directed towards searching for new ones [23]. However, a viable alternative to obtaining hyperbolic dispersion is to use artificial metal-dielectric structures such as continuous film [24] or fishnet-type [25] multilayers or nanorod arrays [18].…”