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.

Natural hyperbolic materials have recently attracted great attention due to their capability of supporting spatial mode frequency much higher than artificial metamaterials and the advantage that they do not require nanofabrication processes. For practical applications, however, hyperbolic bulk materials with lower optical losses in shorter wavelength range should be developed. This work presents the electronic structure and dielectric response of an electride Mg2N, revealing that this material exhibits hyperbolic responses with low optical loss in the visible and plasmonic responses with high‐quality in the near‐infrared range. Negative refraction in the red spectral range has been analytically and numerically demonstrated. In particular, nanoantenna structures of Mg2N generate strong plasmonic resonances in the near‐infrared and the intensity enhancement in the gap region is one order of magnitude higher compared with silver nanoantenna due to its much higher quality factor, which can find potential applications for nanoplasmonic purposes such as single molecule detections by surface‐enhanced hyper‐Raman spectroscopy and nonlinear wavelength generations at the nanoscale.

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This work reports high-harmonic generation (HHG) from 2D monolayer electrides investigated by using ab-initio approach based on time-dependent density functional theory, mainly focusing on monolayer Mg 2 N. Due to low ionization energy and wide potential well, they exhibit distinctively high HHG efficiency and high cutoff energy for out-of-plane pumping, although they also show atomic-like behavior. For out-of-plane pumping with a peak intensity of 40 TW cm −2 at 1550 nm, high harmonics up to 120th order are generated with efficiency 4 orders-of-magnitude higher than monolayer h-BN. For pump intensity below the direct above-threshold ionization (ATI) regime, the tunneling from the ground state and ATI from one-or two-photon resonant state simultaneously contribute to HHG. In the other subnitride monolayers of alkaline earth with larger atomic sizes (Ca 2 N and Sr 2 N), further higher harmonic generation efficiencies are obtained due to the lower work functions and wider potential wells. For in-plane pumping, the intraband transition is the major source of the HHG due to the metallic in-plane responses. The presented results will attract great attention due to the peculiar HHG signature from fundamental aspects and pave the way of obtaining coherent extreme ultraviolet (EUV)/soft-X-ray with high efficiency by using monolayer electrides.

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