Midinfrared one-dimensional photonic crystal constructed from two-dimensional electride material

Abstract

Dicalcium nitride $({\mathrm{Ca}}_{2}\mathrm{N})$ is a layered material that has been recently identified as a two-dimensional electride, based on first-principles calculations, where the electronic property and the optical complex dielectric function are studied using density functional theory. We find that the optical permittivity tensor is highly anisotropic, and ${\mathrm{Ca}}_{2}\mathrm{N}$ possesses metallic characters when the in-plane dielectric function is negative in midinfrared frequency. In addition, the enhanced midinfrared transmission property and the field distribution of one-dimensional photonic crystals composed of alternating ${\mathrm{Ca}}_{2}\mathrm{N}$ layers and the dielectric material are theoretically studied using the transfer-matrix method. It is found that the ${\mathrm{Ca}}_{2}\mathrm{N}$ photonic crystals support a series of passbands and stopbands the numbers of which increase with the thickness and the permittivity of the dielectrics. The field distributions show that the transmission resonances in the passband are attributed to the coupled Fabry-P\'erot resonances of the individual reactively loaded dielectric slabs. It is also noticed that these resonances lie within certain characteristic frequency bands which are independent of the period of the photonic crystal. The low-frequency edge of the passband is highly tunable by the thickness of the electride material and the dielectric material, the permittivity of the dielectric layer, while the high-frequency edge is insensitive to the electride material thickness. Moreover, the first band gap in lower frequency is almost omnidirectional and polarization insensitive. When defect layers are introduced, the twin defect modes are found, and the frequency and the frequency interval of the two defect modes can be tuned just by changing the permittivity, the thickness and the position of the defect layer, respectively. Polarization and angular insensitive absorption bands can also be obtained for the electride-dielectric photonic crystal with a reflective substrate. These properties of electride material photonic crystals have potential applications in tunable multiband filters and absorbers in the midinfrared region.