Stoichiometry-Modulated Dual Epsilon-Near-Zero Characteristics Of Niobium Nitride Films

Metal nitride is an important new type of Epsilon-near-zero (ENZ) materials applicable in visible and near infrared. In this study, NbNx films with different nitrogen content R-n were prepared by magnetron sputtering. The results show that the films are stoichiometry-tunable on the lattice constant, chemical state, and plasmonic properties. Importantly, NbNx films exhibit dual ENZ behavior tunable in the range of 400 to 1000 nm, and the absolute value of the negative real part of epsilon is below 1.0. Increased R-n reduces the screened plasma frequency and narrows the ENZ range by introducing different non-stoichiometric defects. The result of first-principle calculation show that non-stoichiometric defects affect the band structure and the density of states, and the cation vacancies are the main defects tailoring the ENZ behavior of the NbNx films. As examples of the application, we provide conceptual designs of angular filter and perfect absorber, the performances of which can be enhanced by NbNx films. NbNx is proved to be an ENZ material with simple composition, easy preparation and high tunability in a wide range. The tunable dual ENZ characteristics of NbNx films highlight their significant future in visible and near infrared nanophotonic applications.