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, NbN_x 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, NbN_x 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 NbN_x films. As examples of the application, we provide conceptual designs of angular filter and perfect absorber, the performances of which can be enhanced by NbN_x films. NbN_x 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 NbN_x films highlight their significant future in visible and near infrared nanophotonic applications.