Tunable anisotropic plasmons in monolayer Ca₄N₂ induced by orbital-selective transitions

Plasmons in two-dimensional (2D) materials have attracted considerable interest due to their ability to confine light at subwavelength scales. Anisotropic 2D materials, in particular, offer unique opportunities for directional control over plasmon propagation and light-matter interactions. In this study, employing first-principles calculations, we demonstrate that monolayer Ca 4 N 2 can host tunable anisotropic plasmon modes. The electronic band structure of Ca 4 N 2 exhibits pronounced anisotropy, characterized by a pair of saddle-like points. The spatial symmetries of the Bloch wave functions enable orbital-selective interband transitions between these points, which are allowed along the y -direction but forbidden along the x -direction. The anisotropy of plasmons can be enhanced (or diminished) by improving (or reducing) the electron chemical potential, leading to the topological transition of surface plasmon polaritons among elliptical, hyperbolic and isotropic wavefronts. These findings deepen our understanding of anisotropic plasmon behaviors in 2D materials and provide a potential pathway for designing highly tunable plasmonic devices.