Broadband hyperbolic plasmons in aluminum disulfide monolayer and its analogues: The role of orbital anisotropy

Two-dimensional (2D) materials with high in-plane anisotropy offer promising candidates for hosting highly directional surface plasmons, but few of them have been demonstrated to reach this goal thus far. In this paper, we propose a design principle of 2D hyperbolic materials (2D-HMs) based on orbital anisotropy and predict from first-principles calculations a family of 2D-HMs: aluminum disulfide monolayer and its analogues $X{Y}_{2}$ $(X=\mathrm{Al},\mathrm{Ga},\mathrm{In};Y=\mathrm{S},\mathrm{Se},\mathrm{Te})$. These natural 2D-HMs exhibit broadband hyperbolic regimes across the near-infrared to ultraviolet spectrum, enabling the propagation of highly directional hyperbolic surface plasmons. Undamped plasmons emerge along the $x$ direction with the maximum wave vector of $\ensuremath{\sim}0.16\phantom{\rule{0.16em}{0ex}}{\AA{}}^{\ensuremath{-}1}$ and frequency of 4.6 eV, whereas the plasmons along the $y$ direction have low frequency $(<0.6\phantom{\rule{0.16em}{0ex}}\mathrm{eV})$ and decay rapidly to electron-hole pairs for the $\mathrm{Al}{\mathrm{S}}_{2}$ monolayer. By solving Maxwell's equation, we simulate the directional propagation of the surface waves with hyperbolic dispersion relations. We correlate the fascinating plasmonic properties with the unique electronic structures of these highly anisotropic 2D materials, which offers a promising strategy for the design of 2D-HMs.