The Optical Properties and Plasmonics of Anisotropic 2D Materials

In the fast growing 2D materials family, anisotropic 2D materials, with their intrinsic in‐plane anisotropy, exhibit a great potential in optoelectronics. One such typical material is black phosphorus (BP), with a layer‐dependent and highly tunable bandgap. Such intrinsic anisotropy adds a new degree of freedom to the excitation, detection, and control of light. Particularly, hyperbolic plasmons with hyperbolic q‐space dispersion are predicted to exist in BP films, where highly directional propagating polaritons with divergent densities of states are hosted. Combined with a tunable electronic structure, such natural hyperbolic surfaces may enable a series of exotic applications in nanophotonics. Herein, the anisotropic optical properties and plasmons (especially hyperbolic plasmons) of BP are discussed. In addition, other possible 2D material candidates (especially anisotropic layered semimetals) for hyperbolic plasmons are examined. This review may stimulate further research interest in anisotropic 2D materials and fully unleash their potential in flatland photonics. Anisotropic intraband and interband transitions in black phosphorus make it an ideal platform to study anisotropic optical and plasmonic properties in the 2D case. Here, recent development of theoretical and experimental studies on the anisotropic optical excitations and hyperbolic plasmons in black phosphorous are discussed, followed by a perspective on other anisotropic 2D materials for hyperbolic plasmons.