  In the fast growing two-dimensional (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 band gap. 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. In this review, 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.
