  Natural hyperbolic materials (HMs) in two dimensions (2D) have an
extraordinarily high anisotropy and a hyperbolic dispersion relation. Some of
them can even sustain hyperbolic polaritons with great directional propagation
and light compression to deeply sub-wavelength scales due to their inherent
anisotropy. Herein, the anisotropic optical features of 2D natural HMs are
reviewed. Four hyperbolic polaritons (i.e., phonon polaritons, plasmon
polaritons, exciton-polaritons, and shear polaritons) as well as their
generation mechanism are discussed in detail. The natural merits of 2D HMs hold
promise for practical quantum photonic applications such as valley quantum
interference, mid-infrared polarizer, spontaneous emission enhancement,
near-field thermal radiation, and a new generation of optoelectronic
components, among others. These analyses' conclusion outlines existing issues
and potential interesting directions for 2D natural HMs. These findings could
spur more interest in anisotropic 2D atomic crystals in the future, as well as
the quick generation of natural HMs for new applications.
