Optical properties and polaritons of low symmetry 2D materials

Low symmetry 2D materials with intrinsic in-plane anisotropic optical properties and high tunability provide a promising platform to explore and manipulate light–matter interactions. To date, dozens of in-plane anisotropic 2D materials with diverse band structures have been discovered. They exhibit rich optical properties, indicating great potential for novel applications in optics, photonics, and optoelectronics. In this paper, we thoroughly review the anisotropic optical properties and polaritons in many kinds of low symmetry 2D materials, aiming to elicit more research interest in this field. First, the optical properties of anisotropic 2D semiconductors, including interband absorption, photoluminescence, excitons, and band structure engineering for tuning optical responses, are introduced. Then fundamentals and advances in experiments of hyperbolic polaritons in anisotropic 2D materials, including phonon, plasmon, and exciton polaritons, are discussed. Finally, a perspective on promising research directions is given.