Optical properties of black phosphorus

Black phosphorus is a newly rising layered material with tremendous and intriguing physical, chemical, and electronic properties. Recently, it was reintroduced into the community from the perspective of two-dimensional (2D) materials, showing excellent carrier mobility, strong light–matter interactions in the mid-infrared (mid-IR) range, and unique anisotropic physical properties. These features are beyond the existing 2D materials in terms of bandgap range and mobility/bandgap trade-off, making it possible to bridge the application gaps ranging from mid-IR photonics to high-performance field-effect devices. In this paper, we review the status and perspectives of the optical properties of black phosphorus, with a particular emphasis on their broadband tunable bandgap and anisotropic optical properties. We began with a brief introduction on the history, synthesis, crystal/band structures, and basic physical properties of black phosphorus. In Section 3, we discuss the optical properties of black phosphorus, including comparison of the electronic-excitation-induced versus phonon-excitation-induced optical properties, linear versus nonlinear absorption, and static versus dynamic photoresponse, as well as the quasi-particle effect versus the effect. After that, we exemplify typical black phosphorus photonic devices in Section 4. We highlight recent progress in photodetectors covering mid-IR and terahertz wavelength, ultrafast Q-switching and mode-locking pulse lasers and nanophotonic devices. Finally, we present a summary and outlook of black phosphorus regarding current challenges and future application opportunities in Section 5.