Size-dependent nonlinear optical properties of black phosphorus nanosheets and their applications in ultrafast photonics

Multi-layer black phosphorus nanosheets (BPs) with different sizes are synthesized by using a basic solvent exfoliation method in association with controlled gradient centrifugation. Size-dependent nonlinear saturable absorption and Kerr nonlinearity as well as ultrafast carrier dynamics of BPs is systematically studied by using a Z-scan and pump–probe technique. Furthermore, an ultrashort pulse with a pulse duration of about 635 fs centered at a wavelength of 1562 nm is generated by using smaller sized BPs as a saturable absorber. These results directly reveal the physical processes of size-dependent nonlinear optical (NLO) properties of BPs and provide researchers with a viable approach in tailoring the NLO response of BPs through controlling the sizes, paving the way towards BP based electronics and optoelectronics applications such as ultrafast optical switches, modulators, fiber lasers, etc.