Giant nonlinear absorption and excited carrier dynamics of black phosphorus few-layer nanosheets in broadband spectra

Layered black phosphorus nanosheets (BPNs) offer potential uses in nanoelectronic and photonics applications. This work demonstrates giant saturable absorption (SA) and two-photon absorption (TPA) of BPNs under femtosecond visible-to-mid-infrared laser-pulse excitation. In addition, the excited carrier dynamics of BPNs were also studied with ultrafast pump-probe technology. Measurements demonstrated that the nonlinear absorption properties of BPNs strongly depend on excitation wavelength and intensity. BPNs moved from SA to TPA under increasing laser intensities at 800, 1160, 1300, and 1550 nm but showed a purely TPA response at 2000 nm, as gauged by measuring BPN nonlinear absorption coefficients. The BPNs showed broadband photon-induced absorption (PIA) signals from 470 to 700 nm, and the excited carrier recombination time was determined to be 150±10  ps. The excitation intensity dependence of excited carrier dynamics were also studied.