The pump fluence and wavelength-dependent ultrafast carrier dynamics and optical nonlinear absorption in black phosphorus nanosheets

Pump fluence and probe wavelength-dependent ultrafast carrier dynamics and optical nonlinear absorption in black phosphorus nanosheets are investigated by transient absorption spectroscopy and open-aperture Z scan techniques. The decay time becomes longer with larger wavelengths under pump wavelengths of both 400 nm and 800 nm excitation. For 800 nm excitation, pump fluence-dependent lifetime shows complex behaviors, which might be due to the competition between the linear absorption and two photon absorption. For 400 nm excitation, an additional decaying channel is observed at a larger pump fluence, which is explained by an effective subband structure. In open-aperture Z scan measurements, strong saturation absorption is observed in the visible region over a broad band from 450 nm to 700 nm. The saturation intensity shows an increasing trend with increase in the wavelength. Also, the saturation intensities under different pulse widths and solvents are discussed in detail. Our results show that black phosphorus nanosheets have great potential in future ultrathin optoelectronic devices.