Preparation of Black Phosphorus Nanosheets and Research on Nonlinear Optical Response Characteristics

2D black phosphorus nanosheets (BPNSs) are prepared by the ultrasonic-assisted liquid phase stripping method, which improves the preparation yield and monodispersity of the thin-layer BPNSs. Open-aperture Z-scan measurement by nanosecond pulsed laser with low repetition rate (10 Hz) is used to reduce the existence and accumulation of heat accumulation effect. BPNSs have strong saturated absorption in the wideband visible light range from 475 to 625 nm; and with the increase of laser energy, reverse saturated absorption (RSA) occurs at some specific wavelengths in this range. The reverse saturation absorption is observed at 500 nm; and the stronger the excited state absorption is with the increase of laser energy, the more obvious the reverse saturation absorption is. At 525 nm, as the laser excitation energy increases from 750 to 1080 mu J, a transition from saturated absorption to anti-saturated absorption is observed. This shows that laser excitation energy is an important factor in the transition from saturated absorption to anti-saturated absorption. That is to say, by controlling the laser excitation energy and wavelength, BPNSs can have different application values in the field of nonlinear optics, such as saturable absorbers, optical limiters, and optical switches.