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Switch of Optical Nonlinear Absorption in Black Phosphorous

Abstract Nonlinear absorption of multi-layer black phosphorus nanosheets (BPs) were investigated using open aperture Z-scan method with 6 ns nanosecond laser pulses at 532 nm. It was found that, at lower intensities, BPs behave as saturable absorbers, but at higher intensities a shift from saturable absorption to reverse saturable absorption occurs. The transition process was analyzed in terms of the bleaching of ground-state and the excited-state absorption. We investigate the ultrafast dynamics by using pump − probe technique. The investigation implies that BPs has great potential applications in both mode-locking and optical liming in the visible range for ultrafast laser pulses.