Resonant enhancement of nonlinear absorption and ultrafast dynamics of WS 2 nanosheets

The comparative study on nonlinear absorption of WS 2 nanosheets was performed by using nanosecond Z-scan measurements at 532 nm and 500 nm. It was found that, WS 2 nanosheets exhibit saturable absorption (SA) at low excitation intensities, and the saturable intensity at 500 nm is much stronger than that at 532 nm, which implies that resonance plays main role. With the increase of incident intensity, a switch from SA to reverse saturable absorption (RSA) occurs. Besides, ultrafast dynamics of WS 2 nanosheets were investigated by using femtosecond pump-probe technique. Photo-dynamics process was found to contain a double exponential energy relaxation with a fast decay component (14 ps) and a slow one (145 ps).