Investigation of Saturable and Reverse Saturable Absorptions for Graphene by Z-Scan Technique

The optical nonlinear absorption (NLA) property of multilayer graphene was investigated by Z-scan technique with picosecond laser pulse of 532-nm wavelength. Graphene samples were fabricated by chemical vapor deposition (CVD) with different number of layers, which were 1-, 8-, and 16-layer. The dependence of NLA coefficient α(I) on number of layers was investigated under the laser intensity of 7-80 GW/cm
²
. The results showed that α(I) increased as the number of layer increased, and decreased as the laser intensity increased. Nonlinearities of graphene were mainly caused by saturable absorption (SA) effect. The 1-, 8-, and 16-layer graphene exhibited SA nonlinearity. However, reversed SA (RSA) nonlinearity was observed for 16-layer graphene only. The RSA was resulted from two-photon absorption due to layer stacking-induced bandgap opening, confirmed by the α(I) measurement. The RSA may reduce the stability of mode-locking and therefore, it suggests that the lower layer stacking in CVD fabricated graphene is preferable to use as saturable absorbers in laser mode-locking.