▎ 摘　　要

In this work, the nonlinear refractive properties of graphene suspension at different concentration levels are investigated by means of Z-scan technique with continuous wave low power laser beams at 532 nm and 635 nm. Results show that the dispersion of graphene in water exhibits negative nonlinear refractive index in continuous wave regime, which is mainly attributed to thermal nonlinear effect. We find that the absorption coefficient and nonlinear refractive index of the samples increase with increasing the concentration level of the graphene. Moreover, it is obtained that the nonlinear refractive index of the samples at different concentration levels depends on the wavelength of laser beam.