▎ 摘　　要

The objective of this study is to investigate third-order optical nonlinearities of monolayer and randomly stacked bi- and four-layer graphene samples, depending on wavelength and fluence of femtosecond laser pulses in the near-infrared spectral region. Nonlinear refractive indices and absorption coefficients, and in consequence, third-order susceptibilities of graphene samples, are determined by z-scan measurements at four different wavelengths and three fluence regimes categorized by nonlinear transmission measurements. In addition, nonlinear refractive indices are independently investigated by optical Kerr gate experiments, showing good agreement with z-scan results. Our study on third-order nonlinearities of mono-and multi-layer graphene samples is significant for understanding nonlinear optical characteristics of graphene and developing graphene-based nonlinear photonic devices. (C) 2017 Optical Society of America