Two-photon absorption and negative nonlinear refraction in few-layer graphene nanosheets suspensions via liquid-phase exfoliation

In this paper, the nonlinear optical properties of few-layer graphene nanosheets in solution were investigated by the Z-scan technique and femtosecond laser pulses @ 800 nm, using different laser intensities in the ∼0.17−0.22GW/cm 2 range. Few-layer graphene nanosheets were synthesized by the liquid-phase exfoliation method using a mixture of deionized water and Triton X-100, obtaining average sizes of ∼183nm. The nonlinear absorption obtained by an open aperture Z-scan setup showed a two-photon absorption effect compared to those reported in the literature, where the saturable absorption effect is dominant, e.g., in graphene synthesized by chemical vapor deposition and graphene oxide. In contrast, the nonlinear refraction measured by a close aperture setup revealed a negative lens-like (self-defocusing) behavior in all cases. The calculated effective nonlinear absorption coefficient ranged from 2.41×10 −9 to 7.83×10 −9 cm/W, while the nonlinear refraction index was −6.52×10 −13 to −5.01×10 −13 cm 2 /W. We found that as the laser intensities increased, the effective nonlinear absorption coefficient and the absolute value of the nonlinear refraction index also increased. In accordance with the obtained results, we propose few-layer graphene nanosheets for applications where two-photon absorption has significant advantages over conventional one-photon absorption.