Purely coherent nonlinear optical response in solution dispersions of graphene sheets.

We have developed an efficient chemical exfoliation approach for the high-throughput synthesis of solution-processable, high-quality graphene sheets that are noncovalently functionalized by alkylamine. Purely coherent nonlinear optical response of these graphene sheets has been investigated, using near-infrared, visible, and ultraviolet continous wave and ultrafast laser beams. Spatial self-phase modulation has been unambiguously observed in the solution dispersions. Our results suggest that this coherent light scattering is due to a broadband, ultrafast, and remarkably huge third-order optical nonlinearity χ(3), which is a manifestation of the graphene's cone-shaped large-energy-scale band structure. Our experimental findings endow graphene new potentials in nonlinear optical applications.