We demonstrate that the effective third-order nonlinear susceptibility of a graphene sheet can be enhanced by more than two orders of magnitude by patterning it into a graphene metasurface. In addition, in order to gain deeper physical insights into this phenomenon, we introduce a novel homogenization method, which is subsequently used to characterize quantitatively this nonlinearity enhancement effect by calculating the effective linear and nonlinear susceptibility of graphene metasurfaces. The accuracy of the proposed homogenization method is demonstrated by comparing its predictions with those obtained from the Kramers-Kronig relations. This work may open up new opportunities to explore novel physics pertaining to nonlinear optical interactions in graphene metasurfaces.
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