We experimentally demonstrate a negative Kerr nonlinearity for quasi-undopedgraphene. Hereto, we introduce the method of chirped-pulse-pumped self-phasemodulation and apply it to graphene-covered silicon waveguides at telecomwavelengths. The extracted Kerr-nonlinear index for graphene equals n2,gr =-10^(-13) m^2/W. Whereas the sign of n2,gr turns out to be negative in contrastto what has been assumed so far, its magnitude is in correspondence with thatobserved in earlier experiments. Graphene's negative Kerr nonlinearity stronglyimpacts how graphene should be exploited for enhancing the nonlinear responseof photonic (integrated) devices exhibiting a positive nonlinearity. It alsoopens up the possibility of using graphene to annihilate unwanted nonlineareffects in such devices, to develop unexplored approaches for establishing Kerrprocesses, and to extend the scope of the "periodic poling" method often usedfor second-order nonlinearities towards third-order Kerr processes. Because ofthe generic nature of the chirped-pulse-pumped self-phase modulation method, itwill allow fully characterizing the Kerr nonlinearity of essentially any novel(2D) material.
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