Abstract: A non-linear optical device comprising a non-linear element made of a plasmonic material with a periodic structure having a period smaller than the wavelength of a non-linear process intrinsic to the plasmonic material. The plasmonic material is implemented as a gold film which is structured with a periodic array of asymmetric split ring slits. The metamaterial framework of the plasmonic material itself is used as the source of a strong and fast non-linearity. The cubic non-linear response is resonantly enhanced through the effect of the metamaterial structuring by more than two orders of magnitude and its sign and magnitude can be controlled by varying the metamaterial pattern.