  Graphene is known to possess strong optical nonlinearity. Its nonlinear
response can be further enhanced by graphene plasmons. Here, we report a novel
nonlinear electro-absorption effect observed in nanostructured graphene due to
excitation of graphene plasmons. We experimentally detect and theoretically
explain enhanced nonlinear mixing of near-infrared and mid-infrared light in
arrays of graphene nanoribbons. Strong compression of light by graphene
plasmons implies that the effect is non-local in nature and orders of magnitude
larger than the conventional local graphene nonlinearity. The effect can be
used in variety of applications including nonlinear light modulators, light
multiplexors, light logic, and sensing devices.
