  We show that graphene possesses a strong nonlinear optical response in the
form of multi-plasmon absorption, with exciting implications in classical and
quantum nonlinear optics. Specifically, we predict that graphene nano-ribbons
can be used as saturable absorbers with low saturation intensity in the
far-infrared and terahertz spectrum. Moreover, we predict that two-plasmon
absorption and extreme localization of plasmon fields in graphene nano-disks
can lead to a plasmon blockade effect, in which a single quantized plasmon
strongly suppresses the possibility of exciting a second plasmon.
