  Strong circularly polarized excitation opens up the possibility to generate
and control effective magnetic fields in solid state systems, e.g., via the
optical inverse Faraday effect or the phonon inverse Faraday effect. While
these effects rely on material properties that can be tailored only to a
limited degree, plasmonic resonances can be fully controlled by choosing proper
dimensions and carrier concentrations. Plasmon resonances provide new degrees
of freedom that can be used to tune or enhance the light-induced magnetic field
in engineered metamaterials. Here we employ graphene disks to demonstrate
light-induced transient magnetic fields from a plasmonic circular current with
extremely high efficiency. The effective magnetic field at the plasmon
resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong
(~1{\deg}) ultrafast Faraday rotation (~ 20 ps). In accordance with reference
measurements and simulations, we estimated the strength of the induced magnetic
field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ
cm-2.
