  Recently developed plasmonic nanostructures are able to generate intense and
localized magnetic hot spots in a large spectral range from the terahertz to
the visible. However, a direct measurement of the magnetic field at the hot
spot has not been performed yet, due to the absence of magnetic field detectors
that work at those high frequencies and that fit the hot-spot area. We propose
to place a graphene ribbon in the hot spot of a plasmonic nanostructure driven
by a laser beam, such that a current is generated due to both the magnetic
field at the hot spot and the electric field of the laser. We demonstrate that
a nonlinear Hall voltage, which can be measured by standard electrical means,
builds up across the ribbon, making it possible to directly probe the magnetic
field at the hot spot.
