  We demonstrate an experimental method to separately test the optical response
of a single sub-wavelength nano-structure to tailored electric and magnetic
field distributions in the optical domain. For this purpose a highly focused
y-polarized TEM10-mode is used which exhibits spatially separated longitudinal
magnetic and transverse electric field patterns. By displacing a single
sub-wavelength nano-structure, namely a single split-ring resonator (SRR), in
the focal plane, different coupling scenarios can be achieved. It is shown
experimentally that the single split-ring resonator tested here responds
dominantly as an electric dipole. A much smaller but yet statistically
significant magnetic dipole contribution is also measured by investigating the
interaction of a single SRR with a magnetic field component perpendicular to
the SRR plane (which is equivalent to the curl of the electric field) as well
as by analyzing the intensity and polarization distribution of the scattered
light with high spatial resolution. The developed experimental setup as well as
the measurement techniques presented in this paper are a versatile tool to
investigate the optical properties of single sub-wavelength nano-structures.
