  In addition to the electric field E(r), the associated magnetic field H(r)
and current density J(r) characterize any electromagnetic device, providing
insight into antenna coupling and mutual impedance. We demonstrate the optical
analogue of the radio frequency vector network analyzer implemented in
interferometric homodyne scattering-type scanning near-field optical microscopy
(s-SNOM) for obtaining E(r), H(r), and J(r). The approach is generally
applicable and demonstrated for the case of a linear coupled-dipole antenna in
the mid-infrared. The determination of the underlying 3D vector electric
near-field distribution E(r) with nanometer spatial resolution and full phase
and amplitude information is enabled by the design of probe tips with
selectivity with respect to E-parallel and E-perpendicular fabricated by
focused ion-beam milling and nano-CVD.
