Determination of electric-field, magnetic-field, and electric-current distributions of infrared optical antennas: a near-field optical vector network analyzer.

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 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 midinfrared spectral region. 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(∥) and E(⊥) fabricated by focused ion-beam milling and nano-chemical-vapor-deposition methods.