Mapping Electromagnetic Fields Near a Subwavelength Hole

We study, both experimentally and theoretically, the scattering of electromagnetic waves by a subwavelength hole fabricated in a thin metallic film. We employ the scanning near-field optical microscopy in order to reconstruct experimentally the full three-dimensional structure of the electromagnetic fields in the vicinity of the hole. We observe an interference of all excited waves with an incident laser beam which allows us to gain the information about the wave phases. Along with the well-known surface plasmon polaritons propagating primarily in the direction of the incident beam polarization, we observe the free-space radiation diffracted by the hole. We compare the experimental results with the fields of pure electric and pure magnetic dipoles as well as with direct numerical simulations. We confirm that a single hole in a thin metallic film excited at the normal incidence manifests itself as an effective magnetic dipole in the visible spectral range.