Focusing of electric-dipole waves

Diffraction of electromagnetic waves through an aperture in a plane screen is discussed with particular emphasis on the focusing properties of a converging electric-dipole wave. Explicit results for the field in the focal region are given both within the Debye and the Kirchhoff approximations. Within the Debye approximation, the results for a focused electric-dipole wave are compared analytically and numerically with those obtained earlier for the focusing of a linearly polarized plane wave by a rotationally symmetric, aplanatic system. It is shown that at high angular apertures, the converging electric-dipole wave has better energy concentration and less cross-polarization, both in the focal plane and along the optical axis.