Focusing of mixed-dipole waves

A converging mixed-dipole wave is defined as the sum of a converging electric-dipole wave and a converging magnetic-dipole wave. In this paper we study the focusing properties of converging mixed-dipole waves for the case in which the two dipoles are orthogonal. Within the Debye approximation, we compare both analytical and numerical results for the converging mixed-dipole wave with those of the converging electric-dipole wave and also with the results obtained when a linearly polarized plane wave is focused by a rotationally symmetric, aplanatic system. At large angular apertures, the converging electric-dipole wave is shown to give best electric-energy concentration and least cross-polarization, both in the focal plane and along the optical axis. At angular half-apertures less than , the three types of converging waves give indistinguishable electric and total energy distributions around the focus. In applications in which one wishes to maximize the electric energy density and/or minimize the cross-polarization the converging electric-dipole wave is the natural choice as the incident wave.