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 30 degrees, 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.