The field on the Gaussian reference sphere can be expressed as a sum of electric and magnetic multipole components. The electric field at the focus can be maximized by maximizing the electric dipole term.
(050.0050) Diffraction and gratings : Diffraction and gratings
(050.1960) Diffraction and gratings : Diffraction theory
(260.0260) Physical optics : Physical optics
(260.2110) Physical optics : Electromagnetic optics
C. Sheppard, "Polarization and the Focusing of Light," in Frontiers in Optics 2010/Laser Science XXVI, OSA Technical Digest (CD) (Optical Society of America, 2010), paper FWP1.
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