Non-diffracting polarization features around far-field zeros of electromagnetic radiation

Abstract

Light from any physical source diffracts over space, as spherical wavefronts grow and energy density is spread out. Diffractive effects pose fundamental limits to light-based technologies, including communications, spectroscopy, and metrology. Polarization becomes paraxial in the far-field limit, and, by ignoring longitudinal field components, the rich physics of non-paraxial fields that exist in near-fields or a beam’s tight focus is lost. The longitudinal field cannot, however, be ignored when transverse field components vanish (in a transverse field zero) and carry a small non-paraxial region to infinity. We show that a transverse field zero is always accompanied by non-diffracting polarization structures, whose geometries are independent of the distance to the source, including an enclosing intensity ratio tube, and parallel, non-diverging polarization singularities. We illustrate these features in multipole radiation and in double-slit interference, two examples that have time-fixed transverse field zeros. Non-diffracting structures with a changing position are coupled to time-varying zeros, which are present in all far-field radiation.