Non-Diffracting Polarisation Features around Far-Field Zeros of Electromagnetic Radiation

Light from any physical source diffracts and becomes paraxial in the far field region, where polarisation is virtually transverse to the local propagation direction. A longitudinal polarisation component remains and is insignificant unless the transverse field components vanish. Maxwell's equations show that any such transverse field zero, where the longitudinal component dominates, develops non-paraxial features which do not diffract. Non-diffracting structures, independent of the distance to the source, include a zero-enclosing intensity ratio tube, and parallel, non-diverging polarisation singularities. Remarkably, the polarisation's spatial profile appears to be detached from the radiation's intensity profile. While the intensity spreads out in space at larger distances from the source, the polarisation profile maintains a fixed transverse spatial extent. Numerical examples presented for multipole radiation and phased antenna arrays confirm our findings.