Near field spectral dispersion of dipole radiation: The maximum red and blue shifts, and the near and far field boundary

Abstract

Spectral dispersion of dipole radiation in near field zone is studied. Spectral changes are demonstrated using polychromatic incident light of Gaussian spectral profile as test examples. Both red and blue shifts are possible. The maximum redshift can reach upon −20% of the original peak frequency ω0. It is found that there is a boundary at r=0.175λ0 separates the red and blue shift zones. The dipole radiation can be decomposed in two parts, evanescent and propagating parts. The dispersion is studied for the whole radiation, as well as for evanescent and propagating parts. It is discovered that the boundary separates red and blue shift corresponds to the point where the propagating part becomes dominate. Therefore it is proposed to also call the boundary the near and far field boundary. In addition, the spectral changes are different for different dipole orientations, which can be essential for applications of near field antennas.