Contribution of evanescent waves to the far field: the atomic point of view

Evanescent modes of the electromagnetic field are seldom invoked in conventional far-field optics, as their contribution far from the source (a few wavelengths) is negligible. Contradicting this fact, in recent theoretical works, based on a particular decomposition of the free-space Green tensor, it has been asserted that evanescent waves do indeed contribute to the far field, where they appear as an additional ~1/r component of the field. We provide an explicit demonstration that evanescent modes do not contribute to the power radiated to the far field by any dipolar source. First we derive an expression for the free-space field susceptibility in which contributions from evanescent and homogeneous modes are separated, and then we use linear response theory to compute the decay rate for an atomic dipole in vacuum.