Scattering of the evanescent components in the angular spectrum of a tightly focused electromagnetic beam by a spherical particle

•Calculated beam shape coefficients of an evanescent plane wave.•Calculated beam shape coefficients of the evanescent portion of an arbitrary beam.•Predicted when the evanescent portion of a beam could contribute to scattering.

In many previous studies where a tightly focused beam is scattered by a spherical particle, either the experimental conditions were such that evanescent components were absent from the angular spectrum of the incident beam, or if they were present, it was assumed that their contribution to scattering was small with respect to that of the oscillatory components, and could safely be ignored. In this paper the contribution of the evanescent components is explicitly calculated and the validity of their neglect in various situations is assessed. It is demonstrated that when a particle whose size is comparable to the wavelength is located near the plane in which the strength of the evanescent components is maximized, the angular spectrum components just inside the evanescent regime, when added to the contribution of the oscillatory components, can possibly make a significant contribution to the shape coefficients of the incident beam.