Optical response of metal or dielectric nano-objects in strongly convergent light beams

We present a general formalism for describing the optical response of spherical nano-objects located in the focus region of a tightly convergent light beam. The theory can be applied to any polarized light beam emerging from axially symmetric focusing optical devices. The approach is based on the generalized Mie theory equations and the expansion of the angular spectrum plane-wave decomposition of the beam in terms of vector spherical harmonics. The numerical efficiency of the formalism is exemplified in the case of reflecting microscope objectives, in the context of single-particle experiments based on the spatial modulation spectroscopy technique, for spherical gold spheres and pairs of nearly touching gold particles.