  A comprehensive theoretical analysis of photo-induced forces in an
illuminated nanojunction, formed between an atomic force microscopy tip and a
sample, is presented. The formalism is valid within the dipolar approximation
and includes multiple scattering effects between the tip, sample and a planar
substrate through a dyadic Green's function approach. This physically intuitive
description allows a detailed look at the quantitative contribution of multiple
scattering effects to the measured photo-induced force, effects that are
typically unaccounted for in simpler analytical models. Our findings show that
the presence of the planar substrate and anisotropy of the tip have a
substantial effect on the magnitude and the spectral response of the
photo-induced force exerted on the tip. Unlike previous models, our
calculations predict photo-induced forces that are within range of
experimentally measured values in photo-induced force microscopy (PiFM)
experiments.
