  We provide a quantitative explanation of the mechanism of the far-field
intensity modulation induced by a nanoparticle in a focused Gaussian laser
beam, as was demonstrated in several recent direct detection studies. Most
approaches take advantage of interference between the incident light and the
scattered light from a nanoparticle to facilitate a linear dependence of the
signal on the nanoparticle volume. The phase relation between the incoming
field and the scattered field by the nanoparticle is elucidated by the concept
of Gouy phase. This phase relation is used to analyze the far-field
signal-to-noise ratio as a function of exact nanoparticle position with respect
to the beam focus. The calculation suggests that a purely dispersive
nanoparticle should be displaced from the Gaussian beam focus to generate a
far-field intensity change.

Optics;Classical Physics