Mapping 3D Focal Intensity Exposes the Stable Trapping Positions of Single Nanoparticles

The photonic interactions between a focused Gaussian
laser beam
and a nanoscopic particle are highly dependent on exact particle location
and focal intensity distribution. So far, the 3D focal intensity distribution
and the preferred position of a nanoparticle confined within the focal
region were only theoretically predicted. Here, we directly map the
three-dimensional focal intensity distribution, quantify stable trapping
positions, and prove that certain sizes of nanoparticles stably trap
in front of the focus.