Optical tweezers manipulate microscopic objects using forces arising from the subtle
interplay of optical scattering and gradient forces of highly focused laser beams.
Unlike conventional backscatter signal for optical tweezers, the two-photon fluorescence
(TPF) trapping signal from femtosecond optical tweezers (FOTs) shows a slow counterintuitive
decay, when the trapped particles are not entirely within the laser-illuminated volume.
A change in the corner frequency of FOT is also noted with the TPF technique. These
observations are evident even at low average powers. The high peak powers trap not
only single microspheres but also encourage optically directed self-assembly. We use
TPF signatures of trapped particles to show the existence of a directed self-assembly
process and elucidate the structural dynamics during the process of cluster formation.
