Visualizing colloidal aggregation with femtosecond optical tweezers

摘要

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