Optical trapping and tweezing faithfully provide the trapped-particle information without influencing their structure. Furthermore, femtosecond optical tweezers (FOT) with high-repetition-rate lasers provide higher sensitivity due to the capability to generate background free Two-Photon Fluorescence (TPF) signal. The TPF from the trapping signal from the FOTs shows a slow counterintuitive decay when the trapped particles are not entirely within the laser-illuminated volume, which is nonexistent in the conventional backscatter signal for optical tweezers. Such a sensitive FOT, however, due to its high instantaneous photon flux, might influence and optically direct self-assembly of nanoclusters, which we unravel in liquid suspensions and infer the in situ structure and dynamics. Experimentally, we characterize the formation of self-assembled clusters in our FOT by analyzing the back focal plane displacement signals. Subsequently, we explain such experimental results through theoretical models that we derive from the Langevin equation. Measurement of the corner frequency changes in the FOT enables us to recognize the structure and dynamics of the nanocluster formation in condensed media over a broad range of nanoparticle sizes. Due to their distinct shapes under the FOT, we also show that it is possible to control the orientation and structure of these nano-cluster aggregation as a function of the trapping laser polarization. Thus, we utilize the optical gradient forces for the directed self-assembly of nanoclusters with FOT to understand the in situ structure and dynamics, which also serves as a representative analog for the understanding of molecular assembly.
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