Photon Momentum Dictates the Shape of Swarming Gold Nanoparticles in Optical Trapping at an Interface

Optical trapping at an interface mediates the gathering and assembling of particles, inducing the so-called optically evolved assembly that can expand outside the laser focus due to multiple scattering processes. In previous studies, we reported that the so-called Au nanoparticle (NP) dynamically evolving assembly has a dumbbell shape, in which the Au NPs fluctuate in a cooperative fashion, like a swarm of bees. The shape and the size of such an assembly can be controlled from a physicochemical point of view, considering the intrinsic surface plasmon resonance properties of Au NPs. In this work, we will demonstrate that changing the optical conditions of trapping represents an alternative approach for controlling the shape of the swarming NPs. Strikingly, we observe two new appearances of the swarm with elliptical and ring distribution of particles by shifting the axial position of the trapping laser focus with respect to the interface. Indeed, we can modify the outline of the assemblies by displacing the dynamic equilibrium between the different "optically evolved assembly" states. Moreover, the contours of the dumbbell state can be further controlled by the incident and focusing angles of the electromagnetic radiation used in the trapping process. The results are elucidated in terms of the considerable scattering force that arises from the momentum transfer between photons and Au NPs. This work shows the importance of the momentum direction of incident photons at the interface, establishing critical steps to comprehensively control the "optically evolved assembly" phenomena, which has a large potential in research fields such as soft matter or colloidal chemistry.