Optically Evolved Assembling and Swarming of Gold Nanoparticles at Solution Interface Leading to a Plasmonic Machine

In optical trapping of gold (Au) nanoparticles (NPs) at a glass/solution interface, initially NPs align in the manner of optical binding not only inside but also outside of the focused trapping site. Further gathering and assembling of more NPs lead to the formation of their dynamically moving and fluctuating assembly like a flying group of bees in air. Since we found this phenomenon, we have been systematically studying its dynamics and mechanism. It shows clear dependence on trapping laser polarization. The assembling and swarming expand linearly with the direction perpendicular to linear laser polarization, giving a dumbbell shape morphology, while a disk like one is prepared for circularly polarized laser. The morphology and the size of such assemblies can be controlled by tuning optical, physical, and chemical parameters based on the intrinsic surface plasmon resonance properties of Au NPs. We demonstrate new appearance of two swarms with ellipse and ring distribution of NPs by shifting the axial position of the trapping laser focus with respect to the interface. The contours of the dumbbell shape can be controlled by the incident and focusing angles of the trapping laser. Upon using the Au NPs with different size, shape, and structures, surprisingly unique dynamic behavior is observed. Further we introduce nanolithographically fabricated gold nanopattern as a glass interface, showing a new control way of the swarming morphology. These results show high potential of "optically evolved assembling and swarming" in the studies on nanophotonic machine.