A plasmonic machine at solution interface: optical trapping and swarming of gold nanoparticles

We study unique optical trapping phenomenon at glass/solution interface for gold nanoparticles with the diameter of 200 ~ 400 nm. With prolonged irradiation, gathering of many NPs forms a dynamically moving and fluctuating assembly like a flying group of birds in sky. We call this phenomenon as optical trapping and swarming, which extends to a few ten µm, much larger than the focal area of the 1064 nm trapping laser. We have elucidated the swarming dynamics and mechanism in view of optical binding and its expanded network, while here we design new experiments of the swarming toward a plasmonic machine, depending on laser polarization. The morphology and size of the Au NP swarming is intrinsically determined by optical, physical, and chemical parameters of NP, which is demonstrated by utilizing silica-coated gold nanoparticles. Further the swarming can be controlled by performing trapping and swarming on patterned glasses; one is on gold nanodisk pattern fabricated lithographically and the other is on polycaprolactone microchannel prepared by electrospinning writing method. The dumbbell-shaped morphology is switched from bidirectional to unidirectional, and its shape is modified. The dynamically fluctuating Au NPs can induce hydrodynamic flow in solution and give mechanical pressure to the surrounding. Also, the swarming NPs are heated by photo-absorption of the 1064 nm laser. The present findings indicates that the swarming gold nanoparticles can work as a plasmonic machine, and its systematic study will enable various designs of dynamic matter in the few ten micrometer domain.