Unidirectional Optical Swarming of Gold Nanoparticles on Lithographically Fabricated Gold Nanopatterns

Optical trapping has been used to manipulate and assemble small objects within a focused laser beam. Herein, we apply optical trapping with the fabricated gold nanodisk arrangement at the glass/solution interface, where the trapped Au nanoparticles dynamically evolve outside the focus, presenting swarming behavior. At the individual particle level, we observe the trapping and arrangement of Au nanoparticles between the nanodisks with a largely enhanced confinement, which is attributed to immobilization by scattering force and repulsion due to dipole-dipole interactions between the nanoparticles and nanodisks. At the ensemble level, we demonstrate the formation and switch between bidirectional swarming and unidirectional swarming of Au nanoparticles , depending on the laser focusing positions relative to the nanodisks. The fabricated nanodisks give an asymmetric environment for assembly expansion due to its phase-mismatched scattering with nanoparticles, disturbance of correlated motion, and asymmetric far-field scattering. The repeated evolving and scattering processes during assembly evolution amplify the asymmetry, leading to the unidirectional swarming. This can be regarded as dynamically and adaptively evolving optical swarming. Our findings provide valuable insights into optical manipulation techniques coupled with fabricated nanostructures, expanding the possibilities for optical trapping and related research beyond the irradiated area at interfaces.