Strong collective currents of gold nanoparticles in an optical vortex lattice

Self-organized collective behaviour of active units is inspiring new designs of artificial swarms of micron-sized objects. However, active control at the nanoscale remains elusive. We have accurately solved the collective optofluidic dynamics of gold plasmonic 50 nm-radius nanoparticles moving in aqueous solution under a nonconservative optical vortex lattice. Above a critical field intensity and concentration, the interplay between optical forces, thermal fluctuations and hydrodynamic pairing leads to a spontaneous transition towards synchronised motion exhibiting a rich assortment of collective dynamics. The critical phenomenon creates strong unidirectional currents of nanoparticles, reaching speeds of centimeters per second along the diagonals of the lattice.