Light-Driven Ballistic Nanoparticle Swimmers

Directed high-speed motion of nanoscale objects in fluids can have a wide range of applications. However, directed movement and high speed in the nanoscale are rarely compatible. Light is a convenient source that can drive nano objects to move by applying optical pushing forces due to momentum conservation when photons are scattered off the objects. In theory, optical forces from a planewave can also be “negative” that pull objects against the light propagation direction in a homogeneous medium if some unique optical configurations of the objects can be realized, but this is yet to be demonstrated. Nevertheless, these optical forces are too weak to enable fast-moving swimmers in fluids. Here, we report ballistic plasmonic Au nanoparticle (NP) swimmers with unprecedented speeds (~397,000 mm s^-1) realized by both optical pushing and pulling forces from a single Gaussian laser beam. The Au NP excited by the laser at the surface plasmon resonance (SPR) peak interact with the NP both thermally and optically, leading unique conditions for ballistic movements and “negative” optical forces.