Counter-Propagating Optical Trapping of Resonant Nanoparticles Using a Uniaxial Crystal

Abstract

Laser tweezing of optically resonant nanostructures, such as plasmonic nanoparticles and high-index dielectric nanoresonators, is extremely challenging because the enhanced light–matter interaction usually amplifies radiation pressure to an extent where conventional single beam gradient trapping in three dimensions becomes impossible. Such particles are therefore typically trapped off resonance or in two dimensions only. To extend the application potential of optical tweezers to the resonant case, focus splitting inside a uniaxial birefringent crystal and reflection from a mirror is used to develop a counter-propagating beam configuration based on a single microscope objective. The setup allows one to trap and rapidly rotate resonant gold nanorods in water far from any interface, thereby opening a range of possibilities for novel studies of resonantly enhanced optical forces and interactions in uniform environments.