Optical trapping of nanoparticles in superfluid helium

Abstract

Although nanoparticles have been used to study the properties of superfluid helium as fluid tracers, the interaction between nanoparticles and superfluid helium has remained largely unexplored. This is due to the lack of a technique to precisely trap and manipulate nanoparticles in superfluid helium. Optical tweezers, the three-dimensional confinement of a nanoparticle by a strongly focused beam of light, have been widely employed in investigating biomaterial nanomechanics, nanoscopic fluid properties, and ultrasensitive detection in various environments such as inside living cells, at gigapascal pressure, and under high vacuum. However, the cryogenic operation of solid-state-particle optical tweezers is poorly understood. In this study, we demonstrate the optical trapping of metallic and dielectric nanoparticles in superfluid helium below 2 K, which is two orders of magnitude lower than in previous experiments. We prepare the nanoparticles via in situ laser ablation. The nanoparticles are stably trapped with a single laser beam tightly focused in the superfluid helium. Our method provides a new approach for studying nanoscopic quantum hydrodynamic effects and interactions between quantum fluids and classical nanoobjects.