  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
detections 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 the
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 objects.
