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Optical trapping of 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 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.