Inert Shell Coating for Enhanced Laser Refrigeration of Nanoparticles: Application in Levitated Optomechanics

Levitated nanoparticles in vacuum, with their intrinsically low coupling to the environment, hold the promise of precision force sensing and open new avenues for exploring macroscopic quantum physics. Ultimately, the coherence of the levitated oscillator is limited by its blackbody radiation emission and hence its internal temperature. Controlling the internal temperature of a levitated object in vacuum poses a formidable challenge, necessitating contactless cooling methods such as laser refrigeration via anti-Stokes fluorescence. We report on a study exploring the design and synthesis of nanoparticles that can enhance their laser refrigeration efficiency. We developed lanthanide-doped nanocrystals with an inert shell coating and compared their cooling performance with that of bare nanocrystals while optically levitated. We found that the core-shell design shows an improvement in the minimum final temperature: about 31% of the core-shell nanoparticles showed significant cooling compared to a minimal cooling effect for 12% of the bare nanoparticles. Furthermore, we measured a core-shell nanoparticle cooling down to a temperature of 148 +/- 4 K at 26 mbar in the underdamped regime. Our study is a first step toward engineering nanoparticles that are suitable for achieving absolute (center of mass and internal temperature) cooling in levitation, enabling novel prospects for realizing macroscopic quantum superpositions.