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We report on a study exploring the design of nanoparticles that can enhance their laser refrigeration efficiency for applications in levitated optomechanics. In particular, we developed lanthanide-doped nanocrystals with an inert shell coating and compared their performance with bare nanocrystals. While optically levitated, we studied the refrigeration of both types of nanoparticles while varying the pressure. We found that the core-shell design shows an improvement in the minimum final temperature: a fourth of the core-shell nanoparticles showed a significant cooling compared to almost none of the bare nanoparticles. Furthermore, we measured a core-shell nanoparticle cooling down to a temperature of 147 K at 26 mbar in the underdamped regime. Our study is a first step towards engineering nanoparticles that are suitable for achieving absolute (centre-of-mass and internal temperature) cooling in levitation, opening new avenues for force sensing and the realization of macroscopic quantum superpositions., Comment: Any comments are welcome !

Laser-threshold magetometry using the negatively charged nitrogen-vacancy (NV-) centre in diamond as a gain medium has been proposed as a technique to dramatically enhance the sensitivity of room-temperature magnetometry. We experimentally explore a diamond-loaded open tunable fibre-cavity system as a potential contender for the realization of lasing with NV- centres. We observe amplification of the transmission of a cavity-resonant seed laser at 721 nm when the cavity is pumped at 532 nm, and attribute this to stimulated emission. Changes in the intensity of spontaneously emitted photons accompany the amplification, and a qualitative model including stimulated emission and ionisation dynamics of the NV- centre captures the dynamics in the experiment very well. These results highlight important considerations in the realization of an NV- laser in diamond., Comment: 13 pages and 11 figures