  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.
