Hydrothermal morphology control of hexagonal sodium yttrium fluoride microcrystals for solid-state laser refrigeration

Hexagonal sodium yttrium fluoride (β-NaYF) is a promising material for optical refrigeration due to the narrow crystal field splitting of the Yb(III) ion. However, growing single crystals of β-NaYF remains a challenge due to thermal expansion stresses during melt growth. We demonstrate a hydrothermal synthesis of β-NaYF with widely tunable aspect ratios that match computationally predicted cavity resonances. The β-NaYF microcrystals contain 10% Yb(III) cations and are used to build optomechanical laser-refrigeration cantilever devices. Laser refrigeration of these devices shows cooling up to 12.5°C, which is measured using the cantilever’s fundamental eigenfrequency and photoluminescence from the Yb(III) ions.