Chemically Tunable Aspect Ratio Control and Laser Refrigeration of Hexagonal Sodium Yttrium Fluoride Upconverting Materials

Hexagonal sodium yttrium fluoride with Na3xY2-xF6 stoichiometry (beta-NaYF) is a promising material for luminescence upconversion applications due to the narrow crystal field splitting of the Yb(III) ion's lower F-2(7/)2 manifold. However, growing single crystals of beta-NaYF remains an outstanding challenge due to thermal expansion stresses that cause cracking during melt growth. In this paper, we demonstrate a novel hydrothermal synthesis of beta-NaYF with the ability to tune the aspect ratio from microplatelets to microrods with aspect ratios that match computationally predicted cavity (Mie) resonances. These crystals have a root-mean-square roughness below 1 nm after calcination, which makes them ideal for optical cavities. The beta-NaYF microcrystals are doped with 10% Yb(III) cations and are used to build optomechanical laser-refrigeration devices consisting of a hexagonal beta-NaYF crystal located at the end of a cantilever. Laser refrigeration of these devices by >12.5 degrees C is observed using calibrated measurements of both the cantilever's fundamental eigenfrequency and a Boltzmann fit to crystal field luminescence from the Yb(III) ions.