Bridgman Growth of Laser-Cooling-Grade LiLuF4:Yb3+ Single Crystals

The first demonstration of solid-state laser cooling in fluoride crystals grown by the Bridgman method is reported. We present advances in the Bridgman crystal growth of Yb3+-doped LiLuF4 (LLF:Yb) single crystals in a radio-frequency-heated furnace. COMSOL Multiphysics numerical simulations are used to investigate the thermal gradients within the crucible during the crystal growth. Optical spectroscopy and laser-cooling efficiency measurements of three LLF:Yb crystals as well as laser cooling of a LLF:5%Yb crystal in a double-pass geometry from room temperature to 195 K are reported. Solid-state laser cooling is only possible in materials having extremely high chemical purity and crystal quality. The vertical Bridgman method is well suited for the growth of high-quality crystals on the few gram scale, a quantity that is compatible with purification techniques that aim to exceed the 99.999-99.9999% purity that is typical of commercial precursor materials. The results demonstrate that the small-scale Bridgman growth of LLF:Yb in glassy-carbon crucibles is able to produce laser-cooling-grade crystals, opening a new route to produce high-performance materials for solid-state optical refrigerators and radiation-balanced lasers.