Laser cooling of the Yb³⁺-doped LuLiF₄ single crystal for optical refrigeration

A vibration-free, optical cooling refrigerator may imply revolutionary applications in different fields, therefore it is vital to explore new materials suitable for laser cooling below the cryogenic temperature. Here we demonstrate that the Yb³⁺-doped LuLiF₄ crystal is a promising material that could reach the cryogenic temperature limit (123 K) or even lower. With the doping concentration of 7.5%, the Yb³⁺-doped LuLiF₄ crystal can be optically cooled down from the room temperature to 141.8 K in a double-pass setup. Provided with better purity and multiple-pass of laser cooling in the crystal, the temperature limit can even reach as low as 89 K for the 7.5% Yb³⁺-doped LuLiF₄ crystal, thus making it a competitive candidate for laser cooling below the cryogenic temperature.