Bridgman Growth of Laser-Cooling-Grade LiLuF<sub>4</sub>:Yb<sup>3+</sup> Single Crystals
Azzurra Volpi, Karl W. Krämer, Daniel Biner, Brenden Wiggins, Jackson Kock, Alexander R. Albrecht, Eric J. Peterson, M. Spilde, Mansoor Sheik‐Bahae, Markus P. Hehlen
Abstract
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.