Laser Cooling of Yb-Doped KY₃F₁₀ to 170 K

Laser cooling of solids [1] is an emerging technology in which a cycle of optical excitation and fluorescence emission annihilates phonons in solids. This opened the pathway for all-solid-state optical cryocoolers, requiring no cryogenic liquid and no moving part, making them intrinsically vibration-free. To date, the lowest temperature of 87 K by anti-Stokes fluorescence has been demonstrated with Yb-doped LiYF4 (Yb:YLF) as a laser-cooling material. Achieving lower temperatures will require further improvements in crystal quality and purity. To reach the milestone of 77 K, the boiling point of liquid nitrogen, a better-suited laser-cooling material has to be identified. We previously unveiled that Yb-doped KY<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf>F<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</inf> (Yb:KYF) offers an intrinsically higher cooling efficiency than Yb:YLF at temperatures below 200 K, so it is better suited for cryogenic cooling. We successfully grew a Yb:KYF single crystal, with quality indicators for laser cooling among the best reported for any Yb-doped crystals [2]. Here, we present the first high-power laser cooling demonstration of Yb:KYF and its comparison with our Yb:YLF crystal.