Abstract

Laser cooling by anti-Stokes fluorescence is an emerging technology allowing for cooling of solids to the cryogenic temperature regime below 123 K. By exciting optically active ions, e.g., Yb³⁺, in a solid by photons with an energy lower than the mean fluorescence photon energy, the ions emit – enabled by phonon-assisted anti-Stokes processes - more energy than absorbed and hence the solid cools [1]. This technology enables vibrationfree all-solid-state optical refrigerators [2], desired in space and high-precision meteorology applications. High-purity Yb:LiYF₄ (Yb:YLF) is suitable for cooling to below 100 K [1], but the laser cooling characteristics of other materials are not fully revealed. This motivates us to investigate other Yb-doped fluoride crystals to reach unprecedented temperatures for this method, e.g., the boiling point of liquid nitrogen (77 K). We recently identified Yb:KY₃F₁₀ (Yb:KYF) as a promising candidate for optical refrigerators owing to its higher laser cooling figure-of-merit compared with Yb:YLF [3]. Here, we demonstrate the first laser cooling of Yb:KYF using a watt-level pump laser. An excitation laser of 4.1 W at 1013 nm cooled an Yb:KYF sample by 42 K from room temperature.

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