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

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 Yb 3+ -doped LiLuF 4 (LLF:Yb) single crystals in a radio-frequencyheated 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-coolinggrade crystals, opening a new route to produce high-performance materials for solid-state optical refrigerators and radiation-balanced lasers.

Accurate Characterization of the Properties of the Rare-Earth-Doped Crystal for Laser Cooling

“…This study indicated that the background α b does not remain constant but changes dramatically during the cooling process. As an isomorph of the LiYF 4 crystal, the LuLiF 4 crystal displays similar optical behavior [57][58][59][60]. Considering the reduction in α b with decreasing temperature [41], one can expect that the 5% Yb 3+ :LuLiF 4 crystal under our study can be potentially cooled to even lower temperatures than 110 K.…”

“…In a previous work, a temperature drop of ~2.2 K was reported in a Czochralski-grown bulk 5% Yb 3+ -doped LuLiF 4 under 1025 nm pump laser of 220 mW with the corresponding parameters η ext = 99.0 (±0.1), α b = 1.3 (±0.2) × 10 −3 cm −1 [57]. Later, a Czochralski-grown bulk 5% Yb 3+ -doped LuLiF 4 of better purity was cooled to 117.3 K under 1020 nm pump laser of 33 W with estimated cooling parameters being η ext = 99.4% and α b = 4.5 × 10 −4 cm −1 [29] Recently, a Bridgman-grown 5% Yb 3+ -doped LuLiF 4 was cooled to ~195 K under 1020 nm pump laser of 45 W with η ext = 98.9 (±0.1)% and α b = 3.3 (±0.2) × 10 −4 cm −1 [58]. Based on the cooling window from the LITMoS test, the crystal sample under current study can be potentially cooled to ~110 K under 1020 nm pump laser with its measured parameters η ext = 99.4 (±0.1)% and α b = 1.5 (±0.1) × 10 −4 cm −1 .…”

Ytterbium-doped KY3F10 as a promising material for optical cryocoolers

“…The current lowest achieved temperature by this method, using an Yb:YLF crystal, is 87 K [7] which is only 10 K above the boiling point of liquid nitrogen at 77 K. An optical cryocooler prototype based on Yb:YLF demonstrated cooling of a HgCdTe infrared detector as a payload to 135 K [8]. Meanwhile, several other host crystals including LiLuF4 [9], KYF4 [10], BaY2F8 [11] have been tested for laser cooling, but their cooling potential is often not fully explored. Therefore, the choice of materials for cryogenic cooling is still limited to Yb:YLF.…”

Hydrothermal Synthesis and Solid-State Laser Refrigeration of Ytterbium-Doped Potassium-Lutetium-Fluoride (KLF) Microcrystals

“…The first experimental demonstration of solid-state laser cooling involved a Yb-doped ZBLANP glass material in 1995 . To date, net cooling has been observed in a wide variety of glasses − and crystals ,− doped with RE ions.…”