Hydrothermal Synthesis of Yb3+: LuLiF4 Microcrystals and Laser Refrigeration of Yb3+: LuLiF4/Silicon‐Nitride Composite Nanostructures

The hydrothermal synthesis and characterization of 10%Yb³⁺:LiLuF₄ (LLF) microcrystals are reported. A combination of X‐ray diffraction (XRD) analysis, analytical transmission electron microscopy (TEM), scanning TEM (STEM), energy‐dispersive X‐ray (EDX) spectroscopy), temperature‐dependent Fourier‐transform infrared (FTIR) spectroscopy, and photoluminescence (PL) measurements confirm a scheelite (I4₁/a) phase and substitutional doping of Yb³⁺ within the microcrystals. Laser cooling to more than 20 K below room temperature in vacuum (10⁻³ torr) is demonstrated when irradiating individual microcrystals using a near‐infrared pumping wavelength (λ = 1020nm) at a laser power of 40 mW (irradiance of 0.85 MW cm⁻²). The use of these microcrystals is further demonstrated for solid‐state laser refrigeration of an electron‐transparent silicon‐nitride (Si₃N₄) TEM window. A combination of internal luminescence thermometry, heat‐transfer modeling, and control measurements on lithographically patterned Si₃N₄ optical cavities is used to demonstrate successful bulk laser cooling of Si₃N₄ TEM windows by ≈15 K below room temperature, opening new opportunities for contactless in situ TEM refrigeration.