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 (I4(1)/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 (lambda = 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 approximate to 15 K below room temperature, opening new opportunities for contactless in situ TEM refrigeration.