Laser Cooling Of Yb³⁺:Y₃Al₅O₁₂ And Yb³⁺:LuLiF₄ Crystals

The interaction of light with the matter has many fascinating phenomena at the intersection of optics,such as light manipulation,light heating or cooling of objects,which have significant applications in materials science,industry,and medicine.Optical cooling is a new cooling technology after mechanical,liquid,electric,and magnetic cooling.The all-solid-state optical cryocooler based on solid laser cooling technology has the advantages of compactness and vibration-free and can serve various application scenarios that require reliable,vibration-free cryogenic cooling.In addition,solid laser cooling is expected to completely solve the heat dissipation problem of specific optically pumped solid-state lasers by eliminating the thermal gradient of the gain medium and achieving a laser in radiation equilibrium.The motivation behind the continuous progress of laser cooling technology for solid is the pursuit of low temperatures.Over the course of nearly a century,with the development of theoretical research,material growth processes,and laser technology,members of the solid laser cooling family has grown,and high-purity Yb³⁺-doped fluoride crystals have come to the fore.So far,oxide crystals Y₃Al₅O₁₂（YAG）and fluoride crystals Lu Li F₄（LLF）doped with Yb³⁺ions are among the best materials to realize radiation equilibrium lasers and low-temperature optical coolers,respectively.Therefore,this thesis takes Yb³⁺:YAG and Yb³⁺:LLF as laser cooling research objects and conducts system cooling parameters measurement and cooling experiments.The main research contents of the paper are summarized as follows:1.A complete experimental platform for testing the cooling parameters of samples was built,and full laser cooling parameters of solid materials were obtained.The fluorescence spectra of the samples at 80 K-300 K were obtained by performing measurements in a cryostat after calibrating the radiation intensity of the fluorescence collection system using a standard light source,and the absorption spectra of the samples were introduced according to the inverse ease principle.In addition,we designed and built a laser-induced thermal modulation spectroscopy（LITMo S）test system to obtain the crystal’s external quantum efficiency and background absorption coefficient.We use the four-energy-level model to derive an efficiency formula for laser cooling of solid materials and combine the experimentally measured parameters with the theoretical procedure for cooling efficiency to map the"cooling window"of the crystal and characterize the laser cooling performance of the crystal.In addition,the cooling window can be used to predict the optimal cooling wavelength and minimum achievable temperature of the sample,allowing the best cooling performance to be selected without performing complex laser cooling experiments.2.Theoretical and experimental research on YAG crystal laser cooling with 1%-10%Yb³⁺doping concentration was carried out,and the effect of Yb³⁺doping concentration on each cooling efficiency was analyzed.It is shown that YAG crystals with a specific Yb³⁺concentration have the best laser cooling performance.Laser cooling experiments were carried out for a series of Yb³⁺:YAG crystals with different doping concentrations.Under the same experimental conditions,the cooling temperatures of Yb³⁺:YAG crystals with 3%and 5%doping concentrations were significantly lower than those of the other doping concentration samples,with 3%Yb³⁺:YAG crystals achieving a temperature drop of～80 K,which is the most inadequate temperature record obtained for laser cooling of YAG crystals so far.3.A laser cooling study of Yb³⁺:LLF fluoride crystals grown by the Chukrasky method was carried out.By measuring the cooling parameters,the best samples with good optical quality and high purity were selected,and the"clamshell"blackbody radiation shielding structure was designed to optimize the thermal load of the whole experimental system.Using a high-power fiber laser pumping a 7.5%Yb³⁺:LLF crystal in a"clamshell,"the crystal obtained a laser cooling temperature of 121（±1）K,which is below the cryogenic temperature（123 K）defined by the National Institute of Standards and Technology（NIST）for the first time.To date,only the University of New Mexico and Los Alamos National Laboratory in the United States and our group have experimentally broken through the cryogenic temperature using Yb³⁺:YLF crystals as well as Yb³⁺:LLF crystals internationally.In addition,studies have shown that the congruently melting LLF can provide higher purity samples for applications such as laser cooling of solid materials and radiation balancing lasers,making it a more attractive laser cooling material than YLF.