Laser Cooling Of Yb³⁺Doped ZBLANP Glass Based On Microcavity Enhanced Effect

Laser cooling of solids, sometimes also known as optical refrigeration, is a fast developing area of optical science, investigating the interaction of light with condensed matter. The idea to cool solids with anti-Stokes fluorescence was proposed in1929by Peter Pringsheim and first demonstrated experimentally by Epstein’s research team in1995. Since then, researchers have demonstrated laser induced cooling in a broad range of glasses and crystals doped with ytterbium.Firstly, the background and development in material, experiment and application of laser cooling of solids was described. Secondly, two typical theory modes include classical and quantum model was discussed. Cooling power is obtained from the classical model and cooling limit is obtained from the quantum model. Based on these analysis, the factors which affect the cooling efficiency and the methods to develop the experiment were proposed, which include raising quantum efficiency, purifying the material, avoiding fluorescence reabsorption and enhancing the pumping power and absorption factor.According to the factors which affect cooling efficiency, the scheme of using a microsphere cavity which made of doped to cooling is proposed. In this scheme, the trapping light as optical tweezers control the sphere so that it will be suspended and isolated from conduction of heat. Symmetry of sphere can make the photon escape from the cavity easier, thus reduces the reabsorption and heat. Because of the cavity enhancement, we can enhance the absorption and field inside the microsphere. In order to obtain the cooling result, the mode of optical field was discussed firstly. Because of the restriction of sphere cavity, the modes in the sphere come into being whispering gallery modes(WGM). The mode’s analytical solution can be solved by Mie Theory. Then, numerical simulation of optical tweezers, enhancement of absorption factor and field in the sphere were provided. We also use COMSOL software package to do the simulation of the modes inside the microsphere. Finally, we get12times absorption enhancement and more than2300times intensity enhancement, which results almost70K temperature drop from RT in5s.This scheme combine the merits of WGMs to laser cooling of solids, which make it has more attractive prospect. For example, the Radiation Balanced Laser based on laser cooling of solids combined with the Low Threshold Laser based on WGM microsphere cavity can make it more powerful.