Laser Cooling of Yb~(3+) Doped ZBLAN Glass Material
In recent years, laser cooling of solid materials based on the anti-Stokes fluorescence has obtained fast development. Interests in the field of laser cooling of solid are excited by a large number of emergences of new materials and schemes. In this thesis, all of new cooling materials, schemes and results and their recent experimental progresses are first briefly reviewed. Meanwhile, various different temperature measurement techniques are summarized. After that, taking Yb~(3+)-doped ZBLAN as a cooling material, the cooling process is theoretically analyzed and experimentally exploration.First we theoretically analyze the fluorescence cooling process. Our calculation is based on Yb~(3+)-doped material, the cooling limit is also discussed. We propose a two-level model to analyze the absorption and stimulated-emission processes between the Yb~(3+) ~2F_(7/2) ground-state manifold and the ~2F_(5/2) excited-state manifold, and discuss several parameters that influence the cooling power, and find some ways to improve the cooling power. The influences of the doped concentration, pumping power and the effective pump-spot area on laser cooling efficiency are particularly analyzed. At the same time, we discuss the influence of fluorescence reabsorption in cooling cycle. Finally, we make computer simulation for the cooling process and obtain the temperature as a function of the cooling time.We propose a new method to cool the Yb~(3+) doped ZBLAN glass in a standing-wave cavity. Because high power laser sources are needed in the cooling experiment, it is very hard to popularize this technique. There are two advantages of this cavity-enhanced technique: the pumping power is greatly enhanced, and the absorption of the cooling material is greatly increased. Our research shows that the enhanced factor can exceed ten times by using a cavity, we only need to use a pumping laser with a low power of 100mW even a few 10mW, which can be satisfied by using a semiconductor laser diode (LD). Furthermore, we discuss some parameters in this scheme, such as cavity fineness, enhanced factor, optimal reflectivity and absorption. To achieve a higher enhancement, a theoretical study of intra-cavity laser cooling is performed. The results show that for a low intensity, the enhanced factor is more than one hundred, the intracavity configuration is a very efficient method for laser cooling.We propose a new enhanced laser cooling scheme using rare-earth-ions-doped glasses containing small metallic particles, which results from the effective oscillating electric field inside the composite medium after Lorentz local-field correction. Then the dielectric constant of the entire medium is changed, so the transition rate is improved. Meanwhile fluorescence enhancement may occur in the cooling material due to plasma excitation. In general, anti-Stokes cooling is enhanced.From the above theoretical researches, experimental exploration on cavity enhanced laser cooling is performed as well. Using diode laser as light source, we design a plane-concave cavity. Scanning of the cavity is performed, and the resonant cavity is locked by the feedback of lock-in amplifier, and then cavity enhancement is achieved. Moreover, fluorescent spectra of Yb~(3+)-doped ZBLAN in different doped-concentrations and temperatures are performed. The differential luminescence spectrum in different temperatures is obtained. These results are very useful for calibration of the temperature of laser-cooled material.