【作者】 贾佑华；

【导师】 印建平；

【作者基本信息】 华东师范大学 ， 光学， 2008， 博士

【摘要】 近年来,基于反斯托克斯荧光制冷的固体材料激光冷却技术得到了快速发展。新材料的不断发现以及方案的创新激发了人们对该领域研究的兴趣。本文首先综述了各种固体激光冷却的新材料、新方案和新结果及其最新实验进展,介绍了各种荧光制冷的温度测量技术。随后本文以掺杂Yb³⁺的ZBLAN块状玻璃材料为研究对象,开展了相应的理论分析与实验探索。我们首先理论研究了荧光制冷过程,并对Yb³⁺掺杂材料进行了理论计算,讨论了材料的冷却极限。随后采用一个二能级系统模型分析了Yb³⁺离子²F_7/2→²F_5/2能级之间的吸收与受激辐射过程,讨论了影响制冷功率的因素,找到了提高制冷功率的途径,详细分析了掺杂离子浓度、泵浦功率、有效吸收截面等对冷却极限的影响,同时分析了荧光再吸收对制冷的影响。最后对冷却的物理过程进行了理论计算,从而得到了冷却过程中温度随时间的变化曲线。我们提出了腔内增强激光冷却的方案。由于激光制冷实验中要求的激光功率大,激光器成本高,因此该技术距离商业化还有很大的距离。通过把样品材料放在谐振腔中,一方面提高了腔内泵浦功率,同时也增加了样品对泵浦光的吸收。研究表明腔增强后的功率可达到原功率的十多倍,这使得我们可以使用100mw甚至更低的功率来研究激光冷却,半导体激光器就可以满足该要求。此外,我们研究了腔增强方案中腔的精细度、功率增强因子、最佳反射率、最佳的材料吸收等参数。为了更好地实现增强效果,我们还提出了把材料放在激光器腔内冷却的方案。研究表明,在低损耗的情况下,可达到上百倍的增强,制冷效果也得到大大改善。我们提出了掺杂纳米金属粒子材料的激光冷却增强方案。该增强的原理是复合材料中会产生洛仑兹局域场修正的有效电磁场,从而改变材料的介电常数,导致吸收跃迁几率的提高。同时掺杂金属离子会导致基于等离子的荧光增强,从而实现增强的反斯托克斯散射过程。在理论研究的基础上,我们开展了腔增强的实验探索。以半导体激光器为光源,构建了平凹腔,对光学谐振腔进行了扫描,同时利用锁相放大器实现了对腔的伺服控制,把谐振腔锁在共振峰上,从而实现了腔内增强。此外,测量了掺杂Yb³⁺的ZBLAN块状玻璃材料的荧光光谱,研究了掺杂浓度和温度对光谱强度的影响,得到了不同温度下的差分光谱,实现了温度与谱线强度的定标。更多还原

【Abstract】 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 Yb³⁺-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 Yb³⁺-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+ 2}F_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 Yb³⁺ 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 Yb³⁺-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.更多还原