节点文献
超冷极性分子的产生与动力学特性
Generation of Ultracold Polar Molecules and Dynamic Properties
【作者】 钱静;
【导师】 张卫平;
【作者基本信息】 华东师范大学 , 光学, 2010, 博士
【摘要】 玻色-爱因斯坦凝聚是指玻色子在临近绝对零温时,所有的粒子失去了个体特征而相干凝聚到同一量子态的现象。1995年,美国科罗拉多大学的E.A.Cornell和C. E. Wieman小组首次在87Rb原子中观察到了玻色凝聚的现象,而不久后的W. Ketterle及R. G. Hulet小组也分别在23Na及7Li原子中观察到同样的凝聚现象。在超冷原子玻色凝聚的研究不断发展和深入的同时,超冷分子物理也悄然出现。从目前学科的发展趋势我们可以大胆地预见,在今后相当长时间内,超冷原子分子物理将一直是物理学最前沿与最有活力的研究领域之一。从1997年到2001年短短五年时间,该领域就因原子的激光冷却(1997年)和玻色-爱因斯坦凝聚(2001年)连续两次摘取诺贝尔物理学奖的桂冠。如此骄人的成就足以奠定这个新兴领域在物理学发展史上的重要地位。目前用于实现超冷分子有两个途径:一是基于直接减速与冷却化学稳定分子的“直接冷却”技术。这种方法适用的分子样品种类很广,但典型的冷却温度只有几个mK。二是通过磁Feshbach共振或者光缔合,将超冷原子成对地耦合成超冷分子的“间接冷却”技术。该方案能够将分子冷却至几百个nK(尚未到临界温度以下),对应分子的相空间密度达1012cm-3。最近,绝大多数的实验小组都采用磁场Feshbach共振的原理研究如何将分子制备在基态能量最低的量子态上,但是否能够进一步地压缩分子气体来提高相空间密度同时使其冷却至临界温度以下仍然是一个倍受争议的问题。在本论文的工作中,我们致力于思考和研究另一种技术——光缔合产生基态分子的利弊。在光缔合过程中,运用一束泵浦激光可以将自由散射态的原子转换成激发态的分子,但由于该跃迁过程的偶极矩阵元太小,故对泵浦激光功率的要求很高。解决这一问题目前最常用的方法就是另施加可调谐的磁场,人们发现在磁场Feshbach共振附近光缔合的速率可以增大几个数量级。但磁场的引入随之会带来新的问题,即大量粒子由于碰撞驰豫而丢失。基于上述研究进展和存在的问题,我们小组首次提出基于受激拉曼绝热通道原理的全光型原子分子转换模型来克服光缔合技术的缺陷。我们设计了两种新的方案——链式模型和R型系统,利用附加的光场来辅助缔合过程的泵浦激光,从而实现降低泵浦光自身功率的目的。附加的光场作用在分子束缚态能级之间,所以跃迁的Franck-Condon (FC)系数要远远大于原子自由态和分子束缚态间的FC系数。我们的理论设想不依赖于分子的内禀属性,具有非常普适的意义。这些新方案有可能会启发更多的物理学家去思考分子玻色-爱因斯坦凝聚的实验可行性问题,并推动冷分子物理发展成为原子物理、光物理和凝聚态物理学界共同追逐的一个崭新的研究亮点以及科学研究的重要前沿。
【Abstract】 The theoretical prediction of Bose-Einstein Condensation (BEC) dates back to 1925 A. Einstein predicted the occurrence of a phase transition in a gas of non-interacting atoms. In general, a gas of such bosons can develop a macroscopic population of its lowest energy state below a critical temperature when all the particles will lose their individual characteristics. In 1995, E. A. Cornell and C. E. Wieman’s group in Colorado University first observed Bose-Einstein Conden-sation in dilute 87Rb atoms, and later on W. Ketterle in MIT and R. G. Hulet in Rice succeeded in the atomic BECs of 23Na atoms and 7Li atoms, respectively. After the experimental realization of atomic BEC, the study of quantum gases in conditions of high degeneracy has become an emerging field of physics, attract-ing the interest of scientists from different areas. From 1997 to 2001, due to the discoveries of laser cooling and trapping atoms (1997), and atomic Bose-Einstein condensation (2001), this field was awarded by two Nobel Prize in physics, which is not a coincidence although the total history of this area is only more than ten years. In this thesis, we aim to show a variety of theoretical ways to solve the bottlenecks in the production of ultracold polar molecules.Currently, achieving ultracold atomic BEC has been a mature exciting fron-tier in the field of atomic, molecular and optical physics, and meanwhile ultracold molecular BEC, being another challenging direction, is expected to be celebrated as another milestone that promises to greatly spur activities at the forefront of physics research. It is clear that there are many roads to ultracold molecules. Di-rect cooling methods are based on cooling preexisting chemical stable molecules. Advantages include wide applicability and large yield. However, the typical tem-perature is only a few mK. An alternative way is to couple a pair of degener-ate atoms by Feshbach resonance or photoassociation, namely, indirect cooling method, which is proposed to be the most promising way of creating molecules in vibrational ground levels. Then final temperature reaches a few hundred nK (not yet to the critical temperature below), and the corresponding phase-space density is as high as 1012cm-3. Recently, Prof. J. Ye and D. S. Jin from JILA succeeded in achieving fermionic 40K87Rb molecules in the lowest rovibrational level. How-ever, the technique they applied is called”coherent two-photon Raman transfer”, which is based on a magnetic Fano-Feshbach resonance. Whether molecules can be further compressed to increase its phase-space density and cooled down to the critical temperature is still a debated question.In the present work, we focus on how to produce ground-state molecules by photoassociation. In photoassociation process, pairs of free atoms are coupled into excited-state molecules via a pump laser. While generally speaking, due to the small free-bound Franck-Condon(FC) factor, an extremely strong laser power is required for an efficient transfer. To solve this problem, it is possible to apply another magnetic field, which may increase the photoassociation rate by orders of magnitude near Feshbach resonance. However, magnetic induced collision relax-ation will cause a big particle loss. Based on the above achievements, our group first propose to use all-optical stimulated Raman adiabatic passage to overcome the weakness in photoassociation. Two new schemes--chainwise and R-type models are designed for lowering pump laser with the help of extra light fields. To our knowledge, bound-bound FC factor is far larger than free-bound one, so that extra lights are easily controllable. Our scheme is a generalized approach which can be widely used in most of atom-molecule conversion systems. We think these new proposals may bring us one step closer to the final molecular condensation. Cold molecular physics will likely jump to be a new research highlight as well as an important frontier within atomic physics, optics and condensed matter physics community.
【Key words】 ultracold polar molecules; photoassociation; Stimulated Raman Adiabatic Passage;