低溫銣原子的非反轉分佈放大

Abstract

本論文是利用雷射冷卻的方式,將銣原子捕捉並冷卻至10^-4K,並利用 這些超低溫的銣原子,研究非倒轉分佈放大(amplification without population inversion,簡稱AWI)的量子干涉現象.由於原子在極低的溫度 下,使得溫度引起的原子間碰撞可降至最低,對同調性所造成的破壞可減至 最小. 在不考慮碰撞情形下,我們完成了三能階系統的AWI計算.利用微 擾方法解出密度矩陣(density matrix)的每個元素,而得到光的吸收率或 放大率的大小及原子在各能階的分佈. 實驗上,我們觀測到低溫銣原子 的AWI現象.其增益值可高達百分之七十.這些增益是由 光學糖蜜(optical molasses) 作用下的低溫銣原子.磁光陷阱(MOT)中的銣原子由於密度,僅 對光造成吸收而無AWI的現象. Amplification without population inversion (AWI) is aquantum interferenceeffect. In this thesis, we use laser-cooled Rb atoms (at 10^-4K) to study the phenomena. The collisions that destory atomic coference can significantly reduced in these ultracold atoms. We have completed the AWI calculations of a three- level system in the absence of collisions. With the perturbation method, we solve the elements of the density matrix of the three-level system. The elements can determine whether the system exhibit AWI under certain conditions. We have also observed the AWI in cold Rb atoms. The gain can be as high as seventy percent. The gain is resulted from the cold Rb atoms in the optical molasses region. The atom density in magneto- optical trap (MOT) only absorb light and do not contribute AWI phenomena since atoms density in MOT is veryhigh.