- 學位論文
捕捉於光子晶體光纖中低溫原子的數目及生命期之研究
Study of number and life time of cold atoms trapped in hollow-core photonic crystal fibers
摘要
Abstract This thesis reports the construction of the setup for trapping cold atoms with a hollow-core photonic crystal fiber (HC-PCF), and the development of measurement techniques for the number of the trapped atoms inside the HC-PCF core. We studied the optimization of capturing process of atoms and investigated the life time of the cold atoms confined in the center of the HC-PCF core. We used an optical dipole trap (ODT) to load and confine 2,000~3,000 cold atoms with a life time of about 20 ms in the HC-PCF center. A magneto-optical trap (MOT) produced a cloud of 2×107 cold atoms with a temperature of 110 μK right above a vertically positioned HC-PCF. A far red-detune laser beam was focused into the HC-PCF from the lower end and came out of the upper end to form the ODT which guided the cold atoms from the MOT into the fiber core continuously. Inside the HC-PCF, this laser beam also confined the cold atoms in the center without contacting the fiber wall. To study the loading process, we applied the optical pumping method and the absorption method for measuring the atom number captured in the HC-PCF and developed a theoretical model based on the Maxwell-Boltzman thermal distribution of the atom energy to verify the measurement results. Our study shows that the ability of capturing cold atoms is sensitive to the atom cloud position of the MOT and the shape and depth of the ODT potential outside the fiber. We also found that the captured atoms have a rapid-decay behavior which is sensitive to the atom temperature. With the current setup, we were able to trap 3,000 cold atoms resulting in an optical density (OD) of about 3 in the HC-PCF. Reducing the atom temperature to increase the life time of the captured atoms and employing a two-dimensional MOT to enhance the capture efficiency can further improve the atom number as well as the OD. We expect this system can become as a suitable platform for the interaction between few-photon pulses and atoms.
並列摘要
摘要 本論文報告以中空型光子晶體光纖（hollow-core photonic crystal fiber，簡稱HC-PCF）捕捉冷原子的系統架設，與發展HC-PCF內冷原子數的量測技術，並研究被捕捉原子數量的最佳化及原子的生命期（life time），探討影響生命期的因素。 我們利用光偶極位能阱（optical dipole trap，簡稱ODT）導引並捕捉約2,000到3,000顆冷原子進入HC-PCF 的中空核心，並且將這些冷原子束縛在在核心內，被捕捉冷原子在核心內擁有約20 ms的衰減生命期。爲了提供能讓ODT 捕捉的冷原子，首先以磁光陷阱（magneto-optical trap）聚集並維持一團冷原子雲在一條垂直置放的HC-PCF端口附近，此團冷原子的數量約為2×107 顆，溫度約110 μK。一道遠紅調變雷射光束自底端耦合進入HC-PCF，當它自頂端向外輸出時在HC-PCF 端口組成與磁光陷阱交會的光偶極位能阱；此位能阱自磁光陷阱製備的冷原子團中導引了一部分的冷原子進入HC-PCF，構成一個連續性的導引捕捉冷原子系統。在HC-CPF內部，此雷射光同時將冷原子束縛在核心軸線附近，避免冷原子撞上核心內壁而損失。 爲了瞭解被捕捉冷原子的行為，我們採用光抽運法和光吸收法兩種探測機制，來量測被捕捉原子數目，同時以馬克斯威爾－波茲曼分佈為基礎建立了冷原子在HC-PCF 內部的分佈模型，驗證量測結果。量測呈現的結果中，我們可以看到目前的捕捉能力對冷原子團位置，光偶極位能阱雷射光形狀與深度相當敏感；而被捕捉原子的損耗機制方面，則被冷原子溫度強烈影響。 目前這套系統能夠捕捉3,000 顆冷原子，在HC-PCH內部產生相當於3的光學密度（optical density）。爲了增加被捕捉原子的數量和對應的光學密度，進一步降低冷原子溫度來增加被捕捉原子在HC-PCF內的生命期，及架設2 維磁光陷阱取代現存的3 維磁光陷阱以獲得更好的捕捉能力，是可能的改進方向。我們希望未來這套系統能夠作為極少光子與原子交互作用的平台。