| 摘要: | 自2009年美國LCLS (Linac Coherent Light Source)成功打出第一發X射線自由電子雷射(X-ray Free Electron Laser，簡稱XFEL)，人類於利用電磁場探索及控制物質領域進入了嶄新的年代。特別是在原子核物理領域，相較於1970年代以降，利用第三代同步輻射所產生的X射線至多只能激發一顆原子核的狀況，在2018年有團隊利用日本SACLA XFEL成功激發超過50顆的原子核。儘管已有了大於一個數量級的成長，但此數目還不足以媲美，如1966 Nobel獎得主 Alfred Kastler利用光泵浦(optical pumping)技術來極化原子量子態的成就。因此，本論文的主旨是探討透過X射線波導管來幫助進一步聚焦XFEL並激發更多原子核的可能性。
透過數值方法模擬XFEL在波導管中的傳播並與原子核系統耦合的行為。我們首先驗證波導管能侷限X射線以減少耗散，幫助激發更多原子核。接著設計波導管的結構來幫助聚焦X射線，將散射的光聚集起來激發原子核，透過上述波導管的減少耗散以及聚焦，模擬結果發現，以現在European XFEL光源，可以在橢圓曲面的波導管激發將近百萬顆原子核，甚至製造原子核的瞬時居量反轉，對未來要近一步直接利用X射線控制原子核量子態，是不可或缺的一環。另外，我們也模擬波導管的粗糙表面對激發原子核的影響，驗證實驗上製成波導管平滑曲面的誤差對激發原子核的影響不大，以現在的波導管工藝技術，是有辦法達成我們所模擬的結果。
在我們的系統中，未來可以考慮將激發態的原子核當作增益介質，放大入射脈衝，或是更進一步探討製作γ光雷射的可能性。亦或是研究激發態原子核的放光，與波導管的耦合效應，初步發現頻譜上的不對稱現象，與其他團隊的實驗結果相似，近一步證實我們系統的可行性與未來發展性。
;Since the United States LCLS (Linac Coherent Light Source) successfully launched the first X-ray Free Electron Laser (XFEL) in 2009, humans have entered a new era in the field of using electromagnetic field to explore and control substances. Especially in the field of nuclear physics, compared to the 1970s, the X-ray generated by the third generation of synchrotron radiation can only excite at most one nucleus. In 2018, a team successfully used Japan’s SACLA XFEL to excite more than 50 nuclei. Although there has been growth more than one order of magnitude, this number is not yet comparable, for example, Alfred Kastler, the winner of the 1966 Nobel Prize, using optical pumping to polarize atomic quantum states. Therefore, the main purpose of this thesis is to explore the possibility of further focusing XFEL and exciting more nuclei through X-ray waveguides.
Through the numerical simulation, the behavior of XFEL propagating in the waveguide and coupling with the nuclear system is theoretically investigated. We first prove that waveguides can confine X-ray to reduce dissipation and help excite more nuclei. Furthermore, the design of the waveguide structure can help focus X-ray, gather the scattered light to excite nuclei. Through the reduction of dissipation and focusing, simulation results show that with the current European XFEL light source, nearly millions of nuclei can be excited by the elliptic curved waveguide. It can even produce instantaneous population inversion of nuclei, which is an indispensable part to directly use X-ray to control nuclei quantum state in the future. In addition, we also simulate the effect of rough waveguide surface, that is, the error of smooth waveguide surface made in the experiment, on excited nucleus. We verify that rough effect has lightly influence on exciting nuclei, meaning that our simulation results can be achieved by current waveguide technology.
In the future, we can consider using excited nuclei as gain medium to amplify the incident pulse, even exploring the possibility of producing gamma ray laser, or study the emission of excited nuclei coupling with waveguide. Actually, we have found out the asymmetry in the frequency domain, similar to the experimental results of other teams, and further confirm the feasibility and future development of our system. |
