以電漿泡泡產生之中紅外光脈衝光源

Abstract

中紅外光源在波段上包含了 5 μm 到 30 μm，而這個波段目前不僅在醫學顯像上有廣泛的應用更在材料定性上大放異彩。而高強度、小於一皮秒的短脈衝中紅外光光源甚至可以被用在上述兩點的動態學研究。傳統產生中紅外的方法包含了自由電子雷射和非線性光學。前者可以提供數百微焦耳能量、小於一皮秒的中紅外光脈衝。後者雖然僅能產生數微焦耳的能量，但是脈衝時寬卻能夠達到小於一 百飛秒的境界。 在此，我們要介紹如何以十兆瓦鈦藍石晶體輸出的時寬 42 飛秒、中心頻率 810 奈米、205 微焦耳雷射光源來產生中紅外光脈衝。其波段介於 6 到 10 微米且脈衝能量可達 250 微焦耳(相當於目前自由電子雷射達到的極限)，而光束發散角則為 60 mrad。 有趣的是，同樣的實驗環境與參數卻能以雷射電漿波來加速電子達到單能量之 50 MeV 高能電子束。為了釐清這兩者間的關係，我們比對了中紅外光與單能電子束斷層掃描下的強度分布。實驗結果顯示這兩者有強烈的相關性，但這相關性 卻不是因果關係。由 PIC 模擬顯示，中紅外光甚至遠在單能電子束產生前就已經 有成長的跡象。 我們將在第一章介紹中紅外光的應用以及產生方法。並在第二章介紹雷射電漿波的基本原理以及電漿中相關的非線性效應。第三章將介紹以啾頻脈衝放大產 生高強度雷射光源的方法，以及介紹本實驗室 10 TW 雷射系統的架構。第四章則 是實驗設計，包含了參數設計和實驗架設。我們將在第五章討論實驗結果並試圖 去解釋。實驗和模擬結果與我們原先預期的模型相吻合，其機制為雷射電漿加速 器中的光子減速。最後，我們將在第六章總結這篇論文，並計畫未來將完成的全 域定性。

Mid-infrared (MIR) ranging between 5 μm and 30 μm has widespread applications in medical imaging and material characterization. Intense MIR pulses with sub-ps duration is more attractive for resolving dynamics of the detail process. Conventional methods of generating pulsed MIR light source include free electron laser and traditional nonlinear optics. The former can generate MIR pulse of a-few-hundred-μJ energy in sub-ps. The latter is energetically limited below a-few-μJ, but it’s duration can be shorter, said sub-100-fs. Here we demonstrate using a 10 TW Ti:sapphire laser system (42 fs, 810 nm, and 205 mJ) to produce MIR pulses (6 to 10 μm) with 250 μJ (comparable to that of the most intense free electron lasers), and 60 mrad beam divergence. Interestingly, the similar condition had also been used to generate 50- MeV monoenergetic electron beam in the way of laser wakefield acceleration. In order to clarify their relationship, tomographies on the intensities of MIR pulses and monoenergetic electron beams are performed. The experimen- tal result shows a strong correlation, but this correlation is not a causal relationship. A PIC code simulation shows that MIR pulses grow before monoenergetic electron beams are produced. The general applications and generation methods would be introduced in chapter 1. We’ll then focus on basic principles of laser wakefield acceleration and relative plasma nonlinear optics. Chapter 3 is about generation of intense laser pulses by chirped-pulse-amplification and architecture of our 10-TW laser system. Chapter 4 is our experimental design, including parameter design and experimental setup. We will discuss the experimental results and try to explain in chapter 5. Both experimental and simulation results agree that MIR pulses come from self-phase modulation by photon deceleration in the bubble regime of laser wakefield acceleration. Finally, We’ll summarize the thesis and plan future works to finish full characterizations on the MIR light source in chapter 6.