孫啟光汪治平臺灣大學：郭政誠Kuo, Cheng-ChengCheng-ChengKuo2007-11-252018-07-052007-11-252018-07-052007http://ntur.lib.ntu.edu.tw//handle/246246/50799高階諧波產生為時下發展同調性高, 超短脈衝時寬的 X-ray 最有效的 方法, 因此成為非常熱門的研究領域; 根據目前的理論模型及實驗成果, 其 產生可藉由兩種物理機制, 一是藉由光場游離爾後與離子結合產生機制, 為光場先游離出束縛在原子中的電子, 經由光產生的電場加速後與離子 結合而輻射出高階諧波, 二是相對論性諧波產生機制, 藉電子在強電場下 呈現非線性的運動而直接輻射出高階諧波; 然而, 諧波相關研究在應用上 面臨的挑戰仍在於產生更短波長的諧波及達到更高的頻率轉換效率, 為了 使諧波的轉換效率持續增加, 除了增長作用氣體介質長度以提高增益效果 外, 同時更必須抑制因相位反轉造成諧波轉換效率的降低, 此限制可藉由 波導結構使雷射脈衝維持高尖峰強度即使其頻率轉換過程相位匹配, 而達 到大幅提昇諧波轉換效率的目標, 同時亦是本篇論文討論的重點。 第一章 會首先簡述高階諧波的發展演進歷史, 第二章則介紹產生高階諧波的相關 物理原理及目前發展。 受惠於近期在超短脈衝極高功率雷射的進展, 激發脈衝光源具有越 來越高的尖峰強度, 結果促使相對論性諧波產生機制成為發展高強度、短 脈衝時寬、高同調性的 X-ray 光源成為最具潛力的方法; 在光場游離爾 後與離子結合產生機制中, 使用較高的雷射尖峰強度為達到更短波長時, 伴隨而來的高游離率所產生的高密度電子, 其電漿色散效應為達到相位匹 配主要的限制, 導致在頻率轉換過程中相位反轉而令效率大幅降低, 除此 外, 雖利用中空光纖或波狀中空毛細管已被證明可進一步提高此機制產生 的高階諧波, 在高脈衝強度下其相關材料元件卻容易損壞, 且在操作尚可 工調空的元件自由度較單純的利用氣體噴嘴所形成的氣體介質低, 以上的 原因皆限制了未來的應用性, 同時對相對論性諧波產生及相關強場電漿元 件應用並不適合; 這些概念隨著一些產生電漿波導結構及方法的提出, 其 中並討論了如何使雷射脈衝及高階諧波同時在其中傳播的技術並克服相 位反轉問題, 但是尚未藉由實驗驗證, 並證明可應用在相對論性諧波的相 位匹配上, 也提出可用空間中密度調變的結構針對相對論性諧波產生達到 準相位匹配而進一步提高頻率轉換效率, 在此方案中, 在激發脈衝傳播方 向所分佈的高低電漿密度層狀週期性結構, 適當的調整相對寬度可週期性 的調整激發脈衝與諧波之譏的相位差避免相位反轉問題, 另外在高電漿密 渡區域具有的高增益又可同時增強相對論性諧波產生的強度, 於是, 隨著 週期性結構分佈的作用介質長度的增加, 諧波強度可持續的提高, 如前述 目前仍無任何關於準相位匹費的相對論性諧波在實驗上的驗證。 於第三章 會說明如何發展程式化控制液晶波形調變器在氣體上製造出空間暫態電 漿結構的方法及物理原理, 並提出應用在相對論性高階諧波準相位匹配 上的實驗以證明發展的可行性。 第四章列出實驗架設、實驗結果及成果討論, 主要利用一道與激發脈 衝垂直傳播的雷射脈衝, 搭配液晶空間波形調變器以製造空間暫態電漿結 構的方法, 討論實驗中必須注意以達到最佳化的參數, 及此項方法的限制; 而利用此一均勻電漿波導及週期性電漿波導提高相對論性三倍頻諧波的 轉換效率的結果已被首先驗證, 而獻上即時改變調變液晶空間波形調變器 的參數以達到程式化控制製造空間暫態電漿結構, 也觀測到針對三倍頻強 度的共振增強現象, 而針對此週期性電漿結構中的電漿密度及週期條件參 數變化的共振條件, 也證明準相位匹配是造成三倍頻能量提高最主要的影 響因素, 相對論性三倍頻在能量目前可提升 8 倍, 而強度可增加 50 倍, 同時亦配合相關的模擬計算加以佐證。 而預期以非週期性的空間電漿暫態 結構及利用液晶空間波形調變器搭配演算法回溯控制最佳化方法, 可進一 步的提高諧波頻率轉換效率。 第五章會另外提出未來邁向更短波長的 相對論性諧波的理論模擬實驗參數, 同時此項成果也開啟了未來研究其他 電漿物理現象或應用的一種方法或技術, 如在多級電子加速器的發展及質 子加速器的應用。High harmonic generation based on optical-field ionization (OFI) of electrons in atoms followed by recombination and relativistic harmonic generation based on nonlinear quiver motion of free electrons in relativistically intense laser field are active fields of research, as they are the most promising approaches to date for generating coherent ultrashort x-ray pulses. Yet, for a wide range of applications how to produce harmonics with short wavelength and high efficiency is still a challenge. In order to achieve sus- tained growth of harmonics, one need to increase the interac- tion length and at the same time suppress the reverse conversion process. This can be done by guiding the laser pulse and phase matching the frequency conversion process. The first chapter of this thesis gives an introductory to the history of the development of high-order harmonic generation. The second chapter contains the physical background on harmonic generation and current de- velopement. Recent progress in high-power ultrashort-pulse laser technol- ogy has made available pump sources with higher and higher in- tensity. As a result, relativistic harmonic generation is emerg- ing as a promising scheme for generating strong coherent x-rays. However, hollow fiber and corrugated hollow capillary are not suitable for relativistic harmonic generation because optical dam- age occurs under relativistic intensity. A few schemes for pro- ducing a plasma waveguide have been reported, which has the capability of guiding the pump laser pulse and the harmonics si- multaneously, but none of them has been applied to relativistic harmonic generation yet. It has also been proposed that a plasma density modulation may be used to achieve quasi-phase matching of relativistic third harmonic generation. In this scheme period- ically interlacing layers of high- and low-density plasmas in the direction of beam propagation with their widths matching the coherence lengths allow periodic correction of the relative phase between the harmonic wave and the fundamental. With a gain in the high-density layer larger than the inverse gain in the low- density layer, the harmonics can grow with increasing interaction length. To date, no experimental demonstration of quasi-phase- matched relativistic harmonic generation has been reported yet. Technical issues about the programmable fabrication of spatial structures in a gas by laser machining wiht liquid-crystal spatial light modulator and applications to quasi-phase matching rela- tivistic high harmonic generation will be discussed disscussed in chapter 3. The experimental results are discussed in chapter 4. Enhance- ments of relativistic third-harmonic generation (THG) by using a uniform plasma waveguide and a periodic plasma waveguide are demonstrted. For the case of uniform waveguide, enhancement of harmonic generation by optical guiding was verified. For the case of periodic waveguide, resonant dependence of harmonic intensity on plasma density and density modulation parameters was ob- served, which is a distinct characteristic of quasi-phase matching. Under the resonant condition, the energy of the third harmonic was enhanced by a factor of 8, and the intensity was enhanced by a factor of 50. On-line optimization of the resonant enhancement was made possible by the capability of programmable fabrication of longitudinal density modulation using a liquid-crystal spatial light modulator (SLM). Further enhancement is possible by using an aperiodic longitudinal density structure optimized with adap- tive control on the SLM settings to compensate for nonlinear effects in the harmonic growth. Future work based on program- mable fabrication technique and quasi-phase-matching techniques to higher order relativistic harmonic generation is proposed in chapter 5. Application to other feasible laser-plasma phenomena will still be discussed.誌謝...iii 摘要...v abstract...vii 圖目錄...xi 1 緒論...1 1.1 發展高階諧波之重要性...1 1.2 高階諧波發展簡史...2 1.3 十兆瓦雷射驅動光源...5 2 高階諧波產生之物理概念及目前發展...7 2.1 高階諧波產生的原理...7 2.1.1 光場游離爾後與離子結合產生之高階諧波...9 2.1.2 相對論性高階諧波...12 2.2 高階諧波之發展...19 2.2.1 光場游離爾後與離子結合產生諧波之發展...20 2.2.2 相對論性諧波之發展...21 3 增強相對論性高階諧波之提案...23 3.1 空間暫態電漿結構的產生...23 3.1.1 製造空間暫態電漿結構之原理...23 3.1.2 程式化控制並製造空間暫態電漿結構之設計...24 3.2 準相位匹配之相對論性諧波...25 3.2.1 準相位匹配相關物理參數...29 4 增強相對論性低階諧波之實驗...33 4.1 實驗設計及架設...33 4.2 實驗結果與討論...37 4.2.1 程式化製造空間暫態電漿結構的成果...37 4.2.2 相對論性低階諧波隨各種參數之相關性...40 4.2.3 相對論性低階諧波之增強...47 5 結論與展望...57 參考文獻.......613616748 bytesapplication/pdfen-US相對論諧波電漿波導準相位匹配relativisticharmonicplasmawaveguidequasi-phase matchingthesishttp://ntur.lib.ntu.edu.tw/bitstream/246246/50799/1/ntu-96-R93941034-1.pdf