高階諧波是一種從極紫外光延伸至 X 光的高相干性光源， 它是藉由高強度雷射游離氣體原子而產生。高階諧波的轉換效 率與入射光源在介質中作用的相位匹配有關，主要包括四個因 素：中性氣體色散 (neutral gas dispersion)、電漿色散 (plasma dispersion)、光束幾何相位變化 (geometrical phase shift) 與內 在偶極子相位變化 (intrinsic dipole phase variation)。在低游 離率的情況下，可以透過調整雷射強度游離適當比列的原子， 使電漿色散與中性氣體色散達成平衡，藉此達成相位匹配，然 而為了獲得更高階數的高階諧波，需要提高雷射的強度來游離 氣體，以達到更高的截止能量，但是此時游離率會過高而無法 達成相位匹配，導致能量轉換效率降低。 本實驗為了尋找解決能量轉換效率問題的方法，我們針對第 27 階高階諧波進行實驗，詳細紀錄高階諧波產生時的雷射狀 態、氣體狀態、兩者產生作用時的游離狀態，以此直接測量出 高階諧波的相位變化，然後使用一道側向雷射控制作用長度， 得到高階諧波與雷射作用距離的關係，最終統整高階諧波產生 時的詳細條件，以此補助在高游離態下高階諧波產生的研究。;High-order harmonic generation (HHG) is a kind of ultrashort coherent EUV/x-ray source. It is produced by ionizing gas atoms with a high-intensity laser. The conversion efficiency of high-order harmonic generation is determined by the relative phase between the driving field and the harmonic field in the medium. It is determined with four factors: neutral gas dispersion, plasma dispersion, geometrical phase shift, and intrinsic dipole phase variation. In low ionization condition, phase-matching condition can be achieved by balancing the negative plasma dispersion and the positive neutral gas dispersion. In this thesis we focus on the generation of the 27th order harmonic. We made a tomographic measurement of its growing process, and characterized its complete phase-matching condition. We found that the phase-matching condition is nonuniforn in both longitudinal and transverse directions, and the major contribution of the 27th harmonic yield comes from the short-trajectory emission from the central part of the driving pulse. These techniques could be beneficial for pursuing efficient high-harmonic generation in shorter wavelengths.
