利用超短雷射脈衝入射氣態靶產生的高階諧波(High harmonic generation, HHG)是一種高相干性的極紫外光與軟X光光源。這種非線性交互作用的轉換效率，取決於激發雷射電場與高階諧波電場之間的相對相位，因此，達成相位匹配是產生高效率高階諧波的重要條件。這個研究中我們利用調變氣體密度、游離比例以及雷射聚焦點大小，使氣體色散、電漿色散與光源幾何上的相位變化在作用區域內平衡，達到相位匹配的目的。最終在極紫外光(EUV, extreme ultraviolet) 波段，第 23 階諧波輸出脈衝能量達到 1.2 nJ，轉換效率為 1.6*10^(-7)，光譜尖峰亮度為 1.33*10^(23) photons/sec/mm^(2)/mrad^(2)/0.1%BW，能量與指向穩定度分別達 3% 及 16*10^(-6) rad，這樣高亮度、高穩定性的高相干性超快 EUV 光源在材料探測、化學動態反應解析、光電子能譜學、電將非線性光學等研究上有廣泛的應用。

High-harmonic generation (HHG) has been demonstrated as a reliable ultra-short coherent light sources. By focusing intense laser pulses into gas targets, photons ranging from extreme ultraviolet to soft x-ray are produced. we demonstrate the harmonic emission from 24 nm to 38 nm. The output is maximized by adjusting the gas length, gas density, ionization ratio, and the focusing geometry. Phase matching is achieved by the balance of the argon gas dispersion, plasma dispersion, and the geometrical phase shift between the driving laser field and the emitted harmonic field. Such an ultrafast coherent EUV source would be very useful for EUV nonlinear optics and EUV photoelectron spectroscopy.

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