高階諧波是透過高強度雷射脈衝游離氣態原子而產生，所產生的波段從極紫外光 (EUV)至軟X光。高階諧波在時間和空間上同時具有高同調性，是科學研究的有力工具。但其能量轉換效率低，一直是高階諧波在應用上的障礙。為了提高其轉換效率，必須要考慮入射光源與高階諧波在介質中傳播的相位匹配。在低游離率條件下，可透過調節中性氣體密度和雷射強度來達成色散平衡。然而要將高階諧波的截止頻率提高，必須提高雷射強度，或使用高游離態的離子為介質。此時電漿密度大幅升高，無法平衡中性氣體色散與幾何色散而達成相位匹配。 在本計畫中，我們運用中央大學強場雷射設施在雷射強度上的優勢提出使用側向調製的雷射光束來達成高階諧波的準相位匹配，突破轉換效率和頻率延伸之限制。此方法優點如下：(1) 在能量轉換上，高階諧波輸出隨介質長度指數成長，可達成高效率轉換；(2) 藉提升雷射強度，配合使用高游離態的離子為介質，可延伸波段至軟X光；(3) 可藉由準相位匹配的條件來選擇放大單一諧波，大幅提高頻譜的純度和可調性；(4) 實驗架構簡潔，實驗參數可調性高。 為了執行該計畫，我們已經開發了高階諧波斷層掃描技術，這項技術能協助我們觀察高階諧波在作用過程中增長的行為以及其相位的變化。對於計劃來說，該項技術能夠精準地讓我們知道該如何調製側向雷射光束以實現高階諧波的準相位匹配。除此之外，我們也開發了模擬程式，模擬雷射在介質中傳播過程中游離率的分布、以及加入側向雷射光束後高階諧波隨作用距離增加而增長的行為。該模擬程式幫助我們設計實驗參數與了解物理過程。一旦高階諧波的轉換效率與頻率能提升，其在科學與科技發展上有很高的應用潛力，並能對台灣在社會與經濟與國際地位上帶來正面的影響。 ;High-harmonic generation (HHG) from laser-irradiated rare gases has been proven to be a reliable source of ultrashort coherent extreme-ultraviolet (EUV) or soft x-ray. HHG is a powerful tool for science due to the highly spatial and temporal coherence. However, the low conversion efficiency limits the applications. To enhance the conversion efficiency, the phase matching condition between the propagations of the driving pulse and the HHG in the medium. Adjusting the laser intensity and the gas density achieves the dispersion balance at low ionization ratio condition for phase matching. Higher laser intensity or highly ionized ion as the medium can enhance the cut-off frequency, but the unbalanced dispersion results in the phase mismatches. In this project, we propose to utilize a transverse modulated laser beam to achieve the quasi-phase-matching (QPM) of HHG for the enhancement of the conversion efficiency and the cut-off frequency based on the superiority of high intensity at Nation Central University laser facility. This scheme shows merits: (1) HHG yield exponentially grows with the length medium; (2) high intensity laser and highly ionized ion extend the HHG to soft X-ray; (3) the HHG is tunable and selective based on this scheme; (4) the setup is concise. For running this project, we have developed HHG tomography helping us to observe the dynamics of HHG yields during the interaction. This HHG tomography also help us reshape the transverse laser beam precisely for QPM-HHG. We also developed a simulation for the the calculations of the driving pulse propagation and the ionization ratio distribution during the interaction, and the investigations of QPM-HHG by a transverse modulated laser beam. HHG will have the scientific and technologic potential for applications once conversion efficiency and the cut-off frequency can be enhanced, and it will cause the positive impact on society, economy, and international status for Taiwan.
