高相干性高穩定度十兆瓦雷射系統之建造與應用：雷射－原子團交互作用與X光雷射

Construction of a 10-TW Laser of High Coherence and Stability and Its Application in Laser-Cluster Interaction and X-Ray Lasers

朱旭新(Hsu-Hsin Chu)

指導教授 ： 汪治平

國立臺灣大學/理學院/物理研究所/博士(2005年)

https://doi.org/10.6342/NTU.2005.02618

摘要

近十年來飛秒 (fs) 雷射科技以及雷射啾頻脈衝放大技術 (chirped-pulse amplification technique) 的快速進展，已使尖峰功率高達數十兆瓦到百兆瓦 (10~100 TW) 的雷射脈衝能夠由桌上型的小型雷射系統產生，其聚焦之後的光場強度可以超過10^20 W/cm^2。在這麼強的光場之下，物質與光的交互作用達到過去從未能夠探索的境界。以電子為例，其受到的電磁力約為10^-6 Nt，相當於氫原子中質子吸引電子之庫倫力的100倍，其加速度大於10^23 g (地表重力加速度9.8 m/sec^2)，接近黑洞所產生的重力加速度。因此飛秒雷射科技正在開闢物理研究的新領域，並且產生了過去難以想像的應用。例如電漿非線性光學 (plasma nonlinear optics)、高諧頻產生 (high harmonic generation)、桌上型X射線雷射 (table-top x-ray laser)、電漿波式電子加速器 (laser wake-field electron accelerator)、雷射致發核物理 (laser-initiated nuclear physics)、次飛秒光源 (sub-femtosecond light source)、實驗室內天文學 (laboratory astronomy) 以及加速座標與彎曲時空之特性研究。
本論文的第一部份，即在於詳細說明如何建造一套高穩定度並且高時間空間品質的十兆瓦 (10 TW) 鈦藍寶石雷射系統，並且完整檢測其輸出規格。此雷射系統之輸出脈衝能量為 550 mJ，脈衝時寬 50 fs，中心波長 810 nm，頻寬 25 nm。其高時空品質表現如下：時寬－頻寬乘積為傅力葉轉換極限之 1.2 倍，以 F/3.8 與 F/7.7 之偏軸拋物面鏡聚焦，焦點大小分別為 4.3 mm 與 8.5 mm，均為繞射極限之 1.2 倍，同時焦點內之能量分別可達 70% 與 80%，因此光場尖峰強度可達 4 x 10^19 W/cm^2。其高穩定度表現如下：輸出能量擾動只有 1.3%，脈衝時間波形變化與焦點大小變化小於 2.4%，焦點空間位置跳動小於 4 mm。如此品質足以提供作為高強度雷射與物質交互作用之研究。
本論文的第二部份，是將此雷射系統運用於雷射－原子團交互作用之研究中。已知在低溫下氣體分子以 van der Waal’s force 彼此結合所形成的原子團可以有效地吸收高強度雷射脈衝，形成高溫高密度的奈米電漿球，此種奈米電漿球已證實可以輻射出高亮度的X光，放射高能量的帶電粒子 (keV electron, MeV ion)，產生高效率的高階諧波，甚至引發核融合。我們的研究首先著重於利用我們所具備的雷射脈衝時間空間控制能力，達成氬氣奈米電漿球X光輻射之最佳化，其雷射光－X光之轉換效率在 11~12 nm 波段可達 12%，Ar7+ 5d-3p 在 13.8 nm 的輻射亮度達 4.1 x 10^25 photons/cm^2/nm/sec/sr，此強度已足以提供作為X光蝕刻術之光源使用。其次我們研究由奈米電漿球集合形成的奈米電漿球氣體之光學性質，利用奈米電漿球在擴散過程中，其 polarizability 會隨密度下降而改變的特性，我們使用一個預雷射脈衝驅動奈米電漿球的擴散，來調整奈米電漿球氣體整體的折射率變化，而達成對於後續主雷射脈衝之傳播的控制。此項研究驗證了奈米電漿球氣體之折射率能夠由 >1 變化至 <1，這種完全不同於均勻電漿的光學性質能夠成為電漿非線性光學全新的研究方向。
本論文的第三部份是利用此超高功率雷射系統做為氙氣X光雷射之激發光源，完成 Pd-like Xe (Xe8+) 5d-5p 在 41.8 nm 的激發輻射放大。X光雷射在結構探測、活體生物顯微術、內層核電子動力學、高密度電漿診斷、X光蝕刻術等等領域上均有極大的用處，然而過去若要產生波長低於 50 nm 的X光雷射，一般需要能量達數十焦耳甚至數百焦耳的巨型雷射系統做為其激發光源，這種體積與經費的限制使X光雷射的應用難以真正落實，我們利用桌上型超高功率雷射系統達成了 Pd-like Xe 41.8 nm 的飽和輸出，所需能量低於 400 mJ，並澄清 plasma uniformality, ionization induced refraction, above-threshold-ionization heating 等機制對於X光雷射輸出之影響，以做為後續發展波長更短、效率更高之X光雷射之基礎。

關鍵字

X光雷射 ； 雷射原子團交互作用 ； 十兆瓦雷射

並列摘要

Following the progress of high-power laser systems advances in the past decade, the exploration of the interaction between strong electromagnetic field and matter has emerged as a new research frontier called “high-field physics”. The maximum intensity produced at the focus of an intense laser pulse can exceeds 10^20 W/cm^2, which is high enough to drive nonlinear motion of free electrons. Therefore, a new field of nonlinear optics, that of relativistic electrons, has been launched, and many applications such as laser-wakefield electron accelerators, soft x-ray lasers, high-order harmonic generation, and laser fusion are developed.
This thesis records my efforts and accomplishments in the research of high-field physics. Chapter 1 reviews the progress of high-power laser systems, including the mechanisms of femtosecond pulse generation and the principles of chirped-pulse amplification. Chapter 2 describes the construction of a versatile 10-TW laser system that I participated. How to achieve high stability and spatiotemporal quality by robust passive controls are presented, and the design principles and methods of characterization and verification are discussed. The basic physics of laser-plasma interaction is introduced in chapter 3, which provides the background knowledge of the following chapters. Chapter 4 presents the studies on the interaction between intense laser pulses and atomic clusters. Two experiments are described. The first is the maximization of soft x-ray emission form laser-irradiated argon clusters. The conversion efficiency in the 11–20 nm wavelength range reached 12%, and a pulse energy of as high as 0.3 mJ was obtained at 13.8-nm emission line. The brightness of this line emission reached 4.1 × 10^25 photons/cm^2/nm/sec/sr, close to that of synchrotron radiation at the same wavelength. This brightness was high enough for many soft x-ray applications such as x-ray microscopy and photolithography. The second is the control
of laser pulse propagation in a cluster gas, which was the first demonstration in the world. Transient refractive index of ionized cluster gas was verified, and the corresponding variations in the microscopic polarizability and macroscopic refractive index were observed. These unique properties may contribute to research on plasma nonlinear optics, such as phase matching of high-order harmonic generation and plasma waveguide formation. Finally,
chapter 5 presents the first demonstration of an optical-field-ionization (OFI) x-ray laser with a laser-irradiated xenon clustered gas jet. Near saturated amplification was achieved. The output energy reached 95 nJ, and the divergence angle of which was 5.2 mrad. In comparison with previous OFI x-ray lasers which use gas-cell targets, the use of gas jet make contamination-free and long-term operation possible.
The original works described in Chapter 2 and 4 are published in Appl.Phys.B 79, 193 (2004), Opt. Comm. 231, 375 (2004), and Phys. Rev.E 69, 035401(R) (2004). The x-ray laser work in Chapter 5 has been submitted to Phys. Rev. Lett. The international system of units (SI) are adopted for all formulas.

並列關鍵字

10-TW laser ； laser-cluster interaction ； x-ray laser

參考文獻

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IEEE J. Quantum Electron. 24, 398–403 (1988).