利用雷射與凝態物質交互作用產生脈衝粒子與光子

脈衝雷射與凝態物質的交互作用，近幾十年來在科學界，一直是很熱門的研究題材。此交互作用的過程演化以及應用，更是受到關注。而在這眾多的凝態物質當中，我們所感興趣的有兩種，一是原子團，它兼具了局部密度與固體一樣以及平均密度與氣體一樣的特性，使得雷射可以在原子團中傳播又可以讓原子團有效地吸收雷射能量。因為具有此優勢，所以讓原子團成為一種很廣泛被使用的介質，舉凡X光雷射、高階諧波產生、電子加速器與電漿非線性光學等，皆有許多相關的實驗與理論模擬。而另一感興趣的物質為金屬薄片，藉由雷射與薄片交互作用的方式，可產生超過臨界密度的電漿，也就是說相較於氣體，可以產生較高的電子數目�電子流，如此可用來更有效地加速粒子；再加上固態金屬靶本身的重離子，以及可塗佈上含氫或有機分子化合物薄膜的特性，使得近期研究雷射脈衝產生高能量的質子與離子，大部分都著重在於此種方式上。而目前同調光源所最缺少的兆赫茲波段，也有理論模擬指出可以藉由此交互作用產生高強度的輻射。高階諧波的產生，最常利用的方式是原子或原子團與雷射脈衝交互作用。早期的研究已發現，原子團可以增加同種類原子產生高階諧波的最高光子能量，其主要是因為原子團內原子與原子之間的作用力，改變了外層電子的束縛能，進而影響雷射游離外層電子後回撞回來再結合而產生的最高光子能量。而在實驗過程中，我們更發現使用一道預脈衝（雷射的尖峰強度不可超過外層第一顆電子的游離能），可引發原子團週期性的振動。而在此週期性振動的特定時間點導入主脈衝，發現亦可增加高階諧波產生的強度（與只有主脈衝沒有預脈衝所產生的強度相比較）。此種振動，早期在分子結構上已經有相關的研究，但卻從未有人將此應用在原子團的振動與其高階諧波的產生。這是我們第一次在原子團上發現這樣的現象，也是第一個將其應用在增強高階諧波產生。自從啾頻放大技術被應用在雷射放大器上，使得雷射尖峰功率高於兆瓦以上成為可能；而大約在二十年前，Gitomer 等人第一次用使用強場雷射脈衝與固態靶交互作用的方式產生質子束，因此讓此種方式的物理機制以及相關研究，成為近十幾年來科學家很關注的議題。不論是針對不同的靶材或是雷射特性所實驗或模擬出來的定性與定量上的研究，或是針對所產生的質子束應用在癌症治療或是中子產生，皆成為非常重要的題材。在台灣，我們實驗室第一次成功地利用一百兆瓦雷射來產生千萬電子伏特能量的質子束。實驗中所產生的質子最高能量與數目，皆與理論模擬的雷射強度對應質子能量的比例法則相吻合。而在這之中，我們更引入了一道預脈衝，在靶材前方產生一適當的電子密度梯度分布，成功地提高質子的最高能量與數目。此現象在現今的理論雖然有預期，但在實驗上並無顯著的成果，所以這也是一個很令人感興趣的題目。而利用相同的架設，我們也成功地產生具有同調性的兆赫茲與中紅外光源的輻射。參考理論模擬的文獻，經過計算之後，發現理論與實驗有很高的一致性。因此，對於發展具有同調性的兆赫茲與中紅外光源，雷射與固態靶交互作用也是一個非常有潛力的方式。這些實驗顯示出雷射與固態靶交互作用中的物理機制與最後產物，都具有極高的重要性。我將在這篇論文仔細分析與討論，並且檢討其中的優缺點以及未來可以改善的目標。

關鍵字

電漿；原子團振動；質子加速；中紅外光；兆赫茲輻射

並列摘要

In the past decades, the interaction of short-pulse laser with condensed matter has been a very popular research theme. In the thesis, the numerous matters we are interested at are atom cluster and metal foil. The atom clusters have the characteristics of local density as well as solid and average density like gas, which causes the laser to be possible to propagate in the clusters and high absorption rate from the laser to the clusters. These advantages let the atom clusters become a very widely-used medium in application, such as X-ray laser, high-harmonic generation, electron acceleration, plasma nonlinear optics and so on. There are also many related experiments and theoretical simulations on these apllications. In contrast to gas, the metal foil can generate much higher electron density (overdensed) plasma from laser-foil interaction, which causes to produce high electron number (current). Therefore, it can be used to accelerate particle more efficiently. In addition, the foil has the characteristcs of heavy ion in the metal and the hydrogen or organic-molecular coating on the surface, which lets the laser-foil interaction become an important method to produce high-energy proton in the high-field region. Under the similar scheme, generation of tera-hertz radiation from laser-foil interaction have also been reported in theory and simulation. High harmonic generation (HHG) based on optical-field ionization of electrons in atoms followed by recombination is an active field of research, as it is a promising approach for generating coherent ultrashort x-ray pulses. In the simplest case of an atom interacting with a strong laser field, the electron is (tunnel) ionized, accelerated by the ac electric field, and can recombine with the parent ion to generate a high-energy xuv photon. As the process of HHG in individual atoms becomes well understood, exploring HHG in more complex systems has attracted great attention as a new direction of research. In experiment, sharp periodic enhancement of high-harmonic generation synchronized to prepulse-induced cluster vibration is observed. Tenfold enhancement of high-harmonic-generation efficiency is achieved by optimal selection of the prepulse–pump delay. This strong correlation between high-harmonic generation and cluster vibration also provides a new tool for studying the vibrational dynamics of nanometer atomic clusters. This kind of vibration has been researched before in the molecular structure, but never in cluster. For the first time, we observe this cluster vibration induced by laser pulse, and then use it to enhance HHG generation. Since the chirped-pulse amplification (CPA) technique is applied to laser amplifier, the laser peak power above terawatt becomes possible. About twenty years ago, for the first time, Gitomer et al. use the interaction of high-inetnsity-short-pulse laser with solid target to generate proton acceleration. Thereafter, the physics and related research about this interaction become important and noticeable subjects. In taiwan, for the first time, we experimentally generate proton beam with 9-MeV maximum energy by using 100-TW laser system interaction with Al-foil. The results have good agreements with theory and simulation. We also carry out the enhancement of maximum proton energy by using laser prepulse-pump scheme. According to the simulation result proposed by Sheng et al., this enhancement may be due to stochastic heating of electron in the colliding laser fields, where the backward propagating fields are produced in part by the Raman backscattering in the underdense plasma region as well as by reflection from the overdense plasma region. Base on this, the prepulse can be used to optimize electron-density scale length for stochastic heating, which leads to heat the electrons up to a temperature much higher than the corresponding laser ponderomotive potential. Generations of tera-hertz (THz) and mid-infrared (MIR) radiations by using laser-foil interaction are also achieved in the similar setup. The results agree well with the simulation based on linear-mode–conversion model proposed by Sheng et al.. These results reveal that the mechanisms and production from laser-solid interaction are extremely important. In the thesis, I will report my analyses and discussions of the pulsed particle and photon sources generated from the laser interaction with condensed matter, and review their advantages, disadvantages and improvements in the future.

並列關鍵字

Plasma ； Cluster vibration ； Proton acceleration ； Mid-infrared ray ； Tera-hertz radiation

參考文獻

[46] T. Esirkepov, M. Borghesi, S. Bulanov, G. Mourou, and T. Tajima,

[61] R. Sonobe, S. Kawata, S. Miyazaki, M. Nakamura, and T. Kikuchi,

[62] M. Nakamura, S. Kawata, R. Sonobe, Q. Kong, S. Miyazaki, N. Onuma,

J. P. Geindre, E. Lefebvre, and P. Martin, ``Proton Acceleration with

[65] S. Sakabe, T. Mochizuki, T. Yamanaka, and C. Yamanaka, ``Modified

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利用雷射與凝態物質交互作用產生脈衝粒子與光子

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