A Novel Bright Attosecond Light Source Driven by Relativistically Intense Femtosecond Laser Pulses

激光技术的巨大发展使得高重复率少周期的激光脉冲成为可能。近10年来的技术创新更是成功地将这些脉冲放大到100毫焦量级，而且可以同时控制和测量这些高能量少周期激光脉冲的关键参数：例如，对比度和载波包络相位。这些技术的进步使得实验室中能够获得的峰值光强高达10</sup>20</sup> W/cm<\sup>2<\sup>，从而在强场物理研究方面开拓了一系列全新的领域。 激光向其谐波的有效频率上转换是其中的一个非常热门的研究方向。这一过程的重要性最终与高能量极紫外阿秒光脉冲的产生及其在新兴阿秒科学中的作用相关。现今，大多数的阿秒光源都是基于气体介质中的高次谐波产生过程。由于原子中的高次谐波产生过程受到饱和光强的限制，这些光源的亮度有限。超过饱和光强，谐波转换效率因介质耗尽而下降。这严重限制了它们可能的应用范围。获得足够强的阿秒脉冲，是进行阿秒泵浦—阿秒探测光谱学研究的前提。为克服这一困难，人们提出了利用相对论强激光脉冲与高密度等离子体相互作用的方法实现阿秒光源。与原子介质中的谐波产生过程相比，使用等离子体作为非线性介质允许更高的相互作用光强；使得最先进的太瓦和拍瓦激光系统成为超强阿秒光脉冲的理想驱动器。本论文多方面研究相对论飞秒强激光与等离子体相互作用中的高次谐波产生过程，特别关注少周期相对论强激光与固体靶相互作用中的阿秒光脉冲产生，具体内容如下： 1. 在ATLAS激光系统作为驱动器与固体密度等离子体相互作用的实验中，同时测量了由相对论振镜机制产生的高次谐波的效率和发散度。在归一化矢势 a</sub>L</sub>=1.5的条件下，在 17+-3 mrad 的发散锥内测量到30 uJ的高次谐波辐射。这对应于在定向锥内，在17-80 nm范围内的光谱转换效率>=10<\sup>-4<\sup>。 假定测量到的谐波相位锁定合成一个阿秒脉冲链，估算表明，脉冲链中的单个脉冲具有前所未有的高平均功率。而且，基于PIC模拟的研究表明，在更高的激光强度下，有希望获得高达几个百分点的谐波转换效率。 2. 在LWS-20激光系统作为驱动器与固体密度等离子体相互作用产生极紫外准连续谱的原理性验证实验中，清晰地观测到延伸到铝箔滤波片透射窗口截止波长处（17 nm）的谐波信号。在17-42 nm光谱范围测量到的信号傅里叶逆变换为小于60阿秒的单个阿秒脉冲。第22次谐波的光束发散度约为30-40 mrad。估算表明，实验中产生了光子能量>30 eV的0.1 uJ的极紫外单个阿秒脉冲。这一结果比PIC模拟的预期小1至2个数量级。分析了造成这一差距的原因，提出了相应地改进措施。 3. 使用particle-in-cell粒子模拟的方法，系统地研究了少周期相对论强激光脉冲与等离子体相互作用中的阿秒光脉冲产生。发现阿秒光脉冲能量的显著增强依赖于激光脉冲长度，载波包络相位和等离子体标尺长度。基于模拟结果预测，使用当今最先进的激光技术有望获得光子能量大于16 eV、单脉冲能量约为100 uJ的单个孤立的阿秒光脉冲。 4. 使用particle-in-cell粒子模拟的方法，研究了相对论少周期激光脉冲与固体密度等离子体表面相互作用产生的阿秒光脉冲的偏振态。在合适的相互作用条件下，通过控制入射激光脉冲的偏振态可以获得所需的阿秒光脉冲的偏振态。特别地，一个具有特定椭偏率的椭圆偏振激光脉冲能够产生一个几乎为圆偏振的阿秒光脉冲而不会降低谐波转换效率。这一过程可以用来发展新型桌面化的圆偏振极紫外光源，用于以高时间分辨率探测发生在阿秒时间尺度上的物理现象。The prodigious progress in laser technology has made readily available laser systems delivering pulses of a few-cycle duration at high repetition rate. Further innovations have led to successful amplification of these pulses up to 100 mJ level with simultaneous control or characterization over some crucial laser pulse parameters produced by such systems, e.g., contrast level or carrier-envelope phase. These advancements have enabled the generation of enormous peak intensities in the laboratory reaching the 10</sup>20</sup> W/cm</sup>2</sup> level. As a consequence, the road to whole new areas of research in high field physics has been opened. One such area of great interest is the efficient frequency up-conversion of the laser light into harmonics. The significance of this process is ultimately linked to the generation of energetic attosecond XUV bursts and their impact to the emerging field of attosecond science. To date, most of the attosecond light sources are based on high-order harmonic generation in gaseous media. They display however, limited brightness due to the fact that the harmonic generation process in atoms exhibits a saturation intensity over which the conversion efficiency drops due to medium depletion. This severely restricts the scope of applications since the availability of a source delivering rather intense attosecond pulses is the prerequisite for XUV-pump-XUV-probe spectroscopy. To circumvent this limitation, attosecond light source from relativistic interaction of an intense laser pulse with overdense plasma has been suggested. The main advantage over the process of harmonic generation in atomic medium is that the plasma medium allows the use of higher laser intensities available from state-of-the-art multi-TW and PW laser systems, thus rendering them the ideal drivers to a source of intense attosecond pulse trains. The present dissertation investigates high-order harmonic generation from relativistic femtosecond laser pulse interaction with the surface of overdense plasmas, with focus on attosecond light pulse generation from relativistic few-cycle laser pulse interaction with solid density plasma target. It details as following: 1. In the laser and solid density plasmas interaction experiment using the ATLAS laser system as the driver, we report on simultaneous efficiency and divergence measurements for harmonics generated by the relativistic oscillating mirror mechanism. For a value of the normalized vector potential of a</sub> L</sup>= 1.5, we demonstrate the generation of 30 uJ high-harmonic radiation in a 17+-3 mrad divergence cone. This corresponds to a conversion efficiency of >=10</sup>-4</sup> in the 17-80 nm range into a well-confined beam. Presuming phase-locked harmonics, our results predict unprecedented levels of average power for a single attosecond pulse in the generated pulse train. Results of PIC simulations raise the prospect of attaining efficiencies of a few percent at higher laser intensities. 2. In the experiment using the LWS-20 laser system as the driver for the generation of XUV quasi-continuum, harmonic signal up to the aluminum transmission cut off (17 nm) is clearly observed. The measured signal in the wavelength range 17-42 nm is inverse Fourier transformed to one single attosecond pulse with duration shorter than 60 as. The divergence of the 22nd harmonic beam is around 30-40 mrad. Estimation of the experiment results gives 0.1 uJ light energy in a single attosecond pulse for photons >30 eV, which is 1 to 2 orders of magnitude lower than that expected from PIC simulations. The reasons are analyzed with possible solutions. 3. A systematic study is performed through particle-in-cell simulations to investigate the generation of attosecond pulse from relativistic laser plasmas when laser pulse duration approaches the few-cycle regime. A significant enhancement of attosecond pulse energy has been found to depend on laser pulse duration, carrier envelope phase, and plasma scale length. Based on the obtained results, the potential of attaining isolated attosecond pulses with approx. 100 uJ energy for photons >16 eV using state-of-the-art laser technology appears to be within reach. 4. The polarization of attosecond light pulses generated from relativistic few-cycle laser pulse interaction with the surface of overdense plasmas is investigated using particle-in-cell simulation. Under suitable conditions, a desired polarization state of the generated attosecond pulse can be achieved by controlling the polarization of the incident laser. In particular, elliptically polarized laser pulse of suitable ellipticity can generate an almost circularly polarized attosecond pulse without compromising the harmonic generation efficiency. The process is thus applicable as a new tabletop circularly-polarized XUV radiation source for probing attosecond phenomena with high temporal resolution