| 题名 | 阿秒相干辐射产生的若干新机制实验研究 |
| 作者 | 熊辉 |
| 学位类别 | 博士 |
| 答辩日期 | 2008 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 徐至展 |
| 关键词 | 周期量级激光脉冲 高次谐波 三次谐波 X 波 |
| 其他题名 | Experimental study of new mechanisms in attosecond pulse generation |
| 中文摘要 | 超强超短激光的产生及其应用是近年激光物理与技术发展的新前沿。超强超短激光技术已经能够实现聚焦功率密度达到甚至远超过相对论光强量级或者脉宽接近一个光学周期的激光脉冲，从而将激光场与物质的相互作用推进到非微扰、高度非线性的领域，并进而导致一系列新效应，新现象，其中之一就是高次谐波的产生。通常，超强超短激光与原子、分子气体相互作用产生的谐波转化效率在低级次端随着级次的增加而迅速下降；随后下降趋势变得平缓，形成一个宽阔的平台区；最后在谱的末端，谐波效率迅速下降为零，形成陡峭的截止区。实验表明，强场高次谐波的级次可以高达数百，如以800nm波长的驱动激光场为例，产生的谐波光子最高能量已可达到keV量级。 由于高次谐波可望成为可调谐、高强度、台式化的超快（脉宽达到飞秒乃至阿秒时间尺度）极紫外（XUV）相干光源，该领域已经成为国际上强场超快激光物理研究的热点之一。然而因为高次谐波的产生是高度的非线性过程，高次谐波的产生效率比较低。现有的XUV阿秒脉冲产生技术能够获得的单个阿秒脉冲的能量仍然很低，这就需要发展新的高强度的单个阿秒产生技术。另一方面，为了使得产生的XUV阿秒脉冲更短，一个可能的方案是将XUV阿秒脉冲的载波波长推进至更短波段。 本论文重点以产生高强度单个XUV阿秒脉冲，以及获得更短波段的XUV阿秒相干辐射为目标，从实验上分别研究了利用可见－近红外波段的周期量级激光脉冲和多周期激光脉冲组合方案产生可支持单个阿秒脉冲的XUV波段超连续谱、由可调谐红外新波段超强超短激光驱动产生截止区拓展至更短波段的高次谐波以及其在空气中的非线性传输产生具有X波角谱结构的三次谐波等。主要工作和创新性成果如下： 1. 首次采用长短脉冲组合新方案，在实验上成功证实了周期量级激光脉冲对多周期脉冲强场中的电子动力学行为的量子相干控制新机制。实验中采用7fs/45fs 并载波包络相位（CEP）稳定的激光脉冲组合，获得了可支持单个阿秒脉冲产生的XUV波段超连续高次谐波光谱，同时组合脉冲产生的谐波信号比单个长脉冲产生的谐波增强了17倍。该新方案突破了目前周期量级驱动激光脉冲源的能量比较低的限制，可以在采用较小能量的周期量级驱动激光脉冲源的情况下产生高能量的XUV阿秒脉冲。这是国际上首次采用高强度多周期激光脉冲驱动获得高次谐波超连续谱。 2. 首次在实验中观察到了中心波长为1240nm的红外超强超短激光在氩气中产生的高次谐波光谱中的精细干涉结构，系统地研究了随着驱动激光能量的增加高次谐波的干涉结构的变化，并且通过理论模拟证明了这种干涉结构是由于高次谐波产生过程中的量子路径干涉效应所引起。实验结果表明，与传统的~800nm波长的超强超短驱动激光场产生的高次谐波光谱不同，红外波段超强超短驱动激光场产生的高次谐波光谱的对激光脉冲参数更为敏感。 3. 首次在实验中观察到可调谐红外新波段超强超短激光在空气中经成丝过程形成的三次谐波非线性X波，揭示了三次谐波传输过程中形成非线性X波的新机制。实验结果表明，当泵浦激光脉冲能量比较低的时候，基频光和三倍频光之间的相位匹配效应占据主导地位，形成了离轴传输的环状三次谐波结构；当泵浦激光能量进一步提高，基频光和三倍频光之间的群速度匹配效应变得重要并导致三次谐波非线性X波的产生。 4. 改进并建立了紧凑型高次谐波探测系统，并用于载波包络相位可操控的周期量级超强超短激光场驱动产生高次谐波的实验研究。实验中对比了多周期驱动激光场与周期量级驱动激光场中的所产生的高次谐波光谱之间的差异，特别是详细研究了载波包络相位对周期量级激光脉冲驱动产生的高次谐波光谱所带来的影响。实验结果表明当驱动激光脉冲的脉宽大于两个光学周期的时候，载波包络相位对高次谐波产生的影响很小。 |
| 英文摘要 | Generation and application of ultrafast high intensity laser pulses are one of the frontiers in laser technology development and laser physics. The shortest laser pulse generated by state-of-the-art laser technology has approached one optical-cycle. And the laser intensity at the focus is comparable or even much higher than relativistic intensity. The development of ultrafast laser technology makes the study of interaction between laser fields and matters come into nonperturbative and highly nonlinear domain and uncover a lot of new effects and phenomenon, one of which is high-order harmonic generation. Generally, harmonic spectra generated by interaction between high-intensity laser field and atomic or molecular gases contain three parts. There is a sharp decline with orders in conversion efficiency in the lowest order harmonics followed by a broad plateau where the harmonic conversion efficiency varies weakly with orders. And the highest-order harmonics whose conversion efficiency decline rapidly to zero form a cutoff. Experimental studies show that the several-hundredth order harmonic can be generated. For example, with an 800nm wavelength driving laser pulse, harmonic photon energy higher than 1keV was observed. High-order harmonic generated in high-intensity laser field is a candidate for tunable, high-intensity, tabletop, femtosecond/attosecond coherent extreme-ultraviolet (XUV) source, which is currently one of the hottest research topics in high-intensity ultrafast laser physics. However, as a highly nonlinear process, high-order harmonic generation is intrinsically inefficient. New technologies are needed for generating high intensity attosecond XUV pulse on the one hand, because the intensity of isolated single attosecond XUV pulse obtained with current attosecond technology is low. On the other hand, to shorten the current XUV pulse, one of the possible ways is to shorten the carrier wavelength of the attosecond XUV pulse. In this Ph. D. dissertation, for the purpose of generating high intensity isolated attosecond XUV pulse and shorter attosecond XUV pulse, my interests are focused on experimental investigations in super-continuum generation with a synthesized laser field consisting of a visible-infrared few-cycle laser pulse and a multi-cycle laser pulse, extension of high-order harmonic with a tunable infrared high-intensity laser pulse, and third harmonic X-wave generation during nonlinear propagation in air of a tunable infrared high-intensity laser pulse. The major results obtained during my PhD studies are listed below. 1. A synthesized laser field consisting of an intense long (45 fs, multi-cycle) laser pulse and a weak short (7 fs, few-cycle) laser pulse can be used to control the electron dynamics, and to generate extreme ultraviolet super-continuum which could enable production of an intense single isolated attosecond pulse. And the harmonic intensity is increased 17 times in the case of using the synthesized pulse compared with the result obtained with the long pulse only. This new scheme allows for generation of intense single isolated attosecond pulse with low-energy few-cycle driving laser pulses, because the energy of few-cycle laser pulse is still limited by today ultrafast laser technology. To our knowledge, this is the first time that super-continuum was generated with multi-cycle laser pulses. 2. The high-order harmonic generation in argon gas using a driving laser pulse at a center wavelength of 1240nm was studied. Fine interference fringe in the harmonic spectrum was observed. The fringe pattern changes with increasing energy of the driving pulse were systematically examined. Theoretical analysis indicates that the formation of fringes is a result of the quantum path interference. . 3. For the first time, nonlinear angularly resolved X-shaped third harmonic was observed during nonlinear propagation in air of a tunable infrared high-intensity laser pulse. New mechanism of nonlinear X-wave generation in third harmonic generation during propagation was discovered. It shows that at low pump intensity, phase matching between the fundamental and third harmonic waves dominates the nonlinear optical effect and induces a ring structure of the third harmonic beam; whereas at high pump intensity, the dispersion properties of air begins to affect the angular spectrum, leading to the formation of nonlinear X-wave at third harmonic. 4. A compact high-order harmonic diagnostic system was built up for experimental investigation of high-order harmonic generation with carrier envelope wavelength controllable high-intensity few-cycle laser pulse. High-order harmonics generated by few-cycle laser pulse and multi-cycle laser pulse were compared in experiments. The influence of carrier envelope phase of few-cycle laser pulse was particularly studied. The results show that when the laser pulse duration is longer than two optical cycles, the influence of the carrier envelope phase can be ignored. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15237]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 熊辉. 阿秒相干辐射产生的若干新机制实验研究[D]. 中国科学院上海光学精密机械研究所. 2008. |
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