基于超短脉冲强激光与等离子体相互作用的强辐射源产生和应用
文献类型：学位论文
| 作者 | 胡广月 |
| 学位类别 | 博士 |
| 答辩日期 | 2010 |
| 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 ; 雷安乐 |
| 关键词 | 强辐射源，超短脉冲强激光，高能电子源，微结构靶，电离过程，太赫兹辐射，分子指向和取向 |
| 其他题名 | Generation and application of intense radiation sources based on interaction of intense short pulse laser with plasma |
| 中文摘要 | 超短脉冲激光技术的发展已经可以在飞秒至皮秒时间尺度内实现太瓦至拍瓦的峰值激光输出功率，利用高功率的超短脉冲激光与物质相互作用产生强辐射源得到越来越多的关注和应用。等离子体介质没有击穿阈值的特点使得可以加载的瞬时激光功率高至拍瓦量级，极高的激光功率能够保证产生的强辐射源的瞬时功率达到传统的加速器、同步辐射和自由电子激光等大科学装置能够输出的水平，在强辐射源装置的小型化方面，基于超短脉冲强激光与等离子体相互作用的强辐射源具有独特的优势。 本论文重点关注超短脉冲强激光与等离子体相互作用产生的高能电子源和太赫兹辐射源，主要在亚波长光栅微结构靶的超热电子发射和气体电离过程产生太赫兹辐射方面做了若干创新性的实验和理论工作。主要研究成果有： 实验研究了超短脉冲强激光与周期性的亚波长光栅微结构靶相互作用时的超热电子发射特征，发现在共振激发表面等离子体波时，光栅靶产生的超热电子在激光反射方向呈现准直发射，而平面靶的超热电子在靶法线方向和激光反射方向之间以很大的发散角出射。结合实验特征和PIC模拟，认为准直的超热电子发射主要是由共振激发表面等离子体波时特殊结构的表面加速电场所致。 实验演示了亚波长光栅微结构靶对沿靶面出射的“表面加速”电子的增强效应，表面加速电子相对于平面靶增加了三倍以上，总的出靶超热电子数量也增加一倍。PIC模拟发现，使用微结构靶时，产生超热电子的瞬时电场明显增加导致超热电子产额增加，同时表面加速电子相对应的准静态表面电场和磁场显著增强，导致表面电子加速增强。 提出利用中心对称传感介质中太赫兹电场诱导的二次谐波过程和啁啾脉冲探测光来实现对太赫兹脉冲的宽带单发测量，克服了传统的使用电光晶体的探测方法中测量带宽受限的不足。 激光在空气中传输时，会发生成丝现象，激光能够在很远的距离内保持很小的直径而不发散。在丝内激光电离波的尾部，有质动力驱动的等离子体电流能够通过类似高能电子的渡越-切伦轲夫辐射机制产生太赫兹波。但由于电离波速度总是亚光速，同时由于激光成丝时存在最大激光强度限制，导致成丝时能够产生的最大太赫兹辐射能量有限。本论文提出利用特殊聚焦镜在成丝时产生超光速电离波，将最大能够产生的太赫兹辐射强度提高一个量级，同时可以通过操控或调制丝来灵活调整太赫兹辐射的特征。 超短脉冲强激光电离气体时，外加静电场、双色激光场和少周期激光脉冲都可以导致电场不对称，从而产生低频电流并辐射出太赫兹波。通过实验测量偏振依赖性等特征，我们发现外加静电场时太赫兹辐射机制主要是等离子体振荡机制，而不是非线性光学机制。理论发现双色激光场产生的太赫兹辐射强度对相位差的依赖关系在激光强度较高时与半经典“光电流”模型一致，激光强度较低时与量子理论的“单原子偶极振荡”模型一致，解决了两个模型预言的辐射强度对相位差依赖性相差π/2的矛盾。提出将二次谐波或1/2次谐波的长脉冲激光场叠加到少周期激光场上来共同电离气体介质，能够成量级地提高太赫兹辐射强度。 目前电离过程的太赫兹辐射集中于激光场的不对称性的研究上。我们提出利用不对称分子的介质本身的不对称可以进一步提高太赫兹辐射强度。同时，利用太赫兹辐射可以更方便的测量分子指向和取向度的动力学行为，此方法比传统的库伦爆炸方法更快捷方便且更简单。太赫兹辐射可以提供除光电子谱、离子谱和高次谐波谱之外一种新的强场原子分子物理的诊断手段。 |
| 英文摘要 | Benefitted from the rapid development of short pulse laser technology, laser beam with terawatt to petawatt instantaneous power can be achieved in the duration of femetoseconds to petoseconds. Intense radiation source such as high energy electrons, ions, and electromagnetic wave with various wavelengths can be generated in the interaction of intense short pulse laser with matter. Very intense laser beam can be used to generate the radiation sources with plasma media because the plasma has no damage threshold. Even with low conversion efficiency, the intense short pulse laser beam can easily enable the power of radiation sources as high as that of the conventional large facility including accelerator, synchrotron radiation, and free electron laser. Moreover, “table-top” intense radiation source can be obtained by using the short pulse laser beam. In the present thesis, we focus on the high energy electrons source and electromagnetic radiation source with terahertz frequency, especially the effect of subwavelength microstructure target to hot electrons generation and the terahertz radiation emited in the process of gas ionization. The main results are as follows: Collimated hot electron jet was observed in the interaction of intense short pulse laser with subwavelength grating target when the surface plasmon was excited resonantly. The experimental results and 2D PIC simulation indicate that the peculiar configuration of the surface electric field of subwavelength grating target with surface plasmon resonance induces the collimated hot electron jet. The enhancement of surface accelerated hot electrons emitted along the target surface was demonstracted in experiments using subwavelength grating microstructure target. The surface accelerated hot electron was improved more than three times and the total hot electron escaped away from the target was also increased one time. 2D PIC simulation shows that the instantaneous electric field that generates the hot electron was increased significantly. The quasi-static electric field and magnetic field around the target surface were also improved markedly, which induce the increase of surface accelerated hot electron. We present a new scheme using centrosymmetric materials as the sensor medium for the single-shot coherent detection of terahertz wave. Using the mechanism of teraherta wave electric field induced second harmonic of shirped probe laser beam, the new scheme can provide transform limited temporal resolution and cover the entire terahertz spectral region. Plasma electrons current driven by ponderomotive force behind the ionization front can emits terahertz wave in the laser filament through transition-Cherenkov process. The radiation energy is limited by the ionization front with sub-luminous speed and clamping laser intensity in laser filament. Axicon focus lens were proposed to increase the speed of ionization front. It shows that the transition-Cherenkov radiation of terahertz was enhanced by one order with superluminous ionization front. Electron current with low frequency, which produces terahertz wave, can be generated in the process of gas ionization by asysmmetric electric fields. Such asysmmetric electric field can be a static bias electric field, a two-color laser field, or a few-cycle laser pulse. Our experimental results indicated that the terahertz radiation emitted from a static bias electric field is generated by plasma oscillation, not by nonlinear optics mechanism. We found that the different phase dependence of the semi-classical model and the quantum theory, which were used to describe the terahertz radiation generated by two-color laser field, comes from the different laser intensity used in the experiments. The semi-classical transient photocurrent model is only suitable at high laser intensity, while the quantum theory of dipole oscillation of single atom is still applicable at low laser intensity. It is also found that the terahertz radiation generated by a few-cycle laser pulse can be enhanced by one order if adding a weak second or 1/2 harmonic of a long pulse laser beam. We found that asymmetric media, such as molecules or semiconductor quantum well, can also produce electron current with low frequency and emit terahertz radiation. Numerical results show that the terahertz radiation generated by two-color laser field is enhanced significantly by using asymmetric molecules. Moreover, when the oriention of asymmetric molecules is reversed, the intensity of terahertz radiation can be changed from several times to two orders, which makes itself a sensitive method to measure the oriention and alignment of asymmetric molecules. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15633]  |
| 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 | 胡广月. 基于超短脉冲强激光与等离子体相互作用的强辐射源产生和应用[D]. 中国科学院上海光学精密机械研究所. 2010. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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