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题名	中红外激光驱动高次谐波相干辐射控制与阿秒脉冲的产生
作者	李桂存
文献子类	博士
导师	曾志男
关键词	中红外激光，高次谐波，垂直偏振双色场，孤立阿秒脉冲 mid-infrared laser, high-order harmonic generation, orthogonal two-color laser field, isolated attosecond pulse
其他题名	Coherent control of high-order harmonic emission and attosecond pulse generation in mid-infrared laser fields
英文摘要	随着激光脉冲能量放大技术和脉宽压缩技术的发展，超强、超短激光脉冲可聚焦光强已达到 1022 W/cm2 以上，如此强的光强已远远超过原子内部场强对应的光强。即使一般的商品化飞秒脉冲激光器，其输出脉冲可聚焦光强也很容易超过1014 W/cm2。这样的激光脉冲如果与发生物质相互作用，原子或分子库伦势场会被外激光场扭曲，势垒会被压低，电子将以一定的几率穿过势垒，发生隧穿电离。电子一旦电离以后，原子内部库伦势对电子作用基本可忽略，电子可以被当作自由电子，其在激光场中运动可用经典的牛顿方程描述。当电子在激光场驱动下返回与母核发生散射时，会产生诸多的非线性现象，例如背向散射、高次谐波、非顺序双电离、阈上电离等。其中作为广泛应用的是，当电子在激光场作用下运动，有可能返回与母核复合，会辐射出频率为基频光奇数倍的光子，此即高次谐波。 通常地，产生高次谐波的驱动光源是Ti:sapphire 产生的800nm 近红外激光。然而，随着光参量放大（OPA）与光参量啁啾脉冲放大（OPCPA）技术的发展，高强度的中红外飞秒激光光源在实验上已成为可能。由于高次谐波谱截止区频率正比于驱动光波长的平方，利用长波长的中红外驱动激光与气体介质相互作用，高次谐波截止区很容易拓展到keV 甚至数个keV，到达X 射线波段。不幸的是，理论和实验研究结果均表明，高次谐波产率随着驱动激光波长增大而急剧下降。为此，需要探索各种相位匹配技术、波形整形等用于不断提高高次谐波的产率。 关于波形整形，双色场方案是最常用的方案。通过在基频场上叠加一个倍频场，改变双色场的延时可以改变驱动电场波形，从而控制各个半光周期内高次谐波的辐射。利用这种方案,可以有效地对电子轨道进行择优选择，从而在亚光周期时间尺度内控制电子动力学过程。本文主要研究了多光周期中红外激光驱动垂直偏振双色场下高次谐波的辐射，通过在二维平面内控制电子运动及其与母核的复合，发现相比于传统的800nm 近红外激光，在中红外激光驱动下的电子在激光场中运动的时间更长，高次谐波辐射相位也增大，导致谐波频率调制更明显，频率移动在光谱上更容易观测。此外，即使驱动光是多光周期的，在特定延时下电子可返回轨道数大量减少，有利于产生孤立阿秒脉冲。基于以上工作，本论文的主要研究成果以及创新点如下： 1. 研究了在多光周期中红外驱动激光垂直偏振双色场中高次谐波的产生，发现高次谐波谱随着双色场延时有频率调制现象。通过分析不同级次谐波的频率调制，我们能够分别确定谐波內禀相位和传播过程中自相位调制引起的频率调制贡献。理论模拟表明这一现象是由于电子受到垂直偏振双色场的控制，通过改变延时能控制高次谐波的相干辐射过程，实现谐波的精细调谐。该工作为我们提供了一种简单的方法，能够在亚光周 期时间尺度内能够对高次谐波光谱进行动态地精细调谐。 2. 从理论上提出了一种新的方案，利用中红外多光周期垂直偏振双色场驱动下产生高次谐波，发现在某些光谱区域内会出现准连续谱的“驼峰”，这些“驼峰”对应的谐波强度比相邻的正常平台区谐波高一个数量级。经典的电子轨道分析和时频分析表明这些“驼峰”对应于电子的单次碰撞复合，选取这些“驼峰”可以产生高强度、高对比度的单个阿秒脉冲。考虑宏观传播后，发现在松聚焦情况下，这些“驼峰”仍然存在。因此，本方案可以在大能量泵浦、松聚焦实验条件下，利用长波长的中红外激光产生高强度的keV 以及数keV 的孤立阿秒脉冲。; With the rapid development of laser techniques of pulse amplification and pulse compression, the peak intensity of ultraintense, ultrashort laser pulses as high as 1014 W/cm2, which is comparable to the atomic Coulomb field, has been experimentally achieved. When such intense laser fields interact with an atom or molecule, the Coulomb barrier will be suppressed, thus allowing an electron to be tunnel-ionized. Once freed, the motion of the electron can be described by Newton’s equation because the Coulomb effect can be neglected. In this case, when the electron return to the ionic core, many non-perturbative nonlinear phenomena, such as elastic backscattering, high-order harmonic generation (HHG), non-sequential double ionization (NSDI), and above-threshold ionization (ATI), will occur. Among them is HHG which has attracted the most widespread interest of researchers, for it provides an effective way to produce high coherent XUV and X-ray sources. In general, the most widely-used laser source is the near-infrared 800nm pulse produced by Ti: sapphire crystal. However, with the rapid development of frequency down-conversion technique by optical parametric chirped pulse amplification OPCPA), intense mid-infrared (MIR) laser sources with the wavelength up to several microns have been generated. Using the MIR laser pulses for HHG, the spectra can be easily extended to keV and multi-keV X-ray regime because of the famous cut-off law. However, the obtained harmonic yields drop rapidly because of the unfavorable scaling of the single-atom response with laser wavelength. To increase the harmonic efficiency and achieve high-intensity XUV pulses, multi-color waveform synthesizing and phase-matching are frequency used in HHG. Concerning the waveform synthesizing method, the two-color scheme is the most commonly used one. By adding a second harmonic (SH) pulse, the dynamics of the electron trajectories contributing to high harmonic emission can be controlled on a sub-cycle time scale. In this thesis we concentrate on the HHG in an orthogonally polarized two-color (OTC) laser field consisting of a mid-infrared fundamental pulse and its second harmonic pulse. It is found that in the multicycle MIR-OTC field, the number of trajectories are significantly reduced, and the inter-half-cycle interference effect is greatly suppressed. Main scientific results and innovations of this thesis are shown as follows： 1. We have experimentally investigated the frequency modulation of high-order harmonics in an orthogonally polarized two-color laser field consisting of a mid-infrared 1800nm fundamental pulse and its second harmonic pulse. It is demonstrated that the high harmonic spectra can be fine-tuned as we slightly change the relative delay of the two-color laser pulses. Our comprehensive analysis shows that the frequency modulation pattern is a reflection of the average emission time of high-order harmonic generation (HHG), thus offering a simple method to fine-tune the spectra of the harmonics on a sub-cycle time scale. 2. We theoretically propose a new scheme for intense isolated attosecond pulse generation in an orthogonal multicycle midinfrared two-color laser field. It is demonstrated that multiple continuum-like humps, which consist of about twenty orders of harmonics and an intensity of about one order higher than adjacent normal harmonics, are generated when longer wavelength driving fields are used. By filtering these humps, intense isolated attosecond pulses (IAPs) are directly generated without any phase compensation. Our proposal provides a simple technique to generate intense IAPs with various central photon energies covering the multi-keV spectral regime by using multicycle TW or even PW midinfrared lasers with loosely focused conditions in future experiments.
学科主题	光学
内容类型	学位论文
源URL	[http://ir.siom.ac.cn/handle/181231/30918]
专题	中国科学院上海光学精密机械研究所
作者单位	中国科学院上海光学精密机械研究所
推荐引用方式 GB/T 7714	李桂存. 中红外激光驱动高次谐波相干辐射控制与阿秒脉冲的产生[D].

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