阿秒范围电子动力学的直接观测

为研究一个系统的动力学过程，通常采用激光脉冲抽运实验的方法．实验中，用可见波长的抽运脉冲激光来激发该系统，并使用第二个探测脉冲对系统演化与脉冲问延迟的关系进行跟踪．在这种实验中，可达到的时间分辨依赖于激光脉冲的时间精度，但是这些脉冲通常要持续几个飞秒（fs，lfs=10^-15s），因此不能用于研究更快的过程．最近一项很有希望的进展是将脉冲压缩到了200as（as为阿秒，1as=10^-18s）．     

利用阿秒激光追踪和控制原子分子内部电子的运动(英文)

随着强激光技术的快速发展,在物质与激光相互作用下,实验上发现了很多新奇的物理现象。这些现象成功地被各种理论模型和数值模拟所解释和证明。一种很重要的现象就是所谓的高次谐波产生:在原子和分子与强激光相互作用时,能够放出能量为基频光子能量几倍到几百倍的大能量光子。在实验上,人们已经可以通过合成截止频率附近的几个谐波来产生脉冲长度在阿秒量级的激光脉冲(1阿秒=10~(-18)s)。阿秒脉冲的获得开启了超快科学一个全新的领域:人们可以在电子运动的自然时间尺度上去探测和操控原子分子内部电子的运动,这是继飞秒科学后人们操控微观世界物质运动的又一大飞跃。在本文中,我们就最近几年我们在理论上所开展的阿秒物理做一个简单的综述,重点强调利用阿秒光去控制电子的电离动力学以及探测分子内部电子运动。     

采用双脉冲驱动产生高次谐波阿秒脉冲

提出了采用双脉冲机理来产生阿秒脉冲的方法.研究发现采用双脉冲不仅可以产生单个的阿秒脉冲，从而突破目前产生阿秒脉冲的驱动源（几个飞秒的超短激光脉冲）的能量限制，而且在相同的峰值强度下，双脉冲能够产生强度更高的阿秒脉冲. 关键词： 阿秒脉冲 双脉冲机理 高次谐波     

强激光等离子体相互作用驱动高次谐波与阿秒辐射研究进展

对超快过程的探测和控制决定了人类在微观层面认识和改造物质世界的能力.阿秒光源可完成对组成物质的电子运动及其关联效应进行超高时空分辨的探测和操控,为人类认识微观世界提供了全新手段,被认为是激光科学史上最重要的里程碑之一.世界主要科技强国都将阿秒科学列为未来10年重要的科技发展方向.利用强激光与物质相互作用产生高次谐波是突破飞秒极限实现高亮度阿秒脉冲辐射的重要方案之一,成为了近年来激光等离子体领域的研究热点.本文聚焦强激光与等离子体相互作用中的高次谐波和阿秒脉冲辐射,主要介绍其产生机制、研究进展和前沿应用,并对未来的发展趋势和创新突破进行展望.     

利用阿秒激光追踪和控制原子分子内部电子的运动

随着强激光技术的快速发展, 在物质与激光相互作用下,实验上发现了很多新奇的物理现象。这些现象成功地被各种理论模型和数值模拟所解释和证明。一种很重要的现象就是所谓的高次谐波产生：在原子和分子与强激光相互作用时, 能够放出能量为基频光子能量几倍到几百倍的大能量光子。在实验上, 人们已经可以通过合成截止频率附近的几个谐波来产生脉冲长度在阿秒量级的激光脉冲（1阿秒=10-18秒）。阿秒脉冲的获得开启了超快科学一个全新的领域：人们可以在电子运动的自然时间尺度上去探测和操控原子分子内部电子的运动,这是继飞秒科学后人们操控微观世界物质运动的又一大飞跃。在本文中,我们就最近几年我们在理论上所开展的阿秒物理做一个简单的综述,重点强调利用阿秒光去控制电子的电离动力学以及探测分子内部电子运动.     

利用三色组合脉冲激光获得孤立阿秒脉冲发射

孤立阿秒脉冲因可以跟踪和控制原子及分子内电子的运动过程而备受关注.本文从理论上开展了氦原子在3束飞秒脉冲激光组合场辐照下产生的高次谐波和阿秒脉冲辐射的研究.组合激光场由16 fs/1600 nm,15 fs/1100 nm和5.3 fs/800 nm的钛宝石脉冲构成.与前两束脉冲合成的双色场产生谐波谱相比,附加钛宝石脉冲的三色场产生的高次谐波发射谱呈现出高转换效率及宽带超连续特性,超连续谱范围覆盖从230—690次谐波,傅里叶变换后实现了128 as高强度孤立短脉冲的产生.该结果归因于合成的三色场呈现出高功率及少周期的中红外飞秒脉冲激光特性,可以有效控制原子电离以及复合发生在中红外飞秒脉冲的一个有效光学周期内.     

圆偏振激光附加太赫兹场作用下的高次谐波发射和孤立阿秒脉冲的产生

本文通过数值求解二维含时薛定谔方程,研究了圆偏振激光和太赫兹组合场作用下,H+2的高次谐波发射和孤立阿秒(1 as=10-18s)脉冲的产生.研究发现,无论在圆偏振激光场的x或y方向附加一个太赫兹场,都可以产生一个比较平滑的连续谐波谱.通过时频分析我们发现,高次谐波的贡献主要来自于短轨道.适当选取一些级次的连续谐波进行叠加,可以得到129 as或83 as的孤立阿秒脉冲.     

高能电子与超强激光束作用产生的阿秒脉冲列

利用非线性汤姆孙散射的理论，从理论和数值模拟上研究了单电子在横向穿越高斯激光束束 腰时所辐射的x射线阿秒脉冲列的性质. 主要分析了电子以初始能量γ₀=1M eV—100M eV横向穿越激光振幅参数为a₀=1—10的高斯光束束腰获得的阿秒辐射脉冲的 时间 和空间性质. 计算表明，辐射呈现脉冲列的形式. 脉冲列的包络宽度取决于激光强度、束腰 的宽度以及入射电子能量. 电子的初始能量比激光强度对电子辐射脉冲的影响更大. 辐射脉 宽、脉冲间隔和脉冲包络宽度都正比于1/γ²₀，辐射功率正比于 γ⁶₀，辐射能 量正比于γ⁴₀. 当改变激光振幅a₀时，辐射功率正比 于a²₀、辐射包络中单 个脉冲脉宽正比于1/a₀、脉冲之间的间隔正比于a₀. 当保持激光强 度不变，而改变光束 束腰半径w₀时，辐射的脉冲数量、包络和辐射能量正比于w₀. 当 激光功率保 持不变时而改变激光强度和束腰半径时，脉冲包络宽度和最大辐射能量都基本不变. 当激光 振幅参数a₀=1，电子初始能量为10MeV时，激光束腰为两个激光波长时，电子 辐 射脉冲包络宽度只有14×10^-3τ₀（τ₀为入 射激光周期），达到几个阿秒的量级. 关键词： 阿秒脉冲 非线性汤姆孙散射 高斯激光光束     

激光脉冲绝对相位对产生阿秒脉冲的影响

在用超短激光脉冲产生和测量阿秒脉冲的过程中，绝对相位成为一个非常重要的影响因素。讨论了绝对相位对阿秒脉冲的产生和激光振荡的关系的影响，发现高次谐波的产生过程非常稳定，阿秒脉冲和驱动它的激光振荡之间的稳定的相位关系并不会受绝对相位的随机性的破坏，从而为采用交叉相关技术直接测量由强场原子高次谐波过程产生的阿秒脉冲宽度提供了理论依据，正确解释了文献[13]上报道的实验结果。     

强短脉冲辐照叠加态原子增强阿秒脉冲强度

采用强短激光脉冲辐照叠加态原子提高孤立阿秒脉冲的强度.数值研究表明,与强短脉冲(脉冲宽度只有几个光学周期的超短脉冲)辐照基态原子获得孤立阿秒脉冲相比,采用叠加态原子的方案不仅同样可以获得孤立的阿秒脉冲,而且阿秒脉冲的强度得到显著增强.     

超快X射线自由电子激光研究进展北大核心CSCD

现代光源的发展不断推动着人们从更深层次上理解物质的基本结构和动力学行为。X射线自由电子激光作为最先进的光源,其超高的峰值功率、超短的脉冲长度和优良的相干性,为人们以原子级时空分辨率探测和操控物质中的超快过程提供了可能。目前全世界已有多个X射线自由电子激光装置建成并投入使用,在原子分子物理、化学、生命科学、材料科学等各学科应用中都显示出了重要价值。同时大量的研究工作也集中于继续提高X射线自由电子激光的性能,包括把脉冲持续时间从fs量级进一步缩短至as量级,这将为超快科学的发展带来新突破。以超快脉冲产生为主线,综述了近年来超快X射线自由电子激光产生方案的研究进展,从产生原理、方案特性、最新成果等方面介绍了各类产生方案,总结对比了各方案的优缺点,最后对超快X射线自由电子激光的未来发展方向进行了展望。     

Attosecond lighthouses: how to use spatiotemporally coupled light fields to generate isolated attosecond pulses

Under the effect of even simple optical components, the spatial properties of femtosecond laser beams can vary over the duration of the light pulse. We show how using such spatiotemporally coupled light fields in high harmonic generation experiments (e.g., in gases or dense plasmas) enables the production of attosecond lighthouses, i.e., sources emitting a collection of angularly well-separated light beams, each consisting of an isolated attosecond pulse. This general effect opens the way to a new generation of light sources, particularly suitable for attosecond pump-probe experiments, and provides a new tool for ultrafast metrology, for instance, giving direct access to fluctuations of the carrier-envelope relative phase of even the most intense ultrashort lasers.     

Attosecond‐precision ultrafast photonics

We review our recent progress toward attosecond‐precision ultrafast photonics based on ultra‐low timing jitter optical pulse trains from mode‐locked lasers. In femtosecond mode‐locked lasers, the concentration of a large number of photons in an extremely short pulse duration enables the scaling of timing jitter into the attosecond regime. To characterize such jitter levels, we developed new attosecond‐resolution measurement techniques and show that standard fiber lasers can achieve sub‐fs high‐frequency jitter. By leveraging the ultra‐low jitter of free‐running mode‐locked lasers, we pursued high‐precision optical‐optical and optical‐microwave synchronization techniques. Optical signals spanning 1.5 octaves were synthesized by attosecond‐precision timing and phase synchronization of two independent mode‐locked lasers. High‐stability microwave signals were also synthesized from mode‐locked lasers with drift‐free sub‐10‐fs precision. We further demonstrated the attosecond‐precision distribution of optical pulse trains to remote locations via timing‐stabilized fiber links. Finally, the application of optical pulse trains for high‐resolution sampling and analog‐to‐digital conversion is discussed.     

中红外超快光纤激光器研究进展

中红外波段覆盖重要的分子吸收区与多个大气透射窗口,该波段的超快激光器在多个领域具有广泛应用。基于光纤的中红外超快激光器近年来在激光发射与传输、超快脉冲产生与应用等方面发展迅速,为中红外波段超快激光开辟了新的研究手段与应用领域。综述了近十年来中红外超快光纤激光器的发展概况,介绍了近年来中红外波段的激光传输与增益手段。其中,重点回顾了近年来中红外超快脉冲产生技术的研究进展及其代表性工作,包括非线性偏振旋转、可饱和吸收体以及频移反馈锁模技术。此外,还介绍了中红外超快脉冲的压缩放大技术与超连续谱产生应用。最后讨论并总结了中红外超快光纤激光器面临的挑战与可能的发展方向。     

Ultrafast solid-state laser oscillators: a success story for the last 20 years with no end in sight

Ultrashort lasers provide an important tool to probe the dynamics of physical systems at very short time-scales, allowing for improved understanding of the performance of many devices and phenomena used in science, technology, and medicine. In addition ultrashort pulses also provide a high peak intensity and a broad optical spectrum, which opens even more applications such as material processing, nonlinear optics, attosecond science, and metrology. There has been a long-standing, ongoing effort in the field to reduce the pulse duration and increase the power of these lasers to continue to empower existing and new applications. After 1990, new techniques such as semiconductor saturable absorber mirrors (SESAMs) and Kerr-lens mode locking (KLM) allowed for the generation of stable pulse trains from diode-pumped solid-state lasers for the first time, and enabled the performance of such lasers to improve by several orders of magnitude with regards to pulse duration, pulse energy and pulse repetition rates. This invited review article gives a broad overview and includes some personal accounts of the key events during the last 20 years, which made ultrafast solid-state lasers a success story. Ultrafast Ti:sapphire, diode-pumped solid-state, and novel semiconductor laser oscillators will be reviewed. The perspective for the near future indicates continued significant progress in the field.     

高次谐波产生阿秒极紫外和X光脉冲研究新进展

 近年来在可见光谱范围内已经把激光脉冲压缩到接近一个光学周期(2～3 fs)的物理极限,几fs的时间分辨精度可以描述分子化学反应过程,但是要探测远小于可见光周期的电子跃迁过程则需要阿秒(as)量级的光脉冲。利用脉冲间具有相同载波包络相位的阿秒脉冲序列能把可见光波段的光学频率梳向极紫外波段扩展;利用电子和离子碰撞复合过程短于一个光周期这个时间窗,通过测量激光场椭圆极化率对电子轨迹的微扰实现了as精度的分辨率;通过测量碰撞复合过程中的高能电子的辐射谱可以重构阿秒X光脉冲以及探测强场下束缚态和连续态电子动力学。     

Attosecond spectroscopy for filming the ultrafast movies of atoms,molecules and solids

Three decades ago, a highly nonlinear nonpertubative phenomenon, now well-known as the high harmonic generation (HHG), was discovered when intense laser irradiates gaseous atoms. As the HHG produces broadband coherent radiation, it becomes the most promising source to obtain attosecond pulses. The door to the attosecond science was opened ever since. In this review, we will revisit the incredible adventure to the attoworld. Firstly, the progress of attosecond pulse generation is outlined. Then, we introduce the efforts on imaging the structures or filming the ultrafast dynamics of nuclei and electrons with unprecedented attosecond temporal and Angstrom spatial resolutions, utilizing the obtained attosecond pulses as well as the high harmonic spectrum itself.     

Einstein Lecture – Passion for precision

Optical frequency combs from mode‐locked femtosecond lasers have link optical and microwave frequencies in a single step, and they provide the long missing clockwork for optical atomic clocks. By extending the limits of time and frequency metrology, they enable new tests of fundamental physics laws. Precise comparisons of optical resonance frequencies of atomic hydrogen and other atoms with the microwave frequency of a cesium atomic clock are establishing sensitive limits for possible slow variations of fundamental constants. Optical high harmonic generation is extending frequency comb techniques into the extreme ultraviolet, opening a new spectral territory to precision laser spectroscopy. Frequency comb techniques are also providing a key to attosecond science by offering control of the electric field of ultrafast laser pulses. In our laboratories at Stanford and Garching, the development of new instruments and techniques for precision laser spectroscopy has long been motivated by the goal of ever higher resolution and measurement accuracy in optical spectroscopy of the simple hydrogen atom which permits unique confrontations between experiment and fundamental theory. This lecture recounts these adventures and the evolution of laser frequency comb techniques from my personal perspective.     

Reshaping of attosecond x-ray pulses in thin crystals

A method for reshaping and control of the duration of attosecond x-ray pulses in thin crystals is proposed. The finite width of the reflection and transmission curves around the Bragg angle allows one to engineer Fabry-Perot-type filters for the generation of a large variety of attosecond pulse shapes. The method considered here can be used to manipulate attosecond pulses produced by high-harmonic generation and also for shorter wavelengths for attosecond pulses from x-ray free-electron lasers. X-ray pulses with controllable amplitude and phase may find useful applications in the newly emerging area of attosecond time-resolved spectroscopy.