High field physics and extreme nonlinear optics

Despite the fact that high order harmonic generation (HHG) aims at serving as a table-top light source for imaging applications with extremely high spatial resolution, a general lack of accurate conversion efficiency measurements exists in the field. Here, we present such a measurement for a HHG setup with a semi-infinite gas cell. By combining measurements with a calibrated photodiode sensitive in the extreme ultra-violet (XUV) and spatially resolved spectral measurements of HHG spectra, we are able to determine conversion efficiencies of the HHG process as well as brilliance values for individual harmonics. The method is explained in detail and applicable to any target geometry.     

Collaboration of academia and industry for high field science

Close collaboration between academia and industry is essential for opening frontiers of both science and industry. High performance photon detectors developed at industry are playing vital roles in science such as astronomy and high energy physics. Alternatively many advanced industrial and medical products came out of research in basic science. For advancement of high field science, closer collaboration between academia and industry is necessary to develop next generation high power lasers, which will also meet the needs in industry, medicine and energy.     

气象领域极端事件的长程相关性

利用固定阈值法研究了Lorenz系统极端事件序列的长程相关性特征.研究结果表明：不同阈值标定的极端事件序列具有长程相关性,且标度指数α比较接近,但都比Lorenz系统略小.不同的初值对极端事件序列的长程相关性影响不大,即对初值不敏感性,但Lorenz系统的长程相关性随着控制参数的增大而明显减弱.通过与高斯白噪声序列对比研究发现,Lorenz系统极端事件序列具有较好的记忆性特征.最后采用国家气候中心194个测站1957年—2004年日最高气温观测资料进行分析,揭示了实际气象要素中存在类似的规律. 关键词： Lorenz系统 极端事件 长程相关性 记忆性     

The real scalar field in extreme RNdS space

The real scalar field equation between the outer black hole horizon and the cosmological horizon is solved in the extreme Reissner-Nordström de Sitter (RNdS) space. We use an accurate approximation, the polynomial approximation, to approximate the tortoise coordinate x(r) in order to get the inverse function r = r(x) and then to solve the wave equation. The case where the two horizons are very close to each other is discussed in detail. We find that the wave function is harmonic only in the very small regions near the horizons, and the amplitude decreases remarkably near the potential peak because of the effect of the potential. Furthermore, it is found that the height of the potential increases as the cosmological constant Λ decreases, and the wave amplitude will decrease more remarkably with less Λ.     

Approach to extreme ultraviolet supercontinuum in a two-color laser field

In recent years, it has been shown that an intense few-cycle driving pulse combined with a weak control pulse could efficiently extend the harmonic spectrum in the generation of high-order harmonics (HHG). In this paper, we adjust the intensities of the driving and control pulse to generate broader extreme ultraviolet (XUV) supercontinuum. We find that in some special situation when the intensity of the control pulse equals to that of the driving pulse, a clean supercontinuum as long as 80 eV can be realized and a 53-as pulse can be straightforwardly obtained.     

基于广义极值的混沌电场峰值分布研究

通过分析峰值分布及广义极值分布理论关于混沌电场峰值分布的特点,获得一种混沌电场峰值分布的计算方法,结合Bootstrap重采样技术,提出了混沌电场峰值分布参数估计方法,该方法吸取了重采样技术估计的优点,对分布参数置信区间的估计更为准确、实用。以蒙特卡罗仿真和某机械搅拌混响室内电场数据为实例,结果表明,与传统的正态近似法、百分位法、BC-t等方法相比,其计算结果与实际结果更为吻合,证明了该方法具有更高的准确度。     

基于广义极值的混沌电场峰值分布研究

通过分析峰值分布及广义极值分布理论关于混沌电场峰值分布的特点,获得一种混沌电场峰值分布的计算方法,结合Bootstrap重采样技术,提出了混沌电场峰值分布参数估计方法,该方法吸取了重采样技术估计的优点,对分布参数置信区间的估计更为准确、实用。以蒙特卡罗仿真和某机械搅拌混响室内电场数据为实例,结果表明,与传统的正态近似法、百分位法、BC-t等方法相比,其计算结果与实际结果更为吻合,证明了该方法具有更高的准确度。     

Magnetic field dependence of cyclotron resonance linewidth for acoustic polarons in the extreme quantum limit

The magnetic field dependence of cyclotron resonance linewidth (CRLW) due to electron-acoustic phonon interactions in the extreme quantum limit is obtained on the basis of Kubo's formula and Fujita's diagram method. The 2-dimensional electron-piezoelectric phonon interaction generates a finite maximum CRLW as a function of magnetic field while CRLW for all other acoustic polarons increase with the magnetic field.     

Generation of an extreme ultraviolet supercontinuum in a two-color laser field

We theoretically investigate the high-order harmonic generation in a helium atom with a two-color optical field synthesized by an intense 6 fs pulse at 800 nm and a relatively weak 21.3 fs pulse at 400 nm. When the frequency-doubled pulse is properly time shifted with respect to the fundamental pulse, an ultrabroad extreme ultraviolet supercontinuum spectrum with a 148 eV spectral width can be generated which directly creates an isolated 65 as pulse even without phase compensation. We explain this extraordinary phenomenon by analyzing maximum electron kinetic energies at different return times.     

激光诱导击穿光谱在材料表面分析领域中的应用进展

激光诱导击穿光谱作为表面分析工具是近十年发展起来的,对表面分析基础理论、仪器装置及其在材料科学领域中的应用进行详细的综述。重点阐述了激光诱导击穿光谱在国内外冶金、半导体、电子等材料科学领域中的元素分布分析及镀层分析应用进展,指出激光束单个脉冲能量、环境气体及气压、激光束能量分布等参数对分辨率的影响,同时指出在兼顾灵敏度的前提下提高分辨率的途径。与传统经典表面分析工具相比较具有扫描面积大、分析速度快及样品导电与否均可分析等优点,成为传统表面分析工具的有力补充。     

红外激光场中共振结构原子对极紫外光脉冲的压缩效应

通过数值求解一维原子的含时薛定谔方程, 研究了具有共振结构的原子在双色场(红外激光(IR)+极紫外光(XUV)) 驱动下发射高次谐波的特征. 研究结果表明, 具有共振结构的原子所发射的高次谐波与无共振结构原子(简称为一般原子)发射的高次谐波有明显不同, 共振结构的原子除了在某一能量附近(原子的共振能量+电离能)高次谐波的强度有很大提高外, 它还对XUV光的响应较一般原子表现得更为敏感, 即使XUV光的强度较弱, 也能够明显提高XUV光脉冲中心频率附近的谐波强度, 更重要的是通过调节双色场的时间延迟, 能使输入的XUV光的脉宽得到明显的压缩, 通过时间-频率分析给出了发生这种现象的原因. 由此提出了通过滤波-连续反馈的方式可使XUV光的脉冲从200 as压缩至120 as左右.     

双色激光场中极端远紫外超连续谱相干辐射产生

通过脉宽为周期量级的超快强场激光脉冲与惰性气体原子的极端非线性相互作用,能够产生单个的阿秒量级的极端远紫外光脉冲。然而,现有的周期量级的激光脉冲的最短脉宽仍在两个光周期左右,尚不足以支持产生脉宽短于100as的单个极端远紫外脉冲。文章作者首次提出通过在一个周期量级(如6fs)激光脉冲上再叠加一个强度较弱但相对位相精确控制的二倍频激光,能够直接实现单个的65as光脉冲。如进一步经过位相补偿,将有望产生一个脉宽仅为23as的单脉冲,从而将阿秒光脉冲的时间宽度推进到短于一个原子时间单位。     

Expulsion of external fields from extreme horizons: Example of an external gravitational field

In analogy with the Meissner effect in (super) conductors, black holes expel stationary axisymmetric external fields when approaching the extreme state. This has been studied on magnetic fields in the literature. Using the recently obtained solution describing a rotating black hole surrounded by an axisymmetric thin annular disc, we show that the effect also applies to an external gravitational field. This note on “traditional” Prague subject is dedicated to Professor Jiří Bičák on the occasion of his 60th birthday.     

Novel CFT duals for extreme black holes

In this paper, we study the CFT duals for extreme black holes in the stretched horizon formalism. We consider the extremal RN, Kerr-Newman-AdS-dS, as well as the higher dimensional Kerr-AdS-dS black holes. In all these cases, we reproduce the well-established CFT duals. Actually we show that for stationary extreme black holes, the stretched horizon formalism always gives rise to the same dual CFT pictures as the ones suggested by ASG of corresponding near horizon geometries. Furthermore, we propose new CFT duals for 4D Kerr-Newman-AdS-dS and higher dimensional Kerr-AdS-dS black holes. We find that every dual CFT is defined with respect to a rotation in certain angular direction, along which the translation defines a U(1) Killing symmetry. In the presence of two sets of U(1) symmetry, the novel CFT duals are generated by the modular group SL(2,Z), and for n sets of U(1) symmetry there are general CFT duals generated by T-duality group SL(n,Z).     

Plasma devices for focusing extreme light pulses

Since the inception of the laser, there has been a constant push toward increasing the laser peak intensity, as this has lead to opening the exploration of new territories, and the production of compact sources of particles and radiation with unprecedented characteristics. However, increasing the peak laser intensity is usually performed by enhancing the produced laser properties, either by lowering its duration or increasing its energy, which involves a great level of complexity for the laser chain, or comes at great cost. Focusing tightly is another possibility to increase the laser intensity, but this comes at the risk of damaging the optics with target debris, as it requires their placement in close proximity to the interaction region. Plasma devices are an attractive, compact alternative to tightly focus extreme light pulses and further increase the final laser intensity.     

氢原子在红外激光和极紫外脉冲组合场中的阈上电离

通过求解氢原子在强红外(IR)激光和极紫外（XUV）脉冲组合场中的三维含时薛定谔方程(TDSE),理论研究了XUV脉冲的加入对光电子能谱和二维光电子动量分布的影响.计算结果表明,与仅由红外场驱动的情况相比,组合场驱动下的光电子能谱和二维光电子动量分布中呈现出明显的干涉增强结构,干涉结构对XUV脉冲的强度、光子能量和时间延迟都有很强的依赖,该方案可实现高能阈上电离谱的选择性增强.     

Generation of an extreme ultraviolet supercontinuum with a multicycle chirped laser and a static electric field

We theoretically present a method for generating an ultrabroad extreme ultraviolet(XUV) supercontinuum by using the combination of a multicycle chirped laser and a static electric field.At a low laser intensity,the spectral cutoff is extended to the 495th order harmonic,and the bandwidth of the supercontinuum spectrum is broadened to 535 eV.At a high laser intensity,the harmonic cutoff is enlarged to the 667th order,and a supercontinuum covering a bandwidth of 1035 eV is generated.In these two cases,the long quantum path is removed,and the short quantum path is selected.Especially for the relatively high laser intensity,an isolated 23-attosecond pulse with a bandwidth of about 170.6 eV is directly obtained.Finally,we also analyze the influences of the chirp parameter and the duration of the chirped pulse as well as the static field strength on the supercontinuum.