Ultrafast electron diffraction

Ultrafast electron diffraction （UED） technique has proven to be an innovative tool for providing new insights in lattice dynamics with unprecedented temporal and spatial sensitivities. In this article, we give a brief introduction of this technique using the proposed UED station in the Synergetic Extreme Condition User Facility （SECUF） as a prototype. We briefly discussed UED＇s functionality, working principle, design consideration, and main components. We also briefly reviewed several pioneer works with UED to study structure-function correlations in several research areas. With these efforts, we endeavor to raise the awareness of this tool among those researchers, who may not yet have realized the emerging opportunities offered by this technique.

Wavelength-and ellipticity-dependent photoelectron spectra from multiphoton ionization of atoms

We theoretically study the photoelectron momentum distributions from multiphoton ionization of a model lithium atom over a range of laser wavelengths from 500 nm to 700 nm by numerically solving the time-dependent Schr ¨odinger equation. The photoelectron momentum distributions display many ring-like patterns for the three-photon ionization, which vary dramatically with the change of the laser wavelength. We show that the wavelength-dependent photoelectron energy spectrum can be used to effectively identify the resonant and nonresonant ionization pathways. We also find an abnormal ellipticity dependence of the electron yield for the(2+1) resonance-enhanced ionization via the 4d intermediate state, which is relevant to the two-photon excitation probability from the ground state to the 4d state.

Role of excited states in helium-like ions on high-order harmonic generation

We theoretically investigate high-order harmonic generation(HHG) of helium(He), lithium cation(Li+), and beryllium dication(Be2+) using the time-dependent Hartree–Fock method to solve the three-dimensional time-dependent Schr ¨odinger equation. It is found that the intensity of the HHG increases significantly from a certain harmonic order below the ionization threshold, and the initial position of the enhancement does not depend on the intensity or the wavelength of the driving laser field. Further analysis shows that excited states play an important role on this enhancement,consistent with the excited-state tunneling mechanism [Phys. Rev. Lett. 116 123901(2016)]. Our results unambiguously show that excited-state tunneling is essential for understanding the enhancement of HHG. Accordingly, a four-step model is herein proposed to illustrate the multiphoton excitation effect in helium-like ions, which enriches the physics of HHG enhancement.

大面积MCP选通X射线分幅相机的研制

研制大面积微通道板(microchannel plate,MCP)选通X射线皮秒分幅相机.MCP外径为106 mm,微带阴极宽度为12 mm.测量微带阴极的时域反射曲线,得到微带阴极的特性阻抗约为11Ω,相机的时间记录长度约3 ns.对分幅相机进行联调实验,测得其时间分辨率为62 ps,空间分辨率为18.87 lp/mm.

超快光电测量技术

本文综述了近年来趔快光电测量技术的新进展，其中包括扭快光电材料的特性、高速光电器件的基本原理，着重介绍了若干主要的用快光电测量技术．

Time-resolved shadowgraphs and morphology analyses of aluminum ablation with multiple femtosecond laser pulses

Aluminum ablation by multiple femtosecond laser pulses is investigated via time-resolved shadowgraphs and scanning electron microscope （SEM） images of the ablation spot. The spatial distribution of the ejected material and the radius of the shock wave generated during the ablation are found to vary with the increase in the number of pulses. In the initial two pulses, nearly concentric and semicircular stripes within the shock wave front are observed, unlike in subsequent pulses. Ablation by multiple femtosecond pulses exhibits different characteristics compared with the case induced by single femtosecond pulse because of the changes to the aluminum target surface induced by the preceding pulses.

瞬态光学及其进展

扼要介绍了瞬态光学的研究内容及其进展 。

Optically induced abnormal terahertz absorption in black silicon

The absorption responses of blank silicon and black silicon（silicon with micro/nano-conical surface structures） wafers to an 808-nm continuous-wave（CW） laser are investigated at room temperature by terahertz time-domain spectroscopy. The transmission of the blank silicon shows an appreciable change, from ground state to the pump state, with amplitude varying up to 50%, while that of the black silicon（BS） with different cone sizes is observed to be more stable. Furthermore,the terahertz transmission through BS is observed to be strongly dependent on the size of the conical structure geometry.The conductivities of blank silicon and BS are extracted from the experimental data with and without pumping. The non-photo-excited conductivities increase with increasing frequency and agree well with the Lorentz model, whereas the photo-excited conductivities decrease with increasing frequency and fit well with the Drude–Smith model. Indeed, for BS, the conductivity, electron density and mobility are found to correlate closely with the size of the conical structure.This is attributed to the influence of space confinement on the carrier excitation, that is, the carriers excited at the BS conical structure surface have a stronger localization effect with a backscattering behavior in small-sized microstructures and a higher recombination rate due to increased electron interaction and collision with electrons, interfaces and grain boundaries.

Imprint of transient electron localization in H2+ using circularly-polarized laser pulse

Photoelectron momentum distribution of hydrogen molecular ion in a circularly polarized laser pulse is calculated by solving the three-dimensional time-dependent Schrodinger equation(3D-TDSE).At the intermediate internuclear distance,an unusual multi-peak structure is observed in the angular distribution,which is proved to be a signature of the transient localization of the electron upon alternating nucleus.By tracing the time-dependent ionization rate and bound state populations,we provide a clear evidence that the transient electron localization still exists in circularly polarized pulse and the corresponding multiple ionization bursts are directly mapped onto observable angular distributions.In addition,we introduce an intuitive strong-field approximation model which incorporates laser-induced subcycle internal electron dynamics to isolate the effect of the Coulomb potential of the parent ions.In this way,the timing of each ionization burst can be directly read out from the angular distributions.Our results suggest that the ionization time serves as a sensitive tool encoding intramolecular electron dynamics and can be measured using attoclock technique.

线偏振、圆偏振和椭圆偏振的飞秒激光诱导熔融石英损伤(英文)

基于电子分布函数的等离子体模型被广泛应用于熔融石英上飞秒激光诱导损伤过程的研究.在本研究中,等离子体模型被修正以研究不同的偏振状态的损伤过程,包括线偏振、圆偏振和椭圆偏振等.首先,使用扩展的Keldysh理论拟合得到不同偏振下的多光子电离截面.其次,为消除沉积能量梯度对模拟过程的影响,对介电函数进行了"非局部化处理".模拟结果表明:不同偏振下光子电离过程的差异是导致损伤过程差异的主要原因,而材料光学特性的改变导致的负反馈效应会使得该差异减小.修正等离子体模型预测的不同偏振下损伤阈值与实验结果吻合良好.

Ablation and ultrafast dynamics of zinc selenide under femtosecond laser irradiation

The ablation in zinc selenide （ZnSe） crystal is studied by using 150-fs, 800-nm laser system. The images of the ablation pit measured by scanning electronic microscope （SEM） show no thermal stress and melting dynamics. The threshold fluence is measured to be 0.7 J/cm^2. The ultrafast ablation dynamics is studied by using pump and probe method. The result suggests that optical breakdown and ultrafast melting take place in ZnSe irradiated under femtosecond laser pulses.

Dielectric coatings for optimized low-loss saturable absorbers for high-power ultrafast laser

With the development of high power ultrafast laser passively mode-locked by a semiconductor saturable absorber mirror （SESAM）, the damage threshold and degeneration mechanism of the SESAM become more and more important. One way to reduce the maximum electric field inside the active part of the SESAM is the use of a dielectric coating on the top of the semiconductor structure. With Presnel formula, optical transfer matrix, and optical thin film theory, the electric field distribution and reflectance spectrum can be simulated. We introduce the design principles of SESAM including the dependence of reflectance spectrum on dielectric function of absorber, and investigate the dependences of the electric field distribution, modulation depth, reflectance spectrum, and the relative value of incident light power at the top quantum well of SESAM on the number of SiO2/Ta2O5 layers.

Serial time-encoded amplified microscopy for ultrafast imaging based on multi-wavelength laser

In this paper,a serial time-encoded amplified microscopy(STEAM)by employing a multi-wavelength laser as the light source is proposed and experimentally demonstrated.This system achieves ultrafast optical imaging with a tunable frame rate.The measuring range depends on the spectrum width of the multi-wavelength laser.Through tuning the speed of the modulating signal,the frame rate ranges from 100to 250 MHz.In addition,the spatial resolution can be improved by increasing the group velocity dispersion and reducing the wavelength spacing.Finally,with the development of photonic integrate circuits(PIC),the multi-wavelength laser source has the potential for integration on a photonic chip and thus the size of the proposed STEAM could be reduced in the future.

双磁透镜对时间展宽分幅变像管性能的影响

采用双磁透镜设计时间展宽分幅变像管,对其空间分辨特性进行模拟和测试。采用2#分辨率板测试像管空间分辨率,以及大面积阴极测试像管在离轴位置的空间分辨能力。测试结果表明,双磁透镜像管的空间分辨率为74 mm;在图像缩小1倍时,能有效分辨阴极距离中心轴位置30 mm的信息。像管的空间分辨率和在阴极离轴位置的分辨能力都优于单磁透镜像管。因此,双磁透镜能有效提高时间展宽分幅变像管的空间分辨特性。

超短强激光脉冲在大气传播中的自聚焦行为

通过建立涉及克尔效应和多光子电离效应的非线性数学模型 ,利用数值方法研究了超短激光脉冲在大气传播中的自聚焦行为。结果表明由自聚焦导致的成丝现象具有间断性 ,而不是一种稳态的模式。每次轴上的成丝都会由于光场损耗而减弱 ,但同时由于克尔效应会使离轴区域的能量间断地补充到光轴上。

光束成丝的非线性理论

在考虑调制场和本底场之间有能量交换的情况下 ,研究了小尺度调制场的非线性演化过程。结果表明 ,受到小尺度调制的光束在传输过程中有可能经历周期性成丝过程。通过与Bespalov Talanov (B T)关于成丝的线性理论比较 ,发现B T理论给出的最快增长频率和临界频率等结果在非线性演化过程中仍然是适用的 ,但其给出的调制增长率只在调制场的初始增长阶段与本文的非线性理论一致 ,此后 ,调制场的增长速度低于指数形式的增长速率 ,达到某个最大值后将经历周期性变化过程。

超快激光成丝现象研究综述

超快激光成丝是高功率超快激光在透明光学介质中传输时出现的一种独特非线性光学现象,所产生长度远超共焦范围的离子体通道被称为光丝;同时伴随超连续谱产生、荧光辐射和受激放大、脉冲自压缩等丰富的光学效应,在大气遥感、超快激光技术、人工干预天气、激光超精细加工等方面具有广泛应用前景。本文对成丝现象研究的历史发展进行了简单介绍,包括实验技术、基本物理机制和调控方法的主要研究进展,并对成丝在大气远程应用、物理化学机制、强THz波产生等方面的研究所面临的挑战进行了展望。

利用等离子体非线性系数实现超强脉冲的压缩

超短脉冲压缩技术在强场物理研究中有非常重要的作用。但由于强场电离现象在惰性气体自相位调制脉冲压缩技术中限制了脉冲的能量 ,Tempea等人提出可以采用等离子体非线性系数对脉冲进行压缩。本文在考虑毛细管内表面电离的情况下 ,讨论能量为 10mJ左右、脉宽为 5 0fs的脉冲的压缩问题 ,发现可以将脉冲压缩至5fs左右。计算表明频谱展宽可以在气体密度很低的情况下进行 ,这样将可以减小电子对脉冲传播的影响。同时 ,由于毛细管内表面也处于电离状态 ,从而使脉冲能量不会受到电离阈值的限制。

输入光特性对超快非线性干涉仪开关窗口的影响

就超快非线性干涉仪(UNI)的输入光特性对其开关窗口的影响进行了数值模拟和实验研究,输入光包括控制光脉冲和探测光脉冲。在数值模拟中,调节控制光脉冲和探测光脉冲的功率及脉宽,功率越高和脉宽越窄,窗口的形状越好。在调节过程中发现控制光脉冲和探测光脉冲都存在一个最佳的功率点使窗口的形状达到最优。如果继续增大控制光脉冲功率,会使窗口的顶部倾斜,窗口形状恶化;而继续增大探测光脉冲功率,窗口的消光比开始下降。在数值模拟的基础上进行了10 Gb/s的超快非线性干涉仪全光开关实验,在实验中用连续光代替探测光脉冲以观察窗口形状。通过改变控制光脉冲和连续光功率来验证它们对超快非线性干涉仪开关窗口的影响。实验表明,应选用短而强的控制光脉冲和最优功率点的连续光,这与模拟结果吻合。

一种新型加速结构飞秒条纹相机的设计

为提高飞秒条纹相机的性能,设计了一种新型加速结构的条纹相机,它具有加速栅网和加速狭缝通道,平行平板电极结构。考虑实际惯性约束核聚变(ICF)实验中X射线入射Cs I光阴极,光电子能量弥散较大和空间电荷效应作用下,用蒙特卡罗方法抽样2000个光电子能量角度分布值,用四阶龙格-库塔法计算了电子在条纹相机中的飞行轨迹。在成像面上统计电子时空分布得到该条纹相机的时间和空间分辨特性,结果显示使用该加速结构的飞秒条纹相机空间分辨率达到21l lp/mm,时间分辨率达到508.4fs,对比传统平行平板相机,分别提高了90%和29%左右,性能提高明显。该新型条纹相机加速结构简单,具有实用性。