激光等离子体相互作用下高次谐波特性研究

相对论激光与等离子体相互作用中形成的纳米电子束在强激光场中的相干同步辐射是产生相干极紫外线和X射线辐射的独特方式.相对论激光脉冲的宽度和等离子体的各种参数决定了产生单个阿秒脉冲还是阿秒脉冲串.在激光脉冲持续时间只有少数几个光学周期下,其载波包络相位对阿秒脉冲有重要影响.通过控制载波包络相位在合适的范围,可以得到孤立阿秒脉冲.除了驱动激光的载波包络相位,等离子体密度分布梯度和等离子体厚度也会影响阿秒脉冲的特性.     

激光等离子体加速电子与固体靶相互作用产生相对论正电子的模拟计算

通过理论分析,建立了激光等离子体加速电子与固体靶相互作用产生相对论正电子的物理模型,以及Geant4模拟程序.以100 Me V量级的激光等离子体加速电子束参数为输入,模拟研究了不同靶材和靶厚条件下正电子束的产额、能量、角分布等主要物理参数.结果表明:金靶和钽靶是较优秀的电子—正电子转换靶材;对于相同的金属靶材面密度,正电子产额与原子序数Z的四次方成正比,与原子质量数A的平方成反比,即Ne+∝(Z2/A)2;对于不同的靶材,正电子产额有Ne+∝d2,其中d为靶材厚度,但仍存在一个最佳靶厚度.与利用拍瓦、皮秒激光束与固体靶相互作用产生正电子束的方案相比,利用本方案有望获得更高能量以及更小角发散的相对论正电子束,其流强可达107/shot.     

斜入射激光脉冲与稠密等离子体相互作用产生的阿秒脉冲

利用一维粒子模拟程序研究和比较了斜入射和垂直入射激光脉冲与稠密等离子体相互作用得到的阿秒脉冲以及激光强度对阿秒脉冲转换比率的影响.同样参数下,斜入射的阿秒脉冲转换比率明显高于垂直入射的情况,滤波后得到的阿秒脉冲振幅比较大,而脉冲串中阿秒脉冲的个数则是垂直入射时的一半.根据振荡镜面模型对两种情况进行了分析,由镜面振荡方程可以对结果给出解释.保持等离子体密度不变,增大入射激光强度时,随着滤波次数的增加,斜入射与垂直入射的阿秒脉冲的转换比率逐渐趋于相同.300次以上高通滤波后我们得到了处于X射线范围的阿秒脉冲.     

强脉冲X射线场中散射能谱获取方法

通过对X射线与物质相互作用后产生的散射能谱的分析,可实现物质原子序数的提取,并可用于核材料等违禁品的探测。然而,高能X射线的特性对探测系统的屏蔽和时间响应提出了要求。该文提出了一种能够对脉冲X射线的散射能谱进行采集的实验方案:以LaBr3(Ce)晶体为X射线探测器来实现<100ns的散射光子分辨时间,以减少在5μs脉冲出束时间内的脉冲堆积问题;利用120MHz/14位的高采样率ADC(analog-digital converter)电路来采集前放电路的输出脉冲波形,并设计相应的离线算法将该波形数据重建为散射能谱;设计了合适的屏蔽结构,减少了来自加速器靶点的直接透射X射线和环境散射X射线对探测器的影响。利用该方案对11种具有不同原子序数的物质进行了测量,得到了它们的散射能谱,在511keV峰能量分辨率可达到5%左右。     

大功率辐照加速器X射线转换靶设计

为完成可用于木材辐照检疫的大功率辐照加速器项目,设计了其中的X射线转换靶.通过理论分析和Monte Carlo方法模拟计算了几种常用靶材(Au, W, Ta, Cu)的X射线光子剂量产额与靶厚的关系.在确定了各单质靶最佳靶厚的基础上,进行了多层复合转换靶的结构研究.在保证光子剂量产额较高的前提下,兼顾转换靶的冷却效果,选择设计了两种分别以Ta和W为主靶材料、不锈钢为基底的复合靶方案.不锈钢基底中间设置3条水道,在冷却的基础上可对产生的X射线能谱进行硬化.这两种复合靶的热分布计算及产生的X射线均匀性均可满足实际工程的需要.     

用相对论电子束产生低能X射线新方案

本文设计了一种利用已有相对论电子束（REB）装置产生系统电磁脉冲效应研究所需X射线源的新方案．从理论上验证了其可行性．该方案将在随后开展的实验中得到实施．     

利用不对称极化门方案增强阿秒脉冲的强度

理论提出了一种利用不对称极化门方案来增强阿秒脉冲强度的方法.结果表明,当两束圆偏振激光场采用不对称的强度时,不仅谐波干涉减小,而且谐波辐射强度明显增强,呈现了一个带宽在85 eV几乎由单一量子路径贡献产生的超长连续平台区.最后,通过叠加该平台区的谐波辐射光谱可以获得一个半高全宽在52 as的超短单个阿秒脉冲.     

准直准单能靶面电子加速实验研究

实验上使用大能量、亚ps激光脉冲大角度入射固体靶,获得了沿靶面方向定向传播、发散角仅有2°、峰值能量为3–4 Me V的准直、准单能电子束.实验发现激光对比度对靶面电子束的产生起到了至关重要的作用,最佳的对比度为5×10-6.在此最优化条件下,通过背向散射光谱分析发现,共振吸收激发的等离子体波加速可能是电子的主要加速机制.探针光阴影成像及等离子体自发光的精细结构显示,预脉冲与固体靶相互作用中产生了尺度100μm左右的过临界密度预等离子体.这种等离子体的作用类似于等离子体反射镜,使得激光脉冲被限制在预等离子体区与靶面之间,因而最终造成了电子束沿靶面方向的导引.这种靶面电子束因其合适的能量范围、高度的准直性及沿靶面方向定向传播的特性有望在惯性约束聚变尤其是锥靶快点火中得到应用.     

超强光场驱动的高性能高能粒子束的产生及其应用开拓研究

超短超强激光驱动等离子体,可获得电子能量高达1Ge V、质子能量高达60Me V的高性能粒子束,从而在高能加速器、聚变物理、短脉冲高亮度X光源产生、实现小型化自由电子激光等领域都有重大的应用价值。该研究主要研究利用超短超强激光在等离子体中形成稳定的特殊三维尾波结构,即空泡,实现单能电子加速。采用两种控制电子注入的方法,即两束激光对打和纳米细丝扰动,来提高电子加速的稳定性,并控制高能电子的数量和能量。该研究还将通过改变激光传输方向的等离子体密度,来改变空泡中纵向加速静电场的梯度,从而抵消高能电子束本身电荷分离场的梯度,以提高电子束的性能;还将研究高能电子束的细致结构,并考虑其可能的重大应用。该研究将利用靶后鞘层加速实现质子加速,并将利用多层靶来提高加速效率,利用微结构靶获得准单能质子束,同时研究获得高性能高能离子束的其他有效途径。     

Monte Carlo方法在靶尺寸分析中的应用

对于电子直线加速器而言,当电子束的能量一定时,X射线的剂量场分布与很多因素相关,其中靶的尺寸和材料便是重要的因素。目前最经常采用的靶材料是钨,靶尺寸的选取则由加速器的能量等条件来决定。该文采用Monte Carlo通用程序包Geant4,对靶的尺寸进行了一系列的分析。结果表明:靶厚度对X射线剂量场分布影响很大,为得到理想的加速器输出剂量,需要选取合适的靶厚度;相对而言,靶直径的影响则很小。     

Direct observation of electron dynamics in the attosecond domain

Dynamical processes are commonly investigated using laser pump-probe experiments, with a pump pulse exciting the system of interest and a second probe pulse tracking its temporal evolution as a function of the delay between the pulses. Because the time resolution attainable in such experiments depends on the temporal definition of the laser pulses, pulse compression to 200 attoseconds (1 as = 10(-18) s) is a promising recent development. These ultrafast pulses have been fully characterized, and used to directly measure light waves and electronic relaxation in free atoms. But attosecond pulses can only be realized in the extreme ultraviolet and X-ray regime; in contrast, the optical laser pulses typically used for experiments on complex systems last several femtoseconds (1 fs = 10(-15) s). Here we monitor the dynamics of ultrafast electron transfer--a process important in photo- and electrochemistry and used in solid-state solar cells, molecular electronics and single-electron devices--on attosecond timescales using core-hole spectroscopy. We push the method, which uses the lifetime of a core electron hole as an internal reference clock for following dynamic processes, into the attosecond regime by focusing on short-lived holes with initial and final states in the same electronic shell. This allows us to show that electron transfer from an adsorbed sulphur atom to a ruthenium surface proceeds in about 320 as.     

Adaptive subwavelength control of nano-optical fields

Adaptive shaping of the phase and amplitude of femtosecond laser pulses has been developed into an efficient tool for the directed manipulation of interference phenomena, thus providing coherent control over various quantum-mechanical systems. Temporal resolution in the femtosecond or even attosecond range has been demonstrated, but spatial resolution is limited by diffraction to approximately half the wavelength of the light field (that is, several hundred nanometres). Theory has indicated that the spatial limitation to coherent control can be overcome with the illumination of nanostructures: the spatial near-field distribution was shown to depend on the linear chirp of an irradiating laser pulse. An extension of this idea to adaptive control, combining multiparameter pulse shaping with a learning algorithm, demonstrated the generation of user-specified optical near-field distributions in an optimal and flexible fashion. Shaping of the polarization of the laser pulse provides a particularly efficient and versatile nano-optical manipulation method. Here we demonstrate the feasibility of this concept experimentally, by tailoring the optical near field in the vicinity of silver nanostructures through adaptive polarization shaping of femtosecond laser pulses and then probing the lateral field distribution by two-photon photoemission electron microscopy. In this combination of adaptive control and nano-optics, we achieve subwavelength dynamic localization of electromagnetic intensity on the nanometre scale and thus overcome the spatial restrictions of conventional optics. This experimental realization of theoretical suggestions opens a number of perspectives in coherent control, nano-optics, nonlinear spectroscopy, and other research fields in which optical investigations are carried out with spatial or temporal resolution.     

Direct observation of attosecond light bunching

Temporal probing of a number of fundamental dynamical processes requires intense pulses at femtosecond or even attosecond (1 as = 10(-18) s) timescales. A frequency 'comb' of extreme-ultraviolet odd harmonics can easily be generated in the interaction of subpicosecond laser pulses with rare gases: if the spectral components within this comb possess an appropriate phase relationship to one another, their Fourier synthesis results in an attosecond pulse train. Laser pulses spanning many optical cycles have been used for the production of such light bunching, but in the limit of few-cycle pulses the same process produces isolated attosecond bursts. If these bursts are intense enough to induce a nonlinear process in a target system, they can be used for subfemtosecond pump-probe studies of ultrafast processes. To date, all methods for the quantitative investigation of attosecond light localization and ultrafast dynamics rely on modelling of the cross-correlation process between the extreme-ultraviolet pulses and the fundamental laser field used in their generation. Here we report the direct determination of the temporal characteristics of pulses in the subfemtosecond regime, by measuring the second-order autocorrelation trace of a train of attosecond pulses. The method exhibits distinct capabilities for the characterization and utilization of attosecond pulses for a host of applications in attoscience.     

Attosecond control of electrons emitted from a nanoscale metal tip

Attosecond science is based on steering electrons with the electric field of well controlled femtosecond laser pulses. It has led to the generation of extreme-ultraviolet pulses with a duration of less than 100 attoseconds (ref. 3; 1 as = 10(-18) s), to the measurement of intramolecular dynamics (by diffraction of an electron taken from the molecule under scrutiny) and to ultrafast electron holography. All these effects have been observed with atoms or molecules in the gas phase. Electrons liberated from solids by few-cycle laser pulses are also predicted to show a strong light-phase sensitivity, but only very small effects have been observed. Here we report that the spectra of electrons undergoing photoemission from a nanometre-scale tungsten tip show a dependence on the carrier-envelope phase of the laser, with a current modulation of up to 100 per cent. Depending on the carrier-envelope phase, electrons are emitted either from a single sub-500-attosecond interval of the 6-femtosecond laser pulse, or from two such intervals; the latter case leads to spectral interference. We also show that coherent elastic re-scattering of liberated electrons takes place at the metal surface. Owing to field enhancement at the tip, a simple laser oscillator reaches the peak electric field strengths required for attosecond experiments at 100-megahertz repetition rates, rendering complex amplified laser systems dispensable. Practically, this work represents a simple, extremely sensitive carrier-envelope phase sensor, which could be shrunk in volume to about one cubic centimetre. Our results indicate that the attosecond techniques developed with (and for) atoms and molecules can also be used with solids. In particular, we foresee subfemtosecond, subnanometre probing of collective electron dynamics (such as plasmon polaritons) in solid-state systems ranging in scale from mesoscopic solids to clusters and to single protruding atoms.     

Attosecond control of electronic processes by intense light fields

The amplitude and frequency of laser light can be routinely measured and controlled on a femtosecond (10(-15) s) timescale. However, in pulses comprising just a few wave cycles, the amplitude envelope and carrier frequency are not sufficient to characterize and control laser radiation, because evolution of the light field is also influenced by a shift of the carrier wave with respect to the pulse peak. This so-called carrier-envelope phase has been predicted and observed to affect strong-field phenomena, but random shot-to-shot shifts have prevented the reproducible guiding of atomic processes using the electric field of light. Here we report the generation of intense, few-cycle laser pulses with a stable carrier envelope phase that permit the triggering and steering of microscopic motion with an ultimate precision limited only by quantum mechanical uncertainty. Using these reproducible light waveforms, we create light-induced atomic currents in ionized matter; the motion of the electronic wave packets can be controlled on timescales shorter than 250 attoseconds (250 x 10(-18) s). This enables us to control the attosecond temporal structure of coherent soft X-ray emission produced by the atomic currents--these X-ray photons provide a sensitive and intuitive tool for determining the carrier-envelope phase.     

Attosecond metrology.

The generation of ultrashort pulses is a key to exploring the dynamic behaviour of matter on ever-shorter timescales. Recent developments have pushed the duration of laser pulses close to its natural limit-the wave cycle, which lasts somewhat longer than one femtosecond (1 fs = 10-15 s) in the visible spectral range. Time-resolved measurements with these pulses are able to trace dynamics of molecular structure, but fail to capture electronic processes occurring on an attosecond (1 as = 10-18 s) timescale. Here we trace electronic dynamics with a time resolution of 相似文献   

不同啁啾调制对光谱连续区的影响

利用啁啾场调控激光波形,理论研究了不同啁啾场对高次谐波光谱的影响. 结果表明：当采用对称中间啁啾调控时,谐波截止能量的延伸及光谱连续区来自于激光中间区域. 当采用不对称负向啁啾调控时,谐波截止能量的延伸及光谱连续区来自于激光下降区域. 虽然,谐波截止能量在不同啁啾调控下都可以得到延伸,但是,不对称负向啁啾场下光谱连续区的强度要比对称中间啁啾场下光谱连续区强度高2个数量级. 最后,通过叠加光谱连续区上的谐波可以获得2个脉宽在38 as的单个阿秒脉冲. 并且,负向啁啾场下获得脉冲强度要比对称中间啁啾场下获得脉冲强度高2个数量级.     

Probing molecular dynamics with attosecond resolution using correlated wave packet pairs

Spectroscopic measurements with increasingly higher time resolution are generally thought to require increasingly shorter laser pulses, as illustrated by the recent monitoring of the decay of core-excited krypton using attosecond photon pulses. However, an alternative approach to probing ultrafast dynamic processes might be provided by entanglement, which has improved the precision of quantum optical measurements. Here we use this approach to observe the motion of a D2+ vibrational wave packet formed during the multiphoton ionization of D2 over several femtoseconds with a precision of about 200 attoseconds and 0.05 ?ngstr?ms, by exploiting the correlation between the electronic and nuclear wave packets formed during the ionization event. An intense infrared laser field drives the electron wave packet, and electron recollision probes the nuclear motion. Our results show that laser pulse duration need not limit the time resolution of a spectroscopic measurement, provided the process studied involves the formation of correlated wave packets, one of which can be controlled; spatial resolution is likewise not limited to the focal spot size or laser wavelength.     

利用啁啾场与单极场的组合场驱动He+发射高次谐波

数值研究了He~+在啁啾场与单极场下发射高次谐波及阿秒脉冲的特点.计算结果表明,当He~+的初始波函数布局在基态与激发态的叠加态时,其谐波强度比单基态时增强7个数量级.随后在啁啾场及单极控制场的作用下,谐波发射的截止能量明显增强,谐波的干涉结构也明显减小.引入空间非均匀效应,谐波截止能量得到进一步延伸,形成一个320eV的平台区.通过叠加谐波,可获得3个持续时间在45~48as的X射线脉冲.其强度比单基态输出的脉冲增强5~6个数量级.     

调节X2+同位素分子电荷共振增强电离时刻获得高强度阿秒脉冲

利用X_2~+同位素分子(H_2~+、D_2~+、T_2~+)谐波辐射的特点,提出一种有效获得高强度谐波连续区和孤立阿秒脉冲的方法.研究表明,在不同脉宽激光作用下,H_2~+、D_2~+和T_2~+分子可分别进入电荷共振增强电离区域.当激光振幅区域的半个周期正好处于电荷共振增强电离区域时,具有最大辐射能量的谐波能量峰正好具有最佳的辐射强度.随后,在此区域引入半周期单极激光场,被选择出来的谐波能量峰可以继续延伸,进而获得一个仅由单一能量峰贡献而产生的高强度谐波连续区.通过叠加连续区上的谐波可以获得脉宽仅为42 as的孤立阿秒脉冲.