Enhanced harmonic generation from expanding clusters

We report controlled enhancement of optical third harmonic generation (THG) from hydrodynamically expanding clusters of approximately 6x10(5) noble-gas atoms several hundred femtoseconds following ionization and heating by ultrashort pump pulses. This resonant enhancement is more pronounced for orthogonal than for parallel pump-probe polarizations, a consequence of faster cluster expansion along the pump polarization. Simulations show that the nonlinear susceptibility chi(3) of the individual clusters and the coherence length of the clustered plasma medium are optimized nearly simultaneously as the clusters expand, and both contribute to the observed THG enhancement. This dual enhancement mechanism may be scalable to relativistic probe intensity and to generation of high-order harmonics in the soft-x-ray regime.     

DOA estimation for uniform circular array without the source number based on beamspace transform and higher-order cumulant

Fourth-order cumulant is one of most widely used high-order cumulant for direction of arrival (DOA) estimation due to its ability of expanding the virtual array aperture as well as suppressing Gaussian noise. To address the two-dimensional (2D) DOA estimation problem, we propose a modified MUSIC scheme for uniform circular array (UCA) in this paper. Firstly, the fourth-order cumulant of UCA is considered to construct a new propagator, resulting in the elimination of a priori knowledge of the number of signals. Secondly, the UCA is transformed by beamspace transformation, reducing the time computational complexity of the algorithm since the two-dimensional grid search and singular value decomposition are avoided. And finally a low-rank recovery algorithm is adopted to improve the accuracy regarding the limited snapshots scenario. The numerical simulations validate the superiority of the proposed method.     

高阶阈值上电离的量子电动力学理论

用非微扰量子电动力学理论研究一个由再散射产生的高阶阈值上电离（ATI）.利用形式散射理论给出了高阶ATI谱的解析形式，并且讨论了在高阶ATI中不同ATI通道的干涉效应和光电子以不同角度入射时的高阶ATI谱. 关键词： 阈值上电离 再散射 跃迁概率     

数值模拟高阶色散和非线性对光孤子传输的影响

光孤子传输中的高阶色散以及高次非线性效应是光纤通讯发展的重要制约因素。从光孤子在光纤中的一般传输方程出发, 在较大的入纤功率的前提下, 综合考虑了高阶色散、五次非线性和损耗因素, 得到其具体传输方程, 并据此从理论上分析了高阶色散和非线性对光孤子传输性能的影响。本文采用分步傅里叶方法, 以MATLAB为实现工具, 实现高阶色散和非线性对光孤子传输影响的模拟计算, 并深入分析了高阶色散和非线性导致的孤子脉冲频移现象。计算结果表明: 在入射功率较大的时候, 高阶色散效应不可忽略。当五次非线性γ2>0时,孤子脉冲主峰发生微小频移; 而当五次非线性γ2<0时, 孤子脉冲主峰基座产生微小频移; 当高阶色散β3>0时频率出现红移, 而当β3<0时, 频率出现蓝移。     

Influence of Misalignment on High-Order Aberration Correction for Normal Human Eyes

Although a compensation device can correct aberrations of human eyes, the effect will be degraded by its misalignment, especially for high-order aberration correction. We calculate the positioning tolerance of correction device for high-order aberrations, and within what degree the correcting effect is better than low-order aberration （defocus and astigmatism） correction. With fixed certain misalignment within the positioning tolerance, we calculate the residual wavefront rms aberration of the first-6 to first-35 terms along with the 3rd-Sth terms of aberrations corrected, and the combined first-13 terms of aberrations are also studied under the same quantity of misalignment. However, the correction effect of high-order aberrations does not meliorate along with the increase of the high-order terms under some misalignment, moreover, some simple combined terms correction can achieve similar result as complex combinations. These results suggest that it is unnecessary to correct too much the terms of high-order aberrations which are difficult to accomplish in practice, and gives confidence to correct high-order aberrations out of the laboratory.     

Characteristics of high-order harmonic spectrum by using laser-ablated two targets combination

We demonstrated a high-order harmonic generation from lowly charged ions in laser-ablation two-compound plumes (Ag-In, Ag-Mn, and Cr-Te). The high-order harmonic spectra from two-compound plumes were composed of those obtained from each target. Our results show that the control of cutoff energy and intensity of single high-order harmonics can be achieved by using the appropriate target combination.     

双曲型守恒律的一种高精度TVD差分格式

构造了一维双曲型守恒律方程的一个高精度高分辨率的守恒型TVD差分格式.其主要思想是:首先将计算区域划分为互不重叠的小单元,且每个小单元再根据希望的精度阶数分为细小单元;其次,根据流动方向将通量分裂为正、负通量,并通过小单元上的高阶插值逼近得到了细小单元边界上的正、负数值通量,为避免由高阶插值产生的数值振荡,进一步根据流向对其进行TVD校正;再利用高阶Runge KuttaTVD离散方法对时间进行离散,得到了高阶全离散方法.进一步推广到一维方程组情形.最后对一维欧拉方程组计算了几个算例.     

Wave-front phase measurements of high-order harmonic beams by use of point-diffraction interferometry

We report the wave-front phase measurement of high-order harmonics employing point-diffraction interferometry. The high-order harmonics generated in a gas-filled hollow tube showed excellent spatial coherence over nearly the whole cross section of the harmonic beams. Using this coherent harmonic source in the extreme-ultraviolet-soft-x-ray region, we have demonstrated the operation of a point-diffraction interferometer and measured the wave-front phase of high-order harmonics.     

Extension of the spectral volume method to high-order boundary representation

In this paper, the spectral volume method is extended to the two-dimensional Euler equations with curved boundaries. It is well-known that high-order methods can achieve higher accuracy on coarser meshes than low-order methods. In order to realize the advantage of the high-order spectral volume method over the low order finite volume method, it is critical that solid wall boundaries be represented with high-order polynomials compatible with the order of the interpolation for the state variables. Otherwise, numerical errors generated by the low-order boundary representation may overwhelm any potential accuracy gains offered by high-order methods. Therefore, more general types of spectral volumes (or elements) with curved edges are used near solid walls to approximate the boundaries with high fidelity. The importance of this high-order boundary representation is demonstrated with several well-know inviscid flow test cases, and through comparisons with a second-order finite volume method.     

High-order non-uniform grid schemes for numerical simulation of hypersonic boundary-layer stability and transition

The direct numerical simulation of receptivity, instability and transition of hypersonic boundary layers requires high-order accurate schemes because lower-order schemes do not have an adequate accuracy level to compute the large range of time and length scales in such flow fields. The main limiting factor in the application of high-order schemes to practical boundary-layer flow problems is the numerical instability of high-order boundary closure schemes on the wall. This paper presents a family of high-order non-uniform grid finite difference schemes with stable boundary closures for the direct numerical simulation of hypersonic boundary-layer transition. By using an appropriate grid stretching, and clustering grid points near the boundary, high-order schemes with stable boundary closures can be obtained. The order of the schemes ranges from first-order at the lowest, to the global spectral collocation method at the highest. The accuracy and stability of the new high-order numerical schemes is tested by numerical simulations of the linear wave equation and two-dimensional incompressible flat plate boundary layer flows. The high-order non-uniform-grid schemes (up to the 11th-order) are subsequently applied for the simulation of the receptivity of a hypersonic boundary layer to free stream disturbances over a blunt leading edge. The steady and unsteady results show that the new high-order schemes are stable and are able to produce high accuracy for computations of the nonlinear two-dimensional Navier–Stokes equations for the wall bounded supersonic flow.     

High-order polarization vortex spatial solitons

We investigate the formation of high-order polarization vortex spatial solitons. The high-order polarization vortex solitons have novel polarization states which are different from fundamental polarization vortex solitons and have rotational symmetry only in intensity. It is proved that the polarization vortex solitons cannot carry vortex phase. The existence domain and dynamical characteristic of these high-order polarization vortex solitons in Bessel optical lattices are discussed in detail.     

High-order compact finite-difference methods on general overset grids

This work investigates the coupling of a very high-order finite-difference algorithm for the solution of conservation laws on general curvilinear meshes with overset-grid techniques originally developed to address complex geometric configurations. The solver portion of the algorithm is based on Padé-type compact finite-differences of up to sixth-order, with up to 10th-order filters employed to remove spurious waves generated by grid non-uniformities, boundary conditions and flow non-linearities. The overset-grid approach is utilized as both a domain-decomposition paradigm for implementation of the algorithm on massively parallel machines and as a means for handling geometric complexity in the computational domain. Two key features have been implemented in the current work; the ability of the high-order algorithm to accommodate holes cut in grids by the overset-grid approach, and the use of high-order interpolation at non-coincident grid overlaps. Several high-order/high-accuracy interpolation methods were considered, and a high-order, explicit, non-optimized Lagrangian method was found to be the most accurate and robust for this application. Several two-dimensional benchmark problems were examined to validate the interpolation methods and the overall algorithm. These included grid-to-grid interpolation of analytic test functions, the inviscid convection of a vortex, laminar flow over single- and double-cylinder configurations, and the scattering of acoustic waves from one- and three-cylinder configurations. The employment of the overset-grid techniques, coupled with high-order interpolation at overset boundaries, was found to be an effective way of employing the high-order algorithm for more complex geometries than was previously possible.     

Effects of high-order dispersions on dark-bright vector soliton propagation and interaction

The dynamics of dark-bright vector solitons is investigated in a birefringent fiber with the high-order dispersions,and their effects on vector soliton propagation and interaction are analyzed using the numerical method.The combined role of the high-order dispersions,such as the third-order dispersion (TOD) and the fourth-order dispersion (FOD),may cause various deformation of the vector soliton and enhance interaction.These effects depend strictly on the sign of the high-order dispersions.Results indicate that the disadvantageous effects can be reduced effectively via proper mapping of the high-order dispersions.     

A high-order accurate unstructured finite volume Newton–Krylov algorithm for inviscid compressible flows

A fast implicit Newton–Krylov finite volume algorithm has been developed for high-order unstructured steady-state computation of inviscid compressible flows. The matrix-free generalized minimal residual (GMRES) algorithm is used for solving the linear system arising from implicit discretization of the governing equations, avoiding expensive and complex explicit computation of the high-order Jacobian matrix. The solution process has been divided into two phases: start-up and Newton iterations. In the start-up phase an approximate solution with the general characteristics of the steady-state flow is computed by using a defect correction procedure. At the end of the start-up phase, the linearization of the flow field is accurate enough for steady-state solution, and a quasi-Newton method is used, with an infinite time step and very rapid convergence. A proper limiter implementation for efficient convergence of the high-order discretization is discussed and a new formula for limiting the high-order terms of the reconstruction polynomial is introduced. The accuracy, fast convergence and robustness of the proposed high-order unstructured Newton–Krylov solver for different speed regimes is demonstrated for the second, third and fourth-order discretization. The possibility of reducing computational cost required for a given level of accuracy by using high-order discretization is examined.     

弱谐波情况下透射光栅衍射效率标定方法

同步辐射光源中的高次谐波会使透射光栅衍射效率标定精度变差。为了校正光源中的高次谐波对透射光栅衍射效率标定的影响,提出了一种光源存在弱谐波情况下的透射光栅衍射效率标定方法,通过使用谐波X射线的衍射效率修正基波衍射效率标定中谐波的影响,从而得到更为准确的透射光栅衍射效率。使用该标定方法在北京同步辐射光源上开展了透射光栅相对衍射效率标定工作。实验结果表明:在100~800eV存在高次谐波能段,修正后透射光栅一级与零级的相对衍射效率与理论模拟结果吻合较好,修正后光栅二级与一级的相对衍射效率更接近理论模拟结果,但与理论模拟结果仍有较大偏差,该偏差主要来源光栅较弱的二级衍射。     

利用非传统螺旋相位调控高阶涡旋光束的拓扑结构

本文提出一种利用非传统螺旋相位调控高阶涡旋光束拓扑结构的方法.数值模拟并实验研究了具有不均匀旋转梯度的非传统螺旋相位对高阶涡旋光束的调控行为.结果表明, 携带有非传统螺旋相位的高阶涡旋光束在传输过程中, 将退化为沿一条直线排列的多个一阶相位奇点, 并且, 这种非传统螺旋相位对高阶涡旋光束的调控特性, 可抑制相位噪声等扰动所引起的拓扑结构随机退化现象.本文的结论为涡旋光束拓扑结构的调控提供了一种可行的新途径, 在基于涡旋光束的光学通信、光学操控等方面具有潜在应用.     

High-order harmonic generation in sub-one-cycle regime

We theoretically investigate the high-order harmonic generation from the hydrogen atom driven by the laser pulses with the durations less than the optical cycle.It is found that the switching term of the laser field may have an obvious influence on the cutoff,intensity or plateau structure of the high-order harmonic spectrum.Generally speaking,the switching term can shorten the cutoff of the high-order harmonic spectrum for a relatively longer pulse and extend the cutoff for a relatively shorter pulse.     

高阶非完整约束系统嵌入变分恒等式的积分变分原理

本文从高阶非完整系统嵌入变分恒等式的积分变分原理出发, 根据三种不等价条件变分的选取, 得到了高阶非完整系统的三类不等价动力学模型, 即高阶非完整约束系统的vakonomic方程、Lagrange-d'Alembert 方程和一种新的动力学方程. 当高阶非完整约束方程退化为一阶非完整约束时, 利用此理论可以得到一般非完整系统的vakonomic模型、Chetaev模型和一种新的动力学模型. 最后借助于应用实例验证了结论的正确性. 关键词： 高阶非完整约束 变分恒等式 条件变分 vakonomic动力学     

超短激光脉冲中时空耦合效应的研究进展

概述了超短激光脉冲中时空耦合效应的研究进展。将时空耦合效应分为一阶时空耦合和高阶时空耦合两大类,并详细介绍了两类时空耦合效应的研究历史和现状。其中一阶时空耦合效应相对较为简单,对其的理论及实验研究也较为充分。相比一阶时空耦合效应,高阶时空耦合效应的起源更广泛、种类更丰富、时空结构更复杂。目前,对高阶时空耦合效应的研究也刚处于起步阶段。最后,展望了该领域的研究趋势。     

柱透镜聚焦高阶Bessel光束产生焦散光束

研究了高阶Bessel光束经过柱透镜后的光场分布特性.在广义惠更斯-菲涅尔衍射积分理论基础上,推导出高阶Bessel光束透过柱透镜后的衍射光场分布表达式;并利用MATLAB和MATHCAD模拟了不同传播距离处的光强分布.实验上,利用轴棱锥和螺旋相位板产生不同阶数的高阶Bessel光束,并使产生的高阶Bessel光束经过柱透镜,最后用CCD记录下不同距离处的衍射光场.研究结果表明,高阶Bessel光束经过柱透镜形成唇状的焦散光束.