Surface reverberation at sound field focusing in shallow water

A numerical experiment is carried out to study the long-range surface reverberation in the presence of intense surface waves for the case of using vertical transmitting arrays providing sound field focusing at different depths. To focus the field, a phase conjugation of acoustic waves from a probe source positioned at the focusing point is used. It is demonstrated that surface waves considerably affect the focusing quality at a distance of several tens of kilometers from the transmitting array. This prevents the efficient suppression of long-range reverberation by increasing the focusing depth.     

双晶单色器弧矢聚焦晶体面形有限元分析

简要介绍了弧矢聚焦双晶单色器的基本原理、聚焦晶体的压弯以及鞍形形变的抑制,提出了柔性铰链弧矢聚焦晶体压弯机构,对弧矢聚焦晶体衍射面的子午形变和弧矢弯曲面形进行了有限元分析;用光学追踪程序SHADOW得出了弧矢聚焦晶体衍射面面形对单色器的光通量和分辨率的影响。     

A concave photonic crystal waveguide with a corrugated surface for high-quality focusing

A concave two-dimensional(2D) photonic crystal waveguide(PCW) with corrugated surface is theoretically used as a focusing structure.To design this structure,a genetic algorithm is combined with the finite-difference time-domain method.For PCWs with different degrees of concaveness,the power reaches about 80% at different focusing points when the morphology of the concave surface is optimized.More importantly,the focusing location is easily controlled by changing the location of the detector placed in the output field.     

Focusing of intense femtosecond surface plasmon-polaritons

Nonlinear propagation and focusing within a metallic ring of intense (near or higher than metal ablation threshold) surface plasmon-polaritons excited by a femtosecond laser pulse was experimentally and theoretically studied. Fundamental possibility of local fluence increasing at focus of such plasmonic lens (the ring) by two orders of magnitude was shown. Formation of surface nanostructures as a result of the surface plasmon-polaritons focusing was experimentally observed.     

Maximum “focusing” of surface phonons

“Focusing” of surface phonons is studied in detail. Three types of surface phonon “focusing” are found. The asymptotic dependence of the amplitude u of surface wave on the distance r from a point source is obtained for every “focusing” type. The type of maximum “focusing” where $\text{u((r)～ O(r}{}^{\mathrm{<mtext>?1</mtext><mtext>4</mtext>}}\text{)}$ holds is picked out. The “focusing” type is shown to be changed as elastic anisotropy is altered.     

焊缝结构中超声相控阵聚焦声场的数值模拟及分析*

针对焊缝内相控阵声场聚焦问题,建立焊缝结构中多高斯声束法的相控阵声场计算模型,分别用直接聚焦和底面反射聚焦两种方式对焊缝内部的相控阵声场进行数值模拟,分析不同方式下的相控阵聚焦声场特性,讨论不同区域适用的聚焦方式,分析了界面倾斜角度对聚焦效果的影响。结果表明,在相同焊缝结构条件下直接聚焦的方式更适合对厚壁焊缝的中下部进行聚焦扫描,在焊缝上表面附近,当声束偏转角大于80°时,直接聚焦方式无法有效聚焦;反射聚焦的方式更适合对焊缝上表面附近区域进行聚焦扫描,随着焦点深度增加,反射聚焦的声束偏转角增大,当声束偏转角大于55°时,反射聚焦无法形成明显焦点;两种聚焦扫描方式可形成有效的互补。当设置的焦点固定时,直接聚焦的实际焦点随界面倾斜角度增大向预设点远处偏移,而反射聚焦的效果基本不受倾斜角的影响,实际焦点始终在预设焦点附近。     

超声多波聚焦及声偏振方向控制方法*

在超声多波聚焦思想的基础上,通过数值模拟计算,分析了时间反转法的多波聚焦特性以及对声场偏振方向进行控制的可行性。结果表明,在待测目标的不同位置处,时间反转法都能够实现多波聚焦的效果,使具有不同传播速度、不同偏振特性的多种声波自适应聚焦。但是,在介质的近表面处,由于受到表面波的影响,多波聚焦声场仍然具有椭圆偏振特性,无法实现声场偏振方向精确控制的目的;而位于介质内部的多波聚焦点受到表面波影响很小,数值计算结果表明此时多波聚焦声场具有线偏振特性,通过改变声源前后两个脉冲的激发幅度和相位,可以控制声场的偏振方向,达到偏振方向扫描的目的。该文的研究为精确检测裂纹方向或界面性质提供一种可能的途径。     

基于蜷曲空间结构的近零折射率声聚焦透镜

研究基于蜷曲空间结构的近零折射率声聚焦透镜.根据近零折射率材料的声波方向选择机理,采用蜷曲空间结构为基本单元进行排列,设计具有特定入射与出射界面的几何结构,对透射声波的出射方向进行调控,实现了平面声波与柱面声波的聚焦效应,并深入讨论了透镜内部刚性散射体对声聚焦性能的影响.在此基础上,改变近零折射率透镜的出射界面,可以精确调控声波阵面的形状与方向.该类型透镜具有单一的单元结构、高聚焦性能及高鲁棒性等优点.研究结果为设计新型近零折射率声聚焦透镜提供了理论指导与实验参考,同时也为研究声波阵面的调控提供了新思路.     

横向聚焦波荡器磁场的计算与分析

横向聚焦波荡器可用于电子束能量较低、函数较小的情况。在现有横向聚焦波荡器设计方案的基础上,通过倾斜磁块端面和磁极面提出了几种不同结构的横向聚焦波荡器方案。利用三维磁场模拟程序Radia,对这些波荡器的聚焦场特性进行了模拟计算,对其聚焦特性与主要参数的关系进行了研究,并与现有其他横向聚焦波荡器的聚焦特性进行了对比。对弱聚焦和强聚焦两种情况,分别给出较好的结构方案。对于弱聚焦,平均聚焦梯度随中心场强的增大而略有增大,变化不明显;对于强聚焦,平均聚焦梯度随中心场强的增大而明显增大,呈高次曲线关系。     

Surface treatment with linearly polarized laser beam at oblique incidence

An effective method for surface heat treatment with 10.6 μm linear polarized laser beam at oblique incidence is reported. A circular focused laser spot on the workpiece surface, simultaneously with 2.2–4 times increasing of the absorption are obtained in the 70–80° range of the incidence angle. The main element of the experimental setup is the astigmatic focusing head which focalize the laser beam into an elliptical spot of ellipticity >3 at normal incidence. At a proper incidence angle (obtained by the focusing head tilting) the focused laser spot on the work piece surface gets a circular form and p-state of polarization is achieved.We performed laser heat treatment (transformation hardening, surface remelting) of the uncoated surface, as well as the alloying and cladding processes by powder injection. An enhancement of the processing efficiency was obtained; in this way the investment and operation costs for surface treatment with CO₂ laser can be significantly reduced. Several technical advantages concerning the pollution of the focusing optical components, powder jet flowing and reflected radiation by the work piece surface are obtained.     

离焦量对空气中激光诱导铜等离子体光谱特性的影响

利用Nd: YAG脉冲激光在空气中烧蚀金属Cu靶,获得等离子体光谱;采用改变离焦量的方法,研究了离焦量的变化对谱线结构及谱线强度的影响;分析了离焦量分别为1mm、0mm和-2mm时,沿靶面法线方向不同空间距离处电子温度的演化规律;并对等离子体光谱的特性和产生机制进行了讨论. 结果表明,谱线结构、谱线强度和等离子体的电子温度都与离焦量的变化密切相关,聚焦点在-2mm处CuⅠ谱线相对强度出现峰值,电子温度数值最大;聚焦点在-0.5mm和-1.0mm附近谱线相对强度遽然降低的现象是由于等离子体的屏蔽效应造成的.     

Beam focusing from double subwavelength metallic slits filled with nonlinear material surrounded by dielectric surface gratings

Based on the radiation properties of surface plasmon polaritons (SPPs) can be controlled by adjusting the refractive indexes of dielectric materials in the metallic slits, a novel plasmonic focusing structure formed by two subwavelength metal apertures filled with Kerr nonlinear material surrounded by surface dielectric gratings is proposed and demonstrated numerically. Directions of radiation fields are determined by the phase difference of the surface waves at the exit interface and resonance property of each surface grating. Numerical simulations using two-dimensional (2D) Finite-Difference Time-Domain (FDTD) method verify that the deflection angle and focal length can be controlled easily by changing the intensity of incident light, dynamically tunable on-axis and off-axis focusing effects can be achieved.     

高功率激光聚焦均匀照明靶面的验证

由正六边形的球面透镜列阵和一个非球面的主聚焦透镜组成的聚焦系统,用于改善高功率激光(10~(12)W)聚焦照明靶面的均匀性,得到实验上的证明.文中报道了采用这个聚焦系统进行的激光驱动高压冲击波实验,显示出了均匀照明对改善激光冲击波平面性的良好的效果.     

不同样品温度下聚焦透镜到样品表面距离对激光诱导铜击穿光谱的影响

研究了不同温度下聚焦透镜到样品表面距离对激光诱导击穿光谱(laser-induced breakdown spectroscopy,LIBS)强度的影响,使用Nd:YAG脉冲激光激发样品并产生等离子体,探测的等离子体发射的光谱线为Cu(Ⅰ)510.55 nm,Cu(Ⅰ)515.32 nm和Cu(Ⅰ)521.82 nm.使用透镜的焦距为200 mm,测量的聚焦透镜到样品表面距离的范围为170—200 mm,样品温度从25℃升高到270℃,激光能量为26 mJ.总体上,升高样品温度能有效地提高LIBS光谱的辐射强度.在25℃和100℃时,光谱强度随着聚焦透镜到样品表面距离的增加而单调增加;在样品温度更高(150, 200, 250和270℃)时,光谱强度随着距离的增加出现先升高而后又降低的变化.同时,在样品接近焦点附近,随着样品温度的升高,LIBS光谱强度的变化不明显,还可能出现光谱强度随着样品温度升高而降低的情况,这在通过升高样品温度来提高LIBS光谱强度中特别值得我们注意.为了更进一步了解这两个条件对LIBS的影响,计算了等离子体温度和电子密度,发现等离子体温度和电子密度的变化与光谱强度的变化几乎一致,更高样品温度下产生的等离子体温度和电子密度更高.     

Ion focusing and blocking effects of surface scattering at a grazing angle

The effects of ion focusing and blocking on surface scattering at a grazing angle is investigated by computer simulation, where primary ions are bombarded along the semichannel direction. When varying the angle of incidence, the ion focusing effect is clearly observed in reflection coefficients at the specular reflection.     

On the Electron Emission from the Focal Region of a Symmetrically Convergent Surface Plasmonic Wave on the Free Surface of a Metal Film

Optics and Spectroscopy - Excitation and focusing of a radially convergent surface plasmonic wave on the free surface of a gold film in the Kretschmann geometry have been considered based on the...     

On the Focusing of Surface Plasmonic Waves on the Free Surface of a Metal Film

Optics and Spectroscopy - The excitation and focusing of a surface plasmonic wave on the free surface of a metal film in the Kretschmann scheme have been considered based on the theory of...     

New electrostatic cylindrical energy analyzer

The focusing properties of an ideal cylindrical field in full relativistic field have been investigated for charged particles originating from an extended source and an analytical solution for the equations of motion has been provided. This focusing system can be used for production of electrostatic energy analyzer provided surface analysis from an extended area.     

抽运激光聚焦特性对预脉冲诱导的类氖J=0→1 X射线激光近场像的影响

研究了在最佳除焦、预脉冲光束单独散焦及主预脉 时散焦三种不同抽运激光聚焦条件下,预脉冲诱导的类氖铁和锌Ｊ＝０→１Ｘ射线激光的近场像。本实验是在Ａｓｔｅｒｉ ｉｖ磺激光装置上进行的,实验中主、预脉冲间的时间间隔为５ｎｓ。实验发现在抽运激光主、预脉冲最佳聚焦条件下,Ｘ射线激光束在２．５ｃｍ长的线状等离子体输出端沿靶面（垂直）方向分裂成两束;在抽运预脉冲单独散焦条件下,仍然观察到这一分裂现象;但是当抽运     

Three-dimensional effects on the focusing of weak shock waves

The focusing of weak three-dimensional shock waves at an arête is examined. By aligning the co-ordinate system with the principal directions on the initial shock surface, it is shown that the focusing of an arbitrary three-dimensional shock may be regarded as locally two-dimensional. In addition, the scaling relating the two problems is given explicitly.