Overcoming the diffraction limit in wave physics using a time-reversal mirror and a novel acoustic sink

In recent years, time-reversal (TR) mirrors have been developed that create TR waves for ultrasonic transient fields propagating through complex media. A TR wave back propagates and refocuses exactly at its initial source. However, because of diffraction, even if the source is pointlike the wave refocuses on a spot size that cannot be smaller than half a wavelength. Here, by using a TR interpretation of this limit, we show that this latter limitation can be overcome if the source is replaced by its TR image. This new device acts as an acoustic sink that absorbs the TR wave. Here we report the first experimental result obtained with an acoustic sink where a focal spot size of less than 1/14th of one wavelength is recorded.     

Atomic beam focusing by a near-field atomic microlens

Atomic beam focusing by an atomic microlens formed by the optical field diffracted from a circular aperture in a metallic screen is considered for an aperture diameter smaller than the wavelength of the field. Analytic expressions are derived for the dipole gradient force acting on an atom in the field of diffracted radiation. It is shown that the action of the gradient force makes it possible to focus the atomic beam into a spot with a diameter on the order of a few nanometers. Numerical estimates are obtained for the focusing properties of the atomic microlens in the model describing the dipole interaction of Rb atoms with laser radiation in the vicinity of the D line.     

Focusing of an atomic beam by a Fresnel atom microlens

Focusing of an atomic beam by a Fresnel atom microlens formed by an optical field diffracted by an aperture whose size is comparable to or greater than the radiation wavelength is considered. It is shown that the dipole gradient force enables one to focus the atomic beam to a spot of about 10 nm in diameter. The focusing properties of a Fresnel atom microlens are analyzed within a model describing the dipole interaction of rubidium atoms with monochromatic radiation near the D-line.     

Focusing in microlenses close to a wavelength in diameter

Light focused from air into a spherical microlens is affected by diffraction at the lens surface as its diameter approaches the wavelength of light. Through an extension of Mie theory, we show that a converging wave that is incident upon a Si microlens with a diameter less than approximately 4lambda creates a spot as much as 25% smaller than predicted with vector diffraction theory. Si microlenses only a wavelength in diameter are shown to be virtually insensitive to variations in the maximum illumination angle, and changes in index of refraction are not found to cause the proportional changes in spot size that would be expected from vector diffraction theory.     

产生纳米级暗中空光束的方法研究

用时域有限差分法，给出光纤头近场区电场强度模的空间分布.讨论了当光纤纤芯分别取不同尺寸时，对此空间光场强度的影响.要想得到较大范围的暗中空光束，必须增加光纤纤芯尺寸.并且，对光纤纤芯尺寸取一较大值和光纤中空区域尺寸取一较小值时的情况进行讨论，可得在光纤头附近可以出现与光纤中空区域尺寸大小相当的暗斑(10¹ nm量级).但是，暗中空光束中背景光较强.为此，将光纤的空心设计为金属. 发现此时中心暗斑的背景光明显减弱，在近场区域可获得较为理想的暗中空光束.如进一步缩小光纤中空区域尺寸，可以在光纤头附近获得暗斑更小甚至纳米量级的暗中空光束.为获得一种纳米量级的暗中空光束提供一种方法.     

高功率激光器的光谱合束技术研究北大核心CSCD

为了获得高功率激光束,提出利用双色镜对典型波长2种不同类型(脉冲、连续)的高能激光进行合束,以实现高功率高能量激光输出。通过对双色镜的热效应和合束光斑远场激光参数进行仿真分析计算,热效应仿真结果表明,在单束激光10 kW、光斑直径15 mm条件下,双色镜面型热形变量均方根值为0.004λ(λ=632.8 nm),满足光学元件面型小于0.03λ精度要求。搭建了一套基于双色镜的光谱合束系统,并分别进行了高功率连续激光与高功率连续激光、高功率连续激光与高能量脉冲激光的合束试验,合束效率高于95%。试验结果表明,光谱合束可有效应用于高能激光领域。     

Propagation dynamics and optical trapping of a radial Airy array beam

Analytical propagation expression of a radial Airy array beam in coherent and incoherent combination passing through paraxial ABCD system is derived, and used to investigate the effect of combination scheme, array orientation and initial phase of Airy beamlet on propagation dynamics of the resulting beam in free space, where optical spot array and vortex array with different shapes are also found, respectively. And then taking four-beamlet Airy array beam in same array orientation as an example, square optical spot array obtained in focal field can be used for simultaneous trapping multiple Rayleigh particles with relative refractive index larger than 1. The transverse gradient forces serving as restore forces tend to push particles at different initial positions to their individual optical spot center. The analysis of trapping stability indicates that larger input peak intensity of Airy beamlet and smaller particle size are benefit to trapping particle owing to many deeper potential wells. Vortex array produced by coherent combined Airy array beam in this paper is expected to be useful for simultaneous trapping microparticles with relative refractive index smaller than 1.     

紫外示波纸用于列阵相机测光束角分布

用列阵相机测１．０６μｍ波长激光束远场角分布，采用黑白胶片感光，列阵点不易读准，且受显影条件的影响，而采用紫外示波纸就可避免这些缺点，两者测量光束角分布的结果吻合，用紫外示汉纸测量光束７０％能量的光束发散角对误差起伏最大为１０％，而光束７０％能量的平均发莠角处的能量相对起伏在５％以内。     

氧碘化学激光器输出光斑漂移和变形研究

 通过对高能量密度氧碘化学激光器运行过程中存在的远场光斑漂移和变形实验现象的分析和测量,得出在高能量密度激光系统中谐振腔的各反射镜的同心度对远场输出光斑的漂移和变形影响很大。指出提高谐振腔内各反射镜在大气中的同心度及降低在激光系统抽真空过程中光腔盒的不稳定程度, 是显著降低激光远场输出光斑漂移和变形的有效途径。     

A super-resolution Fresnel zone plate and photon sieve

Realizing a smaller or sharper diffractive center spot is a valuable research aim in soft X-ray focus and other related research applications. Fresnel zone plates (FZP) and photon sieves (PS) are often used to focus the X-rays or other wavelength light at present. Here, we show that combination of a super-resolution phase mask (SPM) and an FZP (or PS) as one diffractive optical element can realize a smaller or sharper diffractive center spot without significantly increasing the fabrication difficulty. All these diffractive phase elements can be applied to beam shaping, mask-less lithography, energy congregation in high power lasers, soft X-rays focus, and any other field that requires a smaller or sharper diffractive center spot.     

Study of Nonlinear Optical Properties of Organic Material Utilizing Two－color Z－scan

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Glass processing using the fourth harmonic of nanosecond pulse Nd:YAG laser

Ablation processing of borosilicate glass was carried out using the fourth harmonic of the Q-switch Nd:YAG laser. The dependency of the ablation depth on irradiation pulse energy density and the dependency of the ablation depth on irradiation spot size were investigated. The average ablation depth increased with an increase in irradiation pulse energy density. When the irradiation pulse energy density was the same, the ablation depth of the focused beam with a large spot diameter was deeper than that of the focused beam with a small spot diameter. To shorten the processing time, an increase in the irradiation pulse energy density and use of a large spot size focused beam are effective. Using area scanning together with line scanning, a rectangular through hole (sub-mm size) without cracks or chips was formed in borosilicate glass of 140 μm in thickness. PACS 52.38.Mf; 42.70.Ce; 42.62.Cf     

一种快速测量共聚焦X射线分析装置探测微元尺寸方法

共聚焦X射线荧光技术是一种无损的三维光谱分析技术,在材料,生物,矿物样品分析,考古,证物溯源等领域具有广泛应用。共聚焦X射线荧光谱仪的核心部件为两个多毛细管X光透镜。一个为多毛细管X光会聚透镜（PFXRL）,其存在一后焦点,作用是把X光管所发出的发散X射线会聚成几十微米大小的高增益焦斑。另一透镜为多毛细管X光平行束透镜（PPXRL）,其存在一几十微米大小前焦点,置于X射线能量探测器前端,其作用是接收特定区域的X射线荧光信号。在共聚焦Ｘ射线荧光谱仪中,PFXRL的后焦点与PPXRL的前焦点重合,所形成的区域称作探测微元。只有置于探测微元区域的样品能够被谱仪检测到,使样品与探测微元相对移动,逐点扫描,便能够对样品进行三维无损的X射线分析。探测微元的尺寸决定共聚焦X射线荧光谱仪的空间分辨率,因此精确测量谱仪的探测微元的尺寸是非常重要的。如图1所示,谱仪探测微元可以近似为椭球体,其尺寸可以用水平方向分辨率X, Y,和深度分辨率Z表示。目前,常采用金属细丝或金属薄膜通过刀口扫描的方法测量谱仪探测微元尺寸。为了精确的从三个维度测量探测微元尺寸,金属细丝直径要小于探测微元尺寸。金属细丝和探测微元都是数十微米级别的尺寸大小,很难把金属靠近探测微元。为了得到探测微元在不同X射线能量下尺寸变化曲线,要采用多种金属细丝测量。采用单个金属细丝依次测量比较耗费时间。采用金属薄膜可以很方便地测量探测微元的深度分辨率Z,但是当测量水平分辨率X, Y时,难以准确测量。为了解决以上谱仪探测微元测量中存在的问题,本文提出采用多种金属丝平行粘贴在硬纸片上作为样品用于快速测量探测微元尺寸。附有金属细丝的硬纸片靠近谱仪探测微元,可以将探测微元置于硬纸片所在平面。由于硬纸片与金属细丝在同一水平面,在谱仪摄像头的协助下,可以把金属细丝迅速的靠近探测微元。靠近探测微元后,在全自动三维样品台的协助下,金属细丝沿两个方向对探测微元分别进行一次二维扫描。通过对二维扫描数据的处理便可以获得探测微元尺寸随入射X射线能量变化曲线。采用此方法对实验室所搭建的共聚焦X射线荧光谱仪的探测微元进行了测量。     

Theoretical and experimental investigations on erbium-doped fiber amplifiers

A comprehensive model of erbium-doped fiber amplifiers (EDFA) is proposed in this paper, taking exited-state absorption (ESA), amplified spontaneous emission (ASE), background absorption, mode field, and Erbium ion radial distribution into account. Computer programs capable of handling forward, backward, and different wavelength bidirectional pumping are developed. Among other things, it is found that under typical conditions, Gaussian field approximations may cause considerable errors. The optimum V-value for small signal gain is much smaller than that for minimum Gaussian or Desurvire spot size; ASE propagating opposite to the pimp beam can be several times higher than at propagating along the pump beam, resulting in a faster-than-linear decrease of the pump power at its beginning portion. Experimental results of domestic-made erbium-doped fibers pumped by 980 nm and 1480 nm laser diodes are also given.     

Gold-bead scanning near-field optical microscope with laser-force position control

We have developed a scanning near-field optical microscope with an optically trapped metallic particle that has a small diameter compared to the wavelength of visible light. In this microscope we employed spot illumination to enhance the intensity of light scattered from a probe particle so we could reduce the diameter of the probe particle to 40nm. We detected slight irregularities of the surface of the cover glass near 10-nm depth. Also, we observed gold colloidal particles on the surface of the cover glass.     

Laser forming of a bowl shaped surface with a stationary laser beam

Despite a lot of research done in the field of laser forming, generation of a symmetric bowl shaped surface by this process is still a challenge mainly because only a portion of the sheet is momentarily deformed in this process, unlike conventional sheet metal forming like deep drawing where the entire blank undergoes forming simultaneously reducing asymmetry to a minimum. The motion of laser beam also makes the process asymmetric. To counter these limitations this work proposes a new approach for laser forming of a bowl shaped surface by irradiating the centre of a flat circular blank with a stationary laser beam. With high power lasers, power density sufficient for laser forming, can be availed at reasonably large spot sizes. This advantage is exploited in this technique. Effects of duration of laser irradiation and beam spot diameter on the amount of bending and asymmetry in the formed surface were investigated. Laser power was kept constant while varying irradiation time. While varying laser spot diameter laser power was chosen so as to keep the surface temperature nearly constant at just below melting. Experimental conditions promoted almost uniform heating through sheet thickness. The amount of bending increased with irradiation time and spot diameter. It was interesting to observe that blanks bent towards the laser beam for smaller laser beam diameters and the reverse happened for larger spot diameters (~10 times of the sheet thickness). Effect of spot diameter variation has been explained with the help of coupled thermal-structural finite element simulations.     

激光多普勒测速仪中散射光特性的研究

后向散射光特性与多普勒信号的质量密切相关。为了设计高性能的激光多普勒测速仪,运用散斑理论详细分析了激光多普勒信号的强度与散射光斑大小的关系,结合泛函理论给出了计算多普勒电流的散斑表达式,并通过实验的方法研究了回波信号的偏振特性及其强度分布。理论分析与实验结果表明,激光多普勒信号的强度与接收器件光敏面的直径成正比,与光斑的直径成反比;选用光斑较小的圆偏振激光束,并用光敏面尺寸较小的探测器在镜面反射方向上接收信号光,可以大大提高多普勒信号的信噪比,增强系统的探测能力,为提高系统的测量精度创造有利的条件。     

Laue弯晶聚焦特性的光线追迹研究

由于可有效降低高热负载的影响, Laue弯晶是插入件辐射高通量密度硬X射线(30 keV以上)聚焦、 准直和单色化的最有效的光学元件.研究其聚焦光学特性,对发展高性能、高稳定的Laue弯晶单色器具有重要意义. 采用自行发展的光线追迹软件较为系统地研究了Laue弯晶的聚焦特性, 分析了入射光性质及弯晶参数对聚焦光斑、焦距、发散度等主要光学参数的影响. 结果表明,衍射能量越高,聚焦光斑越小,并趋于稳定值;弯曲半径越小,聚焦光斑越小, 并在其达到一阈值时得到聚焦光斑的极小值,之后随着弯曲半径的变小,由于像差等因素的影响, 聚焦光斑反而变大;晶体越厚,聚焦光斑越大,呈线性正比关系.对于衍射光发散度, 其随着衍射能量的增大而变小,并趋于稳定值;其与晶体曲率呈线性正比关系. 同时通过研究得到弯晶各参数的合理选择范围.     

The effect of the earth's magnetic field on auger analysis

Semi-quantitative analyses of thin films or surfaces are commonly obtained from the peak intensities in the differentiated Auger spectrum. To reduce effects of surface roughness, beam focus and electron current, ratios of peak heights are used rather than absolute values. In performing analysis of CdSe single crystal and thin film samples in a commercial Auger analyzer fitted with a cylindrical mirror analyzer (CMA), the ratio of the Cd(376 eV) to Se(1315 eV) peaks was found to vary by as much as 15% when the diameter of the incident electron beam was increased from 5 to 60 μm. The effect was found to be due to an energy-dependent shift of the electron beam caused by the earth's magnetic field. The electron transmission of the CMA was measured as a function of the primary electron beam spot position on the sample. The transmission decreases rapidly once the spot falls outside an area with a radius ～25 μm. Due to this response, the relative shift in position caused by the magnetic field produces variations in peak ratios when the spot size is changed. This effect will produce inaccurate analysis if the Auger peaks differ significantly in energy and the primary electron beam spot size is large, and accounts for the observed 15% variation in $\text{Cd}\text{Se}$ ratio.     

Spot-scanning beam delivery with laterally-and longitudinally-mixed spot size pencil beams in heavy ion radiotherapy

The three-dimensional(3D) spot-scanning method is one of the most commonly used irradiation methods in charged particle beam radiotherapy.Generally,spot-scanning beam delivery utilizes the same size pencil beam to irradiate the tumor targets.Here we propose a spot-scanning beam delivery method with laterally-and longitudinallymixed size pencil beams for heavy ion radiotherapy.This uses pencil beams with a bigger spot size in the lateral direction and wider mini spread-out Bragg peak(mini-SOBP) to irradiate the inner part of a target volume,and pencil beams with a smaller spot size in the lateral direction and narrower mini-SOBP to irradiate the peripheral part of the target volume.Instead of being controlled by the accelerator,the lateral size of the pencil beam was adjusted by inserting Ta scatterers in the beam delivery line.The longitudinal size of the pencil beam(i.e.the width of the mini-SOBP) was adjusted by tilting mini ridge filters along the beam direction.The new spot-scanning beam delivery using carbon ions was investigated theoretically and compared with traditional spot-scanning beam delivery.Our results show that the new spot-scanning beam delivery has smaller lateral penumbra,steeper distal dose fall-off and the dose homogeneity(1-standard deviation/mean) in the target volume is better than 95%.