Surface segregation of InGaAs films by the evolution of reflection high-energy electron diffraction patterns

Surface segregation is studied via the evolution of reflection high-energy electron diffraction （RHEED） patterns under different values of As4 BEP for InGaAs films. When the As4 BEP is set to be zero, the RHEED pattern keeps a 4x3/（nx3） structure with increasing temperature, and surface segregation takes place until 470 ℃ The RHEED pattern develops into a metal-rich （4x2） structure as temperature increases to 495℃. The reason for this is that surface segregation makes the In inside the InGaAs film climb to its surface. With the temperature increasing up to 515℃, the RHEED pattern turns into a GaAs（2x4） structure due to In desorption. While the As4 BEP comes up to a specific value （1.33 x 10-4 Pa-1.33 x 10-3 Pa）, the surface temperature can delay the segregation and desorption. We find that As4 BEP has a big influence on surface desorption, while surface segregation is more strongly dependent on temperature than surface desorption.

Theoretical solution for wave diffraction by wedge or corner with arbitrary reflection characteristics

An exact analytic solution for wave diffraction by wedge or corner with arbitrary angle (rπ) and reflection coefficients (R0 and Rr) is presented in this paper. It is expressed in two forms-series and integral representations, corresponding recurrence relation and asymptotic expressions are also derived. The solution is simplified for some special cases of rπ. For Rr= R0,r= 1/N and Rr≠R0,r = 1/2N, the solution can be reduced to linear superpositions of incident and partially reflected waves, hence a nonlinear solution of forth order for this problem can be obtained by using the author's theory of nonlinear interaction among gravity surface waves. The given solution is related to inhomogeneous Robin boundary conditions, which include the Neumann boundary conditions usually accepted in wave diffraction theory.

Analysis of restriction factors of widening diffraction bandwidth of multilayer dielectric grating

In order to design a multilayer dielectric grating with wide-bandwidth diffraction spectrum, the restriction factors of both the reflection bandwidth of multilayer dielectric high-reflectivity mirror and the guided-mode resonance phe- nomenon are studied in detail. The reflection characteristics of high-reflectivity mirror in zeroth and -lst transmitted diffraction orders are quantitatively evaluated. It is found that the reflection bandwidth of high-reflectivity mirror in -lst transmitted diffraction order, which determines the final diffraction bandwidth of multilayer dielectric grating, is evidently compressed. Furthermore, it is demonstrated that the reducing of grating period is an effective approach to the elimination of guided mode resonance over a required broad band range both spectrally and angularly. In addition, the expressions for calculating the maximum period ensuring no guided mode resonance in the required bandwidth are derived. Finally, two high-efficiency pulse-compression gratings with broad-band are presented.

Design of a novel correlative reflection electron microscope for in-situ real-time chemical analysis

A novel instrument that integrates reflection high energy electron diffraction(RHEED),electron energy loss spectroscopy(EELS),and imaging is designed and simulated.Since it can correlate the structural,elemental,and spatial information of the same surface region via the simultaneously acquired patterns of RHEED,EELS,and energy-filtered electron microscopy,it is named correlative reflection electron microscopy(c-REM).Our simulation demonstrates that the spatial resolution of this c-REM is lower than 50 nm,which meets the requirements for in-situ monitoring the structural and chemical evolution of surface in advanced material.

A flat-topped etched diffraction grating demultiplexer with low polarization-dependent loss using a tapered MMI structure

A flat-topped etched diffraction grating (EDG) demultiplexer with a low polarization-dependent loss (PDL) is designed. A design and simulation method based on the method of moment (MoM) is proposed. A 65-channcl EDG demultiplexer with channel spacing of 100 GHz is considered as a design example. A tapered multi-mode interferometer (MMI) is used to flatten the passband of the EDG demultiplexer. The numerical results show that the exit width of the tapered waveguide impacts the loss of the TE case more than that of the TM case. Based on this fact, the exit width of the taper is optimized to obtain the lowest PDL. The tapering angle is also optimized where the minimal ripple is obtained. The designed EDG demultiplexer has an excellent flat-topped spectral response and a very low PDL.

Calculation on diffraction aperture of cube corner retroreflector

On the basis of optical property of cube corner retroreflector （CCR）, a new perception and calculation approach for diffraction aperture of CCR in two different forms is presented. The relationship between diffraction apertures and incident light with six different combinations of reflection order and incident angle is established. Far-field diffraction patterns of CCR under various incident conditions are also provided.

Large anisotropic negative thermal expansion in Cu-TDPAT metalorganic framework:A combined in situ X-ray diffraction and DRIFTS study

Cu-TDPAT(H_(6)TDPAT=2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine),a stable nanoporous metal-organic framework with rht topology,has sparked broad interest as an adsorbent for several chemical separation processes.In this work,in situ synchrotron diffraction experiments followed by sequential LeBail refinements reveal that Cu-TDPAT shows unusually large anisotropic negative thermal expansion(NTE).The PASCal crystallography tool,used to analyze the magnitude of the NTE,reveals an average volumetric thermal expansion coefficientαv=-20.3 MK^(-1).This value is significantly higher than the one reported for Cu-BTC(also known as HKUST-1),which contains the same Cu-paddlewheel building unit,αv=-12 MK^(-1).In situ synchrotron single crystal X-ray diffraction and in situ diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS)were employed to shed light on the NTE mechanism.Using these two methods,we were able to elucidate the three main structural motions that are responsible for the NTE effect.The more pronounced NTE behavior of Cu-TDPAT is attributed to the lower symmetry combined with the more complex ligand structure when compared to Cu-BTC.The knowledge obtained in this work is important for understanding the behavior of the adsorbent under transient variable temperature conditions in fixed adsorption beds.

SIMULATION OF BACK-REFLECTED PSEUDO-KOSSEL PATTERNS OF CRYSTAL DIFFRACTION

The method of studying crystals using divergent X-ray beams emitted by a point source is known as the divergent beam method. If the X-ray source is inside a crystal and the characteristic radiation is emitted by the crystal itself, it is called the Kossel method. If the source is outside the crystal and the divergent

Tunability of nonuniform reflection holographic filter

The tunability of nonuniform reflection holographic filter is investigated theoretically and experimentally. It is shown that the reflection holographic filter has not only high optical density and narrow bandwidth, but also good tunability. The coupled wave theoretical model for uniform medium is compared with the model for nonuniform medium. It is identified that the coincidence of the theoretical results of the nonuniform model with the experimental results are better than that of the uniform model.

非视距信号传播场强特性的分析与研究

为探索机场通信路径中障碍物造成非视距传播产生衍射损耗,对信号场强衰减特性进行了研究。首先,基于ITU-R P.1546中不同地形环境对信号传播的影响,结合Deygout模型构建信号场强组合模型。其次,按障碍物数量分析随障碍物高度变化的接收场强传播特性。仿真结果表明,单障碍物越高,占据0.6F1菲涅尔区越大,场强越小;双障碍物路径中,高度增加的障碍物经过由次要转为主要障碍物,直到只剩主要障碍物的过程,使场强逐渐衰减。再次,针对场强上升,分析了次要障碍物高度与0.6F1菲涅尔区的关系。最后,调整覆盖场强最小阈值,确定接收设备是否被障碍物遮挡,从而导致信号被屏蔽。上述研究结果可为未来模拟机场各场景下障碍物分布,预测接收机是否因障碍物受干扰或影响场强覆盖范围提供参考依据。

Influence of interface roughness on reflectivity of tungsten/boron-carbide multilayers with variable bi-layer number by X-ray reflection and diffuse scattering

Infuence of interface roughness on the reflectivity of Tungsten/boron-carbide （W/B4C） multilayers varying with bi-layer number, N, is investigated. For W/B4C multilayers with the same design period thickness of 2.5 nm, a real-structure model is used to calculate the variation of reflectivities with N = 50, 100, 150, and 200, respectively. Then, these multilayers are fabricated by a direct current （DC） magnetron sputtering system. Their reflectivity and scattering intensity are measured by an X-ray diffractometer （XRD） working at Cu Kα line. The X-ray reflectivity measurement indicates that the reflectivity is a function of its bi-layer number. The X-ray scattering measured results show that the interface roughness of W/B4C multilayers increases slightly from layer to layer during multilayer growing. The variation of the reflectivity and interface roughness with bi-layer number is accurately explained by the presented realstructure model.

大气压等离子体射流功能改性硅树脂的工艺研究

[目的]采用大气压等离子体射流(APPJ)技术在聚二甲基硅氧烷(PDMS)表面产生结构色的方法和机制鲜有报道。[方法]通过对处在拉伸状态的PDMS进行1 min的氩气等离子体处理,然后释放拉应力,获得了具有环形渐变结构色的PDMS。[结果]扫描电镜(SEM)照片揭示,APPJ处理后的PDMS表面生成了周期排列的纳米褶皱结构,且褶皱周期是逐渐变化的。X射线光电子能谱(XPS)分析表明,经APPJ处理的PDMS表面发生了进一步交联和氧化,生成了弹性模量比PDMS更高的氧化硅物质层。可见光吸收和反射光谱分析表明,具有结构色的PDMS对可见光表现出了更低的反射率,而其垂直方向的可见光透过率则无显著改变。[结论]通过APPJ技术能够简单、快速地对有机硅化合物表面进行改性而获得精细结构色,该工艺方法有望满足显示、传感、防伪、功能材料等领域的实际需求。

GPR绕射波与反射波分离的变分模态分解方法研究

针对传统绕射波与反射波分离方法易造成有效信号丢失的问题,本文利用二维变分模态分解(2D-VMD)进行探地雷达(GPR)数据的绕射波与反射波分离。对二维GPR数据进行2D-VMD分解后,可得到包含从低频低波数到高频高波数的各阶模态分量及各阶模态分量的F-K谱。通过分析绕射波与反射波在频率-波数域的差异,分别重构绕射波与反射波对应的模态分量,得到分离后的绕射波与反射波数据。数值模拟与实测数据处理结果表明,相比于传统方法,该方法可以有效分离绕射波与反射波,并且不损失任何有效信号。

陆基海洋通信船舶绕射损耗模型研究

海上主要港口和航道上船舶吞吐量大,陆基船舶通信链路被过往船舶遮挡的时间长、频次高,严重影响陆基船舶通信链路的连接质量。针对船岸通信信道测量场景中发生的由过往大型船舶驶过造成遮挡而产生的阴影效应,结合高精度实测数据对这一现象的大尺度信道特性进行了分析和建模。结合阴影效应发生位置的第一菲涅尔半径、离岸距离、收发天线路径距离、天线连线高度和海面曲率引起的相对高度变化等影响因素的内在联系,解析了阴影效应发生时遮挡船只的整个运动过程,并对第一菲涅尔区与遮挡船只相对位置对应的不同时间节点进行了分类。基于ITU-R P.526-15(2019年10月)建议书中的单个刀刃形绕射模型和单个圆形绕射模型,结合船只和第一菲涅尔区的相对关系,构建了基于遮挡船的三维数据和收发端相对位置的船舶绕射损耗模型,针对实测过程中D 06(0.6倍第一菲涅尔区间隙对应的路径距离)范围内和范围外两种情况下的两次船舶遮挡过程进行了模型仿真和实测数据比对分析,并结合分析结果提出了未来的应用方向。

基于数值仿真计算的高速铁路声屏障长度对插入损失的影响

在当前高速铁路降噪措施中,声屏障最为普遍和有效,其中声屏障设置高度是主要设计参数,而声屏障设置长度和位置,则一般根据经验结合简单计算选取,声屏障长度及预测点位置对插入损失变化的影响考虑较少。根据Kurze点声源绕射声衰减理论推导有限长声屏障对列车线声源插入损失的理论算法,对比分析声屏障长度、高度变化对不同位置预测点声屏障插入损失的影响。研究结论如下:(1)随着声屏障长度增加,各预测点位置尤其声屏障两端附近,声屏障插入损失变大;(2)采取加高、延长声屏障措施对于插入损失值及空间分布影响不同,应根据预测点位置情况合理选择声屏障优化措施;(3)半无限长声屏障端部加长50 m后,插入损失变化量较为明显的区域位于声屏障端部水平距离110 m、与线路中心线距离130 m范围内。

Possible Relevance of the Allende Meteorite Conditions in Prebiotic Chemistry: An Insight into the Chondrules and Organic Compounds

The study of the mineral and organic content of the Allende meteorite is important for our understanding of the molecular evolution of the universe as well as the ancient Earth. Previous studies have characterized the magnetic minerals present in ordinary and carbonaceous chondrites, providing information on the evolution of magnetic fields. The interaction of organic compounds with magnetic minerals is a possible source of chemical diversity, which is crucial for molecular evolution. Carbon compounds in meteorites are of great scientific interest for a variety of reasons, such as their relevance to the origins of chirality in living organisms. This study presents the characterization of organic and mineral compounds in the Allende meteorite. The structural and physicochemical characterization of the Allende meteorite was accomplished through light microscopy, powder X-ray diffraction with complementary Rietveld refinement, Raman and infrared spectroscopy, mass spectrometry, scanning electron microscopy, and atomic force microscopy using magnetic signal methods to determine the complex structure and the interaction of organic compounds with magnetic Ni-Fe minerals. The presence of Liesegang-like patterns of chondrules in fragments of the Allende structure may also be relevant to understanding how the meteorite was formed. Other observations include the presence of magnetic materials and nanorod-like solids with relatively similar sizes as well as the heterogeneous distribution of carbon in chondrules. Signals observed in the Raman and infrared spectra resemble organic compounds such as carbon nanotubes and peptide-like molecules that have been previously reported in other meteorites, making the Mexican Allende meteorite a feasible sample for the study of the early Earth and exoplanetary bodies.

近红外光谱对岩矿表面黏土矿物覆层的响应

近红外光谱(NIR)技术具有测试快速、准确和对样品无接触、无损害等诸多优点,因而被广泛应用于材料科学、医学和地球科学等众多领域。在地质学和行星探测领域,近红外光谱常用来原地和遥感识别星球表面矿物组成、分布及相对含量等信息。近红外光谱对含有羟基和水的矿物,特别是黏土矿物有更强的响应,而黏土矿物主要的生成条件和矿物物化性质使其常常以岩石矿物表面覆层形式广泛存在于地表环境中。在实际测试过程中,覆层形式存在的黏土矿物极有可能会对探测目标区的岩石矿物含量和赋存状态等信息产生干扰,其干扰程度和机制尚不十分清楚。为此,本研究针对性地考察了黏土矿物覆层对矿物表面近红外光谱测试的影响。选用绿脱石、绿泥石、高岭石和伊利石4种代表性黏土矿物,覆层的厚度分别设计约为30、10、2和1μm;基体矿物选取角闪石、钾长石和石英为代表。实验结果表明,在位于2100~2370 nm处的近红外光谱吸收波段,不同厚度的黏土矿物覆层均会不同程度地影响基体矿物的反射光谱特征,导致在遥感探测过程中无法准确地获取其光谱信息,且随着覆层厚度增加,基体矿物信息被湮没的程度加剧。与富铁黏土矿物(绿脱石和绿泥石)相比,富铝黏土矿物(高岭石和伊利石)在2100~2370nm波段更易被近红外光谱识别。当因覆层厚度较小(<30μm)而导致黏土矿物的结构羟基特征吸收峰的相对深度与矿物含量相关性较低时,其特征吸收峰的二阶导数强度值可更好地指示覆层矿物的相对含量。本研究所取得的结论可为近红外光谱在矿产勘查和火星遥感探测等领域的应用提供重要参考。

环形通道内爆轰波的起爆机制

旋转爆轰发动机环形燃烧室和预爆轰管的设计是影响发动机点火性能的关键因素。为了获得环形燃烧室中的起爆机制,使用多帧短时开快门摄像法,研究了不同含量氩气稀释的乙炔-氧气爆轰波经直管道沿切向进入环形通道中的传播过程和模式,重点关注爆轰波的失效和重新起爆机制。通过分析胞格模式发现环形通道内爆轰波的传播模式可以分为亚临界、临界和超临界3种状态。环形通道内爆轰波在顺时针和逆时针方向同时传播,根据初始压力和环形管道宽度的不同,会出现完全熄爆模式、熄爆-重新起爆模式和完全不熄爆模式,对应亚临界、临界和超临界3种状态。3种状态在顺时针和逆时针方向出现的顺序并不一致,相比较而言逆时针方向更易熄爆。研究同时也发现重新起爆通过两种方式实现:一种是通过解耦爆轰波与内壁面的反射以及其后的横向爆轰波,另外一种是通过燃烧转爆轰。通过分析直管的临界管径发现,随着环形通道宽度的增大,对于高浓度或低浓度氩气稀释的乙炔-氧气爆轰波,其临界管径均趋近于经典衍射问题中不稳定爆轰波的临界管径。实验研究结论将为旋转爆轰发动机燃烧室和预爆轰管的结构设计提供技术支持。

空气自由场爆炸冲击波数值建模及应用

为建立描述任意时刻、距离下空气自由场爆炸波冲击波压力、密度、粒子速度的经验公式,支撑复杂场景下冲击波载荷的快速计算,采用一维精细数值模拟的方法计算了不同比例距离下的压力、密度、粒子速度时程,并利用曲线拟合方法得到了正相超压峰值等22个冲击波参数与比例距离关系的经验公式,采用改进修正Friedlander方程建立了冲击波压力、密度、粒子速度随时间变化的关系式;利用爆炸冲击波地面反射和建筑后绕射两个典型工况,阐释了提出模型的应用场景,并与试验、数值模拟结果对比。结果表明:压力、密度、粒子速度随比例距离、时间变化的经验关系与数值模拟结果基本吻合;爆炸冲击波地面反射和建筑后绕射两个典型工况下,理论计算与数值模拟的压力云图基本吻合,在相同硬件条件下,理论计算耗时仅为千万级网格数值模拟的5%左右,在计算速度上有明显的优越性。

遮挡效应的太阳射电观测和仿真

遮挡效应是多天线复杂系统设计一个重要考虑因素。国家重大科技基础设施项目子午工程二期建设完成后,明安图台站各类天线总数将达到373套。受限于测试场地、模型复杂度和计算量,特别是对电大尺寸障碍物的遮挡效应通常很难实测和预测。明安图台站太阳射电望远镜(Mingantu Solar Radio Telescope,MST)在2017~2020年S,C和X频段的观测数据显示了以20 m天线和远方丘陵为障碍物的遮挡效应,并可以分为天线、大气和丘陵3个不同特性区域。基于ITU-R P.526-15建议书单刃峰绕射损耗预测方法,进行20 m天线反射面仿真建模和近似计算。观测和仿真结果显示,绕射损耗随频率变化趋势基本一致,遮挡效应与波长、障碍物特性及其相对源和接收天线的距离和角度、接收设备的天线波束和动态范围等有关。