声子晶体的负折射

通过对声子晶体负折射的实验和理论进行梳理,提出可以通过从对称性角度理解声波在周期性结构中的散射,来理解声学负折射现象的物理机制.在此启发下,本文继而运用有限元模拟,通过改变背景介质的声学常数,成功展示了声波在声子晶体中的负折射现象.     

二维蜂窝结构声子晶体的负折射成像研究*

为了得到分辨率更好的平板透镜,对此进行了研究。利用有限元的方法研究了由水/水银材料组合的二维蜂窝结构声子晶体平板透镜的负折射成像特性,通过数值仿真计算了声子晶体的能带结构、等频色散曲线,模拟了点源成像,并且利用高斯波束验证了声子晶体平板透镜的负折射现象,得到了有效折射率neff =-1的成像。对比相同材料的正方晶格和三角晶格而言,该模型的成像效果更佳。为了改善成像分辨率,还在原来的模型上进行了改进,得到了分辨率约为0.37λ的亚波长像。     

声子晶体和声学超构材料

声子晶体和声学超构材料进一步拓展了自然界中声学材料的弹性波性质．这种人工的复合结构材料,由于其周期结构的布拉格散射和局域共振特性,使得其具有奇异的色散特征,在某些频段具有负的有效弹性参数,带来了许多新颖的声学传播效应,例如声子带隙效应、负折射效应、超棱镜效应、超透镜效应、异常透射效应、异常隔声效应等．与此同时,在声子晶体和声学超构材料表面,一类具有亚波长特性的声表面倏逝波也引起了人们的关注,研究其激发、传播、耦合的过程对揭示声子晶体和声学超构材料的奇异声传播效应的物理本质具有重要意义．声子晶体和声学超构材料作为一类新型的人工声学结构材料,在隔声、防振、热控制以及新型声学器件研发等方面具有巨大的应用前景．文章综述了近十几年来国际国内关于声子晶体和声学超构材料的研究进展,并对其未来的研究发展方向做一评述．     

基于二硫化钼的可调一维声子晶体

声子晶体所具有的负折射率、 局域缺陷态与弹性波带隙等特性, 使其在声学隐身、 声学波导以及减震降噪等方向展现了巨大的潜力. 同时, 二硫化钼优异的电学和力学性能使其成为制备纳米机电器件的理想材料. 将单层二硫化钼转移到预先图案化的周期性结构上, 可以制备出纳米尺度的声子晶体器件. 本文设计了一种通过将单层二硫化钼转移贴合在预先制备的周期性沟槽阵列上, 形成一维声子晶体的方案. 有限元分析表明, 这种声子晶体在MHz 范围存在声子能带结构, 可以实现对声波传播的控制. 我们可以通过改变结构参数, 或者通过改变施加在栅电极上的电压, 对能带进行调控. 这种结构为开发基于二维材料的纳米尺度的声子晶体器件提供了可能性.     

无序对二维声子晶体平板负折射成像的影响

利用多重散射理论分析了位置无序对钢/水声子晶体负折射成像的影响.发现声子晶体负折射成像与周期结构中的方向性通路有关;该方向性通路不同于一般意义上的位置波导,它是声子晶体周期散射的结果;通路中的障碍对成像有较大影响;随着散射体位置无序程度的增加,周期性散射减弱,方向性通路被破坏,成像也随之减弱,甚至消失.     

10.6μm波段光子晶体负折射现象的实验验证

介绍了10.6μm波段扫描负折射功率的实时探测系统,在实验上为探测光子晶体光频波段负折射现象提供了实验平台。该系统通过数据采集卡实现计算机对旋转台及平移台的控制,在以光子晶体样品为探测中心的轨道上,对折射光的整个空间功率分布进行实时采集和绘图。这一实验系统能够精确便捷地实现负折射现象在光频波段的探测。根据折射光最大光功率出现的位置,测得光束通过光子晶体样品的负折射现象,而且与理论结果基本吻合。     

云南发现一种具有“三折射”现象的天然晶体

云南发现一种具有“三折射”现象的天然晶体司民真（云南楚雄师专物理系，楚雄６７５０００）武荣国（云南楚雄市环境监测站，楚雄６７５０００）当光在各向异性晶体中传播时，一般只会出现两束折射光线，用检偏器检验，则两束折射光线都是线偏振光，这是众所周知的事实。...     

超元胞方法研究平移群对称性对声子带隙的影响

采用基于超元胞的平面波展开法，计算了由水银(水)正四棱柱体按正方格子排列于水(水银)基体中所组成的两种声子晶体的能带结构.通过改变两相邻的柱体底面边长之比来改变声子晶体的平移群对称性.结果发现，改变相邻柱体的底面边长之比，具有很好的调节声学带隙的作用.研究表明，声子晶体的平移群对称性对于其带隙的形成具有重要影响,同时还表明,超元胞方法也是研究声子晶体平移群对称性影响声子带隙形成的一种有效方法. 关键词： 声子晶体 带隙 对称性     

二维平板光子晶体的等效负折射率测量EI北大核心CSCD

光子晶体特殊的色散特性能导致负折射现象发生。用波束位移法对二维光子晶体平板的负折射现象进行理论计算和实验测量,得到了入射光经平板光子晶体折射后的二维光场分布。测量结果表明,横电(TE)光在光子晶体中发生负折射,出射光沿出射面产生了反方向的横向位移,由此计算出来的负折射率约为-0.44,而横磁(TM)光没有发生负折射。实验结果与仿真结果吻合较好。     

声子晶体板中的第二类狄拉克点和边缘传输

在弹性板波体系中设计了一种具有第二类狄拉克点的声子晶体板.不同于第一类狄拉克点,第二类狄拉克点附近的色散具有大的倾斜,以致于等频面的几何形状由点状变成交叉的线状.微调结构的几何参数破缺该镜面对称性,可打开第二类狄拉克点简并,实现体系的能带反转.能带反转前后的二维声子晶体板属于不同的能谷拓扑相,不同拓扑相之间存在无带隙的拓扑保护界面态.不仅如此,由于弹性板波界面态的特殊应力分布,单一能谷相声子晶体板的边界上同样支持无带隙的弹性波传输.本文拓展类石墨烯体系中的二维狄拉克点和能谷态到第二类情形中,在同一结构中获得了界面和边界上的弹性波无带隙边缘传输.由于结构设计简单,可在微小尺寸下加工获得,为高频弹性波器件的设计和构造提供了可行的途径.     

光子晶体光纤和波导

文章简述了光子晶体光纤和光子晶体波导的类型,导光原理及制作方法等,展示了近年来它们的进展及重要的应用前景。     

一维磁性光子晶体的电磁场分析和数值计算

本文探讨一维磁性光子晶体的概念和结构.提出了一种分析一维磁性光子晶体的法拉第旋转效应的电磁场方法.分析计算表明:嵌于光子晶体中的一层很薄的磁性材料将可获得比单独一层同样厚度的磁性材料大得多的法拉第旋转效应,从而从理论上验证了文献上的相关实验.同时也分析了一维磁性光子晶体用作光子晶体结构下的光隔离器等器件中的法拉第旋转器的可能.     

Growth and Crystallization Habit of a Novel Substrate Crystal LiGaO_2

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Building the pyramids: perfect bubble crystals

Abstract

The use of appropriate flat-sided pyramidal containers to grow ordered foams allows single crystals to be formed. In the case of face-centred cubic crystals, these have been prepared with up to 500 bubbles. Strained and deliberately defective crystals can also be grown. The growth of simple cubic and body-centred cubic crystals is limited by instability; preliminary results are presented for these, as well as ordered bidisperse foams.     

Observations of defect propagation in [100]-oriented opal-type photonic crystals

Charged colloidal suspensions have been used as experimental models for the study of crystal nucleation. Here we propose that the technique of template-assisted colloidal self-assembly can be used to visualize the effects of defect propagation in atomic crystal films produced using epitaxial growth. Templates with periodic line defects were used to grow [100]-oriented three-dimensional photonic crystals by means of the template-assisted colloidal self-assembly method, aided by capillary and gravitational forces. The defect propagation in the [100]-oriented photonic crystal was observed using scanning electron microscopy, both at the surface of the crystal and on cleaved facets. This method is useful in the understanding of defect propagation in the growth of colloidal films on templates - and the same approach may also prove useful for the understanding of atomic crystal growth on substrates with defects. Additionally, the deliberate incorporation of line defects may prove valuable as a way of introducing waveguide channels into three-dimensional photonic crystals.     

新型激光晶体Yb:KY(WO4)2的结构与光谱

采用顶部籽晶提拉法，以K₂W₂O₇为助溶剂，生长了Yb:KY(WO₄)₂新型激光晶体.经热重-差热分析，确定晶体熔点为1045℃，相变温度为1010℃.X射线粉末衍射测试，验证所生长的晶体为β-Yb:KY(WO₄)₂.晶体结构分析确定Yb:KY(WO₄)₂晶体由WO₆八面体连接而成，WO₆八面体是由双氧桥(WOOW)及单氧桥(WOW)构成.晶体粉末样品室温下的红外及拉曼光谱测试，确定WO₆原子基团、双氧桥及单氧桥的振动频率.晶体的吸收峰位于940nm，980nm，发射峰位于989nm—1030nm. 关键词： 晶体结构 光谱 晶体生长     

Flexomagnetoelectric interaction in cubic,tetragonal and orthorhombic crystals

The phenomenological theory of the flexomagnetoelectric coupling in crystals of the cubic, tetragonal and orthorhombic crystal systems has been suggested. Secondary role of the crystal structure chirality was shown. Oppositely, significant role of the crystallographic point group type (symmetric, alternating, dihedral or cyclic) in the flexomagnetoelectric coupling has been derived. It was shown, that conceptually new features of the flexomagnetoelectric effects are expected in the crystals of the cyclic groups (crystal classes 4/m, 4? and 4). Proposed verification of the theory is investigation of the domain wall bend details (changes of the effect symmetry). Special case of such verification near the compensation point is suggested. First-principles mechanisms of the flexomagnetoelectric interaction were discussed.     

Growth Rates of Edge-on Lamellar Crystals Confined in Polymer Thin Films

The growth rates of edge-on lamellar polymer crystals in variable thickness films were investigated in terms of dynamic Monte Carlo (MC) simulations. The growth rates linearly decreased with decreasing film thickness for the thinner films and were nearly constant for the thicker films. The mean stem lengths (crystal thickness) were also constant in different thickness films. The crystal widths parallel to the film thickness increased more slowly with increasing film thickness in the thinner films than that in the thicker films, indicating they were restrained by the film thickness. We propose that the growth rate of edge-on lamellar crystals in thin films is dominanted by the crystal width in the thinner films and by the crystal thickness in the thicker films; the variation of the film thickness can change the three-dimensional shape of the crystal growth front, also affecting the growth rate of the edge-on lamellar crystal.     

Advances in vanadate laser crystals at a lasing wavelength of 1 micrometer

Sapphire, garnet and vanadate crystals are the most prominent optical materials, and vanadates play important roles in optics, especially in lasers and nonlinear optics. Neodymium‐doped yttrium vanadate (Nd:YVO₄) is representative and available commercially. Based on Nd:YVO₄, several vanadate crystals are being developed with the goal of fulfilling the need for differential applications and improvement of certain operational aspects, such as with pulsed lasers or high‐power continuous‐wave lasers. In recent years, some important effects, including energy enhancement, bistability of output performance, self‐Raman frequency shifting, etc., and some novel applications, such as quantum optics, pulsed lasers modulated by the two‐dimensional crystals, etc., have been discovered with vanadates as gain materials. In this paper, the preparation, characterization and laser applications of vanadate laser crystals at the lasing wavelength of 1 micrometer, including YVO₄, GdVO₄, LuVO₄, Gd_xY_1–xVO₄ and Lu_xGd_1–xVO₄ (0 < x < 1) doped with Nd³⁺ and ytterbium (Yb³⁺) are systematically reviewed by highlighting the most recent research progress. Their specific properties are presented, generation mechanisms of novel physical effects are discussed, new applications are given and possible future applications proposed by focusing on some potential strengths.     

氧化锌晶体的水热法生长及性能表征

报道以块状氧化锌陶瓷为培养料,KOH、LiOH和H2O2的混合水溶液为矿化剂体系,采用水热法生长出尺寸为30mm×38mm×8mm的氧化锌晶体。氧化锌晶体 c(0001)和-c(0001)方向的生长速度分别为0.17,0.09mm/day。 c面的颜色为浅绿色,而-c面的颜色为深褐色。在室温下测得 c面的载流子浓度为104cm-3,电阻率为80Ω·cm,迁移率为100cm2/V·s。晶体(0001)面的双晶摇摆曲线的FWHM为45arc-sec。对氧化锌晶体 c面在室温条件下的光致发光谱和吸收光谱进行了测试分析。