Photonic crystals: imaging by flat lens using negative refraction

The positive refractive index of conventional optical lenses means that they need curved surfaces to form an image, whereas a negative index of refraction allows a flat slab of a material to behave as a lens and focus electromagnetic waves to produce a real image. Here we demonstrate this unique feature of imaging by a flat lens, using the phenomenon of negative refraction in a photonic crystalline material. The key advance that enabled us to make this observation lies in the design of a photonic crystal with suitable dispersion characteristics to achieve negative refraction over a wide range of angles.     

左-右手系材料界面上的表面波

利用严格的电磁理论证明2种偏振入射都能在各向同性的负折射材料与正折射材料界面上激发表面波,并推导出了相应表面波的传播波矢和激发条件.针对电磁波在非线性左手系材料中的传播性质,研究左、右手系材料界面上非线性TE表面波的传播行为,分析表明:2种界面上的非线性TE表面波均存在频率通带和禁带.     

一种电波折射订正方法及其在高空风探测中的应用

为提高无线电测风的精度,在根据二维射线追踪法的基础上,建立了适用于球面分层大气的电波折射高精度在线订正方法,并对电波折射误差及高空风探测误差进行了模拟计算和分析.结果表明,电波折射产生的仰角、斜距和高度误差通常为正值,3类误差均随仰角减小和斜距增大而增大;在低仰角和大斜距的条件下,电波折射误差大于定位设备误差,当仰角低...     

Three-dimensional optical metamaterial with a negative refractive index

Metamaterials are artificially engineered structures that have properties, such as a negative refractive index, not attainable with naturally occurring materials. Negative-index metamaterials (NIMs) were first demonstrated for microwave frequencies, but it has been challenging to design NIMs for optical frequencies and they have so far been limited to optically thin samples because of significant fabrication challenges and strong energy dissipation in metals. Such thin structures are analogous to a monolayer of atoms, making it difficult to assign bulk properties such as the index of refraction. Negative refraction of surface plasmons was recently demonstrated but was confined to a two-dimensional waveguide. Three-dimensional (3D) optical metamaterials have come into focus recently, including the realization of negative refraction by using layered semiconductor metamaterials and a 3D magnetic metamaterial in the infrared frequencies; however, neither of these had a negative index of refraction. Here we report a 3D optical metamaterial having negative refractive index with a very high figure of merit of 3.5 (that is, low loss). This metamaterial is made of cascaded 'fishnet' structures, with a negative index existing over a broad spectral range. Moreover, it can readily be probed from free space, making it functional for optical devices. We construct a prism made of this optical NIM to demonstrate negative refractive index at optical frequencies, resulting unambiguously from the negative phase evolution of the wave propagating inside the metamaterial. Bulk optical metamaterials open up prospects for studies of 3D optical effects and applications associated with NIMs and zero-index materials such as reversed Doppler effect, superlenses, optical tunnelling devices, compact resonators and highly directional sources.     

TM polarization characteristics on thermal radiation of a negative refractive index thin film

Thermal radiation TM polarization characteristics of a negative refractive index thin film was studied based on the transmit matrix method and Kirchhoff radiation law. The influence of the thin film parameters on the thermal radiation directional characteristics and spectral property were discussed. And the influence of the evanescent waves was also discussed. The results show that two factors play important roles in the thermal radiation of negative index thin film: one is the interference effect of the thin film structure to the electromagnetic waves; the other is the photon tunneling effect of the negative refractive index material, which was caused by the amplifying evanescent wave. These indicate that spectral and directional characteristics of the thermal emissivity can be modulated by modifying the structure and the physics parameters of the negative refraction index thin film. Supported by the National Natural Science Foundation of China (Grant No. 50606003) and Aeronautic Science Foundation of China (Grant No. 2007ZA51006)     

A terahertz metamaterial with unnaturally high refractive index

Controlling the electromagnetic properties of materials, going beyond the limit that is attainable with naturally existing substances, has become a reality with the advent of metamaterials. The range of various structured artificial 'atoms' has promised a vast variety of otherwise unexpected physical phenomena, among which the experimental realization of a negative refractive index has been one of the main foci thus far. Expanding the refractive index into a high positive regime will complete the spectrum of achievable refractive index and provide more design flexibility for transformation optics. Naturally existing transparent materials possess small positive indices of refraction, except for a few semiconductors and insulators, such as lead sulphide or strontium titanate, that exhibit a rather high peak refractive index at mid- and far-infrared frequencies. Previous approaches using metamaterials were not successful in realizing broadband high refractive indices. A broadband high-refractive-index metamaterial structure was theoretically investigated only recently, but the proposed structure does not lend itself to easy implementation. Here we demonstrate that a broadband, extremely high index of refraction can be realized from large-area, free-standing, flexible terahertz metamaterials composed of strongly coupled unit cells. By drastically increasing the effective permittivity through strong capacitive coupling and decreasing the diamagnetic response with a thin metallic structure in the unit cell, a peak refractive index of 38.6 along with a low-frequency quasi-static value of over 20 were experimentally realized for a single-layer terahertz metamaterial, while maintaining low losses. As a natural extension of these single-layer metamaterials, we fabricated quasi-three-dimensional high-refractive-index metamaterials, and obtained a maximum bulk refractive index of 33.2 along with a value of around 8 at the quasi-static limit.     

复杂环境地区电波折射修正中的折射率精确获取方法

雷达电波射线上大气折射率的准确性是提高电波折射误差修正精度的关键因素之一.对下垫面复杂地区的雷达系统,常用的大气球面分层法因没有考虑大气水平方向变化使得电波射线上的折射率具有较大的误差,从而影响了电波折射误差修正精度.针对下垫面复杂地区的三维大气结构,提出了获得电波射线上大气折射率的组合方法,即在雷达所在地采用直接探测法,在其他电波射线上,先计算出射线点的位置,然后再利用已建立的全国大气剖面模型数据库得到该位置的大气折射率,从而较为精确地获得电波射线上的大气折射率.经实验验证,采用组合法获得的电波射线上的折射率不仅具有较好的精度,而且可有效地提高电波折射误差修正精度,进而提高下垫面复杂地区的雷达探测精度.     

频率对二维声子晶体平板负折射成像的影响

以二维钢/空气三角排列声子晶体为模型,利用多重散射方法分析声子晶体平板负折射成像过程中声波频率与等效负折射率、入射角的变化关系。研究结果表明:平板负折射成像与声波频率及波的入射角度密切相关;对不同的入射波频率,平板的等效负折射率不同,且随着频率增加,等效负折射率绝对值变小;对于确定的晶格结构,只有在确定的频率范围、确定的入射角范围内,才能发生明显的负折射现象;对于某一确定的入射波频率,平板负折射存在一个最大入射角,且随着入射声源频率的增加,这个最大入射角变小,入射波耦合进入声子晶体平板的信号减弱,进而影响像的强度和清晰度。     

层状介质二维井间电磁成像

根据分层介质中波的传播规律,利用均匀介质中的三维张量格林函数推导出二维情况下分层介质的张量格林函数,它是一个对kx的一维Sommerfeld积分。准确求取这一积分后,可以方便地利用迭代等方法反演出井间的电导率。给出了利用上行波和下行波方式表达的界面反射和折射张量函数及其各分量。通过积分方程方法对埋置于有8个分层的两个垂直方向电导率异常体模型进行了计算,反演结果与原模型基本吻合,有效地实现了层状介质中二维井间电磁成像。     

BST负折射特性实验验证

对钛酸锶钡BaxSr（1-x）TiO3（0〈x〈1）（BST）柱阵列异向介质棱镜的负折射进行了实验验证．实验结果表明,该BST柱构成的三维异向介质的负折射特性不依赖于柱体的排列周期,而是BST柱自身的谐振造成了电磁波在该棱镜中的负折射．BST柱排列周期仅对二次负折射的强度产生影响．例如：周期阵列的BST柱异向介质棱镜的斜边是阶梯状时,因具有光栅结构,使得在该斜边上发生的二次负折射被增强;随机阵列的BST棱镜的斜边不具有光栅结构,观察到的二次负折射就比较弱．采用等效介质理论的仿真结果与我们的实验结果符合较好．     

从负折射超材料到光学隐身衣

1964年俄罗斯科学家V.Vesalago从Maxwell方程组出发断言具有负折射率的左手材料(LHM)可以存在,1996年和1999年J.Pendry提出了LHM的初始设计。2000年D.Smith制成首个LHM,它可把微波波束弯折到与常规的不同方向。近年来LHM吸引了人们的兴趣,它也称为超材料,但在自然界并不存在。它可变更电磁波传播,从而造成负折射甚至隐形。一个时期以来,在微波和光频进行的隐身斗篷实验是2D或3D的,它们与通常的隐形方法显著不同。LHM的其他应用如左手传输线、成像技术等。本文首先对负折射研究和隐身衣设计的历史和进展作回顾,进而讨论了这些领域中理论与实验工作的意义。可以看出负折射问题是非常复杂的。还讨论了负折射与负波速、负折射与负GH位移的关系。     

A novel approach to design microwave medium of negative refractive index and simulation verification

In this paper, a novel approach is presented to synthesize microwave medium of negative refractive index by incorporating metallic wire array with negative effective permittivity into the host media such as ferrimagnet-YIG （yttrium iron garnet） applied by external magnetic field whose permeability is negarive. We have designed the composite medium having negative refractive index in C/X band frequencies, analyzed and simulated its electromagnetic （EM） properties by use of EM EDA package based on time-domain finite integration method. The simulation results show that： ① the effective permittivity of the designed metallic wire array is negative in the frequency range from 7.02 GHz to 9.80 GHz; ② the permeability of YIG substrate immersed into an external magnetic field is negative in the frequency range from 5.22 GHz to 8.14 GHz; ③ EM wave can pass through the composite medium synthesized by the above designed metallic wire array and YIG substrate, and ④ the negative refraction behavior occurs on the interface between the composite medium and the normal material with positive refractive index in 7.51-8.13 GHz frequency range, in which the effective permitUvity of the metallic wire array and the permeability of YIG substrate are negative simultaneously. The full wave simulation has demonstrated that the effective refractive index of the designed composite medium is indeed negative and ascertained that the proposed approach to design microwave medium with negative refractive index is viable.     

光子晶体介电常数和波导宽度对负折射现象的频率的影响

为研究光子晶体中的负折射情况,利用时域有限差分法（FDTD）并通过模拟仿真观察了光在左手材料光子晶体中的负折射现象。以硅的圆形介质柱组成的二维六边形排列结构的光子晶体为例,分析了介电常数和波导宽度的变化对负折射现象的频率范围的影响。仿真结果显示：相对介电常数不宜过大,波导宽度根据频率选择的需要可采在0.2~0.5 a范围内选取,从而为制作具有负折射现象的宽带宽光子晶体提供参考。     

二维椭圆形介质柱光子晶体的负折射成像特性

本文利用时域有限差分方法研究了由椭圆形介质柱组成的二维正方晶格光子晶体平板的负折射成像特性.研究表明对于光子晶体第二个能带中的特定频率区域,靠近晶体平板放置的点光源可以经过矩形光子晶体在另一侧形成一个高质量的实像,并且可以实现有效折射率为-1的负折射现象.     

用超声波确定应力强度因子时对波型的选择

利用与超声纵波相应的材料声折射率和应力强度因子的关系，并考虑到横波、纵波在材料中的传播速度及其与入射角、反射角的关系，研究了用超声波确定应力强度因子时对波型的选择。发现：一列超声纵波入射于含裂纹的受力试件表面时，由于反射与折射，将有两列超声纵波透过试件后表面，四列超声纵波穿出试件前表面。要用超声波确定应力强度因子，只能采用透出试件前(或后)表面的第一个纵波信号。     

用惠更斯作图法研究左手材料的负折射现象

针对理解电磁波在左手材料中的传播特性,在详细分析了该介质中电磁波波矢量k的方向、大小和坡印廷矢量s之间关系的基础之上,根据电磁场理论探讨了电磁波在右手、左手材料分界面上的反射和折射(各介质都是各向同性的)现象。并运用惠更斯作图法解释了电磁波由右手材料穿越左手材料时的负折射现象。指出发生负折射现象的根本原因是由于电磁波在左手材料中传播时其波矢量与坡印廷矢量的方向相反。     

Complete photonic bandgaps in 12-fold symmetric quasicrystals

Photonic crystals are attracting current interest for a variety of reasons, such as their ability to inhibit the spontaneous emission of light. This and related properties arise from the formation of photonic bandgaps, whereby multiple scattering of photons by lattices of periodically varying refractive indices acts to prevent the propagation of electromagnetic waves having certain wavelengths. One route to forming photonic crystals is to etch two-dimensional periodic lattices of vertical air holes into dielectric slab waveguides. Such structures can show complete photonic bandgaps, but only for large-diameter air holes in materials of high refractive index (such as gallium arsenide, n = 3.69), which unfortunately leads to significantly reduced optical transmission when combined with optical fibres of low refractive index. It has been suggested that quasicrystalline (rather than periodic) lattices can also possess photonic bandgaps. Here we demonstrate this concept experimentally and show that it enables complete photonic bandgaps--non-directional and for any polarization--to be realized with small air holes in silicon nitride (n = 2.02), and even glass (n = 1.45). These properties make photonic quasicrystals promising for application in a range of optical devices.     

等效折射率模型分析二维光子晶体微谐振腔模态

 采用数值计算方法,利用等效折射率模型研究不同折射率材料制造的二维光子晶体微谐振腔的模态。发现可以通过增加薄膜厚度达到良好的光子局域化。讨论了薄膜层材料折射率分别为n=3.42,2.50,1.80,1.60,薄膜层厚度分别对应为0.1529,0.2182,0.3341,0.4003μm,其等效折射率分别对应于2.81,2.23,1.52,1.38时,达到良好光子局域,说明介质材料与空气折射率差值小的光子晶体微谐振腔结构若要有好的光子局限能力,除了需要靠介质材料本身的全反射效应外,还需有足够厚的膜层厚度。从而找到一个能控制光场分布同时维持单模振荡的机制,揭示微谐振腔体薄膜层厚度对模态影响,为实际研制具有高质量微谐振腔提供了一种较好的预估手段。     

一维正负折射率光子晶体结构禁带及传播特性

采用传递矩阵的方法研究了由正折射率材料和负折射率材料交替排列组成的一维光子晶体结构的透射谱,并对其能带结构和色散关系进行分析,这种正负折射率光子晶体不仅存在一般的布拉格禁带,还存在低频共振禁带.本文也对含左手材料的一维无序结构的局域化进行了分析研究.     

Propagating solitary waves along a rapidly moving crack front

A rapidly moving crack in a brittle material is often idealized as a one-dimensional object with a singular tip, moving through a two-dimensional material. However, in real three-dimensional materials, tensile cracks form a planar surface whose edge is a rapidly moving one-dimensional singular front. The dynamics of these fronts under repetitive interaction with material inhomogeneities (asperities) and the morphology of the fracture surface that they create are not yet understood. Here we show that perturbations to a crack front in a brittle material result in long-lived and highly localized waves, which we call 'front waves' These waves exhibit a unique characteristic shape and propagate along the crack front at approximately the Rayleigh wave speed (the speed of sound along a free surface). Following interaction, counter-propagating front waves retain both their shape and amplitude. They create characteristic traces along the fracture surface, providing cracks with both inertia and a new mode of dissipation. Front waves are intrinsically three-dimensional, and cannot exist in conventional two-dimensional theories of fracture. Because front waves can transport and distribute asperity-induced energy fluctuations throughout the crack front, they may help to explain how cracks remain a single coherent entity, despite repeated interactions with randomly dispersed asperities.