Left-handed metamaterial based superlens for subwavelength imaging of electromagnetic waves

Lenses made of negative index materials have the ability to focus the propagating and evanescent components of electromagnetic waves. Such a possibility enables super resolution, in turn resulting in sharper, subwavelength size images. In this present work, we present subwavelength imaging that was obtained from a one-dimensional left-handed metamaterial (LHM) composed of alternating layers of split-ring resonators and thin wires. We investigated the effect of the thickness of LHM lenses on image size. The left-handed pass band within the negative permittivity and permeability region is shown experimentally and theoretically for different thicknesses of LHM slabs. We also studied the transmission-phase of LHMs with a different number of unit cells along the propagation direction. The phase decreases with the increasing thicknesses of LHM slabs, proving that the phase velocity is negative in the left-handed transmission band. PACS 42.25.-p; 41.20.Jb; 81.05.-t     

二维各向同性异向介质负折射特性仿真验证

提出了一种工作在C波段的二维各向同性异向介质，相比于以往的二维异向介质，它除了具有二维各向同性的优点外，还具有带宽更宽，单元结构的尺寸更小，构造也更简单的特点.通过分别仿真验证异向介质的双负特性和负折射特性最终肯定上述异向介质的存在. 关键词： 各向同性异向介质 宽频带 小单元 双负特性     

Near-sighted superlens

We consider the problem of subwavelength imaging via a slab of a left-handed medium (LHM) in the presence of material losses. We derive the analytical expression for the resolution limit of a LHM-based lens and demonstrate that the area of its subwavelength performance is usually limited to the near-field zone.     

十字环型左手材料单元结构设计与仿真

提出了一种新型十字环型左手材料结构单元.该结构只需在介质板单侧蚀刻, 即可在二维方向上产生等效负介电常数和等效负磁导率.通过理论分析, 提出场-路结合的等效源分析法以计算等效介电常数、磁导率, 并据此对十字环型结构单元的左手特性激发机理进行了论证. 后采用Nicolson-Ross-Weir 等效参数法提取了十字环型单元阵列的相对介电常数和相对磁导率, 并通过棱镜实验对材料的负折射特性进行了验证.实验表明, 该左手材料在6.8-6.9 GHz频段具有二维入射左手特性. 此种左手材料制作工艺相对简单, 为左手材料在微波器件领域的应用提供了一种较为实用的设计方案.     

Wave refraction in negative-index media: always positive and very inhomogeneous

We present the first treatment of the refraction of physical electromagnetic waves in newly developed negative index media (NIM), also known as left-handed media (LHM). The NIM dispersion relation implies that group fronts refract positively even when phase fronts refract negatively. This difference results in rapidly dispersing, very inhomogeneous waves. In fact, causality and finite signal speed always prevent negative wave signal (not phase) refraction. Earlier interpretations of phase refraction as "negative light refraction" and "light focusing by plane slabs" are therefore incorrect, and published NIM experiments can be explained without invoking negative signal refraction.     

Effect of spiral left-handed material with near-zero index on rectangular microstrip antenna

In this paper, spirals are arranged periodically on the surface of the substrate, making a left-handed material (LHM) with a near-zero index of refraction. A radiation patch is etched on the surface of this substrate, making an antenna. The finite difference time domain method is used to study the effect of this LHM with a near-zero index of refraction on the microstrip. At f=13.78 GHz, the equivalent permittivity and permeability of the proposed spiral materials are both negative; at f=28.02 GHz, the equivalent permittivity and permeability are both negative and the refraction index is near zero. Compared with conventional microstrip antenna, the radiation gain of the antenna is much higher and the return loss is much lower due to the near-zero-index effect.     

Three-dimensional subwavelength imaging by a photonic-crystal flat lens using negative refraction at microwave frequencies

We experimentally demonstrate subwavelength resolution imaging at microwave frequencies by a three-dimensional (3D) photonic-crystal flat lens using full 3D negative refraction. The photonic crystal was fabricated in a layer-by-layer process. A subwavelength pinhole source and a dipole detector were employed for the measurement. By point-by-point scanning, we obtained the image of the pinhole source shown in both amplitude and phase, which demonstrated the imaging mechanism and subwavelength feature size in all three dimensions. An image of two pinhole sources with subwavelength spacing showed two resolved spots, which further verified subwavelength resolution.     

Surface-wave mechanism of subwavelength imaging by a flat left-handed superlens

We develop a theory describing the dynamics and interaction of electromagnetic surface waves (ESWs) resonantly excited by an external source in a slab of left-handed material (LHM) with identical negative (equal to −1) values of dielectric permittivity and magnetic permeability that makes up a so-called perfect lens, or a superlens. We show that subwavelength imaging by a superlens is associated with the degeneracy of the spectrum of eigen electromagnetic surface modes at the interfaces of the metamaterial slab, whereas the dynamic response of the superlens is completely determined by the dynamics of these modes and the dispersion properties of the metamaterial. We obtain conditions that enable one to find out when a superlens produces subwavelength images of an external source. We consider the cases of a stationary and a pulse source, as well as of a source that moves with constant velocity or oscillates in space.     

关于左手性介质几何光学的研究（二）

利用几何光学方法研究了由左手性介质制成的简单透镜的光学特性.推理分析了左手性介质透镜处球形界面的负折射规律，并依此表明了常用光学公式仍适用于左手性介质.指出了左手性介质透镜相对于用传统介质制作的普通透镜所具备的固有优点，最后证实了利用单块左手性介质透镜即可完全消球差，并相应提供了零球差左手性介质透镜的设计实例.     

Negative refraction and left-handed properties in two-dimensional photonic crystals

We report on the negative refraction and left-handed behavior of two-dimensional photonic crystals. For the triangular two-dimensional photonic crystals there are two saddle points and a key point in the first Brillouin zone for the second band, which play important role to determine the refraction phenomena. There is no determined relation between the left-handed behavior and the negative refraction. The refraction phenomena depend on the band structure and the configuration of the incident light. For the first configuration, the first Brillouin zone can be divided into three parts. In part I there are negative refraction and left-handed behaviors. In part II there are normal refraction and left-handed behaviors. In part III there are normal refraction and right-handed behaviors. For the second configuration, the first Brillouin zone can only be divided into two parts. For these complex refraction phenomena, it is meaningless to define the phase refractive index, and it is necessary to define the effective refraction index by a Snell-like formula, and the value of the effective index is determined by the configuration and the band structure.     

Experimental verification of negative refraction in a double cross metamaterial

We report on microwave experiments with a metamaterial composed of pairs of metallic crosses. The transmission properties of the structure show a left-handed transmission band at frequencies around 10.2 GHz. The validity of the negative effective index of refraction is verified by a Snell’s law refraction experiment performed on a wedge-shaped sample of the metamaterial. A second measurement of a similar wedge made from blank FR4 boards is done for reference. The results of the measurements show positive refraction over the whole measured frequency band for the FR4 wedge as well as the refraction of the incident radiation to negative angles within the designated left-handed frequency band for the metamaterial sample.     

Lower RCS directional antenna by left-handed material

We theoretically study the radiation of a current line source with a surrounding cylindrical shell of left-handed material (LHM). The far-field radiation pattern is obtained by solving the Maxwell equation in a cylindrical coordinate, which show that this radiation system has good directional emission characteristics and can be used as a directive antenna. The directive emission is explained due to the negative refraction and the focus effect occurring at the interface of the LHM shell. Compared with the parabolic reflector antenna with the same aperture size, our antenna has lower radar cross-section (RCS) in a broadband frequency range, providing a way to realize lower RCS antenna.     

Topology optimization of multilayer left-handed material based on the genetic algorithm

Owing to the special properties, left-handed materials (LHM) have important potential applications in engineering. However, in the current stage, the design of LHM is primarily based on several basic prototypes such as meta-material and transmission line structures. In the present work, the topology optimization technology is introduced to design a type of multilayer LHM. A type of one-dimensional material with periodic unit cell is studied and the topology of its microstructure is designed by using the technology of enumerative search and genetic algorithm (GA), respectively. The material is composed of metal and ferrite films, and several types of negative refraction materials are obtained numerically. These obtained materials with periodic unit cells can exhibit negative refractive index in some frequency spectrums, and the results show that the GA could achieve good designs efficiently in a larger design space.     

Numerical analysis of propagation characteristics of electromagnetic wave in lossy left-handed material media

The propagation characteristics of electromagnetic wave in lossy left-handed materials (LHM) are studied using finite-difference time-domain (FDTD) method base on auxiliary differential equation (ADE) technology. The LHM medium is realized with lossy Drude models for both the negative electric permittivity and the negative magnetic permeability. The discretized ADE-FDTD equations are derived in detail. The incident wave used in the simulation is a multiple cycle m-n-m pulses source. The term of Poynting's vector E_xH_y was calculated. These numerical results demonstrate conclusively that the phase velocity direction of electromagnetic wave propagation and the direction of the Poynting vectors are anti-parallel in LHM. The amplitude of electric field is reduced with the enhancive distance of LHM slab. It is also demonstrated that the energy of electromagnetic wave in the LHM slab is obviously attenuated, and the attenuation of energy becomes stronger with the angular plasma frequency ω_p increasing. These results indicate that LHM stealth is effective in theory, and reasonable selection of the large negative index of refraction can greatly enhance its effectiveness.     

基于非分裂FDTD的左手介质电磁特性的研究

为验证左手介质的电磁特性,采用非分裂时域有限差分方法对左手介质的Drude模型进行建模.这种方法不需要对电场和磁场进行分裂,也不需要对PML空间进行特殊处理,吸收边界PML和工作空间可以通过参数转换来完成,并且构造的PML层为有耗介质,进入PML层的透射波将迅速衰减.它是一种准确而有效的分析色散和各向异性介质的方法,通过此方法有效地验证了左手介质的负折射效应、汇聚效应、相位补偿效应,充分验证了左手介质及其反常特性的存在性和此方法分析左手介质的有效性. 关键词： 左手介质 非分裂 时域有限差分 Drude     

Impacts of Refraction Index Mismatch on Performance of Target Detection and Imaging by Using Flat LHM Lens

Refraction index mismatch between flat left-handed metamaterial (LHM) lens and its surrounding medium generally destroys the focusing of flat LHM lens and degrades the performance of near-field target detection by using flat LHM lens. For LHM lens of refraction index mismatch within ±30%, numerical simulations demonstrate that lenses with large refraction index may suffer less resolution degradation than lenses with small refraction index, and the enhancement of refocused microwave backscattered from target can be subsided by up to approximately 5.5dB. The refraction index mismatch will also shift the target position in the reconstructed image so that theoretical prediction of target position needs to be modified.     

A planar left-handed metamaterial based on electric resonators

A planar left-handed metamaterial(LHM) composed of electric resonator pairs is presented in this paper.Theoretical analysis,an equivalent circuit model and simulated results of a wedge sample show that this material exhibits a negative refraction pass-band around 9.6GHz under normal-incidence and is insensitive to a change in incidence angle.Furthermore,as the angle between the arm of the electric resonators and the strip connecting the arms increases,the frequency range of the pass-band shifts downwards.Consequently,this LHM guarantees a relatively stable torlerence of errors when it is practically fabricated.Moreover,it is a candidate for designing multi-band LHM through combining the resonator pairs with different angles.     

Dual bands of negative refractive indexes in the planar left-handed metamaterials

A kind of planar left-handed metamaterial (LHM) with unique configurations is demonstrated, which offers an approach in building dual-band negative-index materials. Simulated and experimental results predict two left-handed transmission bands near 11.1 and 14.6 GHz. Dual bands of negative refractive indexes are verified using the retrieval procedure. Field and current distribution at the dual magnetic resonance are also examined. The effective electromagnetic parameters show that by carefully adjusting dimensions of the unit cell, electric and magnetic resonances can be coexistent at some frequency ranges with both negative permittivity and negative permeability. The idea can help us designing planar negative-index materials with multibands.     

Enhanced twinkling in left-handed media

A slab of left-handed material (LHM) with refractive index -1 forms a perfect lens that retains subwavelength information about a source or object. Such lenses are highly susceptible to perturbations affecting their performance. It is shown that illuminating a roughened interface between air and an LHM produces a regime for enhanced focusing of light close to the boundary. This generates caustics that are brighter, fluctuate more, and cause Gaussian speckle at distances closer to the interface than in right-handed matter. These effects present fresh challenges for perfecting the perfect lens.     

Negative refraction and focusing of circularly polarized waves in optically active media

Analysis indicates that certain types of optically active media are capable of producing negative refraction and focusing of circularly polarized waves. It is established that a slab of such material acts just as Veselago's hypothetical left-handed media lens, providing subwavelength resolution as Sir Pendry's ideal lens, but for circularly polarized waves.