Homogenization of tensor TL metamaterials

We present a method for homogenizing tensor transmission-line (TL) metamaterials. These are metamaterials consisting of loaded transmission-line networks, that can possess magnetically anisotropic (tensor) effective material parameters. The homogenization employs a local field averaging procedure to compute the effective material parameters. These effective material parameters can be dispersive or non-dispersive. For the tensor metamaterials possessing dispersive effective material parameters, the homogenization method takes advantage of the circuit topology of tensor transmission-line metamaterials to predict material parameters over a frequency range.     

Characteristic properties of electromagnetic wave interaction with uniaxial absorbing metamaterials: A case of the near-zero axial parameter

An exact solution of the boundary electrodynamic problem for a thin layer of uniaxial absorbing metamaterial characterized by the effective permittivity (permeability) tensor with near-zero values of the axial component is investigated. The parameters of optical anisotropy, absorption, and geometry of the problem are determined when the layer exhibits plane electromagnetic wave reflection and transmission that are selective with respect to one or several incident angle ranges. The numerical analysis shows that even small deviations from ideal conditions (losses, non-zero values of the axial component, nonorthogonality of the optical axis to the layer boundaries) can lead to remarkable changes of the metamaterial properties. The low level of losses, required for applications of the material (spatial filtering of the radiation, etc.), seems to be unattainable nowadays for passive metamaterials, so the realization of the concept can be concerned with active metamaterials.     

Extracting the bulk effective parameters of a metamaterial via the scattering from a single planar array of particles

In this paper we present a method for retrieving the effective parameters of a metamaterial composed of a regular rectangular orthorhombic lattice of linear biaxially anisotropic particles suspended in free space. By assuming the point–dipole interaction approximation, equations are derived which extract the electric and magnetic polarizabilities of the individual particles given the measured or simulated scattering parameters of a single planar array of particles. These results are in turn substituted into the Clausius–Mossotti equations to find the bulk effective permittivity and effective permeability. To demonstrate our approach, the extraction method is applied to a metamaterial consisting of a cubic arrangement of magnetodielectric spheres using the scattering parameters obtained by simulating the structure with Ansoft HFSS. Our results show good agreement with a known analytical solution at frequencies in which the Clausius–Mossotti approximation is valid.     

Multipole analysis of meta-atoms

We determine the rigorously calculated scattered field of meta-atoms and decompose it into spherical harmonics with complex amplitudes. Transforming these spherical harmonics into a Cartesian base reveals all multipole moments in this coordinate system, i.e. all electric and magnetic dipole, quadrupole and any higher order moments can be completely determined. We show that these multipole moments provide a deeper understanding on how light interacts with the considered meta-atoms. After sketching the analytical framework, we investigate exemplarily two well-established meta-atoms and show the applicability of our approach.     

External modulators for TeraHertz Quantum Cascade Lasers based on electrically-driven active metamaterials

We have designed, fabricated and measured electrically-driven active metamaterials which operate as external modulators for TeraHertz Quantum Cascade Lasers. The modulation is achieved by applying a voltage to the metamaterial layer which actively displaces carriers from the n-doped layer causing changes in damping and frequency location of the lowest metamaterial response. We demonstrate their operation at 2.4 and 2.8 TeraHertz and obtain a maximum modulation depth of ～60% with a large degree of modulation linearity.     

A discussion on the interpretation and characterization of metafilms/metasurfaces: The two-dimensional equivalent of metamaterials

A metafilm (also referred to as a metasurface) is the surface equivalent of a metamaterial. More precisely, a metafilm is a surface distribution of suitably chosen electrically small scatterers. Metafilms are becoming popular as an alternative to full three-dimensional metamaterials. Unfortunately, many papers in the literature present incorrect interpretations and mischaracterizations of these metafilms. In fact, some of the characterizations presented in the literature result in non-unique parameters for a uniquely defined metafilm. In this paper we discuss an appropriate interpretation and characterization of metafilms and present a correct manner to characterize a metafilm. Additionally, we illustrate the error that results from an incorrect characterization of metafilms. We present various examples to emphasize these points. Finally we present a retrieval approach for determining the uniquely defined quantities (the electric and magnetic susceptibilities of its constituent scatterers) that characterize a metafilm.     

Modeling of active and passive nonlinear metamaterials

We develop general results for nonlinear metamaterials based on simple circuit models that reflect the elementary nonlinear behavior of the medium. In particular, we consider both active and passive nonlinearities which can lead to gain, harmonic generation and a variety of nonlinear waves depending on circuit parameters and signal amplitude. We show that the medium can exhibit a phase transition to a synchronized state and derive conditions for the transformation based on a widely used multiple time scale approach that leads to the well-known Complex Ginzburg–Landau equation. Further, we examine the variety of nonlinear waves that can exist in such systems, and we present numerical results for both active and passive metamaterial cases.     

Metamaterial made of paired planar conductors: Particle resonances, phenomena and properties

The properties and characteristics of a recently proposed anisotropic metamaterial based upon layered arrays of tightly coupled pairs of “dogbone” shaped stripe conductors have been explored in detail. It has been found that a metamaterial composed of such stacked layers exhibits artificial magnetism and may support backward wave propagation. The equivalent network models of the constitutive conductor pairs arranged in the periodic array have been devised and applied to the identification of the specific types of resonances, and to the analysis of their contribution into the effective dielectric and magnetic properties of the artificial medium. The proposed “dogbone” configuration of conductor pairs has the advantage of being entirely realizable and assemblable in planar technology. It also appears more prospective than simple cut-wire or metal-plate pairs because the additional geometrical parameters provide an efficient control of separation between the electric and magnetic resonances that, in turn, makes it possible to obtain a fairly broadband left-handed behaviour of the structure at low frequencies.     

Finite element model updating of a large structure using multi-setup stochastic subspace identification method and bees optimization algorithm

In the present contribution, operational modal analysis in conjunction with bees optimization algorithm are utilized to update the finite element model of a solar power plant structure. The physical parameters which required to be updated are uncertain parameters including geometry, material properties and boundary conditions of the aforementioned structure. To determine these uncertain parameters, local and global sensitivity analyses are performed to increase the solution accuracy. An objective function is determined using the sum of the squared errors between the natural frequencies calculated by finite element method and operational modal analysis, which is optimized using bees optimization algorithm. The natural frequencies of the solar power plant structure are estimated by multi-setup stochastic subspace identification method which is considered as a strong and efficient method in operational modal analysis. The proposed algorithm is efficiently implemented on the solar power plant structure located in Shahid Chamran university of Ahvaz, Iran, to update parameters of its finite element model. Moreover, computed natural frequencies by numerical method are compared with those of the operational modal analysis. The results indicate that, bees optimization algorithm leads accurate results with fast convergence.     

Fabrication of optical negative-index metamaterials: Recent advances and outlook

A status report on optical negative-index-metamaterial fabrication is given. The advantages, drawbacks and challenges of different fabrication techniques including electron-beam lithography (EBL), focused-ion beam (FIB) milling, interference lithography (IL) and nanoimprint lithography (NIL) and direct laser writing are outlined. Since the possibility of creating a truly three-dimensional (3D) metamaterial is critical for real-life applications and the future of this research area, the recent developments on large-scale, multiple-functional-layer metamaterials are discussed in detail, and alternative methods for 3D fabrication of complex structures are mentioned. Throughout the report, main breakthroughs in fabrication of optical negative-index metamaterials are described, as well as challenges facing future manufacturing of optical metamaterials.     

Gain control and diffraction-managed solitons in metamaterials

The question of whether stable, active metamaterials can be created is addressed, both through a discussion of absolute instability and an analysis of a transmission line that produces dispersion analogous to that of the familiar split-ring resonator/wire-based metamaterial. The gain is introduced using negative conductance diodes and it is shown that the frequency bandwidth controls the stability window. The diodes are located as lumped elements in the unit cell. It is demonstrated that the production of an, active, negative phase (left-handed) frequency window, with spatial amplification is possible. The paper then goes on to assume that since loss can be controlled in this way special types of solitons can be created from phase and loss compensation and they could be the basis of future computational devices. The candidates selected are diffraction-managed spatial solitons and their basic properties are displayed numerically.     

Modal interactions in resonant metamaterial slabs with losses

Modal phenomena in planar metamaterial structures modeled by simple scalar frequency-dependent constitutive parameters are investigated. Modal interactions and modal transformations are found in frequency ranges close to the medium resonances when material losses are introduced and their level is continuously varied through certain critical values. Numerical results for TE and TM modes of a grounded metamaterial slab are reported in the form of dispersion curves and pole loci in the complex wavenumber plane for both surface- and leaky-wave propagation regimes. Useful insights into the reported results are provided through the identification of the branch-point singularities in the complex frequency plane that govern the modal interactions.     

二灰砂砾高温快速养生试验研究

二灰砂砾是公路路面基层、底基层常用的无机结合料稳定材料，根据现行规范有关规定，路面结构设计的重要参数劈裂强度和抗压回弹模量需要进行长龄期养生，二灰砂砾养生时间为180 d。由于目前设计周期很短，规范要求的设计参数很难在设计周期内试验出来，直接影响了对设计方案的及时论证。目前，国内外对无机结合料稳定材料高温快速养生研究主要是建立温度和时间的换算公式，从而在实际施工中提高养生温度而在较短的时间内测定材料的强度。该方法是根据经验公式确定材料的强度，存在适用范围小等缺点，实用性有待进一步研究。通过试验研究找出高温养生条件下二灰砂砾的强度增长规律（适宜的曲线形式），基于这种规律确定二灰砂砾标养长龄期（180 d）强度对应的快速养生短龄期大约需要7 d，该方法是对养生条件改变后实测强度，而不是根据经验公式由短龄期强度推算长龄期的结果，所以试验结果的真实性与正常养生相比没有任何区别。在此基础上，通过电镜扫描对二灰砂砾高温养生快速养生短龄期7 d与标准养生180 d龄期时的微结构进行对比分析，发现两种养生条件下试样微结构基本一致，进一步验证高温养生缩短二灰砂砾养生龄期是可行的。     

射线追踪法的现状和展望

射线追踪法作为一种快速有效的波场近似计算方法,不仅对研究波在介质中的传播路径有重要意义,而且能快速反演并成像。在收集、研究国内外相关文献的基础上,介绍射线追踪法的现状及今后发展的展望。     

Analytical modelling of double-negative composites

Analytical model of metamaterials comprising lattices of small resonant scatterers and (optionally) infinite wires is developed for the case when the planar grids of magnetic dipoles alternate with those of electric dipoles (or those of wires) along the direction of the wave propagation. An important question on the possibility to extract material parameters from measurements or simulations of the plane-wave scattering matrix for finite-thickness slabs of left-handed metamaterials is revisited. It is shown that the class of Bloch lattices (for which this extraction makes sense) is broader than one could judge upon the previous works. Also, it is shown that the spatial dispersion that destroys the operation of double-negative media can arise for the oblique propagation with respect to crystal planes even if it is not revealed from the analysis of the normal propagation.     

基于响应面法的单层球壳简谐荷载下的破坏形式判别

为了减小单层球面网壳在简谐荷载作用下的破坏形式判别中的计算量,避免反复试算,节约后处理时间,提出了利用响应面法计算结构可靠指标从而判别失效类型的方法:利用响应面法计算单层球面网壳在简谐荷载下基于位移失效模式的可靠指标;建立可靠指标和破坏形式之间的明确对应关系;获得判别单层球面网壳破坏形式的量化指标。研究表明,该方法能够准确地判别出结果,并具有高效率和实用性。     

Metamirror channel structures: Optical properties

A new kind of metamaterial based on hierarchically organized mirror channels with partially reflecting walls, metamirror structures (MMS), is proposed. Properties of MMS are close to those exhibited by left-handed materials, despite the difference in underlying physical mechanisms. Transmission and reflection of rays are considered for different MMS geometries and types. It is shown that a one-step reflective 2D MMS based on rectangular elementary unit cells, being properly curved, possess the properties of a lens, and a generalized law describing MMS-based lenses is presented. If the MMS have relatively small sizes of unit cells (from one to tens of micrometers), then they may be treated as solid materials with anomalous optical properties in a wide range from visible and near-infrared up to UV wavelengths. It is concluded that MMS features may be observed in metamaterials of other kinds.     

Bloch material parameters of magneto-dielectric metamaterials and the concept of Bloch lattices

Metamaterials comprising lattices of small resonant scatterers and (optionally) infinite wires are considered. The crystal planes of these lattices contain magnetic dipoles and electric dipoles (or wires). The set of so-called Bloch material parameters is discussed. A class of lattices for which these parameters describe the transfer matrix of an individual monolayer is considered. These lattices are called as Bloch lattices. It is shown that for Bloch lattices and only for them the Bloch material parameters can be directly extracted from the S-parameters of a finite-thickness metamaterial slab. Material parameters retrieved in this way in the previous literature are either Bloch material parameters or senseless (not invariant with respect of the number of monolayers in the composite slab). Explicit examples of Bloch and non-Bloch lattices are presented. Some intermediate results (two definitions of the Bloch impedance, frequency bounds of the Lorentz–Lorenz formula for the local material parameters, etc.) having practical and theoretical importance are discussed.