A broadband,polarisation-insensitive and wide-angle coplanar terahertz metamaterial absorber

Broadband terahertz metamaterial absorbers have attracted considerable attention due totheir significant potential for practical applications. These absorbers are usuallystacked in several differently shaped or sized subunits to form a unit cell, making theirfabrication quite troublesome. A simple design for broadband metamaterial absorbers istherefore urgently needed. Herein, we propose a coplanar broadband andpolarisation-insensitive perfect absorber formed by two patterned square metallic ringswith a dielectric layer on top of a metallic ground plane. The full width at half maximum(FWHM) of the device can be up to 42% (with respect to the central frequency), which is 2times greater than that of a single-layered structure. This property is retained well fora very wide range of incident angles. The two patterned square rings resonating atdifferent but similar frequencies leads to the broadband absorption. Moreover, ahybridised resonance model is proposed to analyse the origin of the resonance bandwidth.The results of this metamaterial absorber design appear to be very promising for solarcell, detection and imaging applications.     

Analysis of frequency selective surface absorbers via a novel equivalent circuit method

An equivalent circuit (EC) method for absorbers design is proposed in this paper. Without using full-wave analysis, the EC method can predict the performance of the absorbers. This method is employed to synthesize broadband absorbers by inserting the resistors respectively into the single- and double-square loops structures, then two different prototypes with broadband absorbing frequency bands are manufactured and measured. By comparisons with the results both by using the full-wave analysis and by the measurements, the correctness of the new EC method is verified. Some factors which affect the absorbing bandwidth are also investigated. Due to its fast and accurate characteristics, the EC method which can be theoretically applied to arbitrary FSS is a good candidate for broadband design of the absorbers.     

Realization of wide-angle and wideband absorber using metallic and graphene-based metasurface for mid-infrared and low THz frequency

This article presents a study on mid-infrared and low-THz absorbers based on metallic and graphene metasurface. The absorber is constructed of a periodic array of patterned elements in patch form placed on a quarter-wavelength dielectric film terminated by a metallic reflector. A simple analytical circuit model equivalent to patch array is used for employing the matching impedance approach to realize the wideband absorber. This absorber is polarization independent for normal incident waves owing to its symmetric structure. Simulation and analytical circuit model results show that the graphene and metallic-based absorbers proposed in this paper can operate with an absorption value above 90% in a normalized bandwidth of 100% in the low terahertz (THz) and the mid-infrared regime, respectively. The proposed absorber is wide-angle for both TM and TE polarizations and polarization-insensitive for small incident angles.     

非线性弹簧阻尼减振装置的探究

将汽车底盘减振装置简化为以非线性弹簧、阻尼器为主的被动阻尼式吸振器．利用Matlab软件对其在简谐力作用下的响应进行定量绘图分析,研究其振幅衰减、缓冲周期以及与由线性弹簧构成的简单模型进行比较,解释了减振装置具体的工作过程以及其中的基本工作原理．     

Wideband optical absorber based on plasmonic metamaterial cross structure

The optical absorber with Fano response is valuable for various applications such as solar cells or optical sensors. In this paper, we have modeled an optical plasmonic metamaterial absorber which contains a broken cross as an elementary cell along with four rectangular loads to improve the absorbance and achieve a Fano response within a wide bandwidth at 190–245 THz (25%). The bandwidth of the proposed structure is more than conventional metamaterial absorbers. The prototype absorber has a remarkable enhancement in the electric field in comparison with the simple cross model and the reflection value has reduced to ??47 dB. The parametric studies show how the gap capacitance controls the bandwidth, resonance frequency and the reflection value of the absorber, therefore we can consider this technique as a way to enhance the metamaterial absorber’s bandwidth. The proposed structure can be used as an optical refractive index sensor while the Fano line-shape provides a higher figure of merit (FOM) compared with many others. For this structure, the FOM has obtained as 10,660. The Finite Integration Technique with Perfect Boundary Approximation used for the simulation.     

Sustainable acoustic absorbers from the biomass

There is currently considerable interest in developing sustainable absorbers, either from biomass materials or recycled materials, and it is the former that is the subject of this paper. A number of potential candidate materials are available from the biomass in the form of organic fibres. Non-fibrous materials, such as configurations of whole straw or reed, can also act as sound absorbers. A combination of impedance tube and reverberation chamber measurements have been carried out for a number of biomass materials and the effectiveness of current models for the prediction of the absorptive properties of natural fibres has been investigated. Examination of the acoustical characteristics of a range of natural fibres has confirmed their effectiveness as porous sound absorbers and also the limitations of current models for predicting their performance. Examination of the acoustical performance of materials consisting of different configurations of whole reeds and straws has revealed that these also possess considerable potential for application as broadband sound absorbers with particularly good low frequency absorption characteristics. The combination of natural fibres and whole reeds offer the possibility of developing a range of sustainable absorbers which act very effectively across the complete audio frequency range.     

Large-area broadband saturable Bragg reflectors by use of oxidized AlAs

Broadband saturable Bragg reflectors (SBRs) are designed and fabricated by monolithic integration of semiconductor saturable absorbers with broadband Bragg mirrors. The wet oxidation of AlAs creates low-index AlxOy layers for broadband, high-index-contrast AlGaAs/AlxOy or InGaAlP/AlxOy mirrors. SBR mirror designs indicate greater than 99% reflectivity over bandwidths of 294, 466, and 563 nm for center wavelengths of 800, 1300, and 1550 nm, respectively. Highly strained and unstrained absorbers are stably integrated with the oxidized mirrors. Large-scale lateral oxidation techniques permit the fabrication of SBRs with diameters of 500 microm. Large-area, broadband SBRs are used to self-start and mode lock a variety of laser systems at wavelengths from 800 to 1550 nm.     

Deformable broadband metamaterial absorbers engineered with an analytical spatial Kramers‐Kronig permittivity profile

Electromagnetic (EM) materials with perfect absorption have long been investigated for their important applications in many practical technologies. The trial‐and‐error method has been mostly employed to achieve this target, either by varying the constituent compositions for traditional natural material absorbers or by running computer simulations for general metamaterial (MM) absorbers. In this work, the authors propose a new method with analytical guidance to build omnidirectional perfect absorbers inspired by the recently proposed spatial Kramers‐Kronig (KK) nonreflecting dielectric profile. The subtle combination of the spatial and time dispersions in the metamaterial‐engineered KK profile gives the desired broadband response property. More importantly, these features remain invariant when the sample is uniformly compressed or stretched with large thickness change, i.e., this particular broadband absorber is deformable, which has been firstly reported in the literature. The current results will pave a new way to design high‐efficiency EM absorbers that could also be extended in general to manipulate waves for other fields or applications.

     

Electro-acoustical characterization procedure for cMUTs

A procedure for the electro-acoustical characterization in air of cMUTs is reported. First, the measured input electrical impedance of the transducer is used to calculate the transducer parameters at different bias voltages by fitting it with the Mason model. Then, the single membrane equivalent circuit can be calculated. Second, the cMUT impulse response is obtained through a send-and-receive experiment to be compared with the one predicted by the Mason model. In order to minimize the influence of the emitter in the obtained impulse response an ad hoc broadband piezoelectric transducer centered at the resonant frequency of the cMUT was fabricated. Using this transducer, no deconvolution of the impulse response of the emitter in the cMUT reception pulse is necessary. The procedure is tested for two cMUTs with silicon-rich nitride as structural layer and different membrane diameters (60 and 70 microm).     

SIMPLE TECHNIQUES FOR DETERMINING THE SMALL-SIGNAL EQUIVALENT CIRCUIT OF MESFET’s

Simple techniques for determining the broadband small signal equivalent circuit (SSEC) of MESFETs are presented in this paper. The intrinsic elements are calculated using two-dimensional method from the Y parameters, which are obtained from the Fourier analysis of the device transient response to voltage-step perturbations at the drain and gate electrodes. Whereas, the parasitic external elements are determined by simple approximations used in transmission line modeling. In addition, a new technique is also proposed to determine the source and drain series resistances. A comparison of the SSEC of three different MESFETs technologies shows that the MESFETs on GaN and 4H-SiC are suitable for high power applications. The method used to determine the intrinsic elements is validated with simulated data obtained by Monte Carlo method.     

Broadband and ultra-low reflection metamaterial absorber embedded with magnetic materials

In this study, we design, prepare and characterize a broadband, ultra-low reflectivity and incidence angle-insensitive metamaterial absorber. The design of this absorber not only provides a novel idea for the design of broadband absorbers, but also enhances the application prospects of metamaterial absorbers. By introducing FeSiAlp/epoxy magnetic composite and optimizing the structural parameters, the absorption performance of the metamaterial absorber has been significantly improved. The effective absorption bandwidth (bandwidth with reflectivity less than −10dB) is increased by 3.4 times from 2.19 GHz to 7.49 GHz, and the RL_min (minimum reflection loss) value reaches −38.31 dB at 17.83 GHz, that is the absorption rate reaches 99.99%. Meanwhile, the experimental results also verify the simulation design results. Therefore, the absorber not only plays the characteristics of strong absorption of metamaterial, but also absorbs the advantages of broadband of magnetic material.     

Dynamic analysis and optimal design of a passive and an active piezo-electrical dynamic vibration absorber

This paper is concerned with the dynamic analysis and parameter optimization of both passive and active piezo-electrical dynamic vibration absorbers that are strongly coupled with a single degree of freedom vibrating structure. The passive absorber is implemented by using an R_s∥L_s parallel shunt circuit while the active absorber is implemented by feeding back the acceleration of the structure through a second-order lowpass filter. An impedance-mobility approach is used for the electromechanical coupling analysis of both types of absorbers coupled with the structure. Using this approach it is demonstrated that the passive and active absorbers can be made exactly equivalent. A maximally flat frequency response strategy is used to find the optimal damping ratio of the passive absorber while a robust, optimal control theory is used to find that for the active absorber. It is found that the passive optimization strategy corresponds to an optimal, robust feedback control of 2 dB spillover. Simulations and experiments are conducted to support the theoretical findings.     

Vibro-acoustic noise control treatments for payload bays of launch vehicles: Discrete to fuzzy solutions

Sound transmitted through an enclosing structure into a cavity has been minimised by optimising the locations, numbers and properties of tuned mass dampers placed on the structure and Helmholtz resonators placed in the cavity. Most of the results presented in the literature show the effect of a few well selected acoustic or vibration absorbers on simple vibro-acoustic systems, whereas the results presented here indicate the trends that occur when a few absorbers are replaced by a large number. The results indicate that for low numbers of absorbers, the result of the optimal configuration is to reduce the response of discrete modes, and for a large number of absorbers the result of the optimal configuration is a tendency for the vibro-acoustic energy to be smeared over a range of frequencies, which is similar to the results seen in fuzzy structures. The enormous computational task was achieved using a distributed computing network and an asynchronous parallel genetic algorithm.     

Towards quantitative acousto-optic imaging in tissue

We have investigated the possibilities and limitations of the application of ultrasound modulated coherent light to obtain quantitative information of local absorbers in light-scattering objects, among which tissue. For all objects studied, the combined use of microsecond ultrasound and light pulses enabled us to construct a 3D map of local absorbers with a spatial resolution of ∼2 mm. Moreover, in relatively homogeneous model systems, mimicking a blood vessel embedded in tissue, the use of a calibration procedure allowed for a determination of the local absorbance. Speckle decorrelation times for real tissue containing blood vessels, in which appreciable motion of scatterers can exist, were found to be smaller than 1ms. These relatively short times present a major challenge for acousto-optics to be applied in living tissue systems.     

Modeling for multi-resonant behavior of broadband metamaterial absorber with geometrical substrate

Despite widespread use for extending absorption bandwidth, the coexistence and coupling mechanism of multiple resonance is not well understood. We propose two models to describe the multi-resonant behavior of a broadband metamaterial absorber with geometrical-array substrate(GAS). The multi-resonance coupling of GAS is well described by logarithmic law. The interaction between metasurface and GAS can further broaden the absorption bandwidth by generating a new resonance which coexists with original resonances in substrate. The proposed models can thoroughly describe this multiple-resonance behavior, highlighting guidelines for designing broadband absorbers.     

Temporal response of nonlinear transmission in optically thick saturable absorbers and determination of saturable absorption parameters

The temporal response of nonlinear transmission in saturable absorbers is theoretically investigated on the basis of a four-energy-level model including an excited-state absorption, and is experimentally employed for determining saturable absorption parameters in saturable-dye-doped films. The approximation of optically thin absorbers is shown to be inadequate to this end. In theoretical calculations, optically thick saturable absorbers are treated by a set of simultaneous partial differential equations: a rate equation governing the dynamics of population densities of energy levels and a propagation equation governing the propagation of optical fields in the optically thick absorbers. By comparing experimental results with numerical ones, the saturable absorption parameters can be determined on the basis of the intensity dependence of response time. In numerical calculations, the effect of random orientation is also considered for optically anisotropic molecules rigidly fixed in saturable absorbing media. The characterization of the saturable absorption parameters is conducted by using uranine-doped poly(vinyl alcohol) films.     

动态电路冲激响应初始值的一种简易求法

给出了求解动态电路冲激响应初始值和动态奇异电路响应初始值的一种简易方法.在电路换路后,其电容电压或电感电流的初始状态一般不产生跃变,但有时也可能产生跃变,当一时难于确定时,可先将电路化为其s域模型,并将0-值带进换路后的电路模型,再应用拉氏变换的初值定理,即可在s域解决t域响应的初始状态0 值的问题.该方法广泛适用于各种电路.     

铁镓单晶Janus-Helmholtz换能器

针对深水、低频、宽带换能器的技术需求,结合Janus-Helmholtz换能器的结构特点和铁镓单晶材料低场应变大及机械强度高的特性,提出了铁镓单晶Janus-Helmholtz换能器设计方案。采用永磁偏磁场和环形闭合磁路,建立了一系列铁镓单晶磁致伸缩换能器理论分析模型,包括对磁致伸缩材料参数进行线性化处理,设计了换能器最佳工作点,结合静态磁场和动态磁场分布情况分段细化换能器驱动等效参数,以及利用全阻抗模型通过电感损耗等效计算换能器静态阻抗,然后通过二维有限元分析等效模型,优化分析了换能器的结构参数与电声性能。最后制作了换能器样机,并进行了测试与分析。对比仿真和测试结果表明:全阻抗模型得到的阻抗曲线与样机测试结果相一致,有限元等效模型计算的发送电流响应与样机测试结果良好吻合。换能器样机水中谐振基频为1000 Hz,谐振频率下发送电流响应176.4 dB;在875～2300 Hz频率范围内,发送电流响应起伏不大于6 dB;增加驱动电流有效值到16.2 A,最大声源级可以达到196.2 dB。     

多频与宽频超材料吸收器

提出了多频与宽频超材料吸收器结构模型, 该吸收器是由刻蚀在介质基板两面的金属图案组成, 其正面是由不同大小的金属铜圆片和金属铜圆环结构组合而成, 反面完全覆盖金属铜. 由于不同几何大小的圆片或圆环结构的谐振频率不同, 且是相互独立的. 因此通过将两种不同几何大小的圆片与圆环结构组合, 使其在不同频率谐振, 便可得到双频吸收器; 而将三种不同几何大小的圆片和圆环结构组合便可得到三频吸收器; 同理可得到四频吸收器. 同时, 通过研究吸收器的电场分布, 验证了不同几何大小的圆片或圆环结构在不同频率谐振. 若通过优化组合圆片和圆环结构, 使其谐振频率紧密相连则可得到宽频吸收器. 关键词： 吸收器 超材料 多频 宽频     

An ultra‐black silicon absorber

An ultra‐black (A > 99%) broadband absorber concept on the basis of a needle‐like silicon nanostructure called Black Silicon is proposed. The absorber comprises Black Silicon established by inductively coupled plasma reactive ion etching (ICP‐RIE) on a highly doped, degenerated silicon substrate. Improved absorbers also incorporate an additional oxide capping layer on the nanostructures and reach an absorptance of A > 99.5% in the range of 350 to 2000 nm and A ∼ 99.8% between 1000 and 1250 nm. Fabrication of the absorbers is consistent with CMOS standards and requires no lithography. (Picture: Kasper, Friedrich‐Schiller‐University Jena)