Current strategies for developing multiband electromagnetic metamaterial absorbers in the terahertz range face significant challenges, primarily because of their dependence on inefficient trial-and-error design methods. To address these limitations, in this work, we introduced a deep learning framework incorporating two bidirectional spectrum networks, and build the dataset through the inherent physical mechanisms of multiband terahertz absorbers. This framework established a bidirectional mapping relationship between the structural parameters and the electromagnetic responses. These networks predicted the reflection coefficient and absorption rate of electromagnetic metamaterials in the terahertz range based solely on known structural parameters through forward prediction paths. Analysis of the forward prediction results demonstrated that the deep learning model effectively captured the relationships between the structural parameters of electromagnetic metamaterials and their electromagnetic responses. Furthermore, we evaluated the capability of the inverse design path, which is the process of designing structural parameters to meet specific absorption rate requirements, for spectral curves with two, three, and four peaks generated randomly within the special frequency band. The designed electromagnetic metamaterial not only exhibited reasonable alignment with the peak absorption rates and the designated frequency bands of the target spectral curves but also involved relatively low structural complexity, being based on a prototype structural unit. This finding demonstrated the on-demand inverse design capability of the framework, which allowed for the tailored design of electromagnetic metamaterials with specific absorption peak numbers, thus highlighting its efficiency and flexibility in the design of multiband absorbers.

In this article, a joint topology optimization procedure is executed to design phononic crystals (PnCs) with a maximized band gap. First, the band gap of two-dimensional PnCs is calculated using a finite element model under the plane strain assumption for easy and efficient optimization. In the numerical computation of the band gaps, the band structures of in-plane and out-of-plane modes are analyzed and verified using COMSOL. Then, the density-based and feature-driven methods are sequentially used to design the unit cell of the PnCs with single-phase material. A stiffness constraint is added to ensure the continuity of the material inside the unit cell, and a boundary constraint guarantees continuity amongst the unit cells. The optimization problem is solved through the Method of Moving Asymptotes (MMA), and reconfigurable manufacturing PnC is obtained via joint topology optimization. Lastly, the band structures and vibration modes of the reconstructed model are analyzed. Research results show that the optimal PnCs generate broadband vibrations by localized resonance and have negative refraction characteristics. This research also confirms that the joint topology optimization is an efficient approach for designing novel elastic metamaterials.

本文首先利用有限元仿真软件COMSOL计算了二维手性声子晶体的带隙,分析了散射体参数与韧带涂层参数变化对带隙的影响规律.在此基础上,确定手性声子晶体带隙最优设计的有效参数设计空间;然后基于ISIGHT优化设计平台嵌入遗传算法,开展二维手性声子晶体带隙的最优设计.在带隙的最优设计过程中,先以二维手性声子晶体的有效构型参数为设计变量,相对带隙宽度最大为目标,设计手性声子晶体单胞构型.再以此优化单胞构型为初始构型,以手性声子晶体的有效材料参数为设计变量,相对带隙宽度最大为目标,进一步实现二维手性声子晶体带隙的最优设计.本工作极大限度地挖掘了二维手性声子晶体带隙最优设计潜能,为充分发挥手性声子晶体在减振降噪中的作用提供了可靠有效的分析设计方法.

声子晶体是一种具有声子带隙的周期性结构,通过对其带隙的动态调控设计,进而满足航空航天领域中重大装备对减振降噪性能的特定需求.文章在声子晶体的带隙设计中,引入智能材料,采用拓扑优化方法,开展多功能声子晶体带隙动态调控设计.首先采用有限元方法分析声子晶体的带隙性能,并建立形状记忆合金的温度本构关系模型;其次基于变密度法的拓扑优化方法,在满足特定体分比及强度约束、保证声子晶体单胞之间的连接性约束条件下,以相对带隙最大化为目标函数建立声子晶体带隙设计优化模型;最后根据改进的材料插值模型,分析求解设计灵敏度,采用移动渐进法开展多功能声子晶体带隙结构的拓扑优化设计.优化结果表明:在XY模式下形状记忆合金从马氏体转换到奥氏体带隙拓宽了 103.9%,在Z模式下带宽增大了 3.75 倍.研究结果为声子晶体在复杂环境下实现更为主动带隙调控提供了一种有效的设计方法.

The influences of metasurfaces on the propagation of electromagnetic waves generate several important effects, such as asymmetric transmission and polarization conversion, that are highly useful in optical and microwave communication applications. However, easy method for dynamically controlling the asymmetric transmission of linearly polarized waves with perfect polarization conversion and high efficiency over a wide band in the THz range remain poorly developed. Our work addresses this issue by designing a novel metasurface structure consisting of two outer orthogonal gratings and a central lattice with an optimized chiral graphene monolayer distribution topology sandwiched between dielectric substrates. The frequency-dependent performance of the proposed metasurface is evaluated according to analyses of the asymmetric transmission coefficient, polarization conversion rate, total transmission coefficient, polarization rotation angle, ellipticity, and chirality parameter based on the results of simulations. The results demonstrate that the proposed structure provides highly efficient asymmetric transmission of linearly polarized waves and perfect polarization conversion in the high frequency range from 0.1 to 3.0 THz. The asymmetric transmission and the polarization conversion of the structure are dynamically controllable by changing the Fermi energy of graphene from 0 eV to 1 eV. The results of the analysis reveal that the observed dynamic controllability is a function of the interrelation between the special configuration of the chiral metasurface structure and the special properties of graphene.

In this paper, two-dimensional phononic bandgap materials are designed through multiobjective optimization using the genetic algorithm. Two cases are given. In Case I, 2D phononic crystals (PnCs) with maximum bandgap and minimum mass are optimized. The optimal results show that the third-order relative bandgaps become large, along with the increase in mass. In Case II, 2D local resonance phononic crystals (LRPnCs) simultaneously maximizing the third and sixth relative bandgap widths (RBWs) are designed. The frequency response function of the optimized structures shows that the attenuation of elastic waves is most remarkable at the beginning frequency of the bandgap. Finally, an integrated structure, which accumulates the bandgaps of three optimized substructures, is proposed. These results provide a new approach for bandgap design.

Asymmetric transmission (AT) metamaterials are extensively studied and applied in the fields of polarization converters and photodiodes. In order to further improve the properties of polarization conversion and unidirectional conduction in the high frequency band and to implement their tunability, the novel chiral electromagnetic metamaterials are studied. By the topology optimization technique, a new type of double-layer L-shaped variant metamaterial structure with excellent asymmetric transmission characteristics is designed. The objective function is to maximize the asymmetric transmission coefficient for the linear polarization wave. The rotationally symmetrical design domain is determined by considering polarization conversion and computation efficiency simultaneously. The design domain of upper layer is divided into two parts which are both the 180° rotationally symmetrical. The design domain of the upper layer and lower layer are the 90° rotationally symmetrical around the x and z axis respectively. Therefore, the number of design variables is only 18. Asymmetric transmission of linear polarization wave in the K band and Ka band are implemented. Numerical simulation results and experimental results show that the optimized chiral metamaterial has excellent asymmetric transmission characteristics, and its asymmetric transmission coefficient reaches 0.8562 at a frequency of 21.65 GHz and 0.8175 at a frequency of 28.575 GHz. Its asymmetric transmission mechanism is expounded by analyzing the electric field and surface current distribution at the resonance frequency. Based on the optimized chiral metamatertials, the reasonable geometric parameters are selected and the rotation angle of the metal layer is changed in order to further achieve the tunable AT characteristics. First, the influences of the dielectric substrate layer, the thickness of the metal layer and the side length of the grid on resonance frequency and asymmetric transmission coefficient are analyzed respectively, which provides the basis for the reasonable adjustment of the structural parameters to obtain better asymmetric transmission characteristics. After the reasonable geometric parameters are determined, the rotational angle of the upper metal layer and lower metal layer are changed. The linearly and circularly polarized wave are simultaneously achieved in the K band. In this article, the topology optimization technique is used to design the asymmetric transmission chiral metamaterial structure. The design process has a clear direction. The optimized asymmetric transmission chiral metamaterial has the simple structure type and the easy tunability of its asymmetric transmission characteristics. It can be used widely and easily in the fields of polarization converters and photodiodes. This design method has a broad application prospect in the chiral metamaterial field.

In this paper, chiral metamaterials (CMM) were optimized from conceptual design to fine design with the effective elastic constants unchanged under finite strain. First, through calculation and comparison of examples, the unit cell method was selected to compute the effective elastic properties of the periodic chiral metamaterials under finite strain. Secondly, the conceptual design of chiral metamaterials with prescribed Poisson's ratios under finite strain was realized through density-based and feature-driven topology optimization. Then, the method of moving asymptotes (MMA) was used to solve the optimization problems. Based on the optimal configuration, chiral metamaterials with prescribed Poisson's ratios and Young's moduli under finite strain were carefully designed through shape optimization. Genetic algorithm was used to solve the optimization problem. Finally, the optimal models were fabricated by 3D printing. The optimal design was validated by tensile test results, i.e., the designed chiral metamaterials can maintain effective elastic properties under large deformation, and the invariance of the effective elastic properties depends on the nonlinearity of the flexible chiral metamaterials.

Metamaterial analogue of electromagnetically induced absorption (EIA) has promising applications in spectroscopy and sensing. Here we propose an EIA metamaterial based on hybrid metal/dielectric structures, which are composed of a metallic wire and a dielectric block, and investigate the EIA-like effect by simulations, experiments, and the two-oscillator model. An EIA-like effect emerges in virtue of the near-field coupling between metallic wire and dielectric block, and the dielectric block exhibiting magnetic dipolar resonance makes a major contribution to the resonance absorption. The magnetic flux through the dielectric block engendered by the near filed of the metallic wire determines the coupling between dielectric block and metallic wire. With the variation of the separation between dielectric block and metallic wire, the EIA-like effect is preserved and does not convert into the EIT-like effect although the coupling and consequently the absorbance are altered. Based on the two-oscillator model, the absorption spectrum of the EIA metamaterial is quantitatively analyzed and the parameters of the oscillator system are retrieved.

Flexible chiral honeycomb cores generally exhibit nonlinear elastic properties due to large geometric deformation. The effective elastic moduli and Poisson’s ratio typically vary with an increase in deformation. Here, the size and shape optimization of the chiral hexagonal honeycombs was performed to keep the Young’s moduli and Poisson's ratio unchanged under large deformations. The size of the honeycomb unit cell and the position coordinates of the key points were defined simultaneously as design variables. The equivalent Young's modulus and Poisson’s ratio of chiral honeycombs were calculated through geometric nonlinear analysis. The objective was to minimize the allowable tolerance between the prescribed and actual properties within the range of the target strain. A genetic algorithm was then adopted. The optimal results demonstrate that the chiral honeycombs can maintain effective elastic properties that do not vary under large deformation. These results are meaningful to morphing aircraft designs.

Active control of metamaterial properties is of great significance for designing miniaturized and versatile devices in practical engineering applications. Taking advantage of the highly temperature-dependent permittivity of water, we demonstrate a water-based metamaterial comprising water cubes with thermally tunable Mie resonances. The dynamic tunability of the water-based metamaterial was investigated via numerical simulations and experiments. A water cube exhibits both magnetic and electric response in the frequency range of interest. The magnetic response is primarily magnetic dipole resonance, while the electric response is a superposition of electric dipole resonance and a smooth Fabry–Pérot background. Using temporal coupled-mode theory (TCMT), the role of direct scattering is evaluated and the Mie resonance modes are analyzed. As the temperature of water cube varies from 20 °C to 80 °C, the magnetic and electric resonance frequencies exhibit obvious blue shifts of 0.10 and 0.14 GHz, respectively.

The active control of electromagnetic response in metamaterial and mutual coupling between resonant building blocks is of fundamental importance in realizing high-quality metamaterials. In this work, we propose and experimentally demonstrate the tunabilities of symmetry-broken metasurfaces made of orthogonal electric dipolar resonators. The metasurface with vertical and horizontal wires is integrated with a PIN diode for active control. It is found that the electromagnetically induced transparency (EIT)-like spectrum appears due to the destructive or constructive interferences between the two electric dipolar modes when the structural symmetry broken is introduced to the metasurface. Different from previous works on the EIT-like effect, there is only electric dipole response in our metasuface. The microscopic response of the metasurface is numerically calculated to illustrate the mode coupling between the orthogonal electric dipolar resonators. By applying temporal coupled-mode theory, the interaction between the electromagnetic wave and the symmetry-broken metasurface is described, and the characteristic parameters of the resonator system, which determine the electromagnetic response of the metasurface, are acquired.

Flexible chiral honeycomb cores generally exhibit nonlinear elastic properties in response to large geometric deformation, which are suited for the design of morphing aerospace structures. However, owing to their complex structure, it is standard to replace the actual core structure with a homogenized core material presenting reasonably equivalent elastic properties in an effort to increase the speed and efficiency of analyzing the mechanical properties of chiral honeycomb sandwich structures. As such, a convenient and efficient method is required to evaluate the effective elastic properties of flexible chiral honeycomb cores under conditions of large deformation. The present work develops an analytical expression for the effective elastic modulus based on a deformable cantilever beam under large deformation. Firstly, Euler–Bernoulli beam theory and micropolar theory are used to analyze the deformation characteristics of chiral honeycombs, and to calculate the effective elastic modulus under small deformation. On that basis, the expression for the effective elastic modulus is improved by including the stretching deformation of the chiral honeycomb structure for a unit cell under conditions of large deformation. The effective elastic moduli calculated by the respective analytical expressions are compared with the results of finite element analysis. The results indicate that the analytical expression obtained under consideration of the geometric nonlinearity is more suitable than the linear expressions for flexible chiral honeycomb cores under conditions of high strain and low elastic modulus.

This article presents a thermally tunable Fano resonator obtained by asymmetrically coupling a conductive rubber-based H-shaped split ring resonator (SRR) and a copper C-shaped SRR coated on a Teflon fiberglass slab substrate. Thermal tunability of the Fano resonance is provided by the temperature-sensitive resistance property of the conductive rubber. The thermal tunability of the Fano resonance obtained from the proposed resonator structure is demonstrated by both simulation and experimental results. This study provides a novel tunable approach for designing dynamical electromagnetic modulation devices.

This paper presents the design and related applications of a thermally tunable band-stop filter employing conductive rubber that incorporates electrically conductive particles. The relationship between the environmental temperature and the volume resistivity of conductive rubber materials was experimentally established. The experimental results demonstrated that the conductive rubber has a temperature-sensitive resistance property. The conductive rubber material was employed in the design of a C-shaped split-ring resonator (SRR) structure. The influence of environmental temperature on the electromagnetic properties of the SRR structure was analysed using a numerical simulation and experimental testing. The results demonstrated that the design of thermally tunable metamaterials was possible. As such, an H-shaped conductive rubber SRR was employed in the design of a novel thermally tunable band-stop filter. The results of a numerical simulation and experimental testing verified its band-stop characteristic, and demonstrated that the filtering performance of the H-shaped conductive rubber SRR can be controlled by the environmental temperature. Lastly, by comparison of the effective dielectric properties of the two types of SRRs, it demonstrates the effectiveness of conductive rubber and their temperature-controllable band-stop characteristics.

This paper focuses on the honeycomb cores with absorbing coatings, the effective permittivity is predicted and the absorption property is optimized. The correctness of effective results is verified by experimental measurement, numerical simulation and theoretical analysis. When honeycomb cores with absorbing coatings are regarded as layered multi-phase materials, the upper and lower bounds of effective permittivity are calculated by the way of multi-step equivalence. In order to exhibit the dispersion effect, the improved strong fluctuation theory is proposed. In the second part of works, the size optimization is executed to improve absorption property. The optimization problem is defined that the area T which is surrounded by the effective reflectivity curve and the boundary line (R0 = -10 dB) is maximized under the upper bound constraints of reflectivity and volume fraction. The optimal effective reflectivity curve is obtained within the given frequency range.DOI: http://dx.doi.org/10.5755/j01.ms.22.3.8456

Flexible hexagonal honeycomb cores exhibit nonlinear elastic properties due to the large geometric deformation. To rapidly and efficiently analyze the mechanical properties of honeycomb sandwich structures, it is standard to replace the actual core structure in analyses with a homogenized core material presenting reasonably equivalent elastic properties. As such, a convenient and efficient method is required to evaluate the equivalent elastic properties of flexible hexagonal honeycomb cores. The present work develops analytical expressions based on a deformable cantilever beam under large deformation. On that basis, the equivalence expressions are improved by including the stretching deformations of the honeycomb structure on an infinitesimal section of a unit cell. Finite element analysis and experimental testing are subsequently performed for two examples of flexible hexagonal aluminium and Nomex® honeycomb cores. Both computational and experimental results indicate that their equivalent elastic moduli have the different variation with high strain in the two characteristic orthogonal directions. The analytical predictions demonstrated that the improved analytical expressions are more suitable to flexible honeycomb cores under conditions of high strain and low elastic modulus. It is further revealed that the structure of Nomex honeycomb cores result in different forms of damage during failure along the different load directions, which lead to differing equivalent properties in the orthogonal directions when the strain limit is reached.

In this paper, an electrically reconfigurable split ring resonator (SRR) covered by sessile droplet of nematic liquid crystal (LC) is demonstrated experimentally. The magnetic resonance of single SRR decreases gradually by 237 MHz as external bias voltage is applied, resulting from increasing fringing capacitance due to liquid crystal molecular reorientation along local electric field distribution. The transmission phase can be modulated by more than 100 degrees. Furthermore, frequency tuning range of SRR increases with the droplet height, because of the significant enhancement for SRR capacitance difference between LC states with/without bias voltage. This work will be of interest for the development of reconfigurable metasurface and related application.

In this work, a novel metamaterial absorber (MMA) with perfect wave absorption is designed through the layout optimization. The rotationally symmetrical design domain is determined by considering polarization insensitivity. The symmetrical design variables are related with each other. The optimization objective is to maximize the absorption index which is described by the reflection and transmission coefficients in the frequency range of 8-12GHz with the constraint of volume fraction. The maximum absorptivity of the optimized MMA reaches 99.89%. And it has the polarization insensitivity, the wide range of oblique incident angles and the ideal impedance matching with the free space. Furthermore, the absorption frequencies exhibit regular variations while changing the sizes of unit cell and the thickness of dielectric substrate. Inspired by these analysis results and the existing researches, the multilayer MMA is designed in order to broaden the absorption band efficiently. The absorption frequency band reaches 1.1GHz while the absorptivity is more than 85%.