Magneto-Mechanical Metamaterials with Widely Tunable Mechanical Properties and Acoustic Bandgaps

Mechanical metamaterials are architected manmade materials that allow for unique behaviors not observed in nature, making them promising candidates for a wide range of applications. Existing metamaterials lack tunability as their properties can only be changed to a limited extent after the fabrication. Herein, a new magneto-mechanical metamaterial is presented that allows great tunability through a novel concept of deformation mode branching. The architecture of this new metamaterial employs an asymmetric joint design using hard-magnetic soft active materials that permits two distinct actuation modes (bending and folding) under opposite-direction magnetic fields. The subsequent application of mechanical compression leads to the deformation mode branching where the metamaterial architecture transforms into two distinctly different shapes, which exhibit very different deformations and enable great tunability in properties such as mechanical stiffness and acoustic bandgaps. Furthermore, this metamaterial design can be incorporated with magnetic shape memory polymers with global stiffness tunability, which also allows for the global shift of the acoustic behaviors. The combination of magnetic and mechanical actuations, as well as shape memory effects, impart wide tunable properties to a new paradigm of metamaterials.     

A Fabrication Strategy for Reconfigurable Millimeter-Scale Metamaterials

Rather than depending on material composition to primarily dictate performance metrics, metamaterials can leverage geometry to achieve specific properties of interest. For example, reconfigurable metamaterials have enabled programmable shape transformations, tunable mechanical properties, and energy absorption. While several methods exist to fabricate such structures, they often place severe restrictions on manufacturing materials, or require significant manual assembly. Moreover, these arrays are typically composed of unit cells that are either macro-scale or micro-scale in dimension. Here, the fabrication gap is bridged, and laminate manufacturing is used to develop a method for designing reconfigurable metamaterials at the millimeter-scale, that is compatible with a wide range of materials, and that requires minimal manual assembly. In addition to showing the versatility of this fabrication method, how the use of laminate manufacturing affects the behavior of these multi-component arrays is also characterized. To this end, a numerical model that captures the deformations exhibited by the structures is developed, and an analytic model that predicts the strain of the structure under compressive stress is built. Overall, this approach can be leveraged to develop millimeter-scale metamaterials for applications that require reconfigurable materials, such as in the design of tunable acoustics, photonic waveguides, and electromagnetic devices.     

各向异性超材料中太赫兹波的偏振转换

各向异性超材料可以控制太赫兹波的偏振态,实现入射太赫兹波的偏振转换。为了获得非手性各向异性超材料的透射响应与本征偏振复透射系数的关系及其入射偏振依赖性,在太赫兹时域光谱系统中测量了等臂长L形结构超材料在不同偏振角下的正入射透射谱,获得并分析了透射太赫兹波的偏振转换率和偏振态,所得结果与基于琼斯矩阵和坐标变换计算的结果一致。在0.7 THz~1.3 THz频率范围内可实现约20%的偏振态能量转换效率。在L结构和双L结构的偏振转换透射谱中分别观察到了宽带响应和多频共振响应,表明结构改变对太赫兹波透过特性的敏感性和可操控性。所得到的结果可用于太赫兹功能器件的设计、表征和优化。     

Design, fabrication, and testing of double negative metamaterials

The design, fabrication, and testing of several metamaterials that exhibit double negative (DNG) medium properties at X band frequencies are reported. DNG media are materials in which the permittivity and permeability are both negative. Simulation and experimental results are given that demonstrate the realization of DNG metamaterials matched to free-space. The extraction of the effective permittivity and permeability for these metamaterials from reflection and transmission data at normal incidence is treated. It is shown that the metamaterials studied exhibit DNG properties in the frequency range of interest.     

介质层对超材料太赫兹响应特性的控制规律

作为超材料的重要组成部分,介质层是影响超材料响应特性的关键因素.固定超材料的尺寸和表层金属图形,通过理论推导和仿真模拟详细分析了在太赫兹波段下介质层的介电常数和厚度两个参数对超材料响应特性的控制规律,并首次确定了响应频率与介电常数的关系方程.结果表明,在其他参数不变的情况下,超材料的响应频率主要取决于介质层的介电常数的实部大小,而其吸收率则主要取决于介质层的厚度.据此,提出一种设计超材料的新方法:首先将设计要求的响应频率代入频率方程计算出相应介质层的介电常数,由此挑选合适的介质材料;然后固定介电常数、调节介质层厚度,获得在特定频率具有特定吸收率的超材料.     

太赫兹超常材料及应用

超常材料具有人工设计的结构,并有自然材料所不具备的超常物理性质。超常材料的电磁响应灵活可调,对太赫兹(THz)技术意义非凡。THz超常材料的实现和迅速发展为太赫兹技术的发展和应用带来了新的机遇。总结了THz波段超常材料的研究进展,包括THz波段超常材料的构造及制备、基于超常材料的THz波器件以及超常材料在THz波技术中的其他应用。     

有源超常材料的研究及应用

超常材料是具有自然材料所不具备的超常物理性质的一类人工电磁材料。由于本身具有损耗大、工作带宽窄等缺点严重限制了其应用研究,尤其是在太赫兹和光波频段。在金属微结构单元结合某种非线性材料(半导体、量子结、量子阱等)构成复合型超常材料,或者改变超常材料的外部激励(如温度、光激励、电磁场等)可以控制其整体特性,实现抵消材料损耗、调控它对电磁波的响应强度和频谱范围。本论文总结了有源超常材料在各频段的研究进展及应用。     

3D Transformable Modular Kirigami Based Programmable Metamaterials

Kirigami, the ancient paper art of cutting, has recently emerged as a new approach to construct metamaterials with novel properties imparted by cuts. However, most studies are limited to thin sheets-based 2D kirigami metamaterials with specific forms and limited reconfigurability due to planar connection constraints of cut units. Here, 3D modular kirigami is introduced by cutting bulk materials into spatially closed-loop connected cut cubes to construct a new class of 3D kirigami metamaterials. The module is transformable with multiple degrees of freedom that can transform into versatile distinct daughter building blocks. Their conformable assembly creates a wealth of reconfigurable and disassemblable metamaterials with diverse structures and unique properties, including reconfigurable 1D column-like materials, 2D lattice-like metamaterials with phase transition of chirality, as well as 3D frustration-free multilayered metamaterials with 3D auxetic behaviors and programmable deformation modes. This study largely expands the design space of kirigami metamaterials from 2D to 3D.     

基于智能算法的超材料快速优化设计方法研究进展

目前,超材料研究不断向工程化应用推进,在物理机理与效应、设计理论与方法、加工制备与测试等方面取得了突飞猛进的发展。但是,传统的超材料设计主要依赖人工设计和优化,面对大规模的工程化应用设计时,无法实现数量庞大的超材料结构单元的快速整体设计。近几年,涵盖传统启发式算法和神经网络算法的智能算法在超材料设计中所占的比重逐步上升...     

A Stair-Building Strategy for Tailoring Mechanical Behavior of Re-Customizable Metamaterials

Re-customizable mechanical behavior is critical for versatile materials with tunable functions and applications, but inverse design for varying targets is often hindered by complex coupling between structural topologies and mechanics. In this work, a novel “stair-building” strategy for customizing as well as re-customizing target mechanical behavior for mechanical metamaterials is proposed. Similar to building a stair with bricks, customizing or re-customizing a target stress–strain (force–displacement) curve for the material can be realized by stacking the brick-like loading curves of bistable units visually. The mechanical feasibility of the “stair-building” strategy is firstly physically realized in a type of array-structured multistable mechanical metamaterial and then carefully verified by theoretical mechanics analysis. Accordingly, three specific simple design schemes are further proposed for implementation. The “stair-building” strategy is proved to be rapid, effective, and accurate for mechanical behavior customization by both experiments and finite element simulations. Moreover, re-customization for diverse mechanical behaviors in a wide range can be realized by the same piece of metamaterial. This design strategy provides a novel approach for tailoring metamaterials with re-customizable target mechanical behaviors and applies to a variety of bistable units.     

Iterative reweighted least-squares design of FIR filters

Develops a new iterative reweighted least squares algorithm for the design of optimal L_p approximation FIR filters. The algorithm combines a variable p technique with a Newton's method to give excellent robust initial convergence and quadratic final convergence. Details of the convergence properties when applied to the L_p optimization problem are given. The primary purpose of L_p approximation for filter design is to allow design with different error criteria in pass and stopband and to design constrained L₂ approximation filters. The new method can also be applied to the complex Chebyshev approximation problem and to the design of 2D FIR filters     

太赫兹超材料生物检测应用研究进展

超材料作为一种具备超常物理性质的人工复合材料,能够突破常规材料的限制,为设计先进功能材料开辟一种全新的思路。太赫兹波由于具有光子能量低、对生物物质无电离损害和分子指纹谱等特性,通过与超材料结合,可实现对生物物质高灵敏检测,越来越受到国内外学者的广泛关注。本文总结了近几年来太赫兹超材料传感器在生物分子和细胞检测领域上取得的进展,首先介绍了太赫兹超材料传感器的传感原理和性能指标,其次从超材料结构设计、衬底选择、以及与微流控和新材料结合等方面阐述了太赫兹超材料传感器在生物检测领域的发展。通过对超材料结构进行优化、采用低介电常数薄型衬底、结合微流控技术或在传感器上粘附新材料涂层,可进一步提高超材料传感器的灵敏度,并丰富其在生物医学检测上的功能。最后,对太赫兹超材料传感器的发展趋势和前景进行了展望。     

Pareto Optimal Microwave Filter Design Using Multiobjective Differential Evolution

Microwave filters play an important role in modern wireless communications. A novel method for the design of multilayer dielectric and open loop ring resonator (OLRR) filters under constraints is presented. The proposed design method is based on generalized differential evolution (GDE3), which is a multiobjective extension of differential evolution (DE). GDE3 algorithm can be applied for global optimization to any engineering problem with an arbitrary number of objective and constraint functions. GDE3 is compared against other evolutionary multiobjective algorithms like nondominated sorting genetic algorithm-II (NSGA-II), multiobjective particle swarm optimization (MOPSO) and multiobjective particle swarm optimization with fitness sharing (MOPSO-fs) for a number of microwave filter design cases. In the multilayer dielectric filter design case a predefined database of low loss dielectric materials is used. The results indicate the advantages of this approach and the applicability of this design method.      

Curved Architected Triboelectric Metamaterials: Auxeticity-Enabled Enhanced Figure-of-Merit

Triboelectric generators are integrated into curved architected materials to realize triboelectric metamaterials that simultaneously harvest electricity from wasted mechanical energy and perform mechanical energy absorption. Novel triboelectric mechanical metamaterials (TMMs) of distance-changing, angle-changing, and mixed modes are designed, fabricated, and tested under a cyclic compressive load. The open-circuit voltage and short-circuit current of lightweight TMMs are found to be as high as 40 V and 10 nA. The introduced TMMs can effectively harvest energy under loadings from two distinctive directions. A theoretical model for predicting the energy harvesting properties of TMMs is developed, and the role of auxeticity on the energy harvesting figure-of-merit (FOM_es) is elicited. The introduced TMMs exhibit enhanced FOM_es enabled by a decrease in their negative Poisson's ratio and an increase in their resilience. The FOM_es of curved architected TMMs surpasses by more than 16 times the FOM_es of triboelectric materials with conventional architectures (i.e., triangular, quadrilateral, and hexagonal cell topologies). An intelligent skateboard with integrated TMMs is fabricated as a proof of concept to demonstrate motion sensing, shock-absorbing, and energy harvesting functionalities of multimodal triboelectric metamaterials. The introduced design strategy for triboelectric metamaterials unlocks their applications in self-powered and self-monitoring sports equipment, smart soft robots, and large-scale energy harvesters.     

Decoupling Minimal Surface Metamaterial Properties Through Multi-Material Hyperbolic Tilings

Rapid advances in additive manufacturing have kindled widespread interest in the rational design of metamaterials with unique properties over the past decade. However, many applications require multi-physics metamaterials, where multiple properties are simultaneously optimized. This is challenging since different properties, such as mechanical and mass transport properties, typically impose competing requirements on the nano-/micro-/meso-architecture of metamaterials. Here, a parametric metamaterial design strategy that enables independent tuning of the effective permeability and elastic properties is proposed. Hyperbolic tiling theory is applied to devise simple templates, based on which triply periodic minimal surfaces (TPMS) are partitioned into hard and soft regions. Through computational analyses, it is demonstrated how the decoration of hard, soft, and void phases within the TPMS substantially enhances their permeability–elasticity property space and offers high tunability in the elastic properties and anisotropy at constant permeability. Also shown is that this permeability–elasticity balance is well captured using simple scaling laws. The proposed concept is demonstrated through multi-material additive manufacturing of representative specimens. The approach, which is generalizable to other designs, offers a route towards multi-physics metamaterials that need to simultaneously carry a load and enable mass transport, such as load-bearing heat exchangers or architected tissue-substituting meta-biomaterials.     

The Pareto Optimization of Ultrawideband Polyfractal Arrays

The application of global optimization techniques, such as genetic algorithms, to antenna array layouts can provide versatile design methodologies for highly directive, thinned, frequency agile, and shaped-beam antenna systems. However, these methodologies have their limitations when applied to more demanding design scenarios. Global optimizations are not well equipped to handle the large number of parameters used to describe large- antenna arrays. To overcome this difficulty, a new class of arrays was recently introduced called polyfractal arrays that possess properties well suited for the optimization of large- arrays. Polyfractal arrays are uniformly excited with an underlying self-similar geometrical structure that leads to aperiodic element layouts. This paper expands on polyfractal array design methodologies by applying a robust Pareto optimization technique with the goal of reducing the peak sidelobe levels at several frequencies specified over a wide bandwidth. A recursive beamforming algorithm and an autopolyploidy based mutation native to polyfractal geometries are used to dramatically accelerate the genetic algorithm optimization process. This paper also demonstrates that the properties of polyfractal arrays can be exploited to create designs that possess no grating lobes and relatively low sidelobe levels over ultrawide bandwidths. The best example discussed in this paper maintains a dB peak sidelobe level with no grating lobes from a , to more than a minimum spacing between elements, which corresponds to at least a 40:1 bandwidth for the array.     

A novel dispersive FDTD formulation for modeling transient propagation in chiral metamaterials

Chiral media engineered for applications at microwave frequencies can be described as metamaterials composed of randomly oriented helices (with sizes typically less than a wavelength) embedded within an achiral background that is characterized by its permittivity and permeability. Chiral metamaterials embody properties of magnetoelectric coupling and polarization rotation. Chiral media are also highly dispersive and no effective full-wave time domain formulation has been available to simulate transient propagation through such an important class of metamaterials. A new finite-difference time-domain (FDTD) technique is introduced in this paper to model the interaction of an electromagnetic wave with isotropic dispersive chiral metamaterials, based on the implementation of a wavefield decomposition technique in conjunction with the piecewise-linear recursive convolution method. This formulation represents the first of its kind in the FDTD community. The FDTD model is validated by considering a one-dimensional example and comparing the simulations with available analytical results. Moreover, the FDTD technique is also used to investigate the propagation of electromagnetic waves through multilayered metamaterial slabs that include dispersive chiral and double-negative media. Hence, this model enables the investigation of complex dispersive metamaterials with magnetoelectric coupling and double-negative behavior as well as facilitates the exploitation of their unique properties for a variety of possible applications.     

Volatile Ultrafast Switching at Multilevel Nonvolatile States of Phase Change Material for Active Flexible Terahertz Metadevices

Phase change materials provide unique reconfigurable properties for photonic applications that mainly arise from their exotic characteristic to reversibly switch between the amorphous and crystalline nonvolatile phases. Optical pulse based reversible switching of nonvolatile phases is exploited in various nanophotonic devices. However, large area reversible switching is extremely challenging and has hindered its translation into a technologically significant terahertz spectral domain. Here, this limitation is circumvented by exploiting the semiconducting nature of germanium antimony telluride (GST) to achieve dynamic terahertz control at picosecond timescales. It is also shown that the ultrafast response can be actively altered by changing the crystallographic phase of GST.  The ease of fabrication of phase change materials allows for the realization of a variable ultrafast terahertz modulator on a flexible platform. The rich properties of phase change materials combined with the diverse functionalities of metamaterials and all-optical ultrafast control enables an ideal platform for design of efficient terahertz communication devices, terahertz neuromorphic photonics, and smart sensor systems.     

Ka波段平面Goubau线与超材料相结合的宽带滤波器

平面Goubau线(PGL)可作为单导体波导进行Ka波段表面电磁波传输。通过与超材料共面集成,可抑制PGL特定频点的表面波。在此基础上将超材料组成阵列形式,实现了宽带滤波特性。采用CST对所设计模型进行仿真,通过设置场监视器,重点研究了PGL与超材料的场分布特征与互作用结果。通过不断优化设计,最终获得了20%相对带宽的宽带滤波器。Ka波段平面器件的研究有助于微波毫米波器件小型化、平面化、集成化方向的发展。     

Synthesis of nonuniformly spaced arrays using a general nonlinear minimax optimisation method

Antenna patterns can be synthesized using a new nonlinear minimax optimization method with sure convergence properties. Not requiring derivatives, the proposed method is general and easy to use so that it might be applied to a wide variety of nonlinear synthesis problems for which analytical solutions are not known. To test the algorithm a group of test problems for which exact analytical solutions are known has been considered, namely, optimization of Dolph-Chebyshev arrays by spacing variation. The method is further applied to find the element positions in nonuniformly spaced linear arrays with uniform excitation that produce minimized (equal) sidelobe levels, and comparisons are made with conventional Dolph-Chebyshev arrays.