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Investigation and implementation of Active Willis Metamaterials (AWM) have been done exclusively, in all the available literature, by approaches that do not rely on any solid control theory basis. When coupled with piezoelectric control elements, the available approaches have not included, from the first principles, the exact form of the constitutive relationship of the piezoelectric materials. Furthermore, in all these approaches, stability analysis, robustness, ability to accommodate uncertainty or parameter changes, or consideration of disturbance rejection has not been addressed at all. More importantly, the available formulations have always mixed the flow and effort variables of the AWM, resulting in a form that is totally incompatible for the use in generating, investigating, or even designing any appropriate sensing or control applications of the material. In this paper, the piezoelectric-based AWM is modeled, from the first principles, to develop a constitutive coupling form that enables its use in actuation, sensing, and as an integrated controller that can be analyzed, designed, and optimized using the classical, optimal, and robust control system theories. Lagrange dynamics formulation is used to generate the equations governing the Willis coupling, the piezoelectric coupling, as well as the active robust controller. With this developed controlled-based structure of the AWM, the inherent and powerful capabilities of the AWM that lie in its ability to robustly control the material properties themselves such as the compliance (or stiffness) and specific volume (or density) are demonstrated in great detail via several numerical examples. Controlling these properties enables the AWM to be used in numerous important and imaginative applications such as cloaking, beam shifting, beam focusing, as well as many other applications that are limited only by our imagination. [ABSTRACT FROM AUTHOR]

Transformation elastodynamics can be used to redirect incident waves to mitigate harmful vibrations in the target region. Under the specific Brun–Guenneau–Movchan gauge, the elastic tensor of the transformed domain lacks minor symmetry. Here, we propose a physically realizable seismic cloak composed of lattice-based polar metamaterials capable of exhibiting asymmetric stresses. Both static and dynamic simulations validate the cloaking performance. A metric is calculated to quantify the protective effects, and results demonstrate satisfactory cloaking and aseismic protection across a wide frequency band. [ABSTRACT FROM AUTHOR]

Hiding an isolated object in free space using a transmissive invisibility cloak has become a significant research area, propelled by advancements in metamaterials and transformation optics over the past decade. Despite the availability of various simplified methods for implementing transmissive cloaks, issues such as impedance mismatches and narrow working bandwidths often arise, posing challenges. Achieving a broadband transmissive cloak in free space has proven to be particularly arduous. This study presents a near‐perfect one‐directional broadband transmissive cloak constructed from multilayer metasurfaces of arbitrary shapes, showcasing superior performance across a broadband frequency range. The phase distribution of the metasurfaces and the efficacy of the transmissive cloak are assessed using the generalized Snell's law. An experimental near‐perfect broadband transmissive cloak is successfully demonstrated to operate within the frequency range of 8.5 to 11.2 GHz. This study contributes to reducing the density and mass of cloaks, thereby facilitating the expansion of cloaking capabilities in various directions and across different frequency bands. [ABSTRACT FROM AUTHOR]

Thermal metamaterials are typically achieved by mixing different natural materials to realize effective thermal conductivities (ETCs) that conventional materials do not possess. However, the necessity for multifunctional design of metamaterials, encompassing both thermal and mechanical functionalities, is somewhat overlooked, resulting in the fixation of mechanical properties in thermal metamaterials designed within current research endeavors. Thus far, conventional methods have faced challenges in designing thermal metamaterials with configurable mechanical properties because of intricate inherent relationships among the structural configuration, thermal and mechanical properties in metamaterials. Here, a data‐driven approach is proposed to design a thermal metamaterial capable of seamlessly achieving thermal functionalities and harnessing the advantages of microstructural diversity to configure its mechanical properties. The designed metamaterial possesses thermal cloaking functionality while exhibiting exceptional mechanical properties, such as load‐bearing capacity, shearing strength, and tensile resistance, thereby affording mechanical protection for the thermal metadevice. The proposed approach can generate numerous distinct inverse design candidate topological functional cells (TFCs), designing thermal metamaterials with dramatic improvements in mechanical properties compared to traditional ones, which sets up a novel paradigm for discovering thermal metamaterials with extraordinary mechanical structures. Furthermore, this approach also paves the way for investigating thermal metamaterials with additional physical properties. [ABSTRACT FROM AUTHOR]

Owing to the architected void-filled low-density configurations, metamaterials are prone to defects during the complex manufacturing process, or damages under operational conditions. Recently mechanical cloaking has been proposed to shield the effect of such disorders in terms of homogenized mechanical responses. The major drawback in these studies are that the damage location should be known a priori, and the cloak is designed around that damaged zone before manufacturing. Such postulation does not allow unsupervised damage resilience during the manufacturing and service life of metamaterials by active reconfiguration of the stress field depending on the random and unpredictable evolution of damage. Here, we propose a radically different approach by introducing piezoelectric lattices where the effect of random appearance of any single or multiple disorders and damages with complex shapes, sizes and distributions can be shielded through active multi-physically controlled cloaks by voltage-dependent modulation of the stress fields within the cloaking region. Notably, this can be achieved without breaking periodicity and any additional material in the cloaking region unlike earlier studies concerning mechanical cloaks. The proposed active class of elastic metamaterials will bring a step-change in the on-demand mechanical performance of critically important structural components and unsupervised damage resilience for enhanced durability and sustainability. This article is part of the theme issue 'Current developments in elastic and acoustic metamaterials science (Part 1)'. [ABSTRACT FROM AUTHOR]

A four level chiral medium is considered to analyze and investigate theoretically the reflection/transmission coefficients of right circularly polarized (RCP) beam and left circularly polarized (LCP) beam as well as their corresponding GH-shifts under the effect of compton scattering. Density matrix formalism is used for calculation of electric and magnetic probe fields coherence. The polarization and magnetization are calculated from probes coherence terms in the chiral medium. The electric and magnetic susceptibilities as well as chiral coefficients are related with polarization and magnetization. The refractive indices of RCP and LCP beams under compton scattering effect is modified from the electric/magnetic susceptibilities, chiral coefficients, mass and charge of electron as well as compton scattering angle. The giant positive and negative birefringent Goos–Hänchen (GH) shifts in reflection and transmission beams are investigated in this manuscript under Compton scattering effect. The RCP and LCP beams obey the normalization condition | R (+ , -) | + | T (+ , -) | = 1 at the interface of a lossy chiral medium of | A (+ , -) | ≃ 0 and a thin sheet of balsa wood under the effect of compton scattering angle, incident angle, probe field detuning, control field Rabi frequency, phases of electric and magnetic fields and phase of superposition states. Significant positive/negative giant GH-shifts in reflection and transmission beams are investigated. The results show potential applications in modification of cloaking devices, image coding, polarizing filters and LCD displays. [ABSTRACT FROM AUTHOR]

Traditional optical transform devices are composed of materials with anisotropic magnetic design, which are difficult to manufacture. Based on the idea of bending waveguide devices, invisibility cloak Device Ⅰ and Ⅱ of isotropic materials are designed. According to the existing conformal coordinate transformation, we have derived the perfect parameters required for the conformal devices. Based on the Maxwell equations and finite edge element method, we give the effects of the designed Device Ⅰ and Ⅱ through perfect parameters. In addition, we use the layered idea to design conformal devices and give relevant numerical results to make their manufacturing possible. [ABSTRACT FROM AUTHOR]

• A multifunctional mechanical model with both amplified illusion and cloaking properties is proposed. • The amplified function can make a small object to produce the same displacement distribution as a large object. • Intelligent control system with the detective and control functions is introduced. • Effectively broaden the effective frequency range of structure in 3300Hz×4400 Hz and 1200Hz×3100 Hz. • The cloaking characteristics of the structure are verified through theoretical, numerical and experimental analysis. Illusion device can make an object to produce a field distribution different from its own, offering potential applications in diverse fields. In this work, we propose a multifunctional metamaterial with intelligent control systems, which exhibits amplified illusion and cloaking characteristics for detecting or hiding objects over different frequency ranges. The amplification function allows a small object to generate the same displacement distribution as a larger object, while the cloaking function renders an object invisible and undetectable. Simulation and experimental results demonstrate the exceptional amplification and cloaking properties of the multifunctional configuration. In contrast to single-function structures like cloaking and focusing, which can only fulfill a specific function, designing a multifunctional structure capable of achieving two or more properties within a single configuration presents a significant challenge. [ABSTRACT FROM AUTHOR]

A four-level atomic medium driven by probe and control fields is used to control and modify linear and nonlinear absorption and gain. The normal and anomalous dispersions as well as group index are also controlled by the strength and parameters of driving fields. The angular momentum l of the redefined Rabi frequencies of the control fields intensify the number of absorption and gain peaks according to 2 l 2 condition. Normal dispersion is observed in the region of gain while anomalous dispersion is noted in the region of absorption. The negative group index is studied in the anomalous dispersive region and the positive group index is associated with the normal dispersion region. The normal/anomalous dispersive region and positive/negative group index region also increased by the angular momentum l of the control fields according to 2 l 2 formula. A positive group index of 1000 is calculated, which reduces the group velocity to 3 × 10 5 m/s . An approximate negative group index of - 1000 is calculated, demonstrating superluminality of the group velocity at - 1 / 3 × 10 5 m/s . The modified results of the current study are useful for speeding up computer chips and time cloak technology. [ABSTRACT FROM AUTHOR]

Static and dynamic metamaterials have been extensively studied for their ability to manipulate different physical fields and directed to broad applications. Because the governing equations of heat transfer consist of nonlinear terms with conservation of mass, momentum and energy, the equations can exhibit elliptical, parabolic, hyperbolic and hybrid configurations under different transfer modes. Such multi-mode transfer characteristics intrinsically make thermal metamaterials distinguish themselves from other metamaterials with unique static and dynamic manipulation mechanisms. Therefore, numerous studies have emerged that use the transformation theory and other methods to control static thermal metamaterials. It leads to the development of thermal cloaks, thermal concentrators, thermal diodes, and so on. Originating from the static style, the manipulation of heat transfer has expanded to dynamic systems in recent years. The introduction of hydrodynamics in metamaterial design leads to numerous novel physics effects, such as dynamic cloaking, zero-drag characteristics, topological heat transfer, nonreciprocal diffusion, and non-Hermitian physics. Moreover, the dynamic thermal metamaterials allow accurate control at both time and space dimensions, leading to exciting applications such as adjustable, reconfigurable, and intelligent thermal meta-devices. However, few studies have systematically analyzed thermal metamaterials from the perspective of static and dynamic manipulation. In this review, we aim at clarifying the connection and distinction of static and dynamic manipulation from the scopes of principle, application, and physical effects. We start with the development of static thermal metamaterials and its application. Subsequently, the development of dynamic thermal metamaterials is presented both in fundamental theory and application. Finally, we summarize the research directions and prospect future research challenges for static and dynamic thermal metamaterials. [ABSTRACT FROM AUTHOR]

Reciprocal scatterers necessarily extinguish the same amount of incoming power when excited from opposite directions. This property implies that it is not possible to realize scatterers that are transparent when excited from one direction but that scatter and absorb light for the opposite excitation, limiting opportunities in the context of asymmetric imaging and nanophotonic circuits. This reciprocity constraint may be overcome with an external bias that breaks time-reversal symmetry, posing however challenges in terms of practical implementations and integration. Here, we explore the use of tailored nonlinearities combined with geometric asymmetries in suitably tailored resonant nanoantennas. We demonstrate that, under suitable design conditions, a nonlinear scatterer can be cloaked for one excitation direction, yet strongly scatters when excited at the same frequency and intensity from the opposite direction. This nonreciprocal scattering phenomenon opens opportunities for nonlinear nanophotonics, asymmetric imaging and visibility, all-optical signal processing and directional sensing. [ABSTRACT FROM AUTHOR]

Transformation optics (TO) provides a powerful tool to manipulate electromagnetic waves, enabling the design of invisibility cloaks, which can render objects invisible. Despite many years of research, however, invisibility cloaks experimentally realized thus far can only operate at a single frequency. The narrow bandwidth significantly restricts the practical applications of invisibility cloaks and other TO devices. Here, a general design strategy is proposed to realize a multiband anisotropic metamaterial characterized by two principal permittivity components, i.e., one infinite and the other spatially gradient. Through a proper transformation and combination of such metamaterials, an omnidirectional invisibility cloak is experimentally implemented, which is impedance‐matched to free space at multiple frequencies. Both far‐field numerical simulations and near‐field experimental mappings confirm that this cloak can successfully suppress scattering from multiple large‐scale objects simultaneously at 5 and 10 GHz. The design strategy and corresponding practical realization bring multiband transformation optical devices one step closer to reality. [ABSTRACT FROM AUTHOR]

When an electromagnetic wave is incident onto an object, the scattering from the object leads to its exposure. An invisibility cloak can bend the electromagnetic wave around the object with phase velocities exceeding the light speed in free space, which seems only possible over a narrow bandwidth, as manifested in the existing approaches, such as transformation optics and scattering cancellation. Here, it is experimentally demonstrated that this bandwidth limitation can be overcome by strategically positioning objects at topological nodes, characterized by minimal electromagnetic field amplitudes and indeterminate phases. This is accomplished using all‐dielectric photonic crystal slabs, which are engineered to exhibit a pair of topological nodes at fixed planes across an extensive bandwidth. This bandwidth expansion is facilitated not by resonance, but by the enforcement of mirror and time‐reversal symmetry. These findings thus introduce a novel topological paradigm for the broadband invisibility devices. [ABSTRACT FROM AUTHOR]

Recently, Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT) single crystals present tremendous potential in optical applications such as Q-switches and invisible robotic devices because they possess high electro-optic properties and perfect transparency simultaneously. However, due to the difficulties in testing and sample preparation, the nonlinear optical properties of PMN-PT single crystals have not been reported yet, which poses a bottleneck for exploring its nonlinear application. In this work, a large second-order nonlinear coefficient (d31 ∼ 77.9 pm V−1) of single domain PMN-0.39PT single crystal has been revealed, which is times times higher than the maximum nonlinear coefficient of commercial LiNbO3 single crystal (d33 = 25.4 pm V−1). The complete set of nonlinear coefficients of the sample has been determined by second harmonic generation microscopy. This work presents the promising nonlinear optical property in PMN-PT single crystals and opens a door for designing high-performance nonlinear optical devices. [ABSTRACT FROM AUTHOR]

Droplet encapsulations using liquid or solid shells are of significant interest in microreactors, drug delivery, crystallization, and cell growth applications. Despite progress in droplet-related technologies, tuning micron-scale shell thickness over a large range of droplet sizes is still a major challenge. In this work, we report capillary force assisted cloaking using hydrophobic colloidal particles and liquid-infused surfaces. The technique produces uniform solid and liquid shell encapsulations over a broad range (5–200 μm shell thickness for droplet volume spanning over four orders of magnitude). Tunable liquid encapsulation is shown to reduce the evaporation rate of droplets by up to 200 times with a wide tunability in lifetime (1.5 h to 12 days). Further, we propose using the technique for single crystals and cell/spheroid culture platforms. Stimuli-responsive solid shells show hermetic encapsulation with tunable strength and dissolution time. Moreover, scalability, and versatility of the technique is demonstrated for on-chip applications. Encapsulated liquids are important for several microreactor applications, including (bio)chemistry in confined spaces. Here, the authors report on oil-infused particle shells that allow control of shell thickness, stability, and permeability for applications in crystal growth and cell cultivation. [ABSTRACT FROM AUTHOR]

Skin diseases are a thorny and worrying matter, due to the special nature of the diseases and their similarity in the distinctive features of each disease. This makes doctors face difficulty in arriving at the correct diagnosis. There are two ways to determine the type of skin disease: the surgical method, which is a painful and expensive method and requires special preparations, and the second method is a non-invasive imaging the lesion at an acceptable magnification rate that preserves the important features of the disease and applies one of the diagnostic rules. The non-invasive skin scanning method, which makes it possible to detect microscopically small features that are invisible to the unaided eyes is optical imaging of skin disorders. For efficiently recognizing physical structure and Stages of development of skin disease and its response to therapy, non-invasive, and less painful scanning methods, such as confocal laser microscopy (CLM), multi-photon microscopy (MP), Raman microscopy (RM), photo-acoustic microscopy (PAM), optical coherence microscopy (OCT), and dermatoscopy (DMY), have been enhanced and implemented for skin scanning. Optical imaging is a type of skin surface imaging that offers precise visualization of skin features. The DMY is a non-intrusive human body tool for evaluating skin disorders. The device is portable and auto-illumination, which imaging skin structure that is often invisible to the naked eye. DMY produces digital images and records for re-examination for later use. There are 3-types of DMYs, traditional non-polarized light contact DMY (TNPC), polarized light contact DMY (PC), and polarized light contactless DMY (PCS). TNPC-DMY needs to touch the covered skin with an interfacing liquid. PC provides better penetration than TNPC and interfacing liquid is not required. OCT is a light-reflecting imaging system. It produces direct scanning with a resolution of around 10 µm plus a depth into the skin of up to 2*10−3 m, without needing to print or store. It has been utilized frequently in numerous disorders as well as skin scanning. The technology produces efficient visualization of changes in normal skin tissue. OCT results and histological structure show significant agreement in studies of skin tumors, and inflammatory disorders. OCT is available in a variety of designs, including time-variants (TV-OCT), Fourier-domain (FD-OCT), line field confocal (LC-OCT), spectral domain (SD-OCT), and high-resolution (HR-OCT). Therefore, this study, presents an overview of fundamental concept, design, for many various types of each technique, evaluation, and weaknesses according to practical tests and searches. This is very useful for researchers in the field of skin diseases detection, particularly in the field of automated diagnosis by artificial intelligence where the researcher is intimately familiar with the stages of development of diagnostic mechanisms and how traditional methods have contributed development in computer aided diagnosis. The two majors widely used optical imaging methods for skin are dermatoscopy and OCT, which are improving dermatoscopy diagnostic efficacy by five to thirty percent. The outcomes of this research are summarized in the next points, mentioned techniques rely on the kind of lesion being examined and the dermatologist's skill. Low resolution and a lack of optical sectioning capabilities are two drawbacks of optical imaging techniques, which lead to a restricted degree of diagnostic accuracy. [ABSTRACT FROM AUTHOR]

Transformation optics applied to low frequency magnetic systems have been recently implemented to design magnetic field concentrators and cloaks with superior performance. Although this achievement has been amply demonstrated theoretically and experimentally in bulk 3D macrostructures, the performance of these devices at low dimensions remains an open question. In this work, we numerically investigate the non-monotonic evolution of the gain of a magnetic metamaterial field concentrator as the axial dimension is progressively shrunk. In particular, we show that in planar structures, the role played by the diamagnetic components becomes negligible, whereas the paramagnetic elements increase their magnetic field channeling efficiency. This is further demonstrated experimentally by tracking the gain of superconductor-ferromagnet concentrators through the superconducting transition. Interestingly, for thicknesses where the diamagnetic petals play an important role in the concentration gain, they also help to reduce the stray field of the concentrator, thus limiting the perturbation of the external field (invisibility). Our findings establish a roadmap and set clear geometrical limits for designing low dimensional magnetic field concentrators. [ABSTRACT FROM AUTHOR]

Manipulating heat with thermal metamaterials has garnered increasing attention for enabling underlying physics and promising applications. However, the frequently adopted strategy to fabricate thermal metamaterials is using layered structures, whose design space is limited and, thus, other strategies demand further exploring. Here, we propose the holey engineering strategy as an alternative to design thermal metamaterials based on genetic algorithm optimization. The design procedures are introduced in detail, and two metadevices including the thermal cloak and thermal concentrator, are designed and verified to demonstrate the feasibility and convenience of this strategy. This work proposes a new design method for thermal metamaterials and paves an efficient way for macroscopic heat flow manipulation. [ABSTRACT FROM AUTHOR]

Left-handed materials are known to exhibit exotic properties in controlling electromagnetic fields, with direct applications in negative reflection and refraction, conformal optical mapping, and electromagnetic cloaking. While typical left-handed materials are constructed periodic metal-dielectric structures, the same effect can be obtained in composite guest–host systems with no periodicity or structural order. Such systems are typically described by the effective-medium approach, in which the components of the electric permittivity tensor are determined as a function of individual material properties and doping concentration. In this paper, we extend the discussion on the mixing rules to include left-handed composite systems and highlight the exotic properties arising from the effective-medium approach in this framework in terms of effective values and dispersion properties. [ABSTRACT FROM AUTHOR]

Non-visual auditory camouflage plays a major role in the art of underwater deception. In this work, a hybrid active/semi-active omnidirectional cloaking shell structure composed of alternate complementary piezoelectric and smart viscoelastic (PZT/SVE) actuator layers is proposed that can effectively conceal a three dimensional underwater macroscopic object from broadband incident sound waves. The smart hybrid structure incorporates a finite sequence of fully active parallel-connected multimorph PZT constraining layers inter-stacked with semi-active SVE core layers both of which are collaboratively operative in the framework of a Particle Swarm Optimized (PSO) multiple-input multiple-output active damping control (MIMO-ADC) scheme. The elasto-acoustic modeling of the problem is conducted by coupling the spatial state space methodology based on the classical three-dimensional exact piezoelasticity theory with the wave equations for the inner and outer acoustic domains. The acoustic cloaking performance of proposed configuration is evaluated for four distinct classes of highly functional SVE interlayer materials with tunable (field-dependent) rheological properties, namely, magnetorheological elastomer (MRE), shape memory polymer (SMP), electrorheological fluid (ERF), and magnetorheological shear thickening polishing fluid (MRSTPF). Extensive numerical results reveal significant broadband reductions of the far-field backscattering amplitude in the ( f ∞ θ = π , k ex R ex ) as well as the percentage error of external cloaked field (% Err) by incorporating a sufficient number of smart multimorph PZT/SVE material layers. Furthermore, it is concluded that comparable low frequency acoustic cloaking effects is possible without expenditure of any external energy just by employing the entirely inactive MRSTPF-based cloak as an alternative to the semiactive or fully active multimorph PZT/SVE cloaks. The outcome of proposed study can advantageously serve as the first step towards practical development and experimental implementation of future high performance smart acoustic cloaking devices with expanded broadband near-perfect omnidirectional invisibility for three dimensional objects of diverse geometries. [ABSTRACT FROM AUTHOR]

Metamaterials are artificial materials with properties depending mainly on their designed structures instead of their materials. Pentamode metamaterials are one type of metamaterial. They have solid structures with fluid-like properties, which can only withstand compressive stresses, not shear stresses. Two-dimensional pentamode metamaterials are easier to manufacture than three-dimensional models, so they have received wide attention. In this review, the properties, manufacturing, and applications of two-dimensional pentamode metamaterials will be discussed. Their water-like properties are their most important properties, and their velocities and anisotropy can be designed. They can be processed by wire-cut electrical discharge machining, waterjet cutting, and additive manufacturing techniques. They have a broad application prospect in acoustic fields such as acoustic stealth cloaks, acoustic waveguides, flat acoustic focusing lenses, pentamode acoustic meta-surfaces, etc. [ABSTRACT FROM AUTHOR]

To provide for an ever-increasing need for camouflage, experts in stealth have been envisioning for a few years to use metasurfaces and their unconventional electromagnetic properties to serve as efficient cloaking devices. To prepare for the use of surface-mounted electronic components, useful for frequency-selecting purposes, we take interest in an equivalent circuit analysis of metasurfaces. Indeed, metasurfaces using electronic components invite one to combine the passive behavior of the host structure with that of the lumped elements. The equivalent circuit analysis of the hosting surface deprived of electronic components has so far been dedicated to the case of metasurfaces constituted by rectangular or square unit cells. This work, therefore, aims at extending the former study on Cartesian unit cells to the coaxial geometry of interest. The validity of such a method is then assessed by comparison with the numerical simulation results obtained using a finite-element method. [ABSTRACT FROM AUTHOR]

In this work, we introduce fractal acoustic metamaterials (FAMs), in thicknesses ranging from 5 (λ/69) to 25 mm (λ/18), which are observed to provide multiple narrow-band low-frequency absorptions of acoustic signals. The fractal structures used in this work are carefully designed and fabricated using a side branch Helmholtz resonator design, making these structures easily tunable to multiple frequencies. Using different sizes of the side branches distributed in a fractally oriented configuration onto a plane rigid baseplate, the propagation velocity of acoustic waves is slowed down considerably. There is also a shifting resonating response of the structures toward lower frequencies (<1600 Hz). These FAM structures exhibit no dependence on the acoustic traverse length, as is otherwise commonly seen in coiled meta-structures and others. In order to achieve a near-perfect sound absorption behavior, the geometry of the structure is theoretically ascertained and validated numerically and experimentally. Significant emphasis has been placed on the associated physical mechanism modulating the loss of intensity of the incident acoustic signals. Moreover, with regression analysis performed on a response surface-based optimization scheme (using Design Expert 11 software), the geometric parameters are determined in a way that the absorption demonstrates a narrow-band characteristic at a frequency of 1 K Hz. We have shown in this work the tunability aspect of the various absorption frequency bands through appropriate designs of the FAM. It opens up wide application possibilities of multiple frequency sound absorptions (acoustic cloaking). [ABSTRACT FROM AUTHOR]

Numerous studies have been made to design absorption–diffusion–integrated metasurface (ADIM) for the application of electromagnetic stealth. However, it is still a challenge to achieve an outstanding stealth effect and a simple structure simultaneously. In this article, we propose two simple hybrid copper–graphene units with similar absorptivity and a reflected phase difference of pi, which are utilized to construct a novel ADIM by using a simulated annealing algorithm with a scattering pattern prediction module. A sample with an overall size of 200 × 200 mm2 is designed, fabricated, and measured. Both simulation and experimental results are in good agreement, which fully demonstrate the design method. The proposed ADIM with only a single functional layer exhibits more than 20 dB radar cross section reduction from 7.5 to 13.5 GHz maintained to 20° oblique incidence, which could address this challenge successfully. Power loss density and surface current distributions of the proposed two units are given to provide a physical insight of the mechanisms. The proposed ADIM can enable a wide application in stealth cloak, microwave imaging, wave front manipulation, and so on. [ABSTRACT FROM AUTHOR]

The thermal–electric bifunctional devices with isotropic and anisotropic geometries are designed in this paper, which can realize different control functions (invisible sensors and cloaks) of thermal and electric fields. We first theoretically achieve the bifunctions of invisible sensors and cloaks with isotropic and anisotropic geometries. When the material parameters of the core and matrix and the geometric parameters of the core, inner shell, and outer shell are determined, the thermal–electric invisible sensors could be realized. Meanwhile, the invisible sensors will degrade to invisible cloaks as the inner shell material is insulating. The simulations applying the derived material parameters have also been conducted to verify the thermal–electric bifunctions of different circular and confocal elliptical devices. The results show that the thermal–electric bifunctional devices can effectively control the thermal and electric fields, and these devices exhibit perfect performance of the thermal–electric functions of invisible sensors and cloaks. This work presents a new method to realize thermal–electric bifunctional devices and opens a new window to the development of multi-physics field. [ABSTRACT FROM AUTHOR]

Multilayer printed circuit boards (PCBs) can be produced not only in the traditional way but also additively. Both traditional and additive manufacturing can lead to invisible defects in the internal structure of the electronic component, eventually leading to the spontaneous failure of the device. No matter what kind of technology is used for the production of PCBs, when they are used in important structures, quality control is important to ensure the reliability of the component. The nondestructive testing (NDT) of the structure of manufactured electronic components can help ensure the quality of devices. Investigations of possible changes in the structure of the product can help identify the causes of defects. Different types of manufacturing technologies can lead to diverse types of possible defects. Therefore, employing several nondestructive inspection techniques could be preferable for the inspection of electronic components. In this article, we present a comparison of various NDT techniques for the evaluation of the quality of PCBs produced using traditional and additive manufacturing technologies. The methodology for investigating the internal structure of PCBs is based on several of the most reliable and widely used technologies, namely, acoustic microscopy, active thermography, and radiography. All of the technologies investigated have their advantages and disadvantages, so if high-reliability products are to be produced, it would be advantageous to carry out tests using multiple technologies in order to detect the various types of defects and determine their parameters. [ABSTRACT FROM AUTHOR]

Transformation optics (TO) provides an unprecedented technique to control electromagnetic (EM) waves by engineering the constitutive parameters of the surrounding medium through a proper spatial transformation. In general, ideal transformation optical devices require simultaneous electric and magnetic responses along all three dimensions. To ease the practical implementation, previous studies usually made use of reduced parameters or other simplified approaches, which inevitably introduce extra reflection or unwanted phase shift. Up to today, experimental realizations of full-parameter transformation optical devices in free space are still quite limited. Here, a general design strategy is proposed to solve this problem. As a specific example, a full-parameter spatial-compression TO medium with constitutive parameters taking the diagonal form diag(a, a , 1/ a) for the TM wave incidence was designed and realized experimentally. Such spatial-compression TO media were then applied to the implementation of an ideal omnidirectional invisibility cloak capable of concealing a large-scale object over a wide range of illumination angles. Both the simulation and experiment confirm that the cloak allows for nearly unity transmission of EM waves in the forward direction without introducing extra scattering or phase shift. This work constitutes an important stepping stone for future practical implementation of arbitrary full-parameter omnidirectional transformation optical devices. [ABSTRACT FROM AUTHOR]

This paper concerns the analysis of a passive, broadband approximate cloaking scheme for the Helmholtz equation in R d for d = 2 or d = 3. Using ideas from transformation optics, we construct an approximate cloak by "blowing up" a small ball of radius ϵ > 0 to one of radius 1. In the anisotropic cloaking layer resulting from the "blow-up" change of variables, we incorporate a Drude-Lorentz-type model for the index of refraction, and we assume that the cloaked object is a soft (perfectly conducting) obstacle. We first show that (for any fixed ϵ) there are no real transmission eigenvalues associated with the inhomogeneity representing the cloak, which implies that the cloaking devices we have created will not yield perfect cloaking at any frequency, even for a single incident time harmonic wave. Secondly, we establish estimates on the scattered field due to an arbitrary time harmonic incident wave. These estimates show that, as ϵ approaches 0, the L 2 -norm of the scattered field outside the cloak, and its far field pattern, approach 0 uniformly over any bounded band of frequencies. In other words: our scheme leads to broadband approximate cloaking for arbitrary incident time harmonic waves. [ABSTRACT FROM AUTHOR]

The conventional cloak has been studied in vast space while the open cloaking is explored in a few articles and there is a dire need for further investigations in this field. The open cloaks provide a way to exchange information data from the cloaked region to outside and vice versa. In this work, we have investigated the open cloaking phenomenon in acoustic fields at an audible frequency. This study will be helpful in developing a Multiphysics open cloaking platform and cloaked will enable to transfer or prohibition of the exchange of material from one region to another. [ABSTRACT FROM AUTHOR]

The persistent challenge of adhesion in Fused Filament Fabrication (FFF) technology is deeply rooted in the mechanical and chemical properties of utilized materials, necessitating the exploration of potential resolutions. This involves adjustments targeting the interplay of printing parameters, the mechanical fortification of print beds, and the integration of more adhesive materials, resonating across user levels, from enthusiasts to complex industrial configurations. An in-depth investigation is outlined in this paper, detailing the plan for a systematically designed device. Engineered for FFF device installation, the device facilitates the detachment of printed models, while precisely recording the detachment process, capturing the maximum force, and its progression over time. The primary objective is fabricating a comprehensive measurement apparatus, created for adhesion assessment. The device is adaptable across diverse FFF machines and print bed typologies, conforming to pre-defined conditions, with key features including compactness, facile manipulability, and capacity for recurrent measurements. This pursuit involves evaluating adhesion levels in prints made from diverse materials on varying print bed compositions, aiming to establish a comprehensive database. This repository facilitates judicious material and bed type selection, emphasizing maximal compatibility. Emphasis is placed on operating within a thermally stable context, a pivotal prerequisite for consistent and reproducible results. [ABSTRACT FROM AUTHOR]

Urban surveillance of slow‐moving small targets such as drones and birds in low to medium airspace using radar presents significant challenges. Detecting, locating and identifying such low observable targets in strong clutter requires both innovation in radar hardware design and optimisation of processing algorithms. To this end, the University of Birmingham (UoB) has set‐up a testbed of two L‐band staring radars to support performance benchmarking using datasets of target and clutter from realistic urban environment. This testbed is also providing the vehicle to understand how novel radar architectures can enhance radar capabilities. Some of the challenges in installing the radar at the UoB campus are highligted. Detailed benchmarking results are provided from urban monostatic and bistatic field trials that form the basis for performance comparison against future hardware modification. The solution to the challenge of interfacing the radar to the external oscillators is described and stand‐alone bench tests with the candidate oscillators are reported. The testbed provides a valuable capability to undertake detailed analysis of performance of Quantum photonic‐enabled radar and allows for its comparison with conventional oscillator technology for surveillance of low observable targets in the presence of urban clutter. [ABSTRACT FROM AUTHOR]

Liquid detection has been one of the buzzwords that refer to sensing of various liquids, which include water, chemicals, oils, etc. This detection gains more significance with the advent of metamaterials. Metamaterials are proved well suited in antenna designing, cloaking devices, solar cells, and sensors. Many papers reported metamaterial sensors, which can detect liquid chemicals, oils, etc. through shift in resonance frequency. Very few papers reported the applications of metamaterial sensors in water treatment. Water purification is one of the prime essentials for the existence of healthy humankind. It can be used to remove unwanted chemicals, contaminants and gases from water. The process of water purification can be done with various methods such as thermal, adsorption, distillation, desalination, reverse osmosis, etc. The main aim is to produce water required for specific purpose. In this context, role of metamaterials in water purification is of prime significance. Keeping this in view, an attempt is made to review the importance of metamaterial sensors in liquid detection along with water purification and its importance in comparison with earlier techniques already available. [ABSTRACT FROM AUTHOR]

Structural, optoelectronic, thermoelectric and magnetic properties of layered structured compounds EuFAgX (X = S, Se and Te) are investigated using density functional theory (DFT). Structural properties are in good agreement with the reported outcomes. Electronic properties shows that the understudy compounds are visible light active direct bandgap semiconductors. Electrical conductivity also reveals the semiconducting nature of these compounds. This study reveals that the substitution of Te by S causes reduction in band gape. The outcome demonstrates that EuFAgB (B = S, Se and Te) compounds are optically dynamic in the visible region and suitable for optoelectronic devices and are active aspirants for solar cell applications. Based on their thermoelectric characteristics, they are effective thermoelectric materials for thermoelectric power production for variable systems, nano-thermocouples and thermoelectric generation. Understudy compounds are A-type anti-ferromagnetic, as shown by magnetic susceptibility and magnetic ground state energy. Magnetic probes cannot read anti-ferromagnetic moments because of their anti-ferromagnetic structure, and data is unaffected by magnetic fields. Therefore, these compounds may be excellent candidates for magnetic cloaking devices. [ABSTRACT FROM AUTHOR]

The appearance of metasurfaces provides a new way for the realization of cloaking and illusion devices. However, most proposed metasurface functional devices only display a single cloaking and illusion function. Herein, a dynamic metasurface device with switchable electromagnetic cloaking and illusion functions is fabricated based on a 4D‐printing process. Owing to the shape memory polymer's characteristic of shape change above the glass transition temperature, under the stimulation of temperature, it can control the structural shape change of the metasurface, change the phase distribution law, and realize dynamic cloaking and illusion functions. As proof, the metasurface samples with changeable structure are designed and fabricated by fused deposition modeling of 4D printing combined with liquid metal injection molding. Both the simulation and experimental results confirm that the designed metasurface has excellent cloaking and illusion performance at microwave frequency. This work provides a new idea for the realization of integrated metasurface devices with electromagnetic cloaking and illusion functions. [ABSTRACT FROM AUTHOR]

DEXTER (detection of explosives and firearms to counter terrorism) is a project funded by NATO's Science for Peace and Security (SPS) program with the goal of developing an integrated system capable of remotely and accurately detecting explosives and firearms in public places without impeding the flow of pedestrians. While body scanner systems in secure areas of public places are becoming more and more efficient, the attack at Brussels airport on 22 March 2016, upstream of these systems, in the middle of the crowd of passengers, demonstrated the lack of discreet and real-time security against threats of mass terrorism. The NATO-SPS international and multi-year DEXTER project aims to provide new technical and strategic solutions to fill this gap. This project is based on multi-sensor coordination and fusion, from hyperspectral remote laser to smart glasses, artificial algorithms, and suspect identification and tracking. One of these sensors is dedicated to threat detection (large weapon or explosive belt) using the clothing of pedestrians by means of an active microwave component. This project is referred to as MIC (Microwave Imaging Curtain), also supported by the French SGDSN (General Secretariat of Defense and National Security), and utilizes a radar system capable of generating 3D images in real-time to address non-checkpoint detection of explosives and firearms. The project, led by ONERA (France), is based on a radar imaging system developed by the Fraunhofer FHR institute, using a MIMO architecture with an Ultra-Wide Band waveform. Although high-resolution 3D microwave imaging is already being used in expensive body scanners to detect firearms concealed under clothing, MIC's innovative approach lies in utilizing a high-resolution 3D imaging device that can detect larger dangerous objects carried by moving individuals at a longer range, in addition to providing discrete detection in pedestrian flow. Automatic detection and classification of these dangerous objects is carried out on 3D radar images using a deep-learning network. This paper will outline the project's objectives and constraints, as well as the design, architecture, and performance of the final system. Additionally, it will present real-time imaging results obtained during a live demonstration in a relevant environment. [ABSTRACT FROM AUTHOR]

This paper presents a mantle cloak structure to provide a wideband capability to hide a metallic cylinder over a broad span of angles of obliquely impinging plane waves. A bi-layered dielectric structure and a sheet of metasurface are used to enclose the object to be hidden. At first, the scattered field, including co- and cross-polarized components, is analytically formulated using circumferential waves for a multilayer wrap around the cylinder. A tensor description is also applied to model the impedance of the metasurface for TE and TM waves. Then, a metasurface made of rectangular patches is considered, and its related parameters are optimized to hide a perfect electric conductor (PEC) cylinder. Moreover, a double-layered mantle cloak is presented and numerically investigated using a software package to enhance its cloaking bandwidth. The numerical results show that the clocking bandwidth is enhanced by up to 33% compared to that of a single-layered structure. [ABSTRACT FROM AUTHOR]

Thanks to the pioneering studies conducted on the fields of transformation optics (TO) and metasurfaces, many unprecedented devices such as invisibility cloaks have been recently realized. However, each of these methods has some drawbacks limiting the applicability of the designed devices for real-life scenarios. For instance, TO studies lead to bulky coating layer with the thickness that is comparable to, or even larger than the dimension of the concealed object. In this paper, based on the coordinate transformation, an ultrathin carpet cloak is proposed to hide objects with arbitrary shape and size using a thin anisotropic material, called as infinitely anisotropic medium (IAM). It is shown that unlike the previous metasurface-based carpet cloaks, the proposed IAM hides objects from all viewing incident angles while it is extremely thin compared with the object dimensions. This material also circumvents the conventional transformation optics' complexities and could be easily implemented in practical scenarios. To demonstrate the capability of the proposed carpet cloak, several full-wave simulations are carried out. Finally, as a proof of concept, the IAM is implemented based on the effective medium theory which exhibits good agreement with the results obtained from the theoretical investigations. The introduced material not only constitutes a significant step towards the invisibility cloak but also can greatly promote the practical application of the other TO-based devices. [ABSTRACT FROM AUTHOR]

Optical invisibility, which started in the pages of fiction before becoming an intriguing quest of humankind for over a century, has blossomed into a remarkable scientific journey toward reality over the last two decades. Perfect optical cloaking requires the total scattering of electromagnetic waves around an object at all angles, all polarizations, over a wide frequency range, irrespective of the medium. Such a device is still far-fetched, requiring the transformation of space around a cloaked region such that the phase velocity is faster than other areas to preserve the phase relationships. However, by simplifying the invisibility requirements, pioneering work on spherical transformation cloaks, carpet cloaks, plasmonic cloaks, and mantle cloaks has been realized in narrowband microwave, infrared, and even optical wavelengths. In this Tutorial, we review the theoretical basis for invisibility cloaking, from spherical transformational optics to non-Euclidian cases, and discuss their limitations. Subsequently, we highlight the recent trends in realizing reconfigurable intelligent cloaks to overcome the traditional limitations of wideband operation and parallel efforts in unidirectional cloaking. Because the human eye is insensitive to the phase and polarization of visible light, a class of ray optics cloaking devices has been recently developed by eliminating phase preservation requirements. Notably, we focus on the recent progress achieved on invisibility cloaks that function in natural incoherent light and can be realized using standard optical components. We conclude this Tutorial with a prospective of potential applications and the practicality of optical cloaks in everyday life. [ABSTRACT FROM AUTHOR]

We propose that the propagation of acoustic waves in two-dimensional space can be controlled by the design of gradient embedded paths, which is inspired by a combination of metasurfaces and the inverse method of transformation optics. The phase variation along each gradient embedded path is smooth so that there is almost no reflection below a critical frequency. More importantly, the functionalities of devices designed by our approach are frequency-independent over a pretty broad band. In our work, gradient embedded paths are realized with gradient curved waveguides. Various potential applications such as acoustic directional emitter and focus lens without chromatic aberration are demonstrated. [ABSTRACT FROM AUTHOR]

This paper examines the feasibility of cloaking an obstacle using Plate-type Acoustic Metamaterials (PAMs). We present two distinct strategies to cloak this obstacle, using either the near-zero-density regime of a periodic arrangement of plates or the acoustic doping phenomenon to achieve simultaneous zero-phase propagation and impedance matching. The strong limitations induced by viscothermal and viscoelastic losses that cannot be avoided in such a system are studied. A hiding zone is reported analytically, numerically, and experimentally. In contrast to cloaking, where zero-phase propagation must be accompanied by total transmission and zero reflection, the hiding configuration requires that the scattering properties of the hiding device must not be affected by the presence of the obstacle embedded in it. Contrary to cloaking, the hiding phenomenon is achievable even with a realistic PAM possessing unavoidable losses. [ABSTRACT FROM AUTHOR]

Transparency and cloaking are two typical functions of thermal metamaterials that have attracted intensive research interest. However, two restrictions remain to be solved. The first one is that existing studies considered only one or two modes of heat transfer, which may not be consistent with practical conditions because conduction, radiation, and convection often coexist. The second one is that transparency and cloaking cannot be switched between at will. To solve these problems, we propose an effective medium theory to handle conductive, radiative, and convective (herein called omnithermal) processes simultaneously, which is based on the Fourier law, the Rosseland diffusion approximation, and the Darcy law. With the present theory, we further design an omnithermal metamaterial switchable between transparency and cloaking, which results from the nonlinear properties of radiation and convection. Finite-element simulations indicate that our scheme is robust under different boundary conditions. These results have potential applications such as in thermal camouflage, nonlinear thermotics, and intelligent thermotics. [ABSTRACT FROM AUTHOR]

We propose herein four types of anisotropic pentamode metamaterials (PMs) created by shifting the thin-end contention points along the space-diagonal, x-axis, y-axis, and z-axis directions. The phononic band structure and figure of merit (FOM) of the four types of anisotropic PMs are systematically investigated by using the finite-element method. Compared with isotropic PMs, the first phononic bandgaps appear in low-frequency regions, and the FOM increases by a factor of 6.9 when the thin-end connection area shifts along the z-axis direction. This study investigates how anisotropy affects PM properties and provides a reference to design acoustic cloaks, acoustic waveguides, and acoustic barriers. [ABSTRACT FROM AUTHOR]

The principle of detailed balance states that objects efficiently emitting radiation at a specific wavelength also efficiently absorb radiation at the same wavelength. This principle presents challenges for the design and performance of photonic devices, including solar cells, nanoantennas, and lasers. A design that successfully integrates the properties of an efficient emitter in one state and invisibility in another state is essential for various applications. In this work, we propose a novel nanolaser design based on a semiconductor nanoparticle with gain enveloped by a phase transition material that enables switching between lasing and cloaking (nonscattering) states at the same operating frequency without modifying the pumping conditions. We thoroughly investigate the operational characteristics of the nanolaser to ensure optimal performance. Our nanolaser design can function with both optical and electric pumping and exhibits the features of a thresholdless laser due to its high beta-factor and strong Purcell enhancement in the tightly confined Mie resonance mode. Additionally, we develop a reconfigurable metasurface comprising lasing-cloaking metaatoms capable of transitioning from lasing to a nonscattering state in a fully reversible manner. [ABSTRACT FROM AUTHOR]

Traditional transform optical devices are designed from anisotropic magnetic materials which are difficult to fabricate. In this paper, we design and simulate a conformal carpet invisible cloak and bending waveguide device with isotropic material. According to the existing conformal coordinate transformation, we deduce the perfect parameters required by the conformal device. And the finite element method and error analysis of the model equation and the basic process of adaptive algorithm are given. We first simulated a carpet cloak and bending waveguide device with perfect parameters; then we used the idea of parameter segmentation to design a carpet cloak that requires 35 isotropic materials and use the idea of layering to design the conformal bending waveguide device, and relevant numerical results are presented; finally, we designed the adaptive finite element algorithm to realize the numerical simulation of Maxwell's equations in isotropic material. [ABSTRACT FROM AUTHOR]

Smart materials respond to environmental signals by changing their microstructure and physical properties. Programming multiple behaviors and functions into a single material could increase its utility and adaptability to ever‐changing environmental conditions. A swellable and stretchable metamaterial hydrogel or "metasponge" engineered to morph into customized sizes and shapes that dynamically tune its physical properties and functions is reported. Multiple case studies that take advantage of the morphing properties of the metasponge, including robotic actuation, light guidance, optical and sonic invisibility ("cloaking"), adaptation of propulsion mode, sampling, and multiple biomedical applications, are illustrated. Developing multifunctional smart materials in which logic is programmed into the materials rather than electronic components could pave a new path to autonomy and dynamic responses in soft robots, sensors, and actuators. [ABSTRACT FROM AUTHOR]