Manipulating DC currents with bilayer bulk natural materials

Achieving Environmentally-Adaptive and Multifunctional Hydrodynamic Metamaterials through Active Control

As hydrodynamic metamaterials continue to develop, the inherent limitations of passive-mode metamaterials become increasingly apparent. First, passive devices are typically designed for specific environments and lack the adaptability to environmental changes. Second, their unique functions often rely on intricate structures, or challenging material properties, or a combination of both. These limitations considerably hinder the potential applications of hydrodynamic metamaterials. In this study, an active-mode hydrodynamic metamaterial is theoretically proposed and experimentally demonstrated by incorporating source-and-sink flow-dipoles into the system, enabling active manipulation of the flow field with various functionalities. By adjusting the magnitude and direction of the flow-dipole moment, this device can easily achieve invisibility, flow shielding, and flow enhancing. Furthermore, it is environmentally adaptive and can maintain proper functions in different environments. It is anticipated that this design will significantly enhance tunability and adaptability of hydrodynamic metamaterials in complex and ever-changing environments.

Biphysical undetectable concentrators manipulating both heat flux and direct current via topology optimization

Recent remarkable developments in metamaterials and metadevices manipulating diffusive processes, such as thermal and electrical conduction, have enabled the control of multiple phenomena and the development of multifunctional devices. However, only either multiphysics operations or multiple functionalities are usually implemented on single metadevices. In this paper, we describe a method for the optimal design of metadevices that achieves both cloaking and focusing in the control of both heat flux and direct current by a single device, i.e., biphysical-bifunctional metadevices having four capabilities. Our design scheme performs well in terms of providing cloaking and focusing bifunctionality. Additionally, it assumes bulk natural materials without the use of metamaterials, which improves the manufacturability of the designed metadevices. Moreover, multidirectional metad evices are optimally designed for thermal-electrical conductions transmitted from multiple directions or from heat and voltage sources at various locations.

Mass Diffusion Metamaterials with "Plug and Switch" Modules for Ion Cloaking, Concentrating, and Selection: Design and Experiments

The outstanding abilities of metamaterials to manipulate physical fields are extensively studied in both wave-based and diffusion-based fields. However, mass diffusion metamaterials, with the ability to manipulate diffusion with practical applications associated with chemical and biochemical engineering, have not yet been experimentally demonstrated. In this work, ion cloaking, concentrating, and selection in liquid solvents are verified by both simulations and experiments, and the concept of a "plug and switch" metamaterial is proposed based on scattering cancellation (SC) to achieve switchable functions by plugging modularized functional units into a functional motherboard. Plugging in any module barely affects the environmental diffusion field, but the module choice impacts different diffusion behaviors in the central region. Cloaking strictly hinds ion diffusion, and concentrating increase diffusion flux, while cytomembrane-like ion selection permits the entrance of some ions but blocks others. In addition, these functions are demonstrated in special applications like the catalytic enhancement by the concentrator and the protein protection by the ion selector. This work not only experimentally demonstrates the effective manipulation of mass diffusion by metamaterials, but also shows that the "plug and switch" design is extensible and reconfigurable. It facilitates novel applications including sustained drug release, catalytic enhancement, bioinspired cytomembranes, etc.

Realizing the thinnest hydrodynamic cloak in porous medium flow

Transformation mapping theory offers us great versatility to design invisible cloaks for the physical fields whose propagation equations remain invariant under coordinate transformations. Such cloaks are typically designed as a multi-layer shell with anisotropic material properties, which makes no disturbance to the external field. As a result, an observer outside the cloak cannot detect the existence of this object from the field disturbances, leading to the invisible effect in terms of field prorogation. In fact, for many prorogating fields, at a large enough distance, the field distortion caused by an object is negligible anyway; thus, a thin cloak is desirable to achieve near-field invisibility. However, a thin cloak typically requires more challenging material properties, which are difficult to realize due to the huge variation of anisotropic material parameters in a thin cloak region. For a flow field in a porous medium, by applying the bilayer cloak design method and integrating the inner layer with the obstacle, we successfully reduce the anisotropic multi-layer cloak into an isotropic single-layer cloak. By properly tailoring the permeability of the porous medium, we realize the challenging material parameters required by the ultrathin cloak and build the thinnest shell-shaped cloak of all physical fields up to now. The ratio between the cloak's thickness and its shielding region is only 0.003. The design of such an ultrathin cloak may help to achieve the near-field invisibility and concealment of objects inside a fluid environment more effectively.

Design of an omnidirectional camouflage device with anisotropic confocal elliptic geometry in thermal-electric field

The designed confocal elliptical core-shell structure can realize the omnidirectional camouflage effect without disturbing temperature and electric potential profiles as the directions of heat flux and electric current change. Based on the anisotropy of the confocal ellipse, the anisotropic effective parameters of the confocal elliptical core-shell structure are derived under different heat flux and electric current launching. Then, the matrix material should be anisotropic as the effective parameters to satisfy the omnidirectional camouflage effect, which is demonstrated numerically. In addition, we present a composite structure to realize the anisotropic matrix. The experimental results show that the camouflage device embedded in the composite structure can eliminate the scattering caused by the elliptical core under different directions of heat flux and electric current, thus achieving the omnidirectional thermal-electric camouflage effect experimentally. The omnidirectional camouflage effect in thermal and electric fields can greatly widen the application fields of this device with anisotropic geometry.

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Omnidirectional camouflage device with anisotropic geometry is constructed

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Anisotropic matrix dominates the thermal-electric camouflage effect omnidirectionally

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A multilayered composite structure contributes to the experimental implementation

Metamaterials-Enabled Sensing for Human-Machine Interfacing

Our modern lives have been radically revolutionized by mechanical or electric machines that redefine and recreate the way we work, communicate, entertain, and travel. Whether being perceived or not, human-machine interfacing (HMI) technologies have been extensively employed in our daily lives, and only when the machines can sense the ambient through various signals, they can respond to human commands for finishing desired tasks. Metamaterials have offered a great platform to develop the sensing materials and devices from different disciplines with very high accuracy, thus enabling the great potential for HMI applications. For this regard, significant progresses have been achieved in the recent decade, but haven’t been reviewed systematically yet. In the Review, we introduce the working principle, state-of-the-art sensing metamaterials, and the corresponding enabled HMI applications. For practical HMI applications, four kinds of signals are usually used, i.e., light, heat, sound, and force, and therefore the progresses in these four aspects are discussed in particular. Finally, the future directions for the metamaterials-based HMI applications are outlined and discussed.

dc electric cloak concentrator via topology optimization

We succeeded in simultaneously cloaking and concentrating direct current in a conducting material through topology optimization based on a level-set method. To design structures that perform these functions simultaneously, optimal topology is explored for improving two objective functions that govern separately the cloaking and concentration of current. Our design scheme, i.e., the topology optimization of a direct-current electric cloak concentrator, provides this bifunctionality well despite simple, common bulk materials being used to make up the structures. The materials also rigorously obey the electric conduction equation in contrast to the approximated artificial materials, so-called metamaterials, of other design schemes. The structural features needed for this simultaneous bifunctionality are found by adopting level-set method to generate material domains and clear structural interfaces. Furthermore, robust performances of the bifunctional structures against fluctuations in electrical conductivity was achieved by improving the fitness incorporating multiple objective functions. Additionally, the influence of the size of the current-concentrating domain on the performances of the optimal configuration is investigated.

Reconfigurable sensor and nanoantenna by graphene-tuned Fano resonance

With the rapid developments in compact devices, the multi-function and reconfigurability of nanostructures are highly appreciated, while still very challenging. A majority of devices are usually mono-functional or hard to switch between different functions in one design. In this paper, we proposed graphene-wrapped core-shell nanowires to realize real-time reconfigurable sensors and nanoantenna by tuning the Fermi energies of graphene layers at the surfaces of core and shell, respectively. Owing to the electromagnetic coupling between the two graphene layer, two corresponding Fano resonances of scattering can arise in the Terahertz spectrum, which arises from the interference of bright modes and dark modes. Around the Fano resonances, the scattering can be considerably resonant (as an antenna) or suppressed (as a sensor). Interestingly, the field distributions are distinct at the suppressed scattering states for the two Fano resonances. The presented reconfigurable nanostructures may offer promising potentials for integrated and multi-functional electromagnetic control such as dynamic sensing and emission.

Electromagnetic time-harmonic and static field polygonal rotator with homogeneous materials

We propose a scheme of designing polygonal rotator with homogenous materials by using linear coordinate transformation. Our strategy is available for both time-harmonic electromagnetic field case and static field case. In particular, we found that only one anisotropic material is needed in static field case, and the density of field in the central region can be altered to be denser or sparser, or stay the same. The magnetostatic field rotator can be realized by multilayered structure composed of ferromagnetic materials and superconductor, and the direct current rotator can be realized by metals with different conductivity. Numerical results verify the effectiveness of our strategy in both time-harmonic field case and static field case.

Manipulating cell: flexibly manipulating thermal and DC fields in arbitrary domain

To extend the metamaterial applications on simultaneously regulating multiple fields with transformation optics, we propose a class of manipulative cell here to manipulate thermal and DC fields simultaneously in non-conformal angular schemes. Significant behaviors of thermal cloaking, electrical concentration, and related switched functions are numerically demonstrated with appropriate media. The findings not only present an efficient method for simultaneously manipulating various energy, but also break the limitation of structural profiles in the designs of bi-functional meta-device. Moreover, it may also provide references for efficient energy manipulation and management in conventional energy techniques.

Multi-Physics Bi-Functional Intelligent Meta-Device Based on the Shape Memory Alloys

Transformation theory, succeeding in multiple transportation systems, has enlightened researchers to manipulate the field distribution by tailoring the medium’s dominant parameters in certain situations. Therefore, the science community has witnessed a boom in designing metamaterials, whose abnormal properties are induced by artificial structures rather than the components’ characteristics. However, a majority of such meta-devices are restricted to the particular physical regimes and cannot sense the changes taking place in the surrounding environment and adjust its functions accordingly. In this article we propose a multi-physics bi-functional “intelligent” meta-device which can switch its functions between an invisible cloak and a concentrator in both thermal and DC electric conduction as the ambient temperature or voltage varies. The shape memory alloys are utilized in the design to form a moveable part, which plays the crucial role in the switching effect. This work paves the way for a practicable method for obtaining a controllable gradient of heat or electric potential, and also provides guidance for efficiently designing similar intelligent meta-devices by referring to the intriguing property of shape memory alloys.

DC carpet cloak designed by topology optimization based on covariance matrix adaptation evolution strategy

Optimal designs of direct current (DC) carpet cloaks are obtained using topology optimization based on the covariance matrix adaptation evolution strategy. The cloaking structures are expressed using an immersed boundary-level set method visualized as gray-scale-free configurations and composed simply of homogeneous materials. These cloaks successfully hide bumps made electrically invisible through topology optimization by minimizing the difference in voltage distributions around the cloaked bump and over the flat surface in the absence of the bump. Moreover, reproducing the electric potential field without a bump for DC flowing over a wide angle is achieved by the optimal cloak despite the presence of the bump on the flat surface.

Full-Parameter Omnidirectional Thermal Metadevices of Anisotropic Geometry

Since the advent of transformation optics and scattering cancelling technology, a plethora of unprecedented metamaterials, especially invisibility cloaks, have been successfully demonstrated in various communities, e.g., optics, acoustics, elastic mechanics, dc electric field, dc magnetic field, and thermotics. A long-held captivation is that transformation-optic metamaterials of anisotropic or noncentrosymmetric geometry (e.g., ellipsoids) commonly come along with parameter approximation/simplification or directional functions. Here, a synthetic paradigm with strictly full parameters and omnidirectionality is reported simultaneously to address this long-held issue for molding heat flow and experimentally demonstrate a series of noncentrosymmetric thermal metadevices. It changes the usual perception that transformation thermotic/dc/acoustic metamaterials are just a direct and simplified derivatives of the transformation-optic counterpart. Instead, the proposed methodology solves an intriguingly important and challenging problem that is not possibly achievable for transformation-optic metamaterials. The approach is rigorous, exact, robust, and yet elegantly facile, which may open a new avenue to manipulating the Laplacian and wave-dynamic fields in ways previously inconceivable.

Arbitrarily shaped thermal cloaks with non-uniform profiles in homogeneous media configurations

We propose a novel class of "complete" arbitrary thermal cloaks through rotatory linear maps. Different from the conventionally circular and arbitrary shape cloaks, as well as the unconventionally non-continuous shape cloaks, the proposed cloaking performances are observed in non-uniformly structural devices. Four schemes are demonstrated with homogeneous media configurations, and expected cloaking behaviors are exhibited in the internal regions. Further investigations reveal that the proposed devices perform robustness on the thermal profiles. The findings may also open up a novel avenue to generally achieve novel behaviors in the fields of optics, electromagnetics, and so forth.

Quantized chiral anomaly materials cloak

Chiral anomaly materials (CAM, e.g., axion insulator, topological insulator and some of Weyl semimetal) are new states of quantum matter. Anomalous Hall effect can occur in CAM, the anomalous Hall effect is closely related to the topological magneto-electric effect, i.e., when an electric field is applied to CAM, not only the electric field is induced, but also the magnetic field, vice versa. According to those properties, we design an electric cloak with quantized CAM and conductor, and a magnetic cloak with quantized CAM and superconductor. Simulation and calculation results show that the electric cloak can cloak applied electric field and induce magnetic field, and the magnetic cloak can cloak applied magnetic field and induce electric field. When applied electric field is generated by a point charge, the monopole can be obtained.

Highly efficient manipulation of Laplace fields in film system with structured bilayer composite

Using metamaterials or transformation optics to manipulate Laplace fields, such as magnetic, electric and thermal fields, has become a research highlight. These studies, however, are usually limited to a bulk material system and to single field manipulation. In this paper, we focus on a film system and propose a general practical method applicable for such a system. In this method, the background film is covered with another one to construct a so-called "bilayer composite" to achieve required physical parameters. On the basis of the bilayer composite, a multi-physics cloak and a multi-physics concentrator for electric current and thermal flux are designed, fabricated, and demonstrated. This work provides an efficient way to control and manipulate single/ multi-physics Laplace fields like a dc electric field and a thermal field in a film system, which may find potential applications in IC technology, MEMS, and so on.

Bifunctional metamaterials with simultaneous and independent manipulation of thermal and electric fields

Metamaterials offer a powerful way to manipulate a variety of physical fields ranging from wave fields (electromagnetic field, acoustic field, elastic wave, etc.), static fields (static magnetic field, static electric field) to diffusive fields (thermal field, diffusive mass). However, the relevant reports and studies are usually limited to a single physical field or functionality. In this study, we proposed and experimentally demonstrated a bifunctional metamaterial which could manipulate thermal and electric fields simultaneously and independently. Specifically, a composite with independently controllable thermal and electric conductivity was introduced, on the basis of which a bifunctional device capable of shielding thermal flux and concentrating electric current simultaneously was designed, fabricated and characterized. This work provides an encouraging example of metamaterials transcending their natural limitations, which offers a promising future in building a broad platform for the manipulation of multi-physics fields.

Quasistatic Metamaterials: Magnetic Coupling Enhancement by Effective Space Cancellation

A novel and broadly applicable way to increase magnetic coupling between distant circuits in the quasistatic regime is introduced. It is shown how the use of magnetic metamaterials enhances the magnetic coupling between emitting and receiving coils. Results are experimentally demonstrated by measuring a boost on the efficiency of the wireless transmission of power between distant circuits.

Invisible Sensors: Simultaneous Sensing and Camouflaging in Multiphysical Fields

The first multiphysical invisible sensor is theoretically and experimentally presented. An ultrathin, homogeneous, and isotropic shell is designed to simultaneously manipulate heat flux and DC current and eliminate the multiphysical perturbation, while maintaining the receiving and transmitting properties of the sensor.

Three-dimensional magnetic cloak working from d.c. to 250 kHz

Invisible cloaking is one of the major outcomes of the metamaterial research, but the practical potential, in particular for high frequencies (for example, microwave to visible light), is fatally challenged by the complex material properties they usually demand. On the other hand, it will be advantageous and also technologically instrumental to design cloaking devices for applications at low frequencies where electromagnetic components are favourably uncoupled. In this work, we vastly develop the bilayer approach to create a three-dimensional magnetic cloak able to work in both static and dynamic fields. Under the quasi-static approximation, we demonstrate a perfect magnetic cloaking device with a large frequency band from 0 to 250 kHz. The practical potential of our device is experimentally verified by using a commercial metal detector, which may lead us to having a real cloaking application where the dynamic magnetic field can be manipulated in desired ways.

The development of invisibility cloaks which function at low frequencies are of practical importance, especially for magnetic fields involved in modern technologies. Here, Zhu et al. develop the bilayer approach to create a three-dimensional magnetic cloak able to work in both static and dynamic fields.

Electrostatic Field Invisibility Cloak

The invisibility cloak has been drawing much attention due to its new concept for manipulating many physical fields, from oscillating wave fields (electromagnetic, acoustic and elastic) to static magnetic fields, dc electric fields, and diffusive fields. Here, an electrostatic field invisibility cloak has been theoretically investigated and experimentally demonstrated to perfectly hide two dimensional objects without disturbing their external electrostatic fields. The desired cloaking effect has been achieved via both cancelling technology and transformation optics (TO). This study demonstrates a novel way for manipulating electrostatic fields, which shows promise for a wide range of potential applications.

Simultaneously concentrated electric and thermal fields using fan-shaped structure

In recent years, considerable attention has been focused on transformation optics and metamaterial due to their fascinating properties and wide range of promising applications. Concentrator, one of the most well-known applications of transformation optics and metamaterial, is now limited only to a single physical domain. Here we propose and give the experimental demonstration of a bifunctional concentrator that can concentrate both electric and thermal fields to a given region simultaneously while keeping the external fields undistorted. Fan-shaped structure composed of alternating wedges made of two kinds of natural materials is proposed to achieve this goal. Numerical simulation and experimental results show good agreement, indicating the soundness and feasibility of our scheme.

Diffuse-light all-solid-state invisibility cloak

An ideal invisibility cloak makes arbitrary macroscopic objects within the cloak indistinguishable from its surrounding—for all directions, illumination patterns, polarizations, and colors of visible light. Recently, we have approached such an ideal cloak for the diffusive regime of light propagation using a core-shell geometry and a mixture of water and white wall paint as the surrounding. Here, we present an all-solid-state version based on polydimethylsiloxane doped with titania nanoparticles for the surrounding/shell and on a high-reflectivity microporous ceramic for the core. By virtue of reduced effects of absorption, especially from the core, the cloaking performance and the overall light throughput are improved significantly.

An Optically Controllable Transformation-dc Illusion Device

The concept of multifunctional transformation-dc devices is proposed and verified experimentally. The functions of dc metamaterials can be remotely altered by illuminating with visible light. If the light-induced dc illusion effect is activated, the electrostatic behavior of the original object is perceived as multiple equivalent objects with different pre-designed geometries. The experimental verification of the functional device makes it possible to control sophisticated transformation-dc devices with external light illumination.

Experimental demonstration of a multiphysics cloak: manipulating heat flux and electric current simultaneously

Invisible cloaks have been widely explored in many different physical systems but usually for a single phenomenon for one device. In this Letter we make an experimental attempt to show a multidisciplinary framework that has the capability to simultaneously respond to two different physical excitations according to predetermined scenarios. As a proof of concept, we implement an electric-thermal bifunctional device that can guide both electric current and heat flux "across" a strong 'scatterer' (air cavity) and restore their original diffusion directions as if nothing exists along the paths, thus rendering dual cloaking effects for objects placed inside the cavity. This bifunctional cloaking performance is also numerically verified for a line-source nonuniform excitation. Our results and the fabrication technique presented here will help broaden the current research scope for multiple disciplines and may pave a way to manipulate multiple flows and create new functional devices, e.g., for on-chip applications.