Cloaking Device Research Articles

The FDTD simulation for the performance of dispersive cloak devices

In this paper, we defined the scattering energy intensity based on the Poynting vector to quantitatively study the cloak effect of electromagnetic waves in the time domain. The influences of the effective working frequency bands of four kinds of electromagnetic cloak materials, incidence angle of electromagnetic waves and the number of approximately cloak layers on the cloak effect are studied. To the best of our knowledge, this is the first time to use the time domain method to quantitatively study the effective working frequency band and the scattering energy intensity of cloak materials.

Inverse design method in acoustic wave manipulation

We present here the involvement of inverse design method with the acoustic wave modulation, to simplify how we get the required material parameters and design novel acoustic devices that can perform certain prescribed operation. As an example, the acoustic cloaking will be discussed. The existing acoustic invisibility design based on coordinate transformation method needs negative index media which can be realized through complicate structures. With inverse design method, we have designed a circular cloak device with positive index materials, that successfully make a circular scatterer acoustically invisible. This inverse design method takes a significant step towards realizing practical cloaking devices and would open up new possibilities for manipulating acoustic wavefront with simplified parameter distribution and optimized properties.

Theoretical Analysis of the Magnetic Cloaking Problem Based on an Optimization Method

Control problems are considered for a model of magnetic scattering on a permeable anisotropic obstacle shaped as a spherical layer. Such problems arise in developing technologies for designing magnetic cloaking devices when the corresponding inverse problems are solved by an optimization method. The solvability of the direct and extremal problems for the model in question is proved and the optimality system is derived. Its analysis permits obtaining sufficient conditions on the initial data which ensure the local uniqueness and stability of the optimal solutions.

Self-recording and manipulation of fast long-range hydrogen diffusion in quasifree magnesium

Understanding diffusion of large solutes such as hydrogen and lithium in solids is of paramount importance for energy storage in metal hydrides and advanced batteries. Due to its high gravimetric and volumetric densities, magnesium is a material of great potential for solid-state hydrogen storage. However, the slow hydrogen diffusion kinetics and the deleterious blocking effect in magnesium have hampered its practical applications. Here, we demonstrate fast lateral hydrogen diffusion in quasifree magnesium films without the blocking effect. Massive concomitant lattice expansion leads to the formation of remarkable self-organized finger patterns extending over tens of micrometers. Detailed visualization of diffusion fronts reveals that the fingers in these patterns follow locally the direction of hydrogen diffusion. Thus, the streamlines of the diffusion process are self-recorded by means of the finger pattern. By inclusion of fast hydrogen diffusion objects or local gaps, the resulting streamlines exhibit a clear analogy to optical rays in geometric optics. The possibility to spatially manipulate hydrogen diffusion opens an avenue to build advanced hydrogen storage systems, cloaking and active plasmonic devices, as well as prototype systems for computational models.

Optimization-based design of an elastostatic cloaking device

We present a new method for the design of devices to manipulate the displacement field in Elastic materials. It consists of solving a nonlinear optimization problem where the objective function defines the error in matching a desired displacement field, and the design variables determine the required material distribution within the device. In order to facilitate fabrication, the material at a given point of the device is chosen from a set of predefined materials, giving raise to a discrete optimization problem that is converted into a continuous one using the Discrete Material Optimization technique. The candidate materials maybe simple, isotropic materials, but the device made of them behaves as a whole as a metamaterial, enabling the manipulation of the displacement field in ways that are inconceivable in nature. As an example of application, a device for elastostatic cloaking or unfeelability is designed.

Full-field broadband invisibility through reversible wave frequency-spectrum control

In recent years, a variety of interesting concepts have been proposed to enable the concealment of objects from detection or observation, including invisibility cloaking. For an object to remain truly transparent to an illumination wave, a cloak must restore the exact spatio-temporal profile of the wave, including both amplitude and phase variations across the entire illumination frequency spectrum, i.e., the full field. However, on the basis of their fundamental operating principles, present invisibility solutions force different frequency components of a broadband illumination wave to experience different phase variations, necessarily distorting the wave’s temporal profile and making the cloaking device inherently visible. In this work, we propose a new conceptual approach to the problem, enabling the realization of full-field broadband invisibility, experimentally demonstrated here for the first time to the best of our knowledge. This involves a customized and reversible redistribution of the illumination frequency content, allowing the wave to propagate through the object of interest while preventing any interaction between the wave and the object. We report the experimental concealment of a broadband optical filter from detection with a phase-coherent light pulse of 500 GHz bandwidth, showing full restoration of the complex temporal and spectral profiles of the pulse.

Experimental Evidence of the Thermal Cloak Based on the Path Design of the Heat Flux

We recently showed theoretically that the infinite singularity of the thermal cloak designed by transformation thermodynamics could be eliminated by a new method—the path design of the heat flux without any approximation. In this paper, we present the experimental evidence of such a new strategy of thermal cloak, that is, a truly singularity-free thermal cloak. We fabricate such a transient thermal cloak device without using extreme material parameters. The experimental results show fully controlled, transient cloaking behavior, which are perfectly consistent with the theoretical derivations and simulated results. Since one can flexibly design the path of heat flux in the cloak, it has the large degree-of-freedom to construct thermal cloaks with the specific distributions of material parameters. The new method provides a new blue print for the transient thermal protection of a specific target.

A multidirectional cloak for visible light

A new macroscopic multidirectional cloak scheme for extraordinary rays is proposed by controlling the optical axes of uniaxial crystals. It eliminates the complicated material constraints and can also be utilized to design a cloaking device for ordinary rays or isotropic cloaks after simplification. Numerical ray tracing and full-wave simulation results validate our design. Moreover, if the uniaxial crystals are changed into other materials whose optical axes can be modulated, like liquid crystals, this scheme has the potential to fabricate direction-tunable cloaks.

Use of an Epsilon-Near-Zero Region Comprised of a Graphene Strip-Silica Stack for Designing Cloaking and Reflection Devices

Super cloaking and reflection functionalities are designed using alternating layers of a graphene strip (GS)-silica stack at $1.55~\mu \text{m}$ . First, a zero index material (ZIM) by utilizing the mentioned structure is achieved that according to the effective medium theory, it exhibits an effective permittivity which can be tuned through filling factor, the dielectric material, and Fermi energy of GSs. Afterward, the super cloaking and reflection functionalities for different polarizations are obtained in such a medium depending on the selection of boundary conditions and inclusions. Simulation results reveal that these functionalities exhibit an extremely broadband operation, and additionally, they are robust against the oblique incidence and inclusion shape that the latter feature cannot be achieved through a monolayer graphene. Furthermore, a monolayer graphene provides a moderate bandwidth for the cloaking functionality. Such functionalities using the ZIM provide a much better performance in comparison to other ZIMs made of thin film oxides. In addition, the ZIM property in thin film oxides is controlled chemically, and it is limited to a certain frequency range, whereas the ZIM feature in this paper can be tuned either electrically or chemically, and moreover, it can be set in different frequency ranges.

Carpet cloak for water waves.

Cloaking is a challenging topic in the field of wave motion, and is of significant theoretical value. In this article, a type of carpet cloak has been theoretically designed for water waves by using the effective medium and transformation theory. This carpet cloak device, created by a three-dimensional printer, is composed of a periodic structure which realizes the equivalent anisotropic water depth. We demonstrate its excellent cloaking performance numerically and experimentally in a wide range of frequencies and angles of incidence, with low wave attenuation characteristics and simple device realization of this carpet cloak illustrating that water wave transformation is a powerful method with which to manipulate water waves.

Extinction cross-section cancellation of a cylindrical radiating active source near a rigid corner and acoustic invisibility

Active cloaking in its basic form requires that the extinction cross-section (or energy efficiency) from a radiating body vanishes. In this analysis, this physical effect is demonstrated for an active cylindrically radiating acoustic source in a non-viscous fluid, undergoing periodic axisymmetric harmonic vibrations near a rigid corner (i.e., quarter-space). The rigorous multipole expansion method in cylindrical coordinates, the method of images, and the addition theorem of cylindrical wave functions are used to derive closed-form mathematical expressions for the radiating, amplification, and extinction cross-sections of the active source. Numerical computations are performed assuming monopole and dipole modal oscillations of the circular source. The results reveal some of the situations where the extinction energy efficiency factor of the active source vanishes depending on its size and location with respect to the rigid corner, thus, achieving total invisibility. Moreover, the extinction energy efficiency factor varies between positive or negative values. These effects also occur for higher-order modal oscillations of the active source. The results find potential applications in the development of acoustic cloaking devices and invisibility in underwater acoustics or other areas.

Acoustic cloak by inverse design and gradient based optimization

We present a gradient based minimization of the total scattering cross section (TSCS) from a set of cylindrical obstacles by incrementally repositioning them so that they eventually act as an effective cloaking device. The idea differs from earlier inverse designs that use topology optimization tools and generic algorithms. We use the optical theorem to define the position dependent TSCS in terms of the forward scattering amplitude for an incident plane wave. The gradient-based optimization algorithm reduces the TSCS by evaluating its derivative with respect to the cylinder positions and then perturbatively optimizing the position of each cylinder in the cloaking device while taking into account acoustic the multiple scattering between the cylinders. The method is illustrated by examples of sets of hard cylinders of uniform size.

Invisibility problem in acoustics, electromagnetism and heat transfer. Inverse design method

Two approaches (direct design and inverse design methods) for solving problems of designing devices providing invisibility of material bodies of detection using different physical fields – electromagnetic, acoustic and static are discussed. The second method is applied for solving problems of designing cloaking devices for the 3D stationary thermal scattering model. Based on this method the design problems under study are reduced to respective control problems. The material parameters (radial and tangential heat conductivities) of the inhomogeneous anisotropic medium filling the thermal cloak and the density of auxiliary heat sources play the role of controls. A unique solvability of direct thermal scattering problem in the Sobolev space is proved and the new estimates of solutions are established. Using these results, the solvability of control problem is proved and the optimality system is derived. Based on analysis of optimality system, the stability estimates of optimal solutions are established and numerical algorithms for solving particular thermal cloaking problem are proposed.

Extinction cross-section suppression and active acoustic invisibility cloaking

Invisibility in its canonical form requires rendering a zero extinction cross-section (or energy efficiency) from an active or a passive object. This work demonstrates the successful theoretical realization of this physical effect for an active cylindrically radiating acoustic body, undergoing periodic axisymmetric harmonic vibrations near a flat rigid boundary. Radiating, amplification and extinction cross-sections of the active source are defined. Assuming monopole and dipole modal oscillations of the circular source, conditions are found where the extinction energy efficiency factor of the active source vanishes, achieving total invisibility with minimal influence of the source size. It also takes positive or negative values, depending on its size and distance from the boundary. Moreover, the amplification energy efficiency factor is negative for the acoustically-active source. These effects also occur for higher-order modal oscillations of the active source. The results find potential applications in the development of acoustic cloaking devices and invisibility.

Electromagnetic interior transmission eigenvalue problem for inhomogeneous media containing obstacles and its applications to near cloaking

Abstract This article is concerned with the invisibility cloaking in electromagnetic wave scattering from a new perspective. We are especially interested in achieving the invisibility cloaking by completely regular and isotropic mediums. Our study is based on an interior transmission eigenvalue problem. We propose a cloaking scheme that takes a three-layer structure including a cloaked region, a lossy layer and a cloaking shell. The target medium in the cloaked region can be arbitrary but regular, whereas the mediums in the lossy layer and the cloaking shell are both regular and isotropic. We establish that there exists an infinite set of incident waves such that the cloaking device is nearly invisible under the corresponding wave interrogation. The set of waves is generated from the Maxwell–Herglotz approximation of the associated interior transmission eigenfunctions. We provide the mathematical design of the cloaking device and sharply quantify the cloaking performance.

Bounds on Herglotz functions and fundamental limits of broadband passive quasistatic cloaking

Using a sum rule, we derive new bounds on Herglotz functions that generalize those given in Bernland et al. [J. Phys. A: Math. Theor. 44(14), 145205 (2011)] and Gustafsson and Sjöberg [New J. Phys. 12(4), 043046 (2010)]. These bounds apply to a wide class of linear passive systems such as electromagnetic passive materials. Among these bounds, we describe the optimal ones and also discuss their meaning in various physical situations like in the case of a transparency window, where we exhibit sharp bounds. Then, we apply these bounds in the context of broadband passive cloaking in the quasistatic regime to refute the following challenging question: is it possible to construct a passive cloaking device that cloaks an object over a whole frequency band? Our rigorous approach, although limited to quasistatics, gives quantitative limitations on the cloaking effect over a finite frequency range by providing inequalities on the polarizability tensor associated with the cloaking device. We emphasize that our results hold for a cloak or object of any geometrical shape.

High-transmission acoustic self-focusing and directional cloaking in a graded perforated metal slab

A design strategy and its modeling for high-transmission acoustic self-focusing and directional cloaking in a two-dimensional (2D) and an axisymmetric three-dimensional (3D) gradient-index phononic crystal (GRIN-PC) are reported in this paper. A gradient perforated aluminum slab sandwiched by water is considered. A low-loss directional cloaking device is achieved by controlling the matching coefficient between the slab and the water. The anisotropy coefficient that affects the scattering properties is also introduced. Furthermore, the phase discontinuity for directional cloaking inside and outside the slab is overcome by introducing a non-gradient slab having a lower group velocity behind the GRIN slab as an acoustic delay device. In addition, an axisymmetric 3D directional cloaking structure is obtained by rotating the corresponding 2D structure around the slab axis.

Computational material design for acoustic cloaking

A topology optimization technique based on the topological derivative and the level set function is utilized to design/synthesize the micro-structure of a pentamode material for an acoustic cloaking device. The technique provides a micro-structure consisting of a honeycomb lattice composed of needle-like and joint members. The resulting metamaterial shows a highly anisotropic elastic response with effective properties displaying a ratio between bulk and shear moduli of almost 3 orders of magnitude. Furthermore, in accordance with previous works in the literature, it can be asserted that this kind of micro-structure can be realistically fabricated. The adoption of a topology optimization technique as a tool for the inverse design of metamaterials with applications to acoustic cloaking problems is one contribution of this paper. However, the most important achievement refers to the analysis and discussion revealing the key role of the external shape of the prescribed domain where the optimization problem is posed. The efficiency of the designed micro-structure is measured by comparing the scattering wave fields generated by acoustic plane waves impinging on bare and cloaked bodies.

On isotropic cloaking and interior transmission eigenvalue problems

This paper is concerned with the invisibility cloaking in acoustic wave scattering from a new perspective. We are especially interested in achieving the invisibility cloaking by completely regular and isotropic mediums. It is shown that an interior transmission eigenvalue problem arises in our study, which is the one considered theoretically in Cakoni et al. (Transmission eigenvalues for inhomogeneous media containing obstacles, Inverse Problems and Imaging, 6 (2012), 373–398). Based on such an observation, we propose a cloaking scheme that takes a three-layer structure including a cloaked region, a lossy layer and a cloaking shell. The target medium in the cloaked region can be arbitrary but regular, whereas the mediums in the lossy layer and the cloaking shell are both regular and isotropic. We establish that if a certain non-transparency condition is satisfied, then there exists an infinite set of incident waves such that the cloaking device is nearly invisible under the corresponding wave interrogation. The set of waves is generated from the Herglotz approximation of the associated interior transmission eigenfunctions. We provide both theoretical and numerical justifications.

Evaluation and design of a large-scale cloaking device by the Hamiltonian-based ray-tracing method Part I: full-mesh representation

A Hamiltonian-based ray tracing technique is applied to the evaluation of the performance of cloaking devices with arbitrary shapes based on full-mesh representation. The constitutive parameters of cloaking devices are calculated by the finite element method. The cloaking characteristics of a double cylindrical cloaking device and a huge cloaking device with a completely arbitrary shape are evaluated. The numerical results showed that fine mesh structure is required for better cloaking performance. The full-mesh representation is useful for fabrication design.