Cloaking Device Research Articles

Nonlinearity based wave redirection for acoustic metamaterials

Advances in metamaterials have revealed novel opportunities for controlling wave propagation paths for various applications not realizable with conventional materials. Some prominent examples are schemes for electromagnetic and acoustic cloaking and focusing devices. In the classical approach to the formulations of these devices, one exploits a change of physical coordinates to achieve a desired wave behavior within a finite space. Such a change can be interpreted as a transformation of material properties when the field equations of interest are invariant to coordinate transformations. With regard to acoustics, this approach is constrained to fluid-like metamaterials amenable to the propagation of longitudinal waves only. Complications arise with solid materials because of their inherent ability to sustain both longitudinal and transverse waves, which refract differently in linear isotropic materials because of dissimilar propagation speeds. In this work, the authors explore wave redirection mechanisms that may take advantage of nonlinear wave propagation phenomena in a solid metamaterial. Starting from the classical nonlinear Murnaghan model, a hyper-elastic material is formulated to realize coupling between shear and compressional modes that could lead to a more suitable refractive behavior for acoustic wave redirection in a solid metamaterial. The formulated model is studied using analytical and numerical tools.

Evaluation and design of a large-scale cloaking device by the Hamiltonian-based ray-tracing method Part II: design of the distribution of constitutive parameters

Improvement in cloaking performance by the design of distributions of the constitutive parameters is investigated. The distributions are changed by employing Navier’s equation with various distributions of Young’s modulus as the partial differential equation for the solution of the Jacobian transformation matrix used in transformation optics. The cloaking performance is evaluated by Hamiltonian-based ray tracing. The numerical results show the proposed design method can improve the cloaking performance. The design method can contribute to the realization of large-scale cloaking devices by taking account of the finite resolution of the manufacture.

On regularized full- and partial-cloaks in acoustic scattering

ABSTRACTThe aim of this work is to derive sharp quantitative estimates of the qualitative convergence results developed by Li et al. (2015) for regularized full- and partial-cloaks through transformation optics approach. Let Γ0 be a compact set in ℝ3 and Γδ be a δ-neighborhood of Γ0 for δ∈ℝ+. Γδ represents the virtual domain used for the blow-up construction. By incorporating suitably designed lossy layers, it is shown that if the generating set Γ0 is a generic curve, then one would have an approximate full-cloak within δ2 to the perfect full-cloak, whereas if Γ0 is the closure of an open subset on a flat surface, then one would have an approximate partial-cloak within δ to its perfect counterpart. The estimates derived are independent of the contents being cloaked; that is, the cloaking devices are capable of nearly cloaking an arbitrary content. Furthermore, as a significant by-product, our argument allows the relaxation of the convexity requirement on Γ0 by Li et al. (2015), which is critical for the Mosco convergence argument therein.

Cloaking an Arbitrary Object via Anomalous Localized Resonance: The Cloak is Independent of the Object

In this paper, we present various schemes of cloaking an arbitrary object via anomalous localized resonance and provide their analysis in two and three dimensions. This is a way to cloak an object using negative index materials in which the cloaking device is independent of the object. As a result, we show that in the two dimensional quasi-static regime an annular plasmonic structure of coefficient $-1$ cloaks small but finite size objects nearby. We also discuss its connections with superlensing and cloaking using complementary media. In particular, we confirm the possibility that a lens can act like a cloak and conversely. This possibility was raised about a decade ago in the literature.

CMA-ES based topology optimization for acoustic cloaking devices

CMA-ES based topology optimization for acoustic cloaking devices

Theoretical analysis of the 2D thermal cloaking problem

Coefficient inverse problems for the model of heat scattering with variable coefficients arising when developing technologies of design of thermal cloaking devices are considered. By the optimization method, these problems are reduced to respective control problems. The material parameters (radial and azimuthal conductivities) of the inhomogeneous anisotropic medium, filling the thermal cloak, play the role of control. The model of heat scattering acts as a functional restriction. A unique solvability of direct heat 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 efficient numerical algorithm of solving thermal cloaking problems is proposed.

Full-angle negative reflection realized by a gradient acoustic metasurface

We theoretically demonstrate that full-angle negative reflection can be realized by the gradient acoustic metasurface with a specific surface phase gradient value. A straightforward physical picture is presented here to understand such anomalous phenomena by considering the influence of the non-local effect that originates from the supercell periodicity on the gradient metasurface. Basing on the generalized law of reflection which is modified by a reciprocal lattice vector term, the negative reflection that beyond the critical angle is possible. In this paper, we utilize the coiling-up space structures of deep subwavelength geometrical scale to construct the desired gradient acoustic metasurface and observe the apparent full-angle negative reflection phenomenon. The present work enriches the content of the generalized law of reflection and provide new design methodology for functional acoustic wave modulation devices, such like directional ground acoustic cloaking and acoustic isolation devices.

Kirchhoff's current law as local cloaking condition: theory and applications

Starting from the electric field integral equation (EFIE), it is shown how, for quasi-static cloaking (i.e. zero scattered fields), the EFIE can be reduced to classical Kirchhoff's current law (KCL). The KCL, if considered according to all-dielectric or metal-dielectric structures, is demonstrated to realise devices based on plasmonic or mantle cloaking, respectively. The KCL generalises scattering cancellation theory for arbitrary shape devices in homogeneous backgrounds and it can be extended as a local cloaking condition beyond quasi-static regime. A cloaking device can be seen as a node (i.e. no current source or sink) that ensures all current densities to be compensated even for more complicated low scattering devices.

Analysis of the 3D acoustic cloaking problems using optimization method

Control problems for the 3D model of acoustic scattering which describes scattering acoustic waves by a permeable obstacle with the form of a spherical layer are considered. These problems arise while developing the design technologies of acoustic cloaking devices using the wave flow method. The solvability of direct and control problems for the acoustic scattering model under study is proved. The sufficient conditions which provide local uniqueness and stability of optimal solutions are established.

Theoretical and Numerical Modeling of Acoustic Metamaterials for Aeroacoustic Applications

The advent, during the first decade of the 21st century, of the concept of acoustic metamaterial has disclosed an incredible potential of development for breakthrough technologies. Unfortunately, the extension of the same concepts to aeroacoustics has turned out to be not a trivial task, because of the different structure of the governing equations, characterized by the presence of the background aerodynamic convection. Some of the approaches recently introduced to circumvent the problem are biased by a fundamental assumption that makes the actual realization of devices extremely unlikely: the metamaterial should guarantee an adapted background aerodynamic convection in order to modify suitably the acoustic field and obtain the desired effect, thus implying the porosity of the cloaking device. In the present paper, we propose an interpretation of the metamaterial design that removes this unlikely assumption, focusing on the identification of an aerodynamically-impermeable metamaterial capable of reproducing the surface impedance profile required to achieve the desired scattering abatement. The attention is focused on a moving obstacle impinged by an acoustic perturbation induced by a co-moving source. The problem is written in a frame of reference rigidly connected to the moving object to couple the convective wave equation in the hosting medium with the inertially-anisotropic wave operator within the cloak. The problem is recast in an integral form and numerically solved through a boundary-field element method. The matching of the local wave vector is used to derive a convective design of the metamaterial applicable to the specific problem analyzed. Preliminary numerical results obtained under the simplifying assumption of a uniform aerodynamic flow reveal a considerable enhancement of the masking capability of the convected design. The numerical method developed shows a remarkable computational efficiency, completing a simulation of the entire field in a few minutes on mid-end workstations. The results are re-interpreted in term of boundary impedance, assuming a locally-reacting behavior of the outer boundary of the cloaking layer. The formulation is currently being extended to the analysis of arbitrarily complex external flows in order to remove the limitation of the background uniform stream in the host.

Hamiltonian-based ray-tracing method with triangular-mesh representation for a large-scale cloaking device with an arbitrary shape.

Hamiltonian-based ray-tracing technique with mesh representation is presented for designing large-scale cloaking devices with three-dimensional arbitrary shapes, which have inhomogeneity and anisotropy in their electric permittivity and magnetic permeability. In order to deal with arbitrary shapes, the surfaces of the cloaking devices are represented by triangular meshes. Comparison between the result of cloaking simulations with the mesh representation and those with the rigorous function representation is presented. The numerical results showed that fine-mesh resolution is required for accurate evaluation of cloaking performances.

Open active cloaking and illusion devices for the Laplace equation

We propose open active cloaking and illusion devices for the Laplace equation. Compared with the closed configurations of active cloaking and illusion devices, we focus on improving the distribution schemes for the controlled sources, which do not have to surround the protected object strictly. Instead, the controlled sources can be placed in several small discrete clusters, and produce the desired voltages along the controlled boundary, to actively hide or disguise the protected object. Numerical simulations are performed with satisfactory results, which are further validated by experimental measurements. The open cloaking and illusion devices have many advantages over the closed configurations in various potential applications.

A cloaking device for transiting planets

The transit method is presently the most successful planet discovery and characterization tool at our disposal. Other advanced civilizations would surely be aware of this technique and appreciate that their home planet's existence and habitability is essentially broadcast to all stars lying along their ecliptic plane. We suggest that advanced civilizations could cloak their presence, or deliberately broadcast it, through controlled laser emission. Such emission could distort the apparent shape of their transit light curves with relatively little energy, due to the collimated beam and relatively infrequent nature of transits. We estimate that humanity could cloak the Earth from Kepler-like broadband surveys using an optical monochromatic laser array emitting a peak power of about 30 MW for roughly 10 hours per year. A chromatic cloak, effective at all wavelengths, is more challenging requiring a large array of tunable lasers with a total power of approximately 250 MW. Alternatively, a civilization could cloak only the atmospheric signatures associated with biological activity on their world, such as oxygen, which is achievable with a peak laser power of just around 160 kW per transit. Finally, we suggest that the time of transit for optical SETI is analogous to the water-hole in radio SETI, providing a clear window in which observers may expect to communicate. Accordingly, we propose that a civilization may deliberately broadcast their technological capabilities by distorting their transit to an artificial shape, which serves as both a SETI beacon and a medium for data transmission. Such signatures could be readily searched in the archival data of transit surveys.

Exact analytical solution for fields in a lossy cylindrical structure with hyperbolic tangent gradient index metamaterials

The advent of metamaterials with tailorable frequency dispersion ensured the emergence of transformation optics with proposed applications that include cloaking devices, superconcentrators, superabsorbers, beam shapers and benders, field and polarization rotators, etc. A crucial property of interest for transformation optics is the possibility to custom design the spatial dependence of optical properties, i.e. to attain a gradient of effective refractive index. The cylindrical geometry has been one of the prototypal proposed forms and actually the first theoretical and experimental cloaking devices had radial symmetry. In this contribution we present an exact analytical approach to the solution of Helmholtz’ equations applied to radial propagation of electromagnetic waves through a cylinder containing negative and positive refractive index parts. A hyperbolic tangent gradient of refractive index is assumed. Our approach is inspired by the correspondence between transformation optics and quantum mechanics. A remarkably simple exact analytical solution is derived that takes into account realistic losses in negative-positive refractive index material composites.

A new NZRI metamaterial for electromagnetic cloaking operation

The paper reveals a near zero refractive index (NZRI) property based metamaterial unit cell for electromagnetic cloaking operation. The metamaterial unit cell displays resonance in the frequency of 8.29 GHz and exhibits near zero refractive index property there. The metamaterial was then utilized in designing a single layer rectangular cloaking device. The rectangular cloaking device hides a cylindrical metal object in the C-band region where almost zero scattering cross section is achieved for the object. The metamaterial also exhibits NZRI property there. The commercially available CST Microwave Studio simulation software, was utilized to obtain all the reflection and transmission parameters of the unit cell and the cloak. The experimental results were presented as well for both of the metamaterial and the cloak. It was also revealed that the cloak was object size independent in the certain region of C-band. This is a promising design in the electromagnetic paradigm for the rectangular and C-band cloaking operation.

進化的戦略を用いたカーペットクロークのトポロジー最適化

This paper presents a topology optimization for carpet cloaks by means of an evolutionary strategy. Carpet cloaks are the cloaking devices which can render an bump on a flat plane invisible by imitating the reflection from the flat plane. The objective function is defined as the integrated intensity of the difference between the reflected wave from flat plane and that from carpet cloaks covering the bumps. The structures of carpet cloaks made of dielectrics are expressed by level set functions and the structural boundaries between air and dielectrics are expressed by the iso-surfaces of the level set functions. Level set functions are optimized by CMA-ES which is the most powerful evolutionary algorithm for multimodal problem. Obtained optimal configurations realize good performance under the explicit perimeter constraint.

The Blow-up of Electromagnetic Fields in 3-Dimensional Invisibility Cloaking for Maxwell's Equations

Transformation optics constructions have allowed the design of cloaking devices that steer electromagnetic, acoustic, and quantum waves around a region without penetrating it, so that this region is hidden from external observations. The proposed material parameters are anisotropic, and singular at the interface between the cloaked region and the cloaking device. The presence of these singularities causes various mathematical problems and physical effects on the interface surface. In this paper, we analyze the 3-dimensional cloaking for Maxwell's equations when there are sources or sinks present inside the cloaked region. In particular, we consider nonsingular approximate invisibility cloaks based on the truncation of the singular transformations. We analyze the limit of solutions when the approximate cloaking approaches the ideal cloaking in the sense of distributions. We show that the solutions in the approximate cloaks converge to a distribution that contains Dirac's delta distribution supported on the interface surface. In particular, this implies that the limit of solutions are not measurable functions, making them outside of those classes of functions that have earlier been used in the models of the ideal invisibility cloaks. Also, we give a rigorous meaning for the “extraordinary surface voltage effect" considered in physical literature of invisibility cloaks.

Compact Broadband Terahertz Perfect Absorber Based on Multi-Interference and Diffraction Effects

High absorption over a wide frequency band has attracted considerable interest due to its potential applications in imaging and anti-radar cloaking devices. In order to make these devices more integrated, the thickness of the absorber is crucial. In this paper, a compact, polarization-independent, broadband, omnidirectional terahertz (THz) absorber is proposed, fabricated, and evaluated. This THz absorber is based on the combined effect of the multi-interference and diffraction. It is experimentally demonstrated that over 95% absorption can be obtained in the frequency range from 0.75 to 2.41 THz. This well-designed thinner absorber not only reduces the device thickness to 120 μm, but also introduces more interference peaks so that the absorption spectrum is significantly enlarged. Five successive absorption peaks at 0.88, 1.20, 1.53, 1.96, and 2.23 THz are combined into a broadband THz absorption spectrum.

Fast Adaptive Thermal Camouflage Based on Flexible VO₂/Graphene/CNT Thin Films.

Adaptive camouflage in thermal imaging, a form of cloaking technology capable of blending naturally into the surrounding environment, has been a great challenge in the past decades. Emissivity engineering for thermal camouflage is regarded as a more promising way compared to merely temperature controlling that has to dissipate a large amount of excessive heat. However, practical devices with an active modulation of emissivity have yet to be well explored. In this letter we demonstrate an active cloaking device capable of efficient thermal radiance control, which consists of a vanadium dioxide (VO2) layer, with a negative differential thermal emissivity, coated on a graphene/carbon nanotube (CNT) thin film. A slight joule heating drastically changes the emissivity of the device, achieving rapid switchable thermal camouflage with a low power consumption and excellent reliability. It is believed that this device will find wide applications not only in artificial systems for infrared camouflage or cloaking but also in energy-saving smart windows and thermo-optical modulators.

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.