Passive Cloaking Research Articles

An optimal control strategy to design passive thermal cloaks of arbitrary shape

In this paper, we describe a numerical framework for achieving passive thermal cloaking of arbitrary shapes in both static and transient regimes. The design strategy is cast as the solution of an optimal control problem (OCP) for the heat equation where the coefficients of the thermal diffusivity matrix take the role of control functions, and the distance between the uncloaked and the cloaked field is minimized in a suitable observation domain. The control actions enter bilinearly in the heat equation, thus making the resulting OCP nonlinear, and its analysis non-trivial. We show that optimal diffusivity coefficients exist both for the static and the transient case; we derive a system of first-order necessary optimality conditions; finally, we carry out their numerical approximation using the finite-element method. A series of numerical test cases assess the capability of our strategy to tackle passive thermal cloaking of arbitrarily complex two-dimensional objects.

Acoustic cloaking via a hybrid passive–active technique

Both passive and active methods have been employed extensively in recent times to create the effect of acoustic cloaking, with obstacles being made near-invisible over specific frequency ranges. Passive methods typically rely on metamaterials with complex microstructures, whereas active methods require an external power supply. Furthermore, a large number of active sources and sensors are typically required for active control. In this work, a hybrid method that combines passive and active approaches is developed. The passive acoustic cloak is designed theoretically via layered shells with alternating, functionally graded properties. The layer properties are informed by coupling transformation acoustics with homogenization theory and require unphysical values on the inner cloak wall. As such, some scattering occurs from the obstacle to be cloaked as well as due to the discontinuity in material properties across the shell boundaries. Using distributed monopole control sources, active control is then employed to nullify the non-zero scattered field outside the passive cloak domain. Effective cloaking is achieved using the hybrid passive–active control system.

Mechanism and performance evaluation of a thermoelectric-chip-based cloak for active regulation of heat flow

In the field of heat manipulation, active thermal cloaks based on thermoelectric chips (TECs) have received much less attention than passive thermal cloaks based on metamaterials. This work focuses on active thermal cloaks and explores in-depth their mechanism of heat flow regulation. For a two-dimensional circular object located in a rectangular background plate with uniform temperature gradient, the theoretical design of an active cloak was realized by analytically solving for the target temperature field. A parameter study was conducted numerically for the designed thermal cloak. The results show that the object-to-background thermal conductivity ratio (m) is the key parameter that determines the working mode of the cloak. When m < 1, the TECs work in a heating mode with less power consumption; when m > 1, the TECs work in a chilling/heat pump mode with higher power consumption. If the change of a certain parameter increases the heat flow regulation, it will also cause performance degradation and an increase of the thermal cloak’s power consumption, especially when m is greater than 1. Improvement can be achieved by increasing the number of TECs (N), but the impact of N diminishes rapidly beyond a certain value. The systematic study in this work reveals the high flexibility and wide adaptability of the TE-chip-based thermal cloak in heat flow regulation.

Microwave Metasurface Cloaking for Freestanding Objects

Since the introduction of transformation optics and experimental demonstration using microwave metamaterials in 2006, a variety of electromagnetic invisibility cloaking techniques and physical realizations based on coordinate transformations, scattering cancellation, and control of reflected and transmitted waves have been reported from microwaves to visible. However, existing cloaking methodologies face challenges in reducing cloak thicknesses, concealing large volumes, and cloaking free-standing objects. Here, we design, fabricate, and experimentally validate a unidirectional, single-layer printed metasurface cloak for free-standing cylindrical objects in the microwave regime. Based on a spatially modulated reactance surface, the printed metasurface converts an incident plane wave on the lit side into a surface wave, which carries power to the shadow side before reconstructing the incident wave behind the object. Using a subwavelength-thin printed circuit prototype, the cloaking function is experimentally confirmed, demonstrating a thin, passive cloak that conceals a large, free-standing object.

Acoustic passive cloaking using thin outer resonators

We consider the propagation of acoustic waves in a 2D waveguide unbounded in one direction and containing a compact obstacle. The wavenumber is fixed so that only one mode can propagate. The goal of this work is to propose a method to cloak the obstacle. More precisely, we add to the geometry thin outer resonators of width $\varepsilon$ and we explain how to choose their positions as well as their lengths to get a transmission coefficient approximately equal to one as if there were no obstacle. In the process we also investigate several related problems. In particular, we explain how to get zero transmission and how to design phase shifters. The approach is based on asymptotic analysis in presence of thin resonators. An essential point is that we work around resonance lengths of the resonators. This allows us to obtain effects of order one with geometrical perturbations of width $\varepsilon$. Various numerical experiments illustrate the theory.

Fundamental bounds on the performance of monochromatic passive cloaks.

Fundamental bounds on the performance of monochromatic scattering-cancellation and field-zeroing cloaks made of prescribed linear passive materials occupying a predefined design region are formulated by projecting field quantities onto a sub-sectional basis and applying quadratically constrained quadratic programming. Formulations are numerically tested revealing key physical trends as well as advantages and disadvantages between the two classes of cloaks. Results show that the use of low-loss materials with high dielectric contrast affords the highest potential for effective cloaking.

Enhanced passive thermal stealth properties of VO2 thin films via gradient W doping

Thermal stealth and camouflage have been intensively studied for blending objects with their surroundings against remote thermal image detection. Adaptive control of infrared emissivity has been explored extensively as a promising way of thermal stealth, but it still requires an additional feedback control. Passive modulation of emissivity, however, has been remained as a great challenge which requires a precise engineering of emissivity over wide temperature range. Here, we report a drastic improvement of passive camouflage thin films capable of concealing thermal objects at near room temperature without any feedback control, which consists of a vanadium dioxide (VO2) layer with gradient tungsten (W) concentration. The gradient W-doping widens the metal-insulator transition width, accomplishing self-adaptive thermal stealth with a smooth change of emissivity. Our simple approach, applicable to other similar thermal camouflage materials for improving their passive cloaking, will find wide applications, such as passive thermal camouflage, urban energy-saving smart windows, and improved infrared sensors.

Deep Learning for Robotic Mass Transport Cloaking

We consider the problem of mass transport cloaking using mobile robots. The robots move along a predefined curve that encloses a safe zone and carry sources that collectively counteract a chemical agent released in the environment. The goal is to steer the mass flux around a desired region so that it remains unaffected by the external concentration. We formulate the problem of controlling the robot positions and release rates as a PDE-constrained optimization, where the propagation of the chemical is modeled by the advection-diffusion (AD) PDE. We use a neural network (NN) to approximate the solution of the PDE. Particularly, we propose a novel loss function for the NN that utilizes the variational form of the AD-PDE and allows us to reformulate the planning problem as an unsupervised model-based learning problem. Our loss function is discretization-free and highly parallelizable. Unlike passive cloaking methods that use metamaterials to steer the mass flux, our method is the first to use mobile robots to actively control the concentration levels and create safe zones independent of environmental conditions. We demonstrate the performance of our method in simulations.

Polar Metamaterials: A New Outlook on Resonance for Cloaking Applications.

Rotationally resonant metamaterials are leveraged to answer a longstanding question regarding the existence of transformation-invariant elastic materials and the adhoc possibility of transformation-based passive cloaking in full plane elastodynamics. Combined with tailored lattice geometries, rotational resonance is found to induce a polar and chiral behavior, that is, a behavior lacking stress and mirror symmetries, respectively. The central, and simple, idea is that a population of rotating resonators can exert a density of body torques strong enough to modify the balance of angular momentum on which hang these symmetries. The obtained polar metamaterials are used as building blocks of a cloaking device. Numerical tests show satisfactory cloaking performance under pressure and shear probing waves, further coupled through a free boundary. The work sheds new light on the phenomenon of resonance in metamaterials and should help put transformation elastodynamics on equal footing with transformation acoustics and optics.

Active Scattering-Cancellation Cloaking: Broadband Invisibility and Stability Constraints

While great progress has been made in the theory and realization of invisibility devices, the issue of narrow-bandwidth operation remains unsolved. In particular, physical bounds have been recently derived, based on the Bode–Fano theory of broadband impedance matching, that quantify the inherent difficulty in broadening the bandwidth of any linear, causal, and passive cloak. If we relax the passivity restriction, introducing active media into the system, it is then relevant to ask the following questions: 1) Can the Bode–Fano limit on broadband invisibility be surpassed, at least theoretically, and to what extent? 2) Can we guarantee the stability of a broadband active cloak? In this article, we discuss some steps toward answering these general questions. We first offer an intuitive discussion of why a passive cloak cannot be arbitrarily broadband. Then, we use a 1-D example to show, for the first time, that active cloaks have indeed the potential to operate beyond the Bode–Fano bandwidth limit. Finally, we design a simple 3-D active cloak that exhibits a much broader bandwidth than passive scattering-cancellation cloaks. All the designed active cloaks are stable, and we discuss some relevant conditions to ensure stability. Our results may pave the way for ultrabroadband invisibility devices.

Active Microwave Cloaking Using Parity-Time-Symmetric Satellites

Electromagnetic cloaking is important in various microwave applications, such as stealth technology, interference reduction in multiantenna communication systems, and efficient energy harvesting. Fundamental limitations, though, hinder the operation bandwidth of passive cloaks as the size of the hidden object increases. Inspired by the concept of parity-time symmetry, researchers propose a realistic cloaking scheme based on $a\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}e$ circuitry, which enables a practically realizable, broadband microwave cloak that can satisfy the requirements of real-life applications.

Multipolar passive cloaking by nonradiating anapole excitation

In this paper, we demonstrate the relation between cloaking effect and its nonradiating state by considering the destructive multipolar interaction between near-field scattering by bare object and surrounding coating located in its proximity. This cloaking effect is underpinned by anapole mode excitation and it occurs as destructive interference between the electric dipole moment, generated by a bare object (here a central metallic scatterer) and the toroidal moment, formed inside the cloak (a surrounding cluster of dielectric cylinders). Numerical results show how a cloaking effect based on the formation of the anapole mode can lead to an overall nonradiating metamolecule with all-dielectric materials in the coating region.

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.

Invisibility exposed: physical bounds on passive cloaking

Invisibility cloaks have been one of the major breakthroughs in the field of metamaterials, and several techniques are currently available to suppress the electromagnetic scattering from different objects. So far, however, theoretical and experimental results have consistently shown fundamental challenges in terms of the bandwidth and size of the object to be hidden. Understanding the bandwidth limitations of cloaking therefore becomes important to assess the applicability and potential of cloaking devices in practical scenarios. While it is generally accepted that cloaking is difficult to achieve, no work to date has quantified this issue for general invisibility devices. Here, by applying the Bode–Fano theory of broadband matching, we derive fundamental bounds on bandwidth and performance for general passive cloaking schemes applied to planar objects, determined by the properties of the object to be hidden, and then explore their implications in three-dimensional scenarios. Our results define a general framework to estimate the ultimate performance of passive cloaks and suggest that fundamentally new directions, involving nonlinearities and active metamaterials, become necessary to realize broadband cloaking, opening a new phase in the quest for invisibility.

Invisibility and Cloaking: Origins, Present, and Future Perspectives

Spies and wizards, take note: The authors provide an overview of the recent efforts in metamaterial technology devoted to cloaking, as well as a perspective on the future of this rapidly changing field. The potential of passive cloaking techniques is outlined, as is the possibility of overcoming their limitations using active or nonlinear cloaks.

Multiband and Wideband Bilayer Mantle Cloaks

Using two suitably tailored concentric cloaking metasurfaces, we demonstrate several interesting features of these covers, including significant broadband and/or dual-band scattering reduction. We also show that nearly perfect cloaking for moderately sized targets is enabled by complementary bilayer pairings. We apply these concepts to realistic bilayer cloaks covering a finite-length conductive rod for multiband and wideband operation, offering a $>5-\text{dB}$ (70%) scattering suppression over bandwidths that exceed any passive mantle cloak demonstrated so far by over nine times. Finally, we apply the same covers to more complex geometries, showing that moderate scattering suppression is maintained, despite the change in geometry and increased size.

Physical bounds on electromagnetic invisibility and the potential of superconducting cloaks

The possibility of suppressing the scattering cross section of an object is subject to fundamental physical bounds imposed by causality and passivity. Global cloaking limitations have been recently derived, which imply that any linear, causal and passive cloak necessarily increases the global scattering, integrated over the whole electromagnetic spectrum, compared to the uncloaked object. Here, we expand on this topic, discussing in detail an interesting exception to this limit represented by cloaks with static diamagnetism. In this context, we explore the potential of superconducting materials to realize global and local reduction of the scattering cross section. The concepts of plasmonic and mantle cloaking are extended to superconductors, realizing strong and tunable invisibility, with some unique properties stemming from the peculiar electrodynamics of superconductors. We conclude by qualitatively discussing a possible method to derive more stringent local bounds on cloaking.

Broadening the Cloaking Bandwidth with Non-Foster Metasurfaces

We introduce the concept and practical design of broadband, ultrathin cloaks based on non-Foster, negatively capacitive metasurfaces. By using properly tailored, active frequency-selective screens conformal to an object, within the realm of a practical realization, we show that it is possible to drastically reduce the scattering over a wide frequency range in the microwave regime, orders of magnitude broader than any available passive cloaking technology. The proposed active cloak may impact not only invisibility and camouflaging, but also practical antenna and sensing applications.

Experiments on Active Cloaking and Illusion for Laplace Equation

In recent years, invisibility cloaks have received a lot of attention and interest. These devices are generally classified into two types: passive and active. The design and realization of passive cloaks have been intensively studied using transformation optics and plasmonic approaches. However, active cloaks are still limited to theory and numerical simulations. Here, we present the first experiment on active cloaking and propose an active illusion for the Laplace equation. We make use of a resistor network to simulate a conducting medium. Then, we surround the central region with controlled sources to protect it from outside detection. We show that by dynamically changing the controlled sources, the protected region can be cloaked or disguised as different objects (illusion). Our measurement results agree very well with numerical simulations. Compared with the passive counterparts, the active cloaking and illusion devices do not need complicated metamaterials. They are flexible, in-line controllable, and adaptable to the environment. In addition to dc electricity, the proposed method can also be used for thermodynamics and other problems governed by the Laplace equation.

Towards active dispersionless ENZ metamaterial for cloaking applications

The majority of the designs of electromagnetic cloaks presented in the current literature require a metamaterial, the relative permittivity and/or permeability of which are smaller than one. Due to basic dispersion constraints, such permittivity (or permeability) can exist only within some restricted frequency band. This fact causes a passive electromagnetic cloak to be an inherently narrowband device. Here, it is shown both numerically and experimentally that the basic dispersion constraints of any passive medium might be overcome by using an active transmission line-based metamaterial. This type of metamaterial appears promising for the design of broadband cloaking devices.