Invisibility Cloak Research Articles

A flexible procedure to avoid the boundary materials singularities in 2D transformation-based cloaks

ABSTRACTAn out-of-plane coordinate transformation can avoid singular materials in a two-dimensional electromagnetic cloak. Based on numerical simulations of elliptical cylinders, in this paper we show that the use of two different constants to define the out-of-plane stretching can result in anisotropic materials with lower values and yet with a better scattering reduction for a given incident wave direction.

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.

An iterative domain decomposition, spectral finite element method on non-conforming meshes suitable for high frequency Helmholtz problems

The purpose of this research is to describe an efficient iterative method suitable for obtaining high accuracy solutions to high frequency time-harmonic scattering problems. The method allows for both refinement of local polynomial degree and non-conforming mesh refinement, including multiple hanging nodes per edge. Rather than globally assemble the finite element system, we describe an iterative domain decomposition method which can use element-wise fast solvers for elements of large degree. Since continuity between elements is enforced through moment equations, the resulting constraint equations are hierarchical. We show that, for high frequency problems, a subset of these constraints should be directly enforced, providing the coarse space in the dual-primal domain decomposition method. The subset of constraints is chosen based on a dispersion criterion involving mesh size and wavenumber. By increasing the size of the coarse space according to this criterion, the number of iterations in the domain decomposition method depends only weakly on the wavenumber. We demonstrate this convergence behaviour on standard domain decomposition test problems and conclude the paper with application of the method to electromagnetic problems in two dimensions. These examples include beam steering by lenses and photonic crystal waveguides, as well as radar cross section computation for dielectric, perfect electric conductor, and electromagnetic cloak scatterers.

Nonreciprocity in Antenna Radiation Induced by Space-Time Varying Metamaterial Cloaks

Conventionally, antennas are reciprocal devices, exhibiting the same properties in transmission and in reception. In order to break antenna reciprocity, in this letter, we exploit the properties of space-time varying metamaterials, whose dielectric properties are modulated in both space and time. We propose to surround the antenna by a space-time varying cloak that imparts a momentum bias to the electromagnetic field propagating through it. The propagating wave interacts with the space-time modulation only in one direction, breaking time-reversal symmetry and, thus, the reciprocity of the system. Here, we show that a properly designed space-time varying metamaterial cloak can be used to significantly change the realized gain exhibited by an antenna in its transmission and reception mode. Since the proposed concept is based on weak coupling between propagating modes, electrically long propagation distance in the cloak is required, increasing the overall dimension of the system. However, the cloak thickness can be controlled by modifying the modulation parameters. The proposed concept may enhance the performance of radio-frequency communication systems, reducing the in-band captured noise or interfering signals.

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.

Robust Control of a Multifrequency Metamaterial Cloak Featuring Intrinsic Harmonic Selection

Outside of Hogwarts, experimental verifications of invisibility cloaking have been limited to single-frequency operation for relatively small objects. The authors propose a multifrequency metamaterial cloak for large objects, based on the intrinsic ability of an optimized medium to adjust optical phase delay by the cloak according to the size of the object. This innovative approach to designing multiband systems could have real impact on stealth technology.

Lossless tensor surface electromagnetic cloaking for large objects in free space

Lossless tensor surface electromagnetic cloaking for large objects in free space

The absence of reciprocity in active structures using direct velocity feedback

A variation of direct velocity feedback, often referred to as skyhook damping, is discussed in this paper. Skyhook damping cannot be regarded as collocated control method since only the action force component is collocated with the velocity sensor mounted onto the receiving part of the structure. The reaction control force component reacting off the source part of the structure does not have a collocated sensor. Depending on the characteristics of the passive structure under control, the feedback loop may be quite insensitive to the effects produced by the non-collocated reaction control force component, and maintain stability properties that are otherwise characteristic only for collocated control. Moreover, there exist additional effects related to the response of structures activated by the application of skyhook damping. It is shown in this paper that the structure subjected to such active control, although exhibiting stable response and linear input-output relationships, no longer complies with the reciprocity principle. The absence of reciprocity is interesting given the recent efforts in developing metamaterial cloaks, where one of the critical issues is how to design material structures or systems that demonstrate non-reciprocal behaviour.

Online Masquerade: Redesigning the Internet for Free Speech Through the Use of Pseudonyms.

Anonymity promotes free speech by protecting the identity of people who might otherwise face negative consequences for expressing their ideas. Wrongdoers, however, often abuse this invisibility cloak. Defenders of anonymity online emphasise its value in advancing public debate and safeguarding political dissension. Critics emphasise the need for identifiability in order to achieve accountability for wrongdoers such as trolls. The problematic tension between anonymity and identifiability online lies in the desirability of having low costs (no repercussions) for desirable speech and high costs (appropriate repercussions) for undesirable speech. If we practice either full anonymity or identifiability, we end up having either low or high costs in all online contexts and for all kinds of speech. I argue that free speech is compatible with instituting costs in the form of repercussions and penalties for controversial and unacceptable speech. Costs can minimise the risks of anonymity by providing a reasonable degree of accountability. Pseudonymity is a tool that can help us regulate those costs while furthering free speech. This article argues that, in order to redesign the Internet to better serve free speech, we should shape much of it to resemble an online masquerade.

Frequency domain transformation optics for diffusive photon density waves’ cloaking

We make use of transformation optics technique to realize cloaking operation in the light diffusive regime, for spherical objects. The cloak requires spatially heterogeneous anisotropic diffusivity, as well as spatially varying speed of light and absorption. Analytic calculations of Photon's fluence confirm minor role of absorption in reduction of far-field scattering, and a monopole fluence field converging to a constant in the static regime in the invisibility region. The latter is in contrast to acoustic and electromagnetic cloaks, for which the field vanishes inside the core. These results are finally discussed in the context of mass diffusion, where cloaking can be achieved with a heterogeneous anisotropic diffusivity.

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.

Target illusion by shifting a distance.

A novel homogeneous illusion device with arbitrary polygonal cross section that acts as either invisible cloak or shifting medium has been proposed and designed based on coordinate transformation method. The material parameters of the device are derived and the effectiveness is verified by full-wave simulation. Results show that whether the illusion device acts as invisible cloak or shifting medium depends on a value of shifting distance which is about 2a (ais circum-radius of the outer polygon).When the shifting distance is larger than 2a, the illusion device acts as an invisible cloak, and otherwise it acts as shifting medium. The stealth effect of all kinds of illusion devices are investigated, including identical-size mapped polygonal devices or non-identical size mapped polygonal devices. The results show that the device is a novel interactive cloak and is different from pre-proposed cloaks. The shifting properties of the device are validated by two examples, including moving the target object virtually and generating illusionary image of a line source. The material parameters of the device are homogeneous, which makes it more practicable in reality. It is hoped that our works may open an avenue for designing novel invisible cloaks, and are helpful for speeding up the potential applications of the illusion devices, such as aircraft or military equipment stealth, target objects camouflage or protection.

A Review of Tunable Acoustic Metamaterials

Acoustic metamaterial science is an emerging field at the frontier of modern acoustics. It provides a prominent platform for acoustic wave control in subwavelength-sized metadevices or metasystems. However, most of the metamaterials can only work in a narrow frequency band once fabricated, which limits the practical application of acoustic metamaterials. This paper highlights some recent progress in tunable acoustic metamaterials based on various modulation techniques. Acoustic metamaterials have been designed to control the attenuation of acoustic waves, invisibility cloaking, and acoustic wavefront engineering, such as focusing via manipulating the acoustic impedance of metamaterials. The reviewed techniques are promising in extending the novel acoustics response into wider frequency bands, in that tunable acoustic metamaterials may be exploited for unusual applications compared to conventional acoustic devices.

A hybrid invisibility cloak based on integration of transparent metasurfaces and zero-index materials

The invisibility cloak, a long-standing fantastic dream for humans, has become more tangible with the development of metamaterials. Recently, metasurface-based invisibility cloaks have been proposed and realized with significantly reduced thickness and complexity of the cloaking shell. However, the previous scheme is based on reflection-type metasurfaces and is thus limited to reflection geometry. In this work, by integrating the wavefront tailoring functionality of transparent metasurfaces and the wave tunneling functionality of zero-index materials, we have realized a unique type of hybrid invisibility cloak that functions in transmission geometry. The principle is general and applicable to arbitrary shapes. For experimental demonstration, we constructed a rhombic double-layer cloaking shell composed of a highly transparent metasurface and a double-zero medium consisting of dielectric photonic crystals with Dirac cone dispersions. The cloaking effect is verified by both full-wave simulations and microwave experimental results. The principle also reveals exciting possibilities for realizing skin-thick ultrathin cloaking shells in transmission geometry, which can eliminate the need for spatially varying extreme parameters. Our work paves a path for novel optical and electromagnetic devices based on the integration of metasurfaces and metamaterials.

Light-programmable manipulation of DC field in Laplacian Meta-devices

Impressive progresses have been achieved in the field of metamaterial to mimic the illusion or camouflage effects in nature. These include invisible cloaks and many other cloak-based illusion meta-devices. However, to date many experiments only present single, static or discretized functionalities. The dynamical control of multiple kinds of illusion signals can only be achieved by embedding complex active sources directly connected to external stimuli, leading to limited on/off switching effect in a contact fashion. Here, we experimentally demonstrate a distinct scheme to incorporate multi-functions into one passive Laplacian DC meta-device, assisted by light illumination. It is shown that light-programmable cloaking, full illusion, and partial illusion can be achieved on the same device without physical contact of the heating pads or electric bias, at the cost of only four kinds of natural bulk materials with homogeneous parameters throughout. A DC network is fabricated to demonstrate the proof of concept, with measurement results in good agreement with the numerical simulations. The proposed scheme may open a new avenue to the non-contact multiphysical control of multi-illusion functions for Laplacian fields.

A Microwave Invisibility Cloak: The Design, Simulation, and Measurement of a Simple and Effective Frequency-Selective Surface-Based Mantle Cloak

In recent years, there has been a growing interest in developing invisibility cloaks that can conceal an object. These techniques are often based either on coating a dielectric or conducting object with a homogeneous plasmonic layer of negative permittivity [1] or on creating multilayer structure [2] that cancels the scattering of the cloaked object (i.e., scattering cancelation technique). The technique may also be based on an inhomogeneous layer that bends electromagnetic waves around the region occupied by the cloaked object without interacting with it (i.e., transformation optics technique) [3].

Small-Scale Biological and Artificial Multidimensional Sensors for 3D Sensing.

A vast majority of existing sub-millimeter-scale sensors have a planar, 2D geometry as a result of conventional top-down lithographic procedures. However, 2D sensors often suffer from restricted sensing capability, allowing only partial measurements of 3D quantities. Here, nano/microscale sensors with different geometric (1D, 2D, and 3D) configurations are reviewed to introduce their advantages and limitations when sensing changes in quantities in 3D space. This Review categorizes sensors based on their geometric configuration and sensing capabilities. Among the sensors reviewed here, the 3D configuration sensors defined on polyhedral structures are especially advantageous when sensing spatially distributed 3D quantities. The nano- and microscale vertex configuration forming polyhedral structures enable full 3D spatial sensing due to orthogonally aligned sensing elements. Particularly, the cubic configuration leveraged in 3D sensors offers an array of diverse applications in the field of biosensing for micro-organisms and proteins, optical metamaterials for invisibility cloaking, 3D imaging, and low-power remote sensing of position and angular momentum for use in microbots. Here, various 3D sensors are compared to assess the advantages of their geometry and its impact on sensing mechanisms. 3D biosensors in nature are also explored to provide vital clues for the development of novel 3D sensors.

The effect of geometric parameters of a single-gap SRR metamaterial on its electromagnetic properties as a unit cell of interior invisibility cloak in the microwave regime

Some geometric parameters of a metamaterial unit cell with single-gap Split Ring Resonator (SRR) are modified and the effect of these changes on its electromagnetic properties is examined. In particular, the relative magnetic permeability has been investigated. For unit cells with various geometric parameters the frequency fp in which the relative magnetic permeability is close to zero as well as the resonance frequency f0 are obtained. Finally, as an example, for each of the unit cells, the relative magnetic permeability at 9.5 GHz is obtained and by using these results an electromagnetic invisibility cloak is designed operating at 9.5 GHz which can has three different unit cells with different geometric parameters for each of its layers.

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.

Localized Plasmonic Resonances of Prolate Nanoparticles in a Symmetric Environment: Experimental Verification of the Accuracy of Numerical and Analytical Models

We study the evolution of the surface-plasmon resonances of individual ion-beam-shaped prolate gold nanoparticles embedded in a dielectric SiO 2 environment by electron-energy-loss spectroscopy mapping in a scanning transmission electron microscope. The controlled symmetric dielectric environment obtained through the ion-beam-shaping method allows a direct quantitative comparison with numerical results obtained through simulations (auxiliary differential-equation finite-difference time-domain and boundary-element method) and with theoretical results obtained through analytical models (quasistatic model for prolate nanoellipsoids and waveguide model for infinite one-dimensional plasmonic waveguides), with which our experimental results are in very good agreement. We confirm the accuracy of state-of-the-art numerical tools and analytical theories that establish ion-beam shaping as a very promising method to design metal-dielectric nanocomposites with well-predicted optical properties, and with many possible applications in surface-enhanced Raman spectroscopy and second-harmonic generation, as well as in conventional applications of metamaterials like negative refraction, superimaging, and invisibility cloaking.