Invisibility Cloak Research Articles

Wave generation and transmission in multi-scale complex media and structured metamaterials (part 2)

Wave generation and transmission in multi-scale complex media and structured metamaterials (part 2)

A metamaterial-free omnidirectional invisibility cloak based on thrice transformations inside optic-null medium

A full space omnidirectional cloak with an optic-null medium (ONM) shell and an isotropic homogeneous restoring core is designed by thrice coordinate transformations inside ONM. The designed cloak can be realized by subwavelength dielectric channels enclosed with metallic plates and a uniform dielectric restoring core in air, i.e., no metamaterials or waveguide structures are required. Different from previous studies, the electromagnetic parameters of the designed cloak in this study are strict in accordance with transformation optics (no simplifications), which can achieve a nearly perfect omnidirectional cloaking effect at the design frequency by natural materials. Due to the transformation-invariant feature of the ONM, the shape, size and position of the concealed region can be adjusted freely, while the material parameters of the corresponding cloak remain unchanged.

Acoustic transmissive cloaking with adjustable capacity to the incident direction

Zero-refractive-index (ZRI) phononic crystals (PhCs), in which acoustic waves can be transmitted without phase variations, have considerable potential for engineering wavefronts and thus are applicable to invisibility cloaking. However, the creation of the transmissive cloaking achieved by ZRI-PhCs is challenging under an oblique incidence, which substantially hinders their practical applications. Here, we experimentally demonstrate acoustic transmissive cloaking with the adjustable capacity to the incident direction. Acoustic transmissive cloaking of arbitrarily shaped obstacles can be obtained through a hybrid acoustic structure consisting of one outer layer of a programmable phase-engineered metasurface (PPEM) and one inner layer of a double zero-refractive-index (DZRI)-PhC. The DZRI-PhC is functionally the same as an equiphase area and can guide acoustic waves around the obstacle, a process known as acoustic tunneling. The PPEM perpendicularly transfers the incident acoustic waves to the DZRI-PhC and allows the emergent waves from the DZRI-PhC to transmit along the original incident direction. The DZRI-PhC is made of an array of iron squares in the air. The reciprocal of the effective bulk modulus and the effective mass density is approximately zero at a frequency of 3015 Hz (0.5187 v0/a) originating from the zeroth-order Fabry–Pérot (FP) resonance that possesses infinite phase velocities. Each meta-atom of the outer metasurface consists of a line channel and four shunted Helmholtz resonators, which have effective masses that are engineered by a mechanics system. The amplitude and phase of the sound waves propagating through each meta-atom can be controlled continuously and dynamically, enabling the metasurface to obtain versatile wavefront manipulation functions. Acoustic cloaking is visually demonstrated by experimentally scanning the acoustic field over the hybrid structure at a frequency of 3000 Hz (0.5160 v0/a). Our work may provide applications with great potential, including underwater ultrasound, airborne sound, acoustic communication, imaging, etc.

Multilayer acoustic invisibility cloak based on composite lattice

A concentric cylindrical cloak is showed here to achieve the acoustic cloaking phenomenon. The introduced structure consists of MNE layers and water in MNE substrate in the MHz frequency range. Due to avoiding the incoming acoustic waves by the shell, the object can be hidden inside the cylindrical area of any shape. In order to improve the quality of cloaking, we have optimized the desired shell by considering the manufacturing technology. We show that an optimized, acoustic cloak based on composite lattice structure can reduce the scattering of an object more than a 20-layer realization of acoustic cloak based on multilayer cylindrical structure. This design approach can substantially simplify the fabrication of cloaking shells. In this research, to study the acoustic distribution of the desired structure, finite element method (FEM) has been used to analyze the structure in two dimensions and a cloak of natural materials with isotropic properties has been designed using effective medium theory.

All-angle reflectionless negative refraction with ideal photonic Weyl metamaterials

Negative refraction, an unnatural optical phenomenon in which the incident and the refracted waves reside on the same side of the surface normal, has been demonstrated with the invention of negative index media based on artificially engineered photonic structures called metamaterials. It has received wide attention due to its potential applications in imaging, nonlinear optics, and electromagnetic cloaking. However, it is highly challenging to realize negative refraction operating at all angles and with the perfect transmission. In this work, leveraging the recent development in topological photonics, we propose to realize reflectionless negative refraction for all incident angles with a topological metamaterial. The proposed metamaterial possesses two Weyl points of opposite topological charges. By interfacing the metamaterial with a perfect electric conductor (PEC) or a perfect magnetic conductor (PMC), the Fermi arc connecting the two Weyl points can take the form of a half-circle possessing a positive or a negative refractive index. Importantly, due to the topological protection, there is no reflection at the interface between the PEC and PMC covered areas, leading to the observation of all-angle negative refraction without reflection at the boundary. Our work provides a new platform for manipulating the propagation of surface waves, which may find applications in the construction of integrated photonic devices.

Broadband continuous supersymmetric transformation: a new paradigm for transformation optics

Transformation optics has formulated a versatile framework to mold the flow of light and tailor its spatial characteristics at will. Despite its huge success in bringing scientific fiction (such as invisibility cloaking) into reality, the coordinate transformation often yields extreme material parameters unfeasible even with metamaterials. Here, we demonstrate a new transformation paradigm based upon the invariance of the eigenspectra of the Hamiltonian of a physical system, enabled by supersymmetry. By creating a gradient-index metamaterial to control the local index variation in a family of isospectral optical potentials, we demonstrate broadband continuous supersymmetric transformation in optics, on a silicon chip, to simultaneously transform the transverse spatial characteristics of multiple optical states for arbitrary steering and switching of light flows. Through a novel synergy of symmetry physics and metamaterials, our work provides an adaptable strategy to conveniently tame the flow of light with full exploitation of its spatial degree of freedom.

Switchable ultra-broadband absorption and polarization conversion metastructure controlled by light.

This article proposed a metastructure device that can realize polarization conversion (PC) and absorption function switching in the terahertz (THz) range based on the photoconductivity effect. The photoconductance is formed by exposing silicon to different intensities of light, then the PC and absorption function can be switched. At the same time, the absorption bandwidth is expanded by inserting air resonant cavities into the dielectric substrate, changing the thickness of the dielectric locally, and cutting rectangular slots at the metal bottom plate. When the device works as a polarization converter, linear-to-linear PC with a polarization conversion rate of over 90% at 0.96-1.47 THz can be achieved, and its relative bandwidth is 42%. And when the silicon conductivity is fixed at 3500 S/m through illuminating, the device switches to an ultra-broadband absorber with over 90% absorption at 0.75-1.73 THz and a relative bandwidth of 79%. The designed device can be applied efficiently in many fields, such as electromagnetic cloaking and communication.

Surface Integral Equation Solutions for Electromagnetic Scattering From a Special Class of Anisotropic Media

We present a surface integral equation (SIE) solution for a special class of anisotropic media, whose permittivity and permeability are both symmetric full tensors with the same or multiple values. This kind of media is generated from the invisible cloaking or absorbing material of a perfectly matched layer (PML) design. A new derivation based on the potential functions is presented to obtain the closed-form Green’s function. Detailed and rigorous proofs of some vector identities are given for the first time. Special treatments are detailed to address the singularity of the anisotropic Green’s function for discretizing the SIE. Numerical experiments validate the accuracy, stability, and high efficiency of the proposed solution compared with the analytical solution or the finite element-boundary integral (FE-BI) method.

Experimental Realization of a Superdispersion-Enabled Ultrabroadband Terahertz Cloak.

Invisibility has been a topic of long-standing interest owing to the advent of metamaterials and transformation optics, but still faces open challenges after its tremendous development in recent decades. One of the big challenges is the narrow bandwidth, as the realization of an invisibility cloak is usually based on a metamaterial-an artificial composite material composed of subwavelength resonator structures that are always associated with dispersion. Different from previous works that have tried to eliminate the material dispersion to enhance the bandwidth of aninvisibility cloak, here, it is found that by judiciously harnessing the material dispersion, the bandwidth of the cloak can still be significantly increased. Interestingly, the material dispersion does not violate the law of causality. As a proof of concept, an ultrabroadband terahertz (THz) carpet cloak is experimentally demonstrated through an array of superdispersive microparticles, rendering the target object invisible to detection by both time- and frequency-domain wideband systems. The work presents a feasible invisibility strategy that is closer to practical applications and may pave a brand-new way for the development of dispersion-dominated ultrabroadband metadevices.

The angels and heroes of health care: Justified and appropriate, or harmful and destructive?

The angels and heroes of health care: Justified and appropriate, or harmful and destructive?

3D Printing of A Thermally Programmable Conformal Metasurface (Adv. Mater. Technol. 7/2022)

Curved Metasurfaces In article number 2101479, Qingxuan Liang and co-workers demonstrate a thermally programmable conformal metasurface carpet cloak printed by the composite fused deposition molding. The thermal programming method effectively reduces the complexity of conformal manufacturing and ensures the electromagnetic performances of the metasurface. This work provides a viable implementation strategy for complex curved metasurface and will substantially promote the applications of the conformal invisibility cloaks.

The Man in the Macintosh Is a Man in a Hat

The Man in the Macintosh Is a Man in a Hat John Gordon (bio) The man in the macintosh is a man in a hat, because every man of the time wore headgear. Hence, in "Counterparts," Farrington has a ruse, which depends on the reader's understanding that a man not taking his hat from its place on the hat-rack is not going outdoors.1 True in general, this is doubly true for "Hades." In that episode, men's hats—"[t]hese pots we have to wear"—are a major concern.2 "Hades" begins with Martin Cunningham's "silkhatted head" going into a carriage and ends with the unpleasantness over the dinge in John Henry Menton's hat (U 6.01). In between—in the street, in the chapel, at the gravesite—men's hats are repeatedly being taken off and put back on. The reason for this is obvious. "Hades" is about a funeral, and the doffing and donning of hats is a standard feature of the funeral ceremony, which in turn depends on the understanding that every male present will show up wearing one. So, perforce, the man in the macintosh wears one. If he did not, if he were the only perpetually bareheaded man in the company, he would have been noticed from the outset and not just because of his raincoat. Aside from Menton's dented bowler, there may be another notable hat present in the episode, unusual in one particular: it can render the wearer invisible. In his study of Ulysses Notebook VIII.A.5, Phillip F. Herring calls attention to Joyce's identification (confirmed in the Carlo Linati list of correspondences3) of John O'Connell with Hades, the Greek lord of the underworld, and then adds that in the same notes the German word "Tarnkappe" appears, which Roscher [Herring's source, W. H. Roscher's Ausführliches Lexikon der Griechischen und Römischen Mythologie4] uses to compare Hades' cloak of invisibility with that of Nordic mythology. Joyce's mystery man [the man in the macintosh] has two prominent characteristics: his macintosh and his elusiveness. This evidence suggests that both the caretaker John O'Connell and the Man in the Macintosh are avatars of Hades.5 I think that Herring has a point here but one that calls for some refurbishing. The idea seems to be that M'Intosh's macintosh is or in some way corresponds to the "Tarnkappe," which in turn means "cloak of [End Page 691] invisibility." But if that were so, he should be invisible by virtue of wearing it, and he is not. After all, there he is, in plain sight, macintosh and all. As for O'Connell, there is no indication that he is wearing any sort of remarkable cloak or coat. O'Connell is, however, wearing a hat. In fact, here is how he is introduced: "A portly man, ambushed among the grasses, raised his hat in homage" (U 6.708-09). That is, he comes into the field of vision at or about the moment he takes off his hat. The German "Tarn" means "camouflage." "Kappe" means "hood." A "Tarnkappe" can be either a hood (or cape) that renders the wearer, along with everything else he has on, invisible, or a hood (or hat) that does the same. A hood, after all, can plausibly be taken either as part of an article of outerwear or as a separate article of headwear. Also, "Kappe" sounds like "cap," a coincidence that may help explain why, in a search through Google Books entries for the years from 1850 to 1950, the first ten hits for "Tarnkappe" emerge in translation as "camouflage cap."6 Herring's assumption that the word means "cloak of invisibility," evidently tracing to Roscher, is an inference. All this is a recipe for muddle, and muddles there have been. In Richard Wagner's Ring cycle, Siegfried's "[h]elm" of invisibility—his "Tarnhelm"—can in the same translation both be called a "tarncap," that is, a cap, and described as conferring a "cloak of invisibility." Wikipedia's entry on Alberich, the dwarf whose brother forged the Tarnhelm, calls it, wrongly, a "Tarnkappe" (in fact...

Invisibility Cloaking and Transformation Optics for Three Dimensional Manifolds and Applications in Cosmology

We consider how transformation optics and invisibility cloaking can be used to construct models in subsets $\mathbb R^3$ with a varying metric, where the time-harmonic waves for a given angular wavenumber $k$ are equivalent to the waves in some closed orientable manifold. Our model can be used in two ways. First, the model makes it possible to do mathematical analysis on a compact 3-manifold $M$ by smoothly embedding the set $M\setminus L$, where $L$ is a finite union of closed curves, to an open subset of $\mathbb R^3$. Second, the model could in principle be physically implemented using a device built from metamaterials. In particular the measurements in the metamaterial device given by the Helmholtz source-to-solution operator are equivalent to Helmholtz source-to-solution measurements in a universe given by $(\mathbb{R}_+\times M, -dt^2 +g)$, where $(M,g)$ is a closed, orientable, $C^\infty$-smooth, 3-dimensional Riemannian manifold. Thus the obtained device could be used to simulate cosmological models using metamaterial devices.

To “Throw Off the Cloak of Representation”

To “Throw Off the Cloak of Representation”

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.

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.

Whiteness and Blackness in The Bluest Eye

Toni Morrison has long proposed that the concept of physical beauty is one of the most destructive ideologies of human thought. This essay aims to deal with how Morrison’s 1970 novel The Bluest Eye exposes the dogma that physical beauty and pureness itself are straightforwardly associated with whiteness; allowing whiteness to provide a cloak of invisibility to those who possess it. This then subjects blackness to be the placeholder of what is ugly, bad and dangerous in the world; forcing the trait of blackness to be a conviction of visibility. This essay will follow myself, a white individual, grappling with the fact that I have been given the privilege to go through life unnoticed only because I happened to be born white. Further, the repercussions of this fact are confronted, those who are black are stripped from the privilege of going unnoticed. The Bluest Eye and my mediation of the novel present that even a raised consciousness about the privileges of whiteness fails to prevent racial self-loathing and violence against Blacks due to whiteness taking over the measure of humanness. Despite this, there is still hope in the future of blackness which can only be accomplished by displacing the authority of whiteness and questioning the structures that allow the authority of whiteness to prevail.

Experimental investigation of thermal architected metamaterials for regulating transient heat transfer

Metamaterials have innovative functionalities as a result of artificial design. By arbitrarily designing thermal conductivity, thermal metamaterials have achieved thermal invisible cloaks, camouflages, and deceptions in steady-state. However, density and specific heat capacity have sometimes been overlooked, despite the fact that they are critical for managing transient heat flow and that density are an indicator of creating lightweight materials. The traditional materials are incapable of synchronously controlling thermal conductivity, density, and specific heat capacity. In this research, we experimentally investigate a novel thermal architected metamaterial that can modulate (accelerate or decelerate) transient heat transfer velocity without disrupting the steady-state. The novel type of thermal architected metamaterial achieves efficiency using a topology based on neutral inclusion theory. The transient heat transfer velocity is regulated by altering the effective density and specific heat capacity on the one hand, while the steady-state heat transfer is maintained by keeping the effective thermal conductivity on the other. The density of the architected metamaterial could be adjusted by selecting an appropriate specific heat capacity. The method we provide can be utilized to create lightweight energy-efficient materials (by accelerating transient heat transfer) as well as lightweight engineered thermal protection materials (by decelerating transient heat transfer). Moreover, the proposed thermal architected metamaterial could control heat flow velocity continuously, and this characteristic covers the gaps left by natural materials.

A Review on Metamaterial Absorbers: Microwave to Optical

Metamaterials (MM) are artificially designed materials that possess unique properties due to their geometrical design. They also display some peculiar properties, such as negative refractive index, Snell’s law reversal, Doppler effect reverse, and left-handed behavior. MMs are used in a myriad of applications, including invisibility cloaking, perfect lensing, perfect absorption, and sensing. In this review article, the property of electromagnetic absorption by structures known as metamaterial absorbers (MMAs) is discussed. An MMA is a composite made up of many layers of metallic patterns separated by dielectric. This novel device helps in achieving near-unity absorption by various mechanisms, which are investigated in this article. The MMAs are classified based on their absorption characteristics, such as polarization tunability, broadband operation, and multiband absorption, in different frequency regimes.

Under a cloak of invisibility: Use of books and book chapters published by the Royal Zoological Society of New South Wales since 1970

ABSTRACT Between 1970 and 2013, the Royal Zoological Society of New South Wales (RZS NSW) published 30 books, contributing to many relevant or controversial zoological topics as well as documenting the society’s history and offering perspectives on the development of Australian zoology. Authors from eight countries contributed to books or chapters. Over 97% of authors indicated Australian affiliations in addresses. Approximately 67% of papers citing RZS NSW books or book chapters had at least one author with an Australian affiliation. Ten or more authors with affiliations from each of 22 other countries were also included, indicating that the books have an international profile. This conclusion is supported by an examination of global library holdings in WorldCat; all RZS NSW books are held in libraries with the range between 17 and 152 libraries per book. The range of countries holding each book is 4 to 14. The library holdings are an important reminder that citations are not the sole indicator of use of scientific publications, with the books remaining on library shelves because students and others consult them. Although the books and their chapters were often not listed in either Scopus or Web of Science (Core Collection) or Web of Science (all databases), citations to them could still be retrieved using either a secondary documents search (Scopus) or cited reference search (versions of Web of Science). Chapters from twenty-six books were cited in Scopus in 2020 or 2021, indicating that the books have long citation lives. A list of the 20 most highly cited books or chapters included seven chapters from the two editions of Conservation of Australia’s Forest Fauna, indicating a strong interest in this subject. Eight of the books or chapters on the list covered mammals and a further three covered vertebrates generally, so mammals also engendered strong interest. Benchmarking against similar books from another publisher, CSIRO Publishing, found similar problems of a low profile in Scopus and Web of Science (Core Collection) and Web of Science (all databases), but evidence of extensive library holdings.