Cloaking Device Research Articles

Temporal cloaking via the effect of Compton scattering

In this work we have studied Temporal Cloaking in a 4 level atomic system in the presence of Compton scattering effect. Between the scattered and un-scattered probe light beams multiple time gaps are noted. Furthermore, it is noted that Compton scattering angle has significant influence on the time gap produced in the system. The time gap increased rapidly by increasing the value of φ up to π/2 and then changes slowly. Temporal time gap up to the order of microseconds is noted which might be useful in information hacking technology. Moreover, the time gap is further modified with pulse width, that could have significant applications in secured communication cloaking devices.

Induced effects of a rigid boundary on a pulsating spherical source

This work develops a comprehensive analytical formalism to study the boundary-induced effects on a pulsating spherical radiating source near a rigid planar boundary in an inviscid fluid. Using the modal multipole expansion method, the image method and the translational addition theorem for spherical wave functions, exact closed-form expressions are derived for the dimensionless axial acoustic radiation force function as well as the radiation, amplification and extinction energy efficiency factors. The source-boundary distance and the dimensionless size parameter are especially addressed in the numerical simulations for spherical sources with different vibration modes. It is shown that the axial radiation force is positive or negative, depending on the choice of parameters. Moreover, cases when the extinction acoustic power completely vanishes are predicted theoretically for dipole and quadrupole vibrations, which can be used to achieve acoustic invisibility in acoustic cloaking devices and acoustic stealth technologies. Combination of more than one vibration modes is also investigated to better simulate the acoustic manipulation in practical applications. This work can help us understand the underlying mechanism in manipulating the active spherical radiating sources in a bounded field.

Controlling the thermal and electric fields in isotropic and anisotropic media

In this study, we theoretically propose cloaking and concentration devices allowing simultaneous control of electric and thermal fields in spherically inhomogeneous layered medium with isotropic and anisotropic layers. The above combination of layers (isotropic and anisotropic ones) is obtained by the transformation coordinate approach applied to a spherically inhomogeneous layered medium which contains isotropic and anisotropic layers. It is shown that in steady-state conditions, both thermal and electric fields can pass smoothly around the targeted area while preventing any disturbance in the surrounding medium. The constitutive parameters of both fields have been determined analytically. In this work, we have combined two different methodologies to achieve cloaking in ideal state and in homogenized structure for cylindrical and spherical cases. Numerical validation of the obtained solutions using COMSOL software is performed in this study.

A 4D‐Printed Electromagnetic Cloaking and Illusion Function Convertible Metasurface

AbstractThe appearance of metasurfaces provides a new way for the realization of cloaking and illusion devices. However, most proposed metasurface functional devices only display a single cloaking and illusion function. Herein, a dynamic metasurface device with switchable electromagnetic cloaking and illusion functions is fabricated based on a 4D‐printing process. Owing to the shape memory polymer's characteristic of shape change above the glass transition temperature, under the stimulation of temperature, it can control the structural shape change of the metasurface, change the phase distribution law, and realize dynamic cloaking and illusion functions. As proof, the metasurface samples with changeable structure are designed and fabricated by fused deposition modeling of 4D printing combined with liquid metal injection molding. Both the simulation and experimental results confirm that the designed metasurface has excellent cloaking and illusion performance at microwave frequency. This work provides a new idea for the realization of integrated metasurface devices with electromagnetic cloaking and illusion functions.

Concurrent multiscale topology optimization of metamaterials for mechanical cloak

Mechanical cloaks can hide objects and make them unfeelable by reproducing the surrounding displacement field without the objects and cloaks. In the existing works, mechanical cloaks have been considered for hiding a void, while this work develops a generic method to design cloaks for solids with a given stiffness. As materials with properties beyond natural materials are required to achieve this task, metamaterials with spatially varying microstructures are employed in this work. Similar to the functionally graded material, the present metamaterial has graded stiffness from the cloak to the surrounding region, via graded material volume fractions in the microstructures. Therefore, we present a multiscale topology optimization model that considers the cloak macrostructure and material microstructures, by which concurrent optimization of the structural topology and material properties of mechanical cloaking devices is investigated. The optimization model is implemented via (1) novel mathematical formulations for material microstructures based on the finite element method (FEM) and (2) an extended moving iso-surface threshold (MIST) method with a multi-volume fraction topology update scheme. Numerical examples are provided for multiscale design cases for cloaking a void or stiffer solid (e.g., wood, copper, and diamond), and also include finite element analysis (FEA) of the resulting multiscale metamaterials.

How the secrets of the Black Death give us sustainable meat

Pathogenic bacteria such as Yersinia pestis, causative agent of the plague, have a genetic armoury of proteins they use to defend themselves against the immune system when invading a host. Upon invasion, Y. pestis bacteria deploy a molecular cloaking device, made of a protein called Caf1, which allows them to avoid being eaten by a host’s macrophage cells. Caf1 has several interesting structural properties that allow it to carry out this role, such as its ‘non-stick’, bioinert nature. This provides us with a blank canvas for protein engineering, where we can insert different bioactive signals into the protein structure, allowing us to instruct cells in a defined way, e.g., providing them with attachment sites or behavioural cues. We can also exploit Caf1’s unusual properties to use it as a molecular Lego kit, mixing and matching different bioactive Caf1 modules to make multifunctional biomaterials. We aim to use engineered Caf1 proteins to solve problems in the industrial scale production of cells for technologies such as cell therapy and cultivated meat. For example, by mixing adhesive and growth factor signals in a single material, and displaying multiple copies of each signal at once, we can reduce the number of expensive reagents needed. More generally, Caf1 is an excellent example of how bacterial armaments and defences can be re-engineered and adapted to benefit society, rather than cause disease.

Smart hybrid active/semi-active distributed structural acoustic control of thin- and thick-walled piezo-sandwich bimorph spherical shell cloaks

A novel three dimensional underwater smart piezo-sandwich spherical shell cloak is designed and analyzed based on the hybrid active/semi-active distributed structural acoustic control strategy. The sandwich configuration consists of series- or parallel-connected piezoelectric (PZT) bimorph actuator skin layers collocated with a semi-active electro-rheological fluid (ERF) core layer. Two separate coupled elasto-acoustic formulations are systematically developed. The first model is based on the variational principle of structural mechanics and the approximate Kirchhoff-Love thin shell theory while the standard sliding mode control (SMC) scheme is utilized to activate the acoustic scattering extinction capability of the hybrid structure. The second model is built on the exact linear theory of 3D piezoelasticity and the classical state-space transfer matrix approach along with the active damping control (ADC) strategy. Extensive numerical simulations reveal that the best broadband backscattering cancellation performance can effectively be achieved with the smart hybrid active/semi-active bimorph piezo-sandwich spherical shell cloak operating in parallel connection. To quantify the overall cloaking performance of the proposed design, the percentage error (%Err) of the external cloaked field relative to the background incident pressure field is calculated at selected dominant structural resonance frequencies within eight designated frequency bands (regions I to VIII). The remarkable (near-perfect) cloaking performance of the parallel-connected thick-walled hybrid bimorph piezo-sandwich shell (25%shellcloak) is demonstrated in the intermediate to high frequency regions (i.e., below 1.5% error in regions I, and III to VIII). This excellent invisibility performance is somewhat degraded in a narrow low frequency band (i.e., up to 8% error in region II) which is primarily linked to the elasto-acoustic resonances due to the coherent coupling of various types of highly interacting fluid- and shell-borne peripheral waves. On the other hand, the superior cloaking performance of the parallel-connected thin-walled hybrid bimorph piezo-sandwich shell (7%shellcloak) is observed predominantly in the low frequency range (i.e., below 1.5% error in regions I and II), while this exceptional performance is gradually degraded as the incident wave frequency is increased (i.e., up to 6.5% error in regions III to VIII), where partial or directional cancellation of the scattered field mostly occurs. Also, some important practical issues and limitations regarding the experimental implementation of proposed hybrid active/semi-active acoustic cloaking device are briefly discussed.

Wide-angle high-efficiency absorption of graphene empowered by an angle-insensitive Tamm plasmon polariton.

In recent years, researchers utilized Tamm plasmon polaritons (TPPs) in conventional heterostructures composed of a metal layer, a dielectric spacer layer and an all-dielectric one-dimensional (1-D) photonic crystal (PhC) to achieve high-efficiency absorption of graphene. According to the Bragg scattering theory, photonic bandgaps (PBGs) in all-dielectric 1-D PhC strongly shift toward shorter wavelengths (i.e., blueshift) as the incident angle increases. Therefore, TPPs in conventional heterostructures also show strongly blueshift property. Such strongly blueshift property of TPPs greatly limits the operating angle range of the high-efficiency absorption of graphene. Herein, we realize an angle-insensitive TPP in a heterostructure composed of a metal layer, a dielectric spacer layer and a 1-D PhC containing hyperbolic metamaterial layers. Empowered by the angle-insensitive property of the TPP, we achieve wide-angle high-efficiency absorption of graphene. The operating angle range (A > 80%) reaches 41.8 degrees, which is much larger than those in the reported works based on TPPs and defect modes. Our work provides a viable route to designing cloaking devices and photodetectors.

First principles calculation to investigate the effect of Mn substitution on Cu site in CeCu3-x Mn x V4O12 (x = 0, 1, 2 and 3) system.

Structural, electronic, elastic and magnetic properties of CeCu3-x Mn x V4O12 (x = 0, 1, 2 and 3) system have been carried out through DFT using GGA, GGA+U and HF potential. The investigation of structural optimization reveals that lattice parameters of the understudy system is reliable with the reported results and are increasing with the Mn substitution due to their greater atomic radii as compare to Cu atom. Both the cohesive energy and the enthalpy show that CeCu3V4O12 is the most thermodynamically stable among these compounds. When Mn is replaced by Cu in these compounds, not only it become semi-metals, but the host compound also changes from non-magnetic to anti-ferromagnetic and their electrical resistance provides further credence to their electronic behavior. Mechanical stability, anisotropy, and ductility are all demonstrated through the elastic characteristics of these compounds. Due to anti-ferromagnetic ductile nature of the Mn base compounds, it is expected that the compounds in the system may use for spintronic application and in magnetic cloaking devices.

Foregrounding Women's Safety in Mobile Social Matching and Dating Apps

The design of social matching and dating apps has changed continually through the years, marked notably by a shift to mobile devices, and yet user safety has not historically been a driver of design despite mounting evidence of sexual and other harms. This paper presents a participatory design study with women-a demographic at disproportionate risk of harm through app-use-about how mobile social matching apps could be designed to foreground their safety. Findings indicate that participants want social matching apps to augment women's abilities for self-protection, reflected in three new app roles: 1) the cloaking device, through which the social matching app helps women dynamically manage visibility to geographically nearby users, 2) the informant, through which the app helps women predict risk of harm associated with a recommended social opportunity, and 3) the guardian, through which the app monitors a user's safety during face-to-face meetings and augments their response to risk.

Water wave cloaking using a floating composite plate

The trajectory of surface gravity waves in the potential flow regime is affected by the gravitational acceleration, water density and sea bed depth. Although the gravitational acceleration and water density are approximately constant, the effect of water depth on surface gravity waves exponentially decreases as the water depth increases. In shallow water, cloaking an object from surface waves by varying the sea bed topography is possible, however, as the water depth increases, cloaking becomes a challenge because there is no physical parameter to be engineered and subsequently affects the wave propagation. In order to create an omnidirectional cylindrical cloaking device for finite-depth/deep-water waves, we propose an elastic composite plate that floats on the surface around a to-be-cloaked cylinder. The composite plate is made of axisymmetric, homogeneous and isotropic annular thin rings which provide adjustable degrees of freedom to engineer and affect the wave propagation. We first develop a pseudo-spectral method to efficiently determine the wave solution for a floating composite plate. Next, we optimise the physical parameters of the plate (i.e. flexural rigidity and mass of every ring) using an evolutionary algorithm to minimise the energy of scattered waves from the object and therefore cloak the inner cylinder from incident waves. We show that the optimised cloak reduces the energy of scattered waves as high as 99 % for the target wave number. We quantify the effectiveness of our cloak with different parameters of the plate and show that varying the flexural rigidity is essential to control wave propagation and the cloaking structure needs to be at least made of four rings with a radius of at least three times of the cloaked region. We quantify the wave drift force exerted on the structures and show that the optimised plate reduces the exerted force by 99.9 %. The proposed cloak, due to its structural simplicity and effectiveness in reducing the wave drift force, may have potential applications in cloaking offshore structures from water waves.

Flexible and biocompatible poly (vinyl alcohol)/multi-walled carbon nanotubes hydrogels with epsilon-near-zero properties

• Flexible PVA/MWCNTs hydrogels were fabricated by solution method, and cytotoxicity experiments proved that the process was environmentally friendly and hydrogels were biocompatible. • Epsilon-near-zero properties were realized at radio frequency from PVA/MWCNTs hydrogels, originated from the resonance of the induced electric dipole. • After the frequency achieving epsilon-near-zero, negative permittivity which was different from Drude model but related to interband transition, was observed. Epsilon-near-zero (ENZ) material has been a research hotspot in recent years due to unique physical properties such as inverse Doppler effect and negative refractive index, showing great potentials in the fields of flexible electronics, wearable devices, sensors, etc. The ENZ materials are mostly reported at visible, infrared and terahertz wavelengths, while the report about ENZ materials at radio frequency is rare. In this work, flexible and biocompatible poly (vinyl alcohol)/multi-walled carbon nanotubes (PVA/MWCNTs) hydrogels, which were successfully fabricated by an environmentally friendly method, were used as ENZ materials at radio frequency for the first time. Cytotoxicity experiments proved that the experimental process and products are green, environmentally friendly and biocompatible. The microstructure, crystalline structure, chemical composition and dielectric properties were investigated. Two different water states, which were free water molecules and bound water molecules, coexisted in these PVA/MWCNTs hydrogels and accounted for about 37.5 wt% and 15.9 wt% respectively by analyzing the thermogravimetric analysis curves. When the MWCNTs content reached 12 wt% and 15 wt%, the continuous conductive MWCNTs network was formed in the hydrogel, and ENZ phenomenon was observed at about 760 kHz and 580 kHz respectively, which was attributed to the interband transition. Considering the flexibility and non-toxicity of PVA/MWCNTs hydrogels, the ENZ properties of this structure at the radio frequency can be well used in wearable invisibility cloak, flexible electronics, skin sensors and other wearable devices.

Mechanically Stable Magnetic Metallic Materials for Biomedical Applications.

The structural, electrical, and magneto-elastic properties of lanthanide base nitride (Ln = Dy-Lu) anti-perovskites were investigated using density functional theory (DFT). The reported structural outcomes are consistent with the experiment and decrease from Dy to Lu due to the decrease ofatomic radii of Ln atoms. According to the electronic band profile, the metallic characteristics of these compounds are due to the crossing over of Ln-f states at the Fermi level and are also supported by electrical resistivity. The resistivity of these compounds at room temperature demonstrates that they are good conductors. Their mechanical stability, anisotropic, load-bearing, and malleable nature are demonstrated by their elastic properties. Due to their metallic and load-bearing nature, in addition to their ductility, these materials are suitable as active biomaterials, especially when significant acting loads are anticipated, such as those experienced by such heavily loaded implants as hip and knee endo-prostheses, plates, screws, nails, dental implants, etc. In thesecases, appropriate bending fatigue strength is required in structural materials for skeletal reconstruction. Magnetic properties show that all compounds are G-type anti-ferromagnetic, with the Neel temperatures ranging from 24 to 48 K, except Lu3Nin, which is non-magnetic. Due to their anti-ferromagnetic structure, magnetic probes cannot read data contained in anti-ferromagnetic moments, therefore, data will be unchanged by disrupted magnetic field. As a result, these compounds can be the best candidates for magnetic cloaking devices.

Omnidirectional acoustic cloaking against airborne sound realized by a locally resonant sonic material

We report that a locally resonant sonic material realizes omnidirectional acoustic invisibility in air. To achieve acoustic cloaking in the low-frequency regime, we axisymmetrically placed elastic rods comprised of silicone rubber and lead around a cloaked object. The radii of the rods are designed to minimize their total scattering cross section for a given frequency. The optimization is performed using an algorithm incorporating multiple scattering theory and gradient-based nonlinear programming. We numerically confirmed that the designed cloaking device suppressed the scattering cross section by almost 92% for all incident directions at the target frequency.

Equivalent circuit analysis of a mode-converting metacell in a coaxial transmission line at microwave frequencies

To provide for an ever-increasing need for camouflage, experts in stealth have been envisioning for a few years to use metasurfaces and their unconventional electromagnetic properties to serve as efficient cloaking devices. To prepare for the use of surface-mounted electronic components, useful for frequency-selecting purposes, we take interest in an equivalent circuit analysis of metasurfaces. Indeed, metasurfaces using electronic components invite one to combine the passive behavior of the host structure with that of the lumped elements. The equivalent circuit analysis of the hosting surface deprived of electronic components has so far been dedicated to the case of metasurfaces constituted by rectangular or square unit cells. This work, therefore, aims at extending the former study on Cartesian unit cells to the coaxial geometry of interest. The validity of such a method is then assessed by comparison with the numerical simulation results obtained using a finite-element method.

Broadband cross-circular polarization carpet cloaking based on a phase change material metasurface in the mid-infrared region

In view of the fact that most invisibility devices focus on linear polarization cloaking and that the characteristics of mid-infrared cloaking are rarely studied, we propose a cross-circularly polarized invisibility carpet cloaking device in the mid-infrared band. Based on the Pancharatnam-Berry phase principle, the unit cells with the cross-circular polarization gradient phase were carefully designed and constructed into a metasurface. In order to achieve tunable cross-circular polarization carpet cloaks, a phase change material is introduced into the design of the unit structure. When the phase change material is in amorphous and crystalline states, the proposed metasurface unit cells can achieve high-efficiency cross-polarization conversion, and reflection intensity can be tuned. According to the phase compensation principle of carpet cloaking, we construct a metasurface cloaking device with a phase gradient using the designed unit structure. From the near- and far-field distributions, the cross-circular polarization cloaking property is confirmed in the broadband wavelength range of 9.3–11.4 µm. The proposed cloaking device can effectively resist detection of cross-circular polarization.

Coassembly of elastomeric microfibers and silver nanowires for fabricating ultra-stretchable microtextiles with weakly and tunable negative permittivity

In this work, a type of unique flexible and ultra-stretchable microtextiles was fabricated by coassembly of Ag nanowires (AgNWs) and thermoplastic polyurethane (TPU) microfibers. The microtexiles show tunable and weakly negative permittivity at a low filler content of 1.0–2.4 wt%. The resonance-induced negative permittivity observed in the AgNW/TPU microtextiles below the percolation threshold (fc) is attributed to the electric dipole moment of the isolated AgNWs, and this phenomenon can be explained by the Lorentz model. When the AgNW content exceeds the fc, the low-frequency collective plasma oscillation of the AgNW networks in the composites triggers the tunable plasma-type negative permittivity, which agrees with the Drude model. Additionally, the inductive character of the composites was assessed with the reactance spectra. The as-prepared Ag/TPU microtextiles could be a good candidate for the applications of garment integrated electronic, novel sensors, electromagnetic cloaking, and wearable electronic devices.

New anti-ferromagnetic tri-transition quaternary perovskites for magnetic cloaking and spintronic applications

Magnetism in new tri-transition A-site ordered quaternary perovskites ACu3V4O12 (A = Mn and Cu) are studied using generalized gradient approximation with Hubbard potential (GGA + U) in the domain of density functional theory. The effective Heisenberg model estimates the magnetic exchange coupling constants (J1, J2 and J3) between the spins of Mn and Cu sites. The exchange constants J1 and J3 in MnCu3V4O12 and Cu(1)Cu3(2)V4O12 reveal their long rang interactions through super exchange mechanism. The ground state magnetic order is explained from the optimized energy curves; which reveal the G-type anti-ferromagnetic (AFM) order of MnCu3V4O12 and A-type AFM order of Cu(1)Cu3(2)V4O12. Heisenberg model and magnetic susceptibility are in well agreement in the estimation of Neel temperature 15.1/50 and 15.6/25 K respectively. There is a strong correlation between the magnetism of the A-site atoms (Mn, Cu(1), and Cu(2)), as well as the metallic character of the B-site atoms (V, as well as the tiny contribution of A-site Cu(2) and Mn atoms). Due to AFM metallic nature it is expected that these compounds could be used in spintronic technology and magnetic cloaking devices.

Microfluidic Surface Shields: Control of Flow and Diffusion over Sensitive Surfaces

Selective shielding of surfaces from flow and diffusion is essential in many domains of physics and engineering. We propose and characterize experimentally a framework to shield both flow and diffusion simultaneously within a reshapable fluid volume. To do so, we directly use flow singularities to generate conformal transforms of the diffusion profile, taking inspiration from classical cloaking devices. The properties of microfluidic surface shields as open-space diffusion filters (O-filters) are further investigated. An analytical model is provided, which yields an outstanding match with experimental measurements of both flow and tracer diffusion. Via conformal mapping, these measurements reveal a strong topological link with classical H-filters in microfluidics.

Mechanical unfeelability concentrator through topology optimization

Mechanical cloaks made from elastic metamaterials that make objects unfeelable have been developed, but usually each cloaking device performs only a single function. In this Letter, we present our computational attempts at instilling two different functionalities, cloaking and stress concentration, in a metastructure designed through topology optimizing a mechanical cloak-concentrator for linear elasticity. Solving this optimization problem involved improving two different objective functions, one for cloaking and the other for stress concentration. As an optimizer, the covariance matrix adaptation evolution strategy was adopted to find the optimal level set functions, which express the design configurations with clearly delineated boundaries. Optimized cloak-concentrators reproduce outer displacements as if nothing is present despite the presence of a system concentrating stress on an interior domain. We also present demonstrations of topology optimized configurations for mechanical unfeelability concentrators that perform robustly under loads at any location around the system and under the cloaked region of arbitrary Young's modulus. Our design scheme paves the way for designing metastructures that accommodate various multiple functions via the means of elastostatics.