Omnidirectional Invisibility Research Articles

A novel smart hybrid multimorph piezoelectric spherical shell cloak for broadband near-perfect underwater acoustic camouflage applications

Non-visual auditory camouflage plays a major role in the art of underwater deception. In this work, a hybrid active/semi-active omnidirectional cloaking shell structure composed of alternate complementary piezoelectric and smart viscoelastic (PZT/SVE) actuator layers is proposed that can effectively conceal a three dimensional underwater macroscopic object from broadband incident sound waves. The smart hybrid structure incorporates a finite sequence of fully active parallel-connected multimorph PZT constraining layers inter-stacked with semi-active SVE core layers both of which are collaboratively operative in the framework of a Particle Swarm Optimized (PSO) multiple-input multiple-output active damping control (MIMO-ADC) scheme. The elasto-acoustic modeling of the problem is conducted by coupling the spatial state space methodology based on the classical three-dimensional exact piezoelasticity theory with the wave equations for the inner and outer acoustic domains. The acoustic cloaking performance of proposed configuration is evaluated for four distinct classes of highly functional SVE interlayer materials with tunable (field-dependent) rheological properties, namely, magnetorheological elastomer (MRE), shape memory polymer (SMP), electrorheological fluid (ERF), and magnetorheological shear thickening polishing fluid (MRSTPF). Extensive numerical results reveal significant broadband reductions of the far-field backscattering amplitude in the (f∞θ=π,kexRex)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\left|{f}_{\\infty }\\left(\ heta =\\pi ,{k}_{\ ext{ex}}{R}_{\ ext{ex }}\\right)\\right|)$$\\end{document} as well as the percentage error of external cloaked field (%Err)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$(\\%\ ext{Err})$$\\end{document} by incorporating a sufficient number of smart multimorph PZT/SVE material layers. Furthermore, it is concluded that comparable low frequency acoustic cloaking effects is possible without expenditure of any external energy just by employing the entirely inactive MRSTPF-based cloak as an alternative to the semiactive or fully active multimorph PZT/SVE cloaks. The outcome of proposed study can advantageously serve as the first step towards practical development and experimental implementation of future high performance smart acoustic cloaking devices with expanded broadband near-perfect omnidirectional invisibility for three dimensional objects of diverse geometries.

Multiband Omnidirectional Invisibility Cloak.

Transformation optics (TO) provides a powerful tool to manipulate electromagnetic waves, enabling the design of invisibility cloaks, which can render objects invisible. Despite many years of research, however, invisibility cloaks experimentally realized thus far can only operate at a single frequency. The narrow bandwidth significantly restricts the practical applications of invisibility cloaks and other TO devices. Here, a general design strategy is proposed to realize a multiband anisotropic metamaterial characterized by two principal permittivity components, i.e., one infinite and the other spatially gradient. Through a proper transformation and combination of such metamaterials, an omnidirectional invisibility cloak is experimentally implemented, which is impedance-matched to free space at multiple frequencies. Both far-field numerical simulations and near-field experimental mappings confirm that this cloak can successfully suppress scattering from multiple large-scale objects simultaneously at 5 and 10GHz. The design strategy and corresponding practical realization bring multiband transformation optical devices one step closer to reality.

Full-parameter omnidirectional transformation optical devices.

Transformation optics (TO) provides an unprecedented technique to control electromagnetic (EM) waves by engineering the constitutive parameters of the surrounding medium through a proper spatial transformation. In general, ideal transformation optical devices require simultaneous electric and magnetic responses along all three dimensions. To ease the practical implementation, previous studies usually made use of reduced parameters or other simplified approaches, which inevitably introduce extra reflection or unwanted phase shift. Up to today, experimental realizations of full-parameter transformation optical devices in free space are still quite limited. Here, a general design strategy is proposed to solve this problem. As a specific example, a full-parameter spatial-compression TO medium with constitutive parameters taking the diagonal form diag(a, a, 1/a) for the TM wave incidence was designed and realized experimentally. Such spatial-compression TO media were then applied to the implementation of an ideal omnidirectional invisibility cloak capable of concealing a large-scale object over a wide range of illumination angles. Both the simulation and experiment confirm that the cloak allows for nearly unity transmission of EM waves in the forward direction without introducing extra scattering or phase shift. This work constitutes an important stepping stone for future practical implementation of arbitrary full-parameter omnidirectional transformation optical devices.

Dual-Band Omnidirectional Invisibility of a Thin Slab with Antiscattering Coatings on Both Sides

Making an object invisible means that an incident electromagnetic wave is not scattered with the object being present, which has been widely studied over the past few decades. Nevertheless, rendering omnidirectional invisibility for a large object in free space is still challenging. Here, we present a dual-band antiscattering coating using a double-layered metamaterial. By putting such coatings on both sides of a thin dielectric slab, omnidirectional invisibility is observed. It is shown that the peculiar double-layered metamaterial can provide electric polarization neutralization to the original thin slab, and the Maxwell Garnett mixing rule is safely invoked to guarantee the full elimination of scattering regardless of the incident angle. Full-wave simulations and experimental measurements exhibit that a composite made of a dielectric slab sandwiched by two antiscattering coatings can avoid any scattering in both the C and Ku bands with arbitrary incident angles. The proposed approach is quite simple and nondestructive to the original objects, which can be conveniently scaled to other scenarios including terahertz and optical applications, for boosting various internet of things devices, radar radomes, noninterference isolators in optics, and invisibility cloaks.

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.

Achieving tunable Kerker-type invisibility for a radiation-enhanced electrically small antenna.

Low-temperature gaseous plasmas exhibit great potential in designing tunable and reconfigurable electromagnetic devices. In this paper, based on an overdense-underdense core-shell plasma structure, tunable Kerker-type invisibility for a radiation-enhanced electrically small antenna is achieved, where dominant scattering direction can be mutated between backward and forward while omnidirectional invisibility and signal enhancement are maintained. Moreover, by electromagnetic multipole decompositions, it is shown that the underdense outer plasma with a negative polarizability is able to weaken the strength and modulate the phase of the electric dipolar scattering component (a_{1}), while the magnetic dipolar term (b_{1}) nearly remains unchanged. Consequently, quasi-first and -second Kerker conditions are fulfilled near the cutoff band of a_{1}.

Observation of 2D omnidirectional invisibility from an electrically large cluster under TE polarization.

The concept of perfect invisibility in free space implies an object neither reflects nor refracts optical waves coming from arbitrary directions, regardless of its shape and size. An optimal solution to realize such a peculiar phenomenon is to tune the constitutive parameters of the object to be identical to air. In particular, to render zero extinction from an existing object by covering some additional structures, is of importance for practical implementations, which is challenging. Here, we demonstrate and propose that a thin metallic wire can be tuned to be air-like under TE polarization, with the aid of an external enclosure. This is achieved through a precise dispersion engineering with independently controllable electric and magnetic responses. Consequently, an electrically large cluster composed of multiple thin wires can be safely hidden in free space, without any macroscopic cloaking structure, which is verified by full-wave simulations and experiments. The measured results on an electrically large airplane-like sample show the excellent performance of 2D omnidirectional invisibility at the designed frequency. This proposed metamaterial would be helpful in enhancing the mechanical stability, electrical conduction, and heat dissipation of a device (or system) by extra wires (or pipes), without disturbing its electromagnetic characteristics.

Three-dimensional large-scale acoustic invisibility cloak with layered metamaterials for underwater operation

A class of three-dimensional acoustic cloak for underwater operation is designed by using a multi-area coordinate transformation method. To overcome the difficulty of achieving materials with the ideal parameters of the cloak in nature, layered metamaterials consisting of mercury and water are designed based on effective medium theory. The acoustic properties of the layered metamaterials can be characterized by the anisotropic densities and isotropic modulus, which can satisfy the material requirements of the cloak shell. This design has many advantages over previous designs of acoustic cloak, including a large cloaking area, omnidirectional invisibility, broad working frequency and easier design methodology. Our transformation strategy and method of metamaterials design offer a cost-effective way and efficient technique for fabricating a large-scale cloak. Excellent invisibility is achieved by using simulations based on the finite element method. The good cloaking performances have demonstrated great potential in the promotion of the practical applications of a large-scale acoustic cloak, especially for low-frequency sound manipulation underwater.

Transformation-Invariant Metamaterials.

The fundamental semiconductor concept of doping has recently been transplanted to photonics in the platform of epsilon-near-zero (ENZ) media. By doping nonmagnetic impurities, ENZ media can exhibit almost arbitrary magnetism. However, this original photonic doping approach results only in isotropic media and thus cannot achieve impedance matching for all incident angles. We extend the photonic doping approach of ENZ media by adding anisotropy, which enables full transparency with omnidirectional impedance matching. More importantly, such anisotropically doped ENZ media preserve their material parameters under arbitrary coordinate transformations, thereby providing a powerful platform to construct various ideal transformation optical devices. As an example, a full-parameter omnidirectional invisibility cloak is demonstrated to hide objects from a wide range of incident angles. The transformation-invariant material proposed not only supplements and extends the rising technologies of ENZ media but also constitutes a significant step towards the practical implementation of ideal transformation optical devices.

Perfect broadband invisibility in isotropic media with gain and loss.

We offer a simple route to perfect omnidirectional invisibility in a spectral band of desired width. Our approach is based on the observation that in two dimensions a complex potential v(x,y) is invisible for incident plane waves with a wavenumber not exceeding a pre-assigned value α, provided that its Fourier transform with respect to y, which we denote by v˜(x,Ky), vanishes for Ky≤2α. We can fulfill this condition for potentials modeling the permittivity profile of an optical slab. Such a slab is perfectly invisible for any transverse electric wave whose wavenumber is in the range [0,α]. Our results also apply to transverse magnetic waves propagating in a medium with a relative permittivity ϵ^(x,y) that is a smooth bounded function with a positive real part.

Theory and Implementation of Scattering-Dark-State Particles at Microwave Frequencies

Recently, a special scattering phenomenon due to nanostructured metallic–dielectric particles was reported. With the plasmonic dispersion of metals at optical frequencies, these nanoparticles can exhibit a “scattering dark state,” showing a theoretically zero Rayleigh scattering. In this paper, aiming to implement such particles at microwave frequencies, we explain this scattering state using antenna theory. The analytical result shows that when the distance between two short dipoles approaches zero, their far-field radiations can be mutually canceled at the frequency where the derived condition is satisfied. Furthermore, simulation and experiments are performed to verify that either a single particle or a random collection of such particles could have omnidirectional invisibility. The derived theory also provides a new understanding to conventional microwave structures such as frequency-selective surfaces. Our method can be applied to a broad spectrum from radio frequency to optical regime, and can potentially inspire new applications such as perfect antenna radomes.

Digital integral cloaking

Toward the goal of achieving broadband and omnidirectional invisibility, we propose a method for practical invisibility cloaking. We call this “digital cloaking,” where space, angle, spectrum, and phase are discretized. Experimentally, we demonstrate a two-dimensional (2D) planar, ray optics, digital cloak by using lenticular lenses, similar to “integral imaging” for three-dimensional (3D) displays. Theoretically, this can be extended to a good approximation of an “ideal” 3D cloak. With continuing improvements in commercial digital technology, the resolution limitations of a digital cloak can be minimized.

Invisible metallic mesh

A solid material possessing identical electromagnetic properties as air has yet to be found in nature. Such a medium of arbitrary shape would neither reflect nor refract light at any angle of incidence in free space. Here, we introduce nonscattering corrugated metallic wires to construct such a medium. This was accomplished by aligning the dark-state frequencies in multiple scattering channels of a single wire. Analytical solutions, full-wave simulations, and microwave measurement results on 3D printed samples show omnidirectional invisibility in any configuration. This invisible metallic mesh can improve mechanical stability, electrical conduction, and heat dissipation of a system, without disturbing the electromagnetic design. Our approach is simple, robust, and scalable to higher frequencies.

Perfect conformal invisible device with feasible refractive indexes

Optical conformal mapping has been used to construct several isotropic devices with novel functionalities. In particular, a conformal cloak could confer omnidirectional invisibility. However, the maximum values of the refractive indexes needed for current designs are too large to implement, even in microwave experiments. Furthermore, most devices designed so far have had imperfect impedance matching and therefore incomplete invisibility functionalities. Here we describe a perfect conformal invisible device with full impedance matching everywhere. The maximum value of refractive index required by our device is just about five, which is feasible for microwave and terahertz experiments using current metamaterial techniques. To construct the device, we use a logarithmic conformal mapping and a Mikaelian lens. Our results should enable a conformal invisible device with almost perfect invisibility to be made soon.

A THIRD WAY TO CLOAK AN OBJECT: COVER-UP WITH A BACKGROUND OBJECT (Invited Paper)

Based on a space folding transformation, we propose a new way to hide an object in full space, namely, to cover-up the scattering of the hidden object with the scattering of a background object so that only the scattering of the background object can be detected by an outside observer and the hidden object disappears electromagnetically (a very weak \ghost or perturbation may appear inside the strong background object image in an experiment). The present method is essentially difierent from previous methods of cloaking an object, namely, the optically isolated cloak and the scattering cancellation cloak, and thus provide a third way to cloak an object. Unlike all the previous methods of cloaking, the present full-space omni-directional invisibility simultaneously has the following features: (i) the hidden object can \see the outside world without being detected; (ii) the cloak can still work when some characteristics (e.g., shape and medium) of the hidden object change; and (iii) there is no need to know the information of the incident wave in advance. The present work furthers efiorts to achieve invisibility and conceal an object in a real environment in full space.