Unidirectional Cloak Research Articles

Fourier metasurface cloaking: unidirectional cloaking of electrically large cylinder under oblique incidence.

The existence of a non-electrically-small scatterer adjacent to the source can severely distort the radiation and lead to a poor electromagnetic compatibility. In this work, we use a conducting hollow cylinder to shield a cylindrical scatterer. The cylinder is shelled with a single dielectric layer enclosed by an electromagnetic metasurface. The relationship between the scattering field and the surface impedance is derived analytically. By optimizing the Fourier expansion coefficients of the surface impedance distribution along ϕ-dimension, the scattering cross-section can be effectively reduced. This unidirectional cloaking method is valid for both TM/TE and non-TM/TE incident field and is not limited to a plane-wave incident field. The accuracy and effectiveness of the method are verified by four cloaking scenarios in microwave regime. We demonstrate that with the surface impedance obtained by the proposed method, a metasurface is designed with physical subwavelength structures. We also show a cloaking scenario under a magnetic dipole radiation, which is closer to the case of a realistic antenna. This method can be further applied to cloaking tasks in terahertz and optical regimes.

Modelling Optical Invisibility in a Lenticular Microlens array with methods of Geometrical Optics and Weber's Contrast

Though many advances have been made in exploring the ‘cloaking’ capabilities of metamaterials and optical systems in a wide range of the light spectrum, e.g. electromagnetic cloaking or paraxial ray optics, the phenomenon of unidirectional cloaking associated with ‘woodpile’ optical structures has not yet been widely discussed in the scope beyond experimental findings or digital integral cloaking; moreover, the relatively low cost of a lenticular array renders the investigation of its concealment properties in the visible regime of great interest for practical purposes. Thus, the following research paper provides an comprehensive theoretical model and analytical criterion for the phenomenon of unidirectional cloaking in the visible regime associated with a lenticular microlens array within the methods of computational ray optics (computing lens’s spherical aberration using governing ray optics equations) and Weber’s contrast (as perceived by the human eye) by considering the concealed object’s virtual image as seen through a lenticular lens. Having shown that the criterion for practical invisibility adheres to our experimental results in determining the boundary case of invisibility, we employed it to explore the effect system parameters have on the array’s cloaking capabilities. We established that the critical parameter for the array’s cloaking capability is the diverging angle of an array’s lenticular lens, the value of which positively correlates with the refraction index and the lens concave segment’s curvature. Therefore, a lenticular array with most prominent cloaking capabilities consists of lenses with the greatest admissible index of refraction and a arc measure for the lens’s concave segment.

Dynamic cloaking of a diamond-shaped hole in elastic plate

Invisibility cloaks for flexural waves have mostly been examined in a continuous-wave regime, while invisibility is likely to deteriorate for short pulses. Here, we propose the practical realization of a unidirectional invisibility cloak for flexural waves based on an area-preserving coordinate transformation. Time-resolved experiments reveal how the invisibility cloak deviates a pulsed plane wave from its initial trajectory and how the initial wavefront perfectly recombines behind the cloak, leaving the diamond-shaped hole invisible, notwithstanding the appearance of a forerunner. Three-dimensional full-elasticity simulations support our experimental observations.

Strictly conformal transformation optics for directivity enhancement and unidirectional cloaking of a cylindrical wire antenna

Using conformal transformation optics, a cylindrical shell made of an isotropic refractive index material is designed to improve the directivity of a wire antenna while making it unidirectionally invisible. If the incident wave comes from a specific direction, it is guided around the wire. Furthermore, when an electrical current is used to excite the wire, the dielectric shell transforms the radiated wave into two lateral beams, improving directivity. The refractive index of the dielectric shell is calculated using the transformation optics recipe after establishing a closed-form conformal mapping between an annulus and a circle with a slit. The refractive index is then modified and discretized using a hexagonal lattice. Ray-tracing and full-wave simulations with COMSOL Multiphysics are used to validate the functionality of the proposed shell.

Transforming Space with Non-Hermitian Dielectrics.

Coordinate transformations are a versatile tool to mold the flow of light, enabling a host of astonishing phenomena such as optical cloaking with metamaterials. Moving away from the usual restriction that links isotropic materials with conformal transformations, we show how nonconformal distortions of optical space are intimately connected to the complex refractive index distribution of an isotropic non-Hermitian medium. Remarkably, this insight can be used to circumvent the material requirement of working with refractive indices below unity, which limits the applications of transformation optics. We apply our approach to design a broadband unidirectional dielectric cloak, which relies on nonconformal coordinate transformations to tailor the non-Hermitian refractive index profile around a cloaked object. Our insights bridge the fields of two-dimensional transformation optics and non-Hermitian photonics.

Inverse design of invisibility cloaks using the optical theorem

We develop and apply an optimization method to design unidirectional invisibility cloaks. Our method is based on minimizing the forward scattering amplitude of the cloaked object which, by the optical theorem, is equivalent to the total cross section. The use of the optical theorem circumvents the need to evaluate and integrate the scattering amplitude over an angle at each iteration, and thus provides a simpler, more computationally efficient objective function to optimize structures. We implement the approach using gradient descent optimization and present several gradient-permittivity unidirectional cloaks that reduce scattering by metallic targets of different sizes and shapes.

Optical cloaking and invisibility: From fiction toward a technological reality

Optical invisibility, which started in the pages of fiction before becoming an intriguing quest of humankind for over a century, has blossomed into a remarkable scientific journey toward reality over the last two decades. Perfect optical cloaking requires the total scattering of electromagnetic waves around an object at all angles, all polarizations, over a wide frequency range, irrespective of the medium. Such a device is still far-fetched, requiring the transformation of space around a cloaked region such that the phase velocity is faster than other areas to preserve the phase relationships. However, by simplifying the invisibility requirements, pioneering work on spherical transformation cloaks, carpet cloaks, plasmonic cloaks, and mantle cloaks has been realized in narrowband microwave, infrared, and even optical wavelengths. In this Tutorial, we review the theoretical basis for invisibility cloaking, from spherical transformational optics to non-Euclidian cases, and discuss their limitations. Subsequently, we highlight the recent trends in realizing reconfigurable intelligent cloaks to overcome the traditional limitations of wideband operation and parallel efforts in unidirectional cloaking. Because the human eye is insensitive to the phase and polarization of visible light, a class of ray optics cloaking devices has been recently developed by eliminating phase preservation requirements. Notably, we focus on the recent progress achieved on invisibility cloaks that function in natural incoherent light and can be realized using standard optical components. We conclude this Tutorial with a prospective of potential applications and the practicality of optical cloaks in everyday life.

Shear-wave manipulation by embedded soft devices

In accordance with hyperelastic transformation theory, a range of shear-wave manipulation devices can be designed with neo-Hookean materials pre-deformed properly. However, how such devices fit the background medium remains elusive. In this study, a systematic formula is developed in terms of elastic wave transmission and reflection between un-deformed and pre-deformed hyperelastic materials. By both theoretical analyses and numerical simulations, the shear-wave propagation from an un-deformed neo-Hookean material to the pre-deformed one is investigated. Among the three typical deformations, “constrained” uniaxial tension and simple-shear are found to be able to ensure total transmission, whereas ordinary uniaxial tension and hydrostatic compression could cause reflection. Thus, three embedded shear-wave manipulation devices are proposed, namely, a unidirectional cloak, a splicing beam bend and a concave lens; their performance is verified by numerical simulations. This study may underpin the design and realization of soft-matter-based wave control devices. Potential applications can be expected in nondestructive testing, impact protection, biomedical imaging, as well as soft robotics.

A Unidirectional TM-Wave Cloak With Full Parameters

Unidirectional cloaks or carpet cloaks have become an essential branch of invisibility cloaks due to non-extreme parameters that are relatively easy for realization and potential applications. So far, a unidirectional cloak for transverse electric (TE) polarized wave has been successfully realized to hide an object in the air along a single direction with almost ideal performance. However, a practical method to achieve a unidirectional cloak for transverse magnetic (TM) polarization is still missing. In this paper, we successfully have designed, fabricated, and measured a full-parameter unidirectional TM-wave cloak in free space. In comparison to the previously-realized TE-wave counterpart, our cloak uses a perfect electric conductor boundary to obey the free-space symmetry condition of the TM incident wave and is therefore much more convenient to fabricate. The cloaking performance of the fabricated sample has been demonstrated via both full-wave numerical simulation and near-field measurement.

Thermal surface transformation and its applications to heat flux manipulations.

A new method to control heat flux, called thermal surface transformation (TST), is introduced from transformation thermodynamics. Compared with transformation thermodynamics, TST has many advantage. First, there is no mathematical calculation during the whole process in TST (novel thermal devices can be designed graphically in a surface-to-surface way). Second, all thermal devices of various functions, shapes and sizes designed by TST only require one homogenous anisotropic thermal medium, i.e., thermal-null medium (TNM). With the help of the effective medium theory, TNM can be realized by layered copper and expanded polystyrene, whose performance on controlling heat flux by TST is verified by numerical simulations. Many examples are given, including thermal imaging devices, thermal unidirectional cloak, concentrator, rotator and thermal focusing devices.

Beam splitting and unidirectional cloaking using anisotropic zero-index photonic crystals

We propose an anisotropic photonic crystal (PC) structure with close-to-zero effective parameters. The anisotropic zero-index property of PCs provides complex iso-frequency contours, where their shapes can change from ellipses or hyperbolae to a linear crossing. A mechanism of light beam splitting can be achieved by utilizing the linear crossing shaped iso-frequency curve. The manipulation of light beam propagations is verified by both numerical simulations and microwave experiments. By using pure dielectrics and scaling down to optical wavelengths, we also propose a design of nonperiodic PC structures to achieve optical unidirectional cloaking without size or shape limitation. Our design provides a promising platform for cloaking applications in the optical regime.

New devices for unidirectional acoustic cloaking for large objects in infinite frequency bandwidth

For years, researchers struggled to realize the acoustic invisibility cloaks with simple for large objects in broadband. In this paper, a new-type unidirectional acoustic cloak is proposed and designed by using eight especial triangles region filled with a certain refractive index inert gas. The acoustic wave can be manipulated to pass through the model but “by-pass” the cloaked region according to the traditional Snell’s law. The incident plane waves will remain without any change. Therefore, the cloaked region cannot be detected. Our designed cloak can realize the infinite length of the invisibility region along the propagation direction and offer good cloaking effect in any frequency range. The simulation results agree well with the theoretical predictions and verify the nondetectability effect of the unidirectional acoustic cloak. Our mechanism may be useful for further actual applications in hiding large objects acoustically.

Broadband unidirectional acoustic cloak based on phase gradient metasurfaces with two flat acoustic lenses

Narrow bandwidth and bulky configuration are the main obstacles for the realization and application of invisible cloaks. In this paper, we present an effective method to achieve broadband and thin acoustic cloak by using an acoustic metasurface (AMS). In order to realize this cloak, we use slitted unit cells to design the AMS due to the advantage of less energy loss, broad operation bandwidth, and subwavelength thickness. According to the hyperboloidal phase profile along the AMS, the incident plane waves can be focused at a designed focal spot by the flat lens. Furthermore, broadband acoustic cloak is obtained by combining two identical flat lenses. The incident plane waves are focused at the center point in between of the two lenses by passing through one lens, and then recovered by passing through the other one. However, they cannot reach the cloaked regions in between of the two lenses. The simulation results can verify the non-detectability effect of the acoustic cloak. Our study results provide an available and simple approach to experimentally achieve the acoustic cloak, which can be used in acoustic non-detectability for large objects.

Broadband unidirectional cloak designed by eikonal theory.

A method for designing optical device is derived based on the eikonal theory, which could obtain the eikonal distribution on a curved surface according to the propagation characteristics of the subsequent light wave. Then combining with the phase matching condition, we designed a broadband unidirectional cloak. Different from the reported unidirectional cloaks, the proposed one could be used for coherent wave and has continuous broadband performance. Moreover, it has three cloaked regions. Full-wave simulation results verify the properties of the cloak.

Large‐Scale Far‐Infrared Invisibility Cloak Hiding Object from Thermal Detection

Natural animals such as crotaline and boid snakes have the ability to detect infrared radiation, which is emitted by any living object while it is invisible to human eyes. To evade thermal detection, various attempts, such as the stealth or low observability technologies, are generally made to reduce the infrared signature. However, the validity of these technologies is background dependent. In this paper, the first, large‐scale, far‐infrared, unidirectional invisibility cloak in air environment is experimentally demonstrated. The cloak constructed from germanium can smoothly guide the infrared wave around the hidden object when facing the incident wave direction. The cloak works for full polarization in a broad far‐infrared spectrum. This work provides a new way to manipulate infrared radiation for future applications.

Broadband unidirectional cloaks based on flat metasurface focusing lenses

Bandwidth extension and thickness reduction are now the two key issues of cloaks. In this paper, we propose to achieve broadband, thin uni-directional electromagnetic (EM) cloaks using metasurfaces. To this end, a wideband flat focusing lens is firstly devised based on high-efficiency transmissive metasurfaces. Due to the nearly dispersionless parabolic phase profile along the metasurface in the operating band, incident plane waves can be focused efficiently after passing through the metasurface. Broadband unidirectional EM cloaks were then designed by combining two identical flat lenses. Upon illumination, the incident plane waves are firstly focused by one lens and then are restored by the other lens, avoiding the cloaked region. Both simulation and experiment results verify the broadband unidirectional cloak. The broad bandwidth and small thickness of such cloaks have potential applications in achieving invisibility for electrically large objects.

Unidirectional Cloaking Based on Metasurfaces with Balanced Loss and Gain

Rendering things invisible has turned out to be in the domain of science, not magic. The authors offer an approach to cloaking based on a simple conformal surface wrapped around the object of interest. Their theory shows that by covering the object with a stealth surface on the illuminated side, and a time-reversed version of the same surface on the shadowed side, one can realize ideal cloaking, independent of the size of the object, in a robust, broadband, ultrathin, and lossless technology.

A unidirectional acoustic cloak for multilayered background media with homogeneous metamaterials

The acoustic cloak, which can make an object hard to detect acoustically in a homogeneous background, has attracted great attention from researchers in recent years. The inhomogeneous background media were considered in this paper. The relative constitutive parameters were derived for acoustic cloaks working in multilayered media. And a unidirectional acoustic cloak for layered background media was proposed, designed and implemented successfully in a wide frequency range. In water and NaCl aqueous solution, the acoustic cloak was designed and realized with homogeneous metamaterials which were composed of steel and porous materials. The effective parameters of the unit cells of the cloak were determined by using the effective medium theory. Numerical results demonstrated excellent cloaking performance and showed that such a device could be physically realized with natural materials which will greatly promote the real applications of an invisibility cloak in inhomogeneous backgrounds.

Terahertz carpet cloak based on a ring resonator metasurface

In this work we present the concept and design of an ultrathin (\ensuremath{\lambda}/22) terahertz (THz) unidirectional carpet cloak based on the local phase compensation approach enabled by gradient metasurfaces. A triangular surface bump with center height of 4.1 mm (1.1\ensuremath{\lambda}) and tilt angle of 20\ifmmode^\circ\else\textdegree\fi{} is covered with a metasurface composed of an array of suitably designed closed ring resonators with a transverse gradient of surface impedance. The ring resonators provide a wide range of control for the reflection phase with small absorption losses, enabling efficient phase manipulation along the edge of the bump. Our numerical results demonstrate a good performance of the designed cloak in both near field and far field, and the cloaked object mimics a flat ground plane within a broad range of incidence angles, over 35\ifmmode^\circ\else\textdegree\fi{} angular spectrum centered at 45\ifmmode^\circ\else\textdegree\fi{}. The presented cloak design can be applied in radar and antenna systems as a thin, lightweight, and easy to fabricate solution for radio and THz frequencies.

Geometric optics-based multiband cloaking of large objects with the wave phase and amplitude preservation.

The geometric optics principles are used to develop a unidirectional transmission cloak for hiding objects with dimensions substantially exceeding the incident radiation wavelengths. Invisibility of both the object and the cloak is achieved without metamaterials, so that significant widths of the cloaking bands are provided. For the preservation of wave phases, the λ-multiple delays of waves passing through the cloak are realized. Suppression of reflection losses is achieved by using half-λ multiple thicknesses of optical elements. Due to periodicity of phase delay and reflection suppression conditions, the cloak demonstrates efficient multiband performance confirmed by full-wave simulations.