A recently published theory has suggested that a cloak of invisibility is in principle possible, at least over a narrow frequency band. We describe here the first practical realization of such a cloak; in our demonstration, a copper cylinder was "hidden" inside a cloak constructed according to the previous theoretical prescription. The cloak was constructed with the use of artificially structured metamaterials, designed for operation over a band of microwave frequencies. The cloak decreased scattering from the hidden object while at the same time reducing its shadow, so that the cloak and object combined began to resemble empty space.

/pdf/metamaterial-electromagnetic-cloak-at-microwave-frequencies-1qj4zdgl8e.pdf

Metamaterial Electromagnetic Cloak at Microwave Frequencies

Recently, artificially constructed metamaterials have become of considerable interest, because these materials can exhibit electromagnetic characteristics unlike those of any conventional materials. Artificial magnetism and negative refractive index are two specific types of behavior that have been demonstrated over the past few years, illustrating the new physics and new applications possible when we expand our view as to what constitutes a material. In this review, we describe recent advances in metamaterials research and discuss the potential that these materials may hold for realizing new and seemingly exotic electromagnetic phenomena.

/pdf/metamaterials-and-negative-refractive-index-5ehczes1hu.pdf

Metamaterials and negative refractive index.

Metamaterials are typically engineered by arranging a set of small scatterers or apertures in a regular array throughout a region of space, thus obtaining some desirable bulk electromagnetic behavior. The desired property is often one that is not normally found naturally (negative refractive index, near-zero index, etc.). Over the past ten years, metamaterials have moved from being simply a theoretical concept to a field with developed and marketed applications. Three-dimensional metamaterials can be extended by arranging electrically small scatterers or holes into a two-dimensional pattern at a surface or interface. This surface version of a metamaterial has been given the name metasurface (the term metafilm has also been employed for certain structures). For many applications, metasurfaces can be used in place of metamaterials. Metasurfaces have the advantage of taking up less physical space than do full three-dimensional metamaterial structures; consequently, metasurfaces offer the possibility of less-lossy structures. In this overview paper, we discuss the theoretical basis by which metasurfaces should be characterized, and discuss their various applications. We will see how metasurfaces are distinguished from conventional frequency-selective surfaces. Metasurfaces have a wide range of potential applications in electromagnetics (ranging from low microwave to optical frequencies), including: (1) controllable “smart” surfaces, (2) miniaturized cavity resonators, (3) novel wave-guiding structures, (4) angular-independent surfaces, (5) absorbers, (6) biomedical devices, (7) terahertz switches, and (8) fluid-tunable frequency-agile materials, to name only a few. In this review, we will see that the development in recent years of such materials and/or surfaces is bringing us closer to realizing the exciting speculations made over one hundred years ago by the work of Lamb, Schuster, and Pocklington, and later by Mandel'shtam and Veselago.

An Overview of the Theory and Applications of Metasurfaces: The Two-Dimensional Equivalents of Metamaterials

A new type of cloak is discussed: one that gives all cloaked objects the appearance of a flat conducting sheet. It has the advantage that none of the parameters of the cloak is singular and can in fact be made isotropic. It makes broadband cloaking in the optical frequencies one step closer.

Hiding under the Carpet: A New Strategy for Cloaking

The possibility of cloaking an object from detection by electromagnetic waves has recently become a topic of considerable interest. The design of a cloak uses transformation optics, in which a conformal coordinate transformation is applied to Maxwell's equations to obtain a spatially distributed set of constitutive parameters that define the cloak. Here, we present an experimental realization of a cloak design that conceals a perturbation on a flat conducting plane, under which an object can be hidden. To match the complex spatial distribution of the required constitutive parameters, we constructed a metamaterial consisting of thousands of elements, the geometry of each element determined by an automated design process. The ground-plane cloak can be realized with the use of nonresonant metamaterial elements, resulting in a structure having a broad operational bandwidth (covering the range of 13 to 16 gigahertz in our experiment) and exhibiting extremely low loss. Our experimental results indicate that this type of cloak should scale well toward optical wavelengths.

http://science.sciencemag.org/content/sci/323/5912/366.full.pdf

Broadband Ground-Plane Cloak

The guidance conditions of modes with both real and imaginary transverse wave numbers inside a dielectric slab with negative permittivity and permeability are solved. We show that for real transverse wave numbers, cutoffs exist for all modes, unlike in conventional media where the first even mode is always propagating. In addition, we also show that guided modes exist for imaginary transverse wave numbers, with the power concentrated at the interfaces instead of having maxima inside the slab. A graphical method of determining the imaginary transverse wave numbers of the guided modes is introduced which clearly identifies the different conditions of propagation depending on the properties of the slab. In particular, propagation of guided waves inside less dense negative media is shown to be possible.

Guided modes with imaginary transverse wave number in a slab waveguide with negative permittivity and permeability

The mathematical solution for Cerenkov radiation in a novel medium, left-handed medium (LH medium), which has both negative permittivity and permeability, is introduced in this paper. It is shown that the particle motion in the LH medium generates power that propagates backward. In this paper, both dispersion and dissipation are considered for the LH medium. The results show that in such a material, both forward power and backward power exist. In addition, we show that the losses will affect the Cerenkov angle. The idea of building a Cerenkov detector using LH medium is introduced, which could be useful in particle physics to identify charged particles of various velocities.

Čerenkov radiation in materials with negative permittivity and permeability

The coordinate transformation by using form-invariant transformations of Maxwell equations has led to an approach for designing devices with anisotropic metamaterial. In this paper, we present the design methodology for a low profile planar focusing antenna based on the transformation of a parabolic antenna. The electromagnetic behavior of the planar antenna is simulated by a two-dimensional finite element method and the results show that the planar antenna has the same performances as the parabolic antenna. The coordinate transformation technology provides an alternative design method to the conventional antennas.

Planar focusing antenna design by using coordinate transformation technology

Based on electromagnetic wave scattering theory, we demonstrate that magnetic and electric surface currents are induced at the inner boundary of a cylindrical cloak by incoming waves. These surface currents have no counterparts in the coordinate transform theory. In addition, electromagnetic fields can penetrate into the concealed region under specified conditions, but carry no power. The field distribution can be unsymmetrical when a circularly polarized wave is obliquely incident onto the cloak. The far-field scattering for a nonideal cylindrical cloak is also theoretically addressed.

/pdf/response-of-a-cylindrical-invisibility-cloak-to-19cia70vb9.pdf

Response of a cylindrical invisibility cloak to electromagnetic waves

The electromagnetic field solution for a spherical invisibility cloak with an active device inside is established. Extraordinary electric and magnetic surface voltages are induced at the inner boundary of a spherical cloak, which prevent electromagnetic waves from going out. The phase and handness of polarized waves obliquely incident on such boundaries are kept in the reflected waves. The surface voltages due to an electric dipole inside the concealed region are found equal to the auxiliary scalar potentials at the inner boundary, which consequently gain physical counterparts in this case.

/pdf/extraordinary-surface-voltage-effect-in-the-invisibility-ejgmlwhdv8.pdf

Bae-Ian Wu

Papers

Guided modes with imaginary transverse wave number in a slab waveguide with negative permittivity and permeability

Čerenkov radiation in materials with negative permittivity and permeability

Planar focusing antenna design by using coordinate transformation technology

Response of a cylindrical invisibility cloak to electromagnetic waves

Extraordinary surface voltage effect in the invisibility cloak with an active device inside.