Nonmagnetic Cloak Research Articles

Cloaking in shallow-water waves via nonlinear medium transformation

A major obstacle in designing a perfect cloak for objects in shallow-water waves is that the linear transformation media scheme (also known as transformation optics) requires spatial variations of two independent medium properties. In the Maxwell’s equation and for the well-studied problem of electromagnetic cloaking, these two properties are permittivity and permeability. Designing an anisotropic material with both variable permittivity and variable permeability, while challenging, is achievable. On the other hand, for long gravity waves, whose governing equation maps one-to-one to the single polarization Maxwell’s equations, the two required spatially variable properties are the water depth and the gravitational acceleration; in this case changing the gravitational acceleration is simply impossible. Here we present a nonlinear transformation that only requires the change in one of the medium properties, which, in the case of shallow-water waves, is the water depth, while keeping the gravitational acceleration constant. This transformation keeps the governing equation perfectly intact and, if the cloak is large enough, asymptotically satisfies the necessary boundary conditions. We show that with this nonlinear transformation an object can be cloaked from any wave that merely satisfies the long-wave assumption. The presented transformation can be applied as well for the design of non-magnetic optical cloaks for electromagnetic waves.

Fundamentals of designing cylindrical high-order transformation optics invisibility cloaks using silver–silica metamaterials

Metamaterials have effective properties that are distinct from their composites as they consist of engineered-designed properties that are not in nature. In order to be able to design a metamaterial, we should establish sufficient understanding of the properties of the constituents. This will enable us to engineer new effective parameters of the metamaterial. We shall perform a detailed analytical study for the effective parameters and the constituents’ parameters of silver–silica metamaterial. This will define the optical response of the mixture at different sizes of the inclusions’ and different volume fractions of the silver and silica. Also an optimum value of the volume fraction is proposed to achieve a broadened resonance optical response. Finally, we propose the design technique and constraints of a non-magnetic optical cloaking device, based on high-order transformation optics with different volume fractions of silver and silica.

Near-perfect nonmagnetic invisibility cloaking

We suggest a way to obtain a near-perfect nonmagnetic invisibility cloak structure. Within the particular example of cylindrical geometry and transverse magnetic polarization of light, we show that such a cloak can operate for a cylindrical object of very large diameter, keeping the total scattering cross section limited by $2\ensuremath{\lambda}/\ensuremath{\pi}$. We also demonstrate that proposed cloak does not require extremely high values of dielectric permittivity, in contrast to the cloaks designed according to a transformation optics technique.

Photorealistic ray tracing of free-space invisibility cloaks made of uniaxial dielectrics

The design rules of transformation optics generally lead to spatially inhomogeneous and anisotropic impedance-matched magneto-dielectric material distributions for, e.g., free-space invisibility cloaks. Recently, simplified anisotropic non-magnetic free-space cloaks made of a locally uniaxial dielectric material (calcite) have been realized experimentally. In a two-dimensional setting and for in-plane polarized light propagating in this plane, the cloaking performance can still be perfect for light rays. However, for general views in three dimensions, various imperfections are expected. In this paper, we study two different purely dielectric uniaxial cylindrical free-space cloaks. For one, the optic axis is along the radial direction, for the other one it is along the azimuthal direction. The azimuthal uniaxial cloak has not been suggested previously to the best of our knowledge. We visualize the cloaking performance of both by calculating photorealistic images rendered by ray tracing. Following and complementing our previous ray-tracing work, we use an equation of motion directly derived from Fermat's principle. The rendered images generally exhibit significant imperfections. This includes the obvious fact that cloaking does not work at all for horizontal or for ordinary linear polarization of light. Moreover, more subtle effects occur such as viewing-angle-dependent aberrations. However, we still find amazingly good cloaking performance for the purely dielectric azimuthal uniaxial cloak.

Scattering characteristics of nonmagnetic invisibility cloak with minimized scattering

A kind of transformation functions is proposed to realize the nonmagnetic invisibility cloak with minimized scattering on the basis of generalized transformation. By matching the impedance at the outer surface of the cloak, the transformations with two parameters are determined. The good performance of the cloak is indicated by the full wave simulation based on the finite element method. Furthermore, based on the calculation of total scattering cross section, it is shown that the scattering cross section is very sensitive to the different parameters even though the impedance at the exterior boundary matches perfectly with the free space. In addition, from the effective media theory, an alternating layered system composed of two isotropic materials is proposed to realize experimentally the cloak.

Optimized cylindrical invisibility cloak with minimum layers of non-magnetic isotropic materials

We present optimized design of cylindrical invisibility cloak with minimum layers of non-magnetic isotropic materials. Through an optimization procedure based on genetic algorithm, simpler cloak structure and more realizable material parameters can be achieved with better cloak performance than that of an ideal non-magnetic cloak with a reduced set of parameters. We demonstrate that a cloak shell with only five layers of two normal materials can result in an average 20 dB reduction in the scattering width for all directions when covering the inner conducting cylinder with the cloak. The optimized design can substantially simplify the realization of the invisibility cloak, especially in the optical range.

Generalized transformation for nonmagnetic invisibility cloak with minimized scattering

We propose a generalized transformation for a nonmagnetic invisibility cloak with minimized scattering. In comparison with the previous transformation, it has been demonstrated that the unphysical singularity at the inner boundary and the limitation of the thickness of the cloak shell can be removed by choosing the appropriate transformation function. Furthermore, our generalized transformation is used to design two different nonmagnetic cloaks without considering the unphysical singularity and the limitation of the thickness. Based on the effective medium theory, an alternating layered system composed of two isotropic materials is employed to realize these cloaks. Moreover, through calculating the scattering cross section of the cloak, it is found that such a nonmagnetic cloak can only work in a narrow frequency range.

Molding the light flow at desire

Prof. Shi-ning ZHU How to manipulate light at will, either in space or in time, is one of the central problems in the optics community. In the past decade, researchers have demonstrated the ability to control light in unprecedented ways with the aid of artificially engineered materials, among which metamaterials have attracted world-wide interest. Metamaterials are composites of subwavelength artificial structures. From effective medium theory, a metamaterial can be characterized as a homogeneous material, whose EM properties (i.e., electric permittivity and magnetic permeability) mainly rely on the internal structures rather than the chemical elements of the metamaterial. Some unusual material properties unavailable in nature, such as optical magnetism and negative refractive index, have been realized by metamaterials. With the full freedom offered by metamaterials, researchers have demonstrated negative refraction, reversed Cherenkov radiation, super lenses, and recent invisibility cloaks, which bring revolutionary impacts on modern optics. Metamaterials have become a very active research area involving many other disciplines. For instance, at the early stage of metamaterial research, negative refraction and superlensing were concurrently explored in photonic crystals. Nowadays, as metamaterials enter the optical regime, the plasmonic effect of metals can play an important role. Indeed, the investigation on coupling between metallic metamaterial elements and biosensing by metamaterials has been rapidly developing. It is expected that the interplay between the communities of photonic crystals, metamaterials, and plasmonics will continue to stimulate exciting ideas and discoveries. Frontiers of Physics in China gathers together experts in these three communities for this special issue. Ten excellent papers present recent progress and a future outlook in artificial wave-functional materials. On the subject of photonic crystals, Y. Liu et al. present ultrafast optical switching from Kerr nonlinearity; J. F. Liu and X. H. Wang demonstrate the manipulation of spontaneous emission by photonic crystal or metallic micro-/nanostructures; while recent research on surface mode optical microcavities in photonic crystals is reviewed by J. Wang and M. Qiu. T. R. Zhai et al. introduce the holographic method to fabricate photonic crystals with low refractive index materials and apply it to the design of microlasers. In terms of metamaterials, H. Liu et al. present a review on the hybridization effect due to the coupling between individual metamaterial elements, which is beyond the traditional effective medium theory. J. M. Hao et al. show how to manipulate light polarizations by microwave and optical metamaterials. Two papers focus on transformation optics. One is by Y. Lai et al., who present the remote illusion device that can transform the stereoscopic image of an object into the illusion of another different object; while in the other paper, D. Bao et al. propose a broadband nonmagnetic dielectric cloak. Sub-diffraction-limited far-field imaging in infrared is investigated by P. Chaturvedi and N. X. Fang. Finally, in plasmonics, A. Garcia-Etxarri et al. reports chemical sensing based on plasmonic nanoparticle aggregation. This special issue summarizes some of the exciting results obtained in the past few years. Without any doubt, we envision that this field will grow, and many more will emerge in the near future.

Minimizing the scattering of a nonmagnetic cloak

Nonmagnetic cloak offers a feasible way to achieve invisibility at optical frequencies using materials with only electric responses. In this letter, we suggest an approximation of the ideal nonmagnetic cloak and quantitatively study its electromagnetic characteristics using a full-wave scattering theory. It is demonstrated that the forward scattering of the impedance matched cloak increases dramatically as the thickness of the cloak decreases. Nevertheless, it is still possible to effectively reduce the total scattering cross section with a very thin cloak whose impedance is not matched to the surrounding material at the outer boundary. Our analysis also provides the flexibility of reducing the scattering in an arbitrary direction.

Experimental demonstration of a nonmagnetic metamaterial cloak at microwave frequencies

Metamaterials have paved the way to unprecedented control of the electromagnetic field1,2. The conjunction with space coordinate transformation has led to a novel "relativity inspired" approach for the control of light propagation. "Invisibility cloak" is the most fascinating proposed devices3,4. However, the realized structures up to now used a graded "metamagnetic" so as to achieve the cloaking function11. Artificial magnetism is however still very challenging to obtain in optics despite the currently promising building blocks13-17, not suited for optical cloaking. We report here the first experimental demonstration of non-magnetic cloak at microwave frequencies by direct mapping of the magnetic field together with the first experimental characterization of a cloak in free space configuration. The diameter of the concealed region is as big as 4.4 in wavelength units, the biggest reported experimentally so far. The principle can be scaled down to optical domain while keeping the compatibility with current nanofabrication technologies.

Nearly perfect nonmagnetic invisibility cloaking: Analytic solutions and parametric studies

Coordinate-transformation approaches to invisibility cloaking rely on the design of an anisotropic, spatially inhomogeneous ``transformation medium'' capable of suitably rerouting the energy flux around the region to conceal without causing any scattering in the exterior region. It is well known that the inherently magnetic properties of such medium limit the high-frequency scaling of practical ``metamaterial'' implementations based on subwavelength inclusions (e.g., split-ring resonators). Thus, for the optical range, nonmagnetic implementations, based on approximate reductions of the constitutive parameters, have been proposed. In this paper, we present an alternative approach to nonmagnetic coordinate-transformation cloaking, based on the mapping from a nearly transparent, anisotropic and spatially inhomogeneous virtual domain. We show that, unlike its counterparts in the literature, our approach is amenable to exact analytic treatment, and that its overall performance is comparable to that of a nonideal (lossy, dispersive, parameter truncated) implementation of standard (magnetic) cloaking.

One-Directional Perfect Cloak Created With Homogeneous Material

A one-directional invisibility cloak is presented in this letter. Perfect invisibility can be achieved for TM waves along certain directions. The parameters are just homogeneously anisotropic and relatively easy for practical realizations. Nonmagnetic cloak is also studied to further reducing the complexity of the cloak parameters. Thin layered systems with alternating isotropic materials are then designed to model the parameters of such kind of cloaks.

Designing the coordinate transformation function for non-magnetic invisibility cloaking

An optical invisibility cloak based on a transformation approach has recently been proposed by a reduced set of material properties due to their easier implementation in reality and little need for an inhomogeneous permeability distribution, but the drawback of undesired scattering caused by the impedance mismatching at the outer boundary is unavoidable in such a cloak. By properly designing the coordinate transformation function to ensure impedance matching at the outer surface, we show that the performance of a nonmagnetic cylindrical cloak could be improved with minimized scattering fields. Using either a single high order power function or an optimized piecewise continuous power function, a cylindrical non-magnetic cloak has been designed with nearly perfect cloaking performance, which is better than those generated with a linear or a quadratic function. Due to the monotonicity of the designed power functions, the resulting cloak has no restriction on the size of the cloaking shell, therefore is suitable for both thick and thin cloaking structures.

A higher‐order optical transformation for nonmagnetic cloaking

AbstractIn coordinate‐transformation‐based approaches to electromagnetic concealment (“cloaking”) of objects, use of higher‐order (quadratic) mappings has been proposed as an effective device to obtain satisfactory responses without the use of magnetic materials that are complicated to synthesize at optical frequencies. In this article, we explore a new higher‐order algebraic transformation, which allows, in principle, for a broader range of applicability and further parametric optimization. Via full‐wave numerical studies of near‐ and far‐field observables, we assess its performance by comparison with various reference cases (nonreduced parameters, quadratic transformation, no cloak). © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 3186–3190, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23905

Two-dimensional metamaterial structure exhibiting reduced visibility at 500 nm

Metamaterials provide unprecedented freedom and flexibility in the creation of new structures, which control electromagnetic wave propagation in very unusual ways. Very recently various theoretical designs for an electromagnetic cloak were suggested and an experimental realization of a partial cloak operating in a two-dimensional cylindrical geometry were reported in the microwave frequency range. We report on an experimental two-dimensional reduced visibility structure that approximates the distribution of the radial component of the dielectric permittivity necessary to achieve nonmagnetic cloaking in the visible frequency range.

Designs for optical cloaking with high-order transformations

Recent advances in metamaterial research have provided us a blueprint for realistic cloaking capabilities, and it is crucial to develop practical designs to convert concepts into real-life devices. We present two structures for optical cloaking based on high-order transformations for TM and TE polarizations respectively. These designs are possible for visible and infrared wavelengths. This critical development builds upon our previous work on nonmagnetic cloak designs and high-order transformations.

Semiclassical description of non magnetic cloaking

We present a semiclassical description of non-magnetic cloaking. The semiclassical result is confirmed by numerical simulations of a gaussian beam scattering from the cloak. Further analysis reveals that certain beams penetrate the non-magnetic cloak thereby degrading the performance.

Nonmagnetic cloak with minimized scattering

In an electromagnetic cloak based on a transformation approach, reduced sets of material properties are generally favored due to their easier implementation in reality, although a seemingly inevitable drawback of undesired scattering exists in such cloaks. Here, the authors suggest the use of high-order transformations to create smooth moduli at the outer boundary of the cloak, therefore completely eliminating the detrimental scattering within the limit of geometric optics. The authors apply this scheme to a nonmagnetic cylindrical cloak and demonstrate that the scattered field is reduced substantially in a cloak with optimal quadratic transformation as compared to its linear counterpart.

Optical cloaking with metamaterials

Artificially structured metamaterials have enabled unprecedented flexibility in manipulating electromagnetic waves and producing new functionalities, including the cloak of invisibility based on coordinate transformation. Here we present the design of a non-magnetic cloak operating at optical frequencies. The principle and structure of the proposed cylindrical cloak are analyzed, and the general recipe for the implementation of such a device is provided. The cloaking performance is verified using full-wave finite-element simulations.