Two-dimensional Cloak Research Articles

Hybrid nonsingular mappings in the multi-objective design of a transverse-magnetic reduced cloak

This paper describes the integrated use of two design strategies for controlling the permittivity and permeability of a transformation electromagnetics two-dimensional cloak. This hybrid process permits more freedom in the choice of the constitutive properties, which could simplify the implementation with metamaterials. Because simplicity of materials and scattering performance are inherently conflicting characteristics, a multi-objective genetic algorithm is applied to optimize them. According to the presented results for a transverse-magnetic layered cloak, it is possible to have a good performance in terms of scattering even with an approximated reduced set of parameters with unitary axial permeability.

Optimal control for electromagnetic cloaking metamaterial parameters design

In this paper, we develop an optimal control problem to optimize the permittivity and permeability of the metamaterial which can have the invisible cloaking effect. The inverse problem is calibrated as an optimization problem constrained by the time-harmonic Maxwell’s equations. Proper objective functional is introduced with coefficients of the Maxwell’s equations as control variables. We adopt the discontinuous Galerkin method to solve the state equation. Several two-dimensional cloaking benchmark problems are used to test our optimal control method. Numerical results demonstrate that the optimized permittivity and permeability of the metamaterial can achieve the cloaking effectively.

Impact of Radii Ratios on a Two-Dimensional Cloaking Structure and Corresponding Analysis for Practical Design at Optical Wavelengths

This work is an extension to the evaluation and analysis of a two-dimensional cylindrical cloak in the Terahertz or visible range spectrum using Finite Difference Time-Domain (FDTD) method. It was concluded that it is possible to expand the frequency range of a cylindrical cloaking model by careful scaling of the inner and outer radius of the simulation geometry with respect to cell size and/or number of time steps in the simulation grid while maintaining appropriate stability conditions. Analysis in this study is based on a change in the radii ratio, that is, outer radius to inner radius, of the cloaking structure for an array of wavelengths in the visible spectrum. Corresponding outputs show inconsistency in the cloaking pattern with respect to frequency. The inconsistency is further increased as the radii ratio is decreased. The results also help to establish a linear relationship between the transmission coefficient and the real component of refractive index with respect to different radii ratios which may simplify the selection of the material for practical design purposes. Additional performance analysis is carried out such that the dimensions of the cloak are held constant at an average value and the frequency varied to determine how a cloaked object may be perceived by the human eye which considers different wavelengths to be superimposed on each other simultaneously.

A revisit to imperfect acoustic cloak of multi-layered shell structures considering sound speed and impedance matching

Abstract An imperfect multi-layered acoustic cloak is proposed for a two-dimensional cloaking zone based on feasible material properties. In this model, the matching of sound speed and acoustic impedance has been investigated, and the effects of material and geometric properties on the imperfect cloak have been studied for better design of the imperfect cloak. The imperfect cloak could be improved using appropriate changes in the design parameters. By increasing the thickness of the high density layer and with some changes in the sound speeds between the high density and the low density layers, the imperfect cloaking model showed better cloaking performance than Cummer–Schurig cloak. Also, present results show that the sound speed matching is more important for acoustic cloaking than the impedance matching. These results can be applied as a practical design guide for two-dimensional cloaks using multilayered structures composed of naturally existing materials.

Level set based topology optimization for optical cloaks

This letter presents a level set-based topology optimization method that provides discrete cloaking configurations with superior performance. In some cases, the amount of light scattered around the two-dimensional cloaking structures is less than one-tenth that obtained in previous studies. Optimal configurations that express different geometrical characteristics can be obtained by adjusting a regularization parameter. The obtained configurations are free from grayscales, areas of intermediate density between that of dielectric materials or air, and the use of a level set method provides clear structural boundaries. The level set functions are given on grid points and the dielectric boundaries are interpreted as lines on the iso-surface of the level set functions. The finite element method is used for light scattering analyses, computations of the adjoint variable field, and when updating the level set functions.

Dispersion Engineering for Multifunctional Photonic Crystal Based Nanophotonic Devices at Infrared Wavelengths

TIn this paper, we report the design, the fabrication and the near field optical microscopy of Negative Index Material (NIM) and GRadient INdex (GRIN) photonic crystal based flat lenses. They were fabricated on the basis of an InPbased photonic crystal technological platform including hole and pillar networks fabrication at nanometer scale. They show the possibility of sub-wavelength focusing by all dielectric periodic or quasi-periodic crystals. Particular attention is paid to the analysis of SNOM images using three-dimensional simulations. Finally, in order to demonstrate the versatility of our approach, a two-dimensional cloaking device mixing hole and pillar arrays is evaluated to pave the way for future integrated nanophotonic devices with complex functionalities.

Obtaining a nonsingular two-dimensional cloak of complex shape from a perfect three-dimensional cloak

A general method is proposed to construct an approximate nonsingular two-dimensional (2D) cloak by taking constitutive parameter tensors (CPTs) of a three-dimensional perfect cloak on a mirror-symmetric cross section in order to avoid any singularity at the inner boundary of a perfect 2D cloak. As two examples, reduced 2D models for an elliptic cloak and a bow-tie cloak are derived. The reduced 2D models are characterized explicitly with CPT of finite values and good cloaking effects are verified numerically.

Imperfect Cloaking Devices Based on Metamaterials

Cloaking devices designed using the coordinate transform approach were shown to be realizable, at least in principle, within the realm of electromagnetic metamaterials. In this paper we investigate the strictness of conditions imposed on the parameters of metamaterial cloaks by calculating the degree of wave scattering when those parameters have variations with respect to theoretically ideal values. A simple idea is used to obtain analytic results for the case of the nonideal two-dimensional cloaking cylinder. Also, results of realistic flnite element simulations of the Helmholtz equation are presented and it is found that they are in excellent agreement with the analytic results.

Quasistatic cloaking of two-dimensional polarizable discrete systems by anomalous resonance

Discrete systems of infinitely long polarizable line dipoles are considered in the quasistatic limit, interacting with a two-dimensional cloaking system consisting of a hollow plasmonic cylindrical shell. A numerical procedure is described for accurately calculating electromagnetic fields arising in the quasistatic limit, for the case when the relative permittivity of the cloaking shell has a very small imaginary part. Animations are given which illustrate cloaking of discrete systems, both for the case of induced dipoles and induced quadrupoles on the interacting particles. The simulations clarify the physical mechanism for the cloaking.