Cloak Region Research Articles

Realizing the thinnest hydrodynamic cloak in porous medium flow

Transformation mapping theory offers us great versatility to design invisible cloaks for the physical fields whose propagation equations remain invariant under coordinate transformations. Such cloaks are typically designed as a multi-layer shell with anisotropic material properties, which makes no disturbance to the external field. As a result, an observer outside the cloak cannot detect the existence of this object from the field disturbances, leading to the invisible effect in terms of field prorogation. In fact, for many prorogating fields, at a large enough distance, the field distortion caused by an object is negligible anyway; thus, a thin cloak is desirable to achieve near-field invisibility. However, a thin cloak typically requires more challenging material properties, which are difficult to realize due to the huge variation of anisotropic material parameters in a thin cloak region. For a flow field in a porous medium, by applying the bilayer cloak design method and integrating the inner layer with the obstacle, we successfully reduce the anisotropic multi-layer cloak into an isotropic single-layer cloak. By properly tailoring the permeability of the porous medium, we realize the challenging material parameters required by the ultrathin cloak and build the thinnest shell-shaped cloak of all physical fields up to now. The ratio between the cloak’s thickness and its shielding region is only 0.003. The design of such an ultrathin cloak may help to achieve the near-field invisibility and concealment of objects inside a fluid environment more effectively.

Polarization-insensitive 3D conformal-skin metasurface cloak

Electromagnetic metasurface cloaks provide an alternative paradigm toward rendering arbitrarily shaped scatterers invisible. Most transformation-optics (TO) cloaks intrinsically need wavelength-scale volume/thickness, such that the incoming waves could have enough long paths to interact with structured meta-atoms in the cloak region and consequently restore the wavefront. Other challenges of TO cloaks include the polarization-dependent operation to avoid singular parameters of composite cloaking materials and limitations of canonical geometries, e.g., circular, elliptical, trapezoidal, and triangular shapes. Here, we report for the first time a conformal-skin metasurface carpet cloak, enabling to work under arbitrary states of polarization (SOP) at Poincaré sphere for the incident light and arbitrary conformal platform of the object to be cloaked. By exploiting the foundry three-dimensional (3D) printing techniques to fabricate judiciously designed meta-atoms on the external surface of a conformal object, the spatial distributions of intensity and polarization of its scattered lights can be reconstructed exactly the same as if the scattering wavefront were deflected from a flat ground at any SOP, concealing targets under polarization-scanning detections. Two conformal-skin carpet cloaks working for partial- and full-azimuth plane operation are respectively fabricated on trapezoid and pyramid platforms via 3D printing. Experimental results are in good agreement with numerical simulations and both demonstrate the polarization-insensitive cloaking within a desirable bandwidth. Our approach paves a deterministic and robust step forward to the realization of interfacial, free-form, and full-polarization cloaking for a realistic arbitrary-shape target in real-world applications.

Numerical Assessment of the Performance of Elastic Cloaks for Transient Flexural Waves

A relevant application of transformation elastodynamics has shown that flexural waves in a Kirchhoff-Love plate can be diverted and channeled to cloak a region of the ambient space. To achieve the goal, an orthotropic meta-structural plate should be employed. However, the corresponding mathematical transformation leads to the presence of an unwanted strong compressive prestress, likely beyond the buckling threshold of the structure, with a set of in-plane body forces to warrant equilibrium. In addition, the plate must possess, at the same time, high bending stiffnesses, but a null twisting rigidity. With the aim of estimating the performance of cloaks modelled with approximate parameters, an in-house finite element code, based on a subparametric technique, is implemented to deal with the cloaking of transient waves in orthotropic thin plates. The tool allows us to explore the sensitivity of specific stiffness parameters that may be difficult to match in a real cloak design. In addition, the finite element code is extended to investigate a meta-plate interacting with a Winkler foundation, to confirm how the subgrade modulus should transform in the cloak region.

Controlling Out-of-Plane Elastic Shear Wave Propagation With Broadband Cloaking

A major challenge in designing a perfect invisibility cloak for elastic waves is that the mass density and elasticity tensor need to be independent functions of its radius with a linear transformation medium. The traditional cloak for out-of-plane shear waves in elastic membranes exhibits material properties with inhomogeneous and anisotropic shear moduli and densities, which yields a poor or even negative cloaking efficiency. This paper presents the design of a cylindrical cloak for elastic shear waves based on a nonlinear transformation. This excellent broadband nonlinear cloak only requires variation of its shear modulus, while the density in the cloak region remains unchanged. A nonlinear ray trajectory equation for out-of-plane shear waves is derived and a parameter to adjust the efficiency of the cylindrical cloak is introduced. Qualities of the nonlinear invisibility cloak are discussed by comparison with those of a cloak with the linear transformation. Numerical examples show that the nonlinear cloak is more effective for shielding out-of-plane elastic shear waves from outside the cloak than the linear cloak and illustrate that the nonlinear cloak for shear waves remains highly efficient in a broad frequency range. The proposed nonlinear transformation in conjunction with the ray trajectory equation can also be used to design nonlinear cloaks for other elastic waves.

Two-dimensional invisibility anti-cloak structured by a homogeneous anisotropic medium

A two-dimensional invisibility anti-cloak constructed from a positive homogeneous anisotropic medium is proposed that is easier to implement compared with previously reported invisibility anti-cloak structures. Theoretical analysis and numerical simulations show that the proposed invisibility anti-cloak can allow external electromagnetic waves to penetrate into the cloak region, while the concealing effect of the invisibility cloak is not affected for outside viewers. Design details and full-wave simulation results are provided.

Influence of nonlinear coordinate transformation on thermal cloaking performances of multilayered cylindrical thermal cloaks

Layered discretization is an efficient way of realizing thermal cloaks. The aim of this paper is to investigate the effect of nonlinear coordinate transformation on the thermal cloaking performance of multilayered cylindrical thermal cloak. Two indexes are defined to quantify the thermal cloaking performance. Nonlinear coordinate transformations with different forms are established to design the multilayered structures and the thermal cloaking performances are analyzed by full-wave simulations. The results show that the thermal cloaking performances of multilayered structures are directly influenced by the form of coordinate transformation which determines the distributions of heat flows in the cloak region. Through the active changing of the form of coordinate transformation, the thermal cloaking performance of multilayered structure can be controlled and optimized. In addition, a three-piecewise linear coordinate transformation is designed to concentrate the heat flux into the middle region of the cloak and the corresponding multilayered structure is demonstrated to have near-perfect thermal cloaking performance. These results may provide a beneficial guidance to achieve performance improvements for multilayered structures of thermal cloaks.

Broadband Carpet Cloak Designed using Nanocomposite Metamaterials for 3- 5��������Wavelength Range

In this paper, a broadband carpet cloak using nanocomposite layers to operate in whole 3–5-μm wavelength band is designed successfully. For this purpose, spherical nanometals or doped semiconductors with desired plasma frequency are proposed. In this way, the refractive index of the cloak as an analytic function is considered by optical conformal mapping. In a two-dimensional structure, we approximated the obtained refractive index inside each pixel to be constant. Each pixel is considered as an effective medium that possesses nanoparticles with desired plasma frequency. Therefore, plasma frequency of nanoparticles should be adjusted proportional to the desired value of refractive index in each two-dimensional pixels, which is approximated to be constant (two-dimensional layers approximation). The cloak region is compacted for broadband performance. To compact the cloak region, refractive index values should be greater than 1 and realization of these required values is possible by designing nonresonant structures.

Midinfrared Invisibility Cloak Design Using Composite Optical Materials

In this paper, conventional transformation optics is applied to design an optical cloak in mid-IR region (5 μm) for TM polarization to conceal cylindrical object using composite optical structures. Effective medium theory in this method will be used. Based on required permittivity and permeability for TM radiation, we approximate the cloak region by a 25 polymeric composite cylindrical layers. Each layer is a polymeric medium (Poly Tetra Fluor Ethylene) with random distribution of plasmonic spherical nanoparticles, which possess desired plasma frequency in order to achieve required permittivity at operational wavelength. The results show that by changing the plasma frequency of spherical particles in each layer, there is possibility of cloak design at other operational wavelengths in midinfrared range. In order to calculate the effective permittivity of each layer, Clausius–Mossotti formula is used. Based on this idea, an infrared cloak design is shown and completely simulated.

Design of Cylindrical Metamaterial Invisible Cloak with Non-Singular Electromagnetic Tensors

Based on line transformation, cylindrical metamaterial invisible cloak with non-singular electromagnetic tensors is designed. The results of simulation software based on finite element methods (FEM) show that the singularity in the electromagnetic tensors of the cloak has been eliminated, comparing with the cylindrical cloak with ideal tensors. From the simulation results, although there is electromagnetic perturbation in the free-space of the solution region, this cloak exhibits nice electromagnetic invisible performance. The bending wave front is clearly seen when the incoming wave transmits into the cloak region. Because of the non-singular electromagnetic tensors, the designed cloak may have more engineering application than the cloak with idea tensors.

Ray-optics cloaking devices for large objects in incoherent natural light

A cloak that can hide living creatures from sight is a common feature of mythology but still remains unrealized as a practical device. To preserve the wave phase, the previous cloaking solution proposed by Pendry and colleagues required transformation of the electromagnetic space around the hidden object in such a way that the rays bending around the object inside the cloak region have to travel faster than those passing it by. This difficult phase preservation requirement is the main obstacle for building a broadband polarization-insensitive cloak for large objects. Here we propose a simplified version of Pendry’s cloak by abolishing the requirement for phase preservation, as it is irrelevant for observation using incoherent natural light with human eyes, which are phase and polarization insensitive. This allows for a cloak design on large scales using commonly available materials. We successfully demonstrate the cloaking of living creatures, a cat and a fish, from the eye.