Good Cloaking Performance Research Articles

Performance investigation of nanoscale thermal cloak by the perforated silicon film

In recent years, nanoscale thermal cloak, as a representative of nanoscale heat flux regulation devices, has attracted a lot of attention from researchers. However, the existing design methods are relatively complicated and all adopt constant temperature boundaries, the temperature changes constantly in the real environment, which greatly hinders its engineering applications. In this paper, inspired by phonon localization theory, we construct a nanoscale thermal cloak by a perforated silicon membrane and evaluate its cloaking performance and dynamic response. Results show that when the perforated area is fixed, the more the number of holes, the better the cloaking performance. In addition, the nanoscale thermal cloak still exhibits good cloaking performance in the dynamic environment. Finally, the cloaking mechanism is analyzed by calculating the phonon density of states (PDOS) and mode participation rate (MPR), and the reduction of thermal conductivity in the functional region is attributed to phonon localization.

Ultrathin Conformal Magnetic Invisible Cloak for Irregular Objects.

Magnetic invisible cloaking has been previously demonstrated but only limited to objects with rotational geometries either in spherical or cylindrical shapes, for which the classic analytical bilayer scheme could be strictly applied to design the hiding coat. In this work, we show that a quasi-static cloaking effect could be achieved for irregular objects, e.g., metals with sharp edges, using a numerical optimization scheme. In the quasi-static limit, it is unambiguously proved that the disturbance of the irregular geometries could be well compensated by the inhomogeneous distribution of the soft ferromagnetic (FM) layer either in permeability values or in shapes under the framework of a bilayer cloak. An FM mesh coat with a constant thickness of 0.5 mm was successfully engineered to meet the specific requirements. Experimentally, good cloaking performance with a field disturbance of less than 0.5% has been achieved for a 2 × 2 × 5 cm3 brass bar in a wide frequency range from ∼10 to 250 kHz. A commercial metal scanner was also applied to verify the practical potential. The general strategy to hide almost arbitrary objects was discussed in the end. In principle, the numerical conformal coat engineered by the composite material proposed here could be broadly extended for objects with various geometries.

Topology-optimized thermal carpet cloak expressed by an immersed-boundary level-set method via a covariance matrix adaptation evolution strategy

Our proposal, a topology optimization method for thermal carpet cloaks, eliminates the disturbance caused by a bump obstacle on a flat boundary and, despite the presence of the bump, reproduces the temperature distribution as if no bump is present. Structural topologies are expressed using an immersed-boundary level-set method and the optimal topologies corresponding to the optimal sets of the level-set functions are explored using the covariance matrix evolution strategy for minimizing the difference between the temperature field under a flat boundary and that for a cloaked bump on the boundary. The difference is evaluated as an objective function and, in the best instance, this function value for the optimal cloak approaches a value 0.000214% of that when no carpet cloak is present around the bump. By moving the location of the heat source, we investigated the performance response of the topology-optimized thermal carpet cloaks for different angles of heat flow. The thermal carpet cloaks presented exhibit good cloaking performance for heat flowing over a wide angle.

Scattering suppression and cloak for electrically large objects using cylindrical metasurface based on monolayer and multilayer mantle cloak approach

The authors proposed analytical models applicable to monolayer and multilayer mantle cloaks for suppressing the scattering fields of electrically large conducting objects using printed ultrathin metasurfaces. To analyse the proposed structure, the two-sided impedance surface method is used and the scattering cross section (SCS) is obtained using Mie theory. It is shown that using surface impedance expression obtained from the analytical model for monolayer mantle cloak they can design a proper unit cell of a metasurface to reduce significantly total scattering from the cylinder. The proposed mantle cloaks in this work are mainly designed to minimise simultaneously several scattering multi-poles of a relatively large object around considerably broad bandwidth. Numerical results and full-wave simulation show good cloaking performance in designed frequency. The simulated results using their analytical models as well as the full-wave results show that the multilayer mantle cloak is an optimised candidate for cloaking of electrically large objects. The proposed multilayer mantle cloak confirms 3.6 dB SCS gain to cancel the scattering fields of a perfect electric conductor cylinder with a radius of 1.425λ0 at the designed frequency. Finally, they consider a cluster of closely spaced and overlapping three cloaked cylinders for suppression of electromagnetic fields.

Two-Dimensional Arbitrarily Shaped Acoustic Cloaks With Triangular Patterns of Homogeneous Properties

Acoustic cloaking is an intriguing phenomenon that has attracted lots of attention. The required inhomogeneous and anisotropic properties of acoustic cloaks derived with transformation acoustics make them difficult to realize. In this paper, a new mapping relation is presented. An acoustic cloak can be divided into any number of arbitrary triangular patterns, which are mapped from similar patterns in virtual space. Transformation from one triangular domain to another leads to homogeneous properties using transformation acoustics. The resulting cloak is composed of homogeneous triangular parts, each having just two alternating layers of material. The manner of division of the cloak affects the properties of each triangular part dramatically, which can be leveraged to vary the properties of each triangular part for more realistic material properties. Simulations of models based on this method show good cloaking performance at reducing the reflected and scattered waves due to the cloaked obstacle.

Two-dimensional cylindrical thermal cloak designed by implicit transformation method

As a new-type technology of heat management, thermal metamaterials have attracted more and more attentions recently and thermal cloak is a typical case. Thermal conductivity of thermal cloak designed by coordinate transformation method is usually featured by inhomogeneity, anisotropy and local singularity. Explicit transformation method, which is commonly used to design thermal cloak with the coordinate transformation known in advance, has insufficient flexibility, making it hard to proactively reduce the difficulty of device fabrication. In this work, we designed the thermal conductivity of two-dimensional (2D) cylindrical thermal cloak using the implicit transformation method without knowledge of the coordinate transformation in advance. With two classes of generation functions taken into consideration, this study adopted full-wave simulations to analyze the thermal cloaking performances of designed thermal cloaks. Material distributions and simulation results showed that the implicit transformation method has high flexibility. The form of coordinate transformation not only influences the homogeneity and anisotropy but also directly influences the thermal cloaking performance. An improved layered structure for 2D cylindrical thermal cloak was put forward based on the generation function g ( r ) = r 15 , which reduces the number of the kinds of constituent materials while guaranteeing good thermal cloaking performance. This work provides a beneficial guidance for reducing the fabrication difficulty of thermal cloak.

Design and analysis of the trapeziform and flat acoustic cloaks with controllable invisibility performance in a quasi-space

We present the design, implementation and detailed performance analysis for a class of trapeziform and flat acoustic cloaks. An effective large invisible area is obtained compared with the traditional carpet cloak. The cloaks are realized with homogeneous metamaterials which are made of periodic arrangements of subwavelength unit cells composed of steel embedded in air. The microstructures and its effective parameters of the cloaks are determined quickly and precisely in a broadband frequency range by using the effective medium theory and the proposed parameters optimization method. The invisibility capability of the cloaks can be controlled by the variation of the key design parameters and scale factor which are proved to have more influence on the performance in the near field than that in the far field. Different designs are suitable for different application situations. Good cloaking performance demonstrates that such a device can be physically realized with natural materials which will greatly promote the real applications of invisibility cloak.

Bandwidth enhancement of transformation optics-based cloak with reduced parameters

In this paper, dispersive cloak design with broad bandwidth and minimal scattering cross section is proposed by appropriately selecting a radial permeability for each shell in a discretized reduced cloak. The dispersive medium is constructed by artificially varying the inner radius of the cloak with frequency, and this variation results into unique material properties at every frequency. The variation of inner radius of the cloak with frequency is artificial since the actual physical dimension of inner radius remains invariant. The relation between bandwidth and geometrical parameters of cloak is obtained by ensuring that transformation media must satisfy the condition that group velocity must remain less than the speed of light along every direction for a finite frequency range. The proposed cloak provides $$8.9\,\%$$ bandwidth with respect to the center frequency for $$50\,\%$$ reduction in total scattering cross section, and at the design frequency, the minimum scattering cross section obtained is $$0.266$$ . The proposed dispersive cloak design is verified by numerical full-wave simulations results which also confirm good cloaking performance.

Mechanical cloak design by direct lattice transformation

Spatial coordinate transformations have helped simplifying mathematical issues and solving complex boundary-value problems in physics for decades already. More recently, material-parameter transformations have also become an intuitive and powerful engineering tool for designing inhomogeneous and anisotropic material distributions that perform wanted functions, e.g., invisibility cloaking. A necessary mathematical prerequisite for this approach to work is that the underlying equations are form invariant with respect to general coordinate transformations. Unfortunately, this condition is not fulfilled in elastic-solid mechanics for materials that can be described by ordinary elasticity tensors. Here, we introduce a different and simpler approach. We directly transform the lattice points of a 2D discrete lattice composed of a single constituent material, while keeping the properties of the elements connecting the lattice points the same. After showing that the approach works in various areas, we focus on elastic-solid mechanics. As a demanding example, we cloak a void in an effective elastic material with respect to static uniaxial compression. Corresponding numerical calculations and experiments on polymer structures made by 3D printing are presented. The cloaking quality is quantified by comparing the average relative SD of the strain vectors outside of the cloaked void with respect to the homogeneous reference lattice. Theory and experiment agree and exhibit very good cloaking performance.

Design of a broadband ultra-large area acoustic cloak based on a fluid medium

A broadband ultra-large area acoustic cloak based on fluid medium was designed and numerically implemented with homogeneous metamaterials according to the transformation acoustics. In the present work, fluid medium as the body of the inclusion could be tuned by changing the fluid to satisfy the variant acoustic parameters instead of redesign the whole cloak. The effective density and bulk modulus of the composite materials were designed to agree with the parameters calculated from the coordinate transformation methodology by using the effective medium theory. Numerical simulation results showed that the sound propagation and scattering signature could be controlled in the broadband ultra-large area acoustic invisibility cloak, and good cloaking performance has been achieved and physically realized with homogeneous materials. The broadband ultra-large area acoustic cloaking properties have demonstrated great potentials in the promotion of the practical applications of acoustic cloak.

An elasto-mechanical unfeelability cloak made of pentamode metamaterials.

Metamaterial-based cloaks make objects different from their surrounding appear just like their surrounding. To date, cloaking has been demonstrated experimentally in many fields of research, including electrodynamics at microwave frequencies, optics, static electric conduction, acoustics, fluid dynamics, thermodynamics and quasi two-dimensional solid mechanics. However, cloaking in the seemingly simple case of three-dimensional solid mechanics is more demanding. Here, inspired by invisible core-shell nanoparticles in optics, we design an approximate elasto-mechanical core-shell 'unfeelability' cloak based on pentamode metamaterials. The resulting three-dimensional polymer microstructures with macroscopic overall volume are fabricated by rapid dip-in direct laser writing optical lithography. We quasi-statically deform cloak and control samples in the linear regime and map the displacement fields by autocorrelation-based analysis of recorded movies. The measured and the calculated displacement fields show very good cloaking performance. This means that one can elastically hide objects along these lines.

Metamaterials. Invisibility cloaking in a diffusive light scattering medium.

In vacuum, air, and other surroundings that support ballistic light propagation according to Maxwell's equations, invisibility cloaks that are macroscopic, three-dimensional, broadband, passive, and that work for all directions and polarizations of light are not consistent with the laws of physics. We show that the situation is different for surroundings leading to multiple light scattering, according to Fick's diffusion equation. We have fabricated cylindrical and spherical invisibility cloaks made of thin shells of polydimethylsiloxane doped with melamine-resin microparticles. The shells surround a diffusively reflecting hollow core, in which arbitrary objects can be hidden. We find good cloaking performance in a water-based diffusive surrounding throughout the entire visible spectrum and for all illumination conditions and incident polarizations of light.

Tunable ultrathin mantle cloak via varactor-diode-loaded metasurface.

We propose a tunable strategy for the ultrathin mantle cloak via metasurface. The tunable cloak is implemented by loading varactor diodes between two neighboring horizontal metallic strips which constitute the metasurface. We demonstrate that the varactor diodes enable the capacitive reactance of the metasurface to be tunable from -157 Ω to -3 Ω when the DC bias voltage is properly changed. The active metasurface is then explored to cloak conformally a conducting cylinder. Both numerical and experiment results show that the cloaking frequency can be continuously controlled from 2.3 GHz to 3.7 GHz by appropriately adjusting the bias voltage. The flexible tunability and good cloaking performance are further examined by the measured field distributions. The advanced features of tunability, low profile, and conformal ability of the ultrathin cloak pave the way for practical applications of cloaking devices.

Homogeneous anisotropic dielectric invisibility carpet cloak made of Al2O3 plates

In this paper, we report the fabrication of a homogeneous anisotropic invisibility carpet cloak that works for transverse magnetic (TM) wave in free space. Linear coordinate transformation is employed for the design of the dielectric tensors, and the cloak is made of Al2O3 slabs with a Teflon background. The triangular invisible region has a height of 32.4 mm (one third of the height of the whole device) and a width of 200 mm. The cloaking effect is examined in far-field free-space transmission experiments by measuring the scattering parameters. The results show our device has very good cloaking performance in a wide frequency range from 8 to 12 GHz.

Experimentally demonstrated a unidirectional electromagnetic cloak designed by topology optimization

Electromagnetic invisible devices usually designed by transformation optics are rather complicated in material parameters and not suitable for general applications. Recently a topology optimized cloak based on level-set method was proposed to realize nearly perfect cloaking by Fujii et al [Appl. Phys. Lett. 102, 251106 (2013)]. In this work we experimentally implemented this idea and fabricated a unidirectional cloak with a relative large invisible region made of single dielectric material. Good cloaking performance was verified through measurement which consists very well with numerical simulation. The advantages and disadvantages of this optimization method are also discussed.

First experimental demonstration of an isotropic electromagnetic cloak with strict conformal mapping

In the past years quasi-conformal mapping has been generally used to design broadband electromagnetic cloaks. However, this technique has some inherit practical limitations such as the lateral beam shift, rendering the device visible or difficult to hide a large object. In this work we circumvent these issues by using strict conformal mapping to build the first isotropic cloak. Microwave near-field measurement shows that our device (with dielectric constant larger than unity everywhere) has a very good cloaking performance and a broad frequency response. The present dielectric approach could be technically extended to the fabrication of other conformal devices at higher frequencies.

A Negative Conductivity Material Makes a dc Invisibility Cloak Hide an Object at a Distance

AbstractThe theoretical design and the first experimental verification of an exterior dc invisibility cloak that can hide an object from dc detection at a distance are presented. Based on the transformation optics theory, the exterior dc cloak requires negative conductivity material to create folded geometry, which will cancel the real geometry of detected object in distance and make it invisible. Negative conductivities are designed and realized using active devices, together with resistor networks, to generate the equivalent conductivity materials required by the exterior dc cloak. An experimental sample of the dc cloak is fabricated on the printed circuit board and the measured result has excellent agreement with numerical and circuit simulations, showing very good cloaking performance at a distance.

Far field free-space measurement of three dimensional hole –in –Teflon invisibility cloak

In this paper, we report a fabrication of a three-dimensional (3D) carpet cloak that works for any polarization in free space. Two-dimensional (2D) conformal mapping is first employed and the 3D structure is generated by a rotation of the 2D cloak. The structure of the cloak is hole-in-dielectric. The triangular invisible region has a height of 36 mm (one third of the height of the whole device) and a width of 240 mm. The cloaking effect is examined in free space by measuring the scattering parameters. The results show our device has very good cloaking performance in a wide frequency range from 4 to 10 GHz.

Route towards cylindrical cloaking at visible frequencies using an optimization algorithm

We derive a model based on the Maxwell-Garnett effective-medium theory that describes a cylindrical cloaking shell composed of metal rods which are radially aligned in a dielectric host medium. We propose and demonstrate a minimization algorithm that calculates for given material parameters the optimal geometrical parameters of the cloaking shell such that its effective optical parameters fit the best to the required permittivity distribution for cylindrical cloaking. By means of sophisticated full-wave simulations we find that a cylindrical cloak with good performance using silver as the metal can be designed with our algorithm for wavelengths in the red part of the visible spectrum ($623\phantom{\rule{0.28em}{0ex}}$nm $<\ensuremath{\lambda}<773\phantom{\rule{0.28em}{0ex}}$nm). We also present a full-wave simulation of such a cloak at an exemplary wavelength of $\ensuremath{\lambda}=729\phantom{\rule{0.28em}{0ex}}$nm ($\ensuremath{\hbar}\ensuremath{\omega}=1.7\phantom{\rule{0.28em}{0ex}}$eV) which indicates that our model is useful to find design rules of cloaks with good cloaking performance. Our calculations investigate a structure that is easy to fabricate using standard preparation techniques and therefore pave the way to a realization of guiding light around an object at visible frequencies, thus rendering it invisible.

Dispersive full-wave finite-difference time-domain analysis of the bipolar cylindrical cloak based on the effective medium approach

A dispersive full-wave finite-difference time-domain model is used to study the performance of bipolar cylindrical invisibility cloaking devices. We have considered two different cloaking structures generated by the mapping of the σ axis and the mapping of the τ axis of bipolar coordinates. The permittivity and permeability tensors for the cloaking devices are obtained from an effective medium approach in general relativity. The σ-axis mapped bipolar cylindrical cloak is found to be imperfect, and the cloaking performance is found to depend on the polarization of the incident waves, the direction of propagation of the waves, and the loss tangents of the metamaterial. Only the case of TM waves for the specific propagation direction shows good cloaking performance. On the other hand, the τ-mapped cloaking device shows good cloaking performance for all polarizations and directions of propagation. However, this structure has a singular boundary at the inner radius. Realistic cloaking materials with loss still show a cloak that is working, but attenuated backscattering waves exist.