Invisibility Cloak Research Articles

A multilayered homogeneous hydrodynamic cloak realized in a free fluid flow

In recent years, hydrodynamic invisibility cloaks have attracted significant attention from the scientific and engineering communities due to their potential in applications such as fluid flow manipulation, drag reduction, submarine stealth, and biomedical engineering. However, cloaks based on transformation mapping theory typically require porous medium flow, limiting practical implementation. To address this, we draw inspiration from Hele-Shaw flow and develop a homogeneous hydrodynamic cloak composed of multiple layers in free fluid flow at low Reynolds numbers (Re≪1). Through structural optimization, we construct a cloaking shell with ten concentric layers of alternating heights, achieving near-perfect cloaking as demonstrated by simulations and experiments. This non-porous cloak design offers valuable insights into the rapidly advancing field of hydrodynamic metamaterials and enables methods for controlling fluid flow at the microscale, with promising applications in microfluidics.

Self-Adaptive Intelligent Metasurface Cloak System with Integrated Sensing Units.

Metasurfaces, which are ultrathin planar metamaterials arranged in certain global sequences, interact uniquely with the surrounding light field and exhibit unusual effects of light modulation. Many interesting applications have been discovered based on metasurfaces, particularly in invisibility cloaks. However, most invisibility cloaks are limited to working in specific directions. Achieving effectiveness in multiple directions requires the metasurface to be designed with both perception and modulation capabilities. Current multi-directional metasurface cloak systems are implemented with discrete components rather than an integrated sensing component. Here, we propose an intelligent metasurface cloak system that integrates sensing units, resulting in the cloaking effect with the help of a real-time direction sensor and an adaptive feedback control system. A reconfigurable reflective meta-atom based on phase modulation is presented. Sensing units replace parts of the meta-atoms in the designed tunable metasurface, integrating with an FPGA responsible for measuring the direction and frequency of the incident wave, constituting a closed-loop system together with the feedback parts. Experimental results demonstrate that the metasurface cloak system can recognize the different directions of the incoming wave, and can adaptively manipulate the reflected phase of EM waves to conceal objects without any human participation.

Antenna Optimization Using Metamaterials

Abstract Metamaterials are artificial structures engineered to exhibit electromagnetic properties not found in natural materials. These materials offer a diverse range of characteristics, including negative refractive index, reverse radiation, and electromagnetic cloaking. These unique properties find applications in microwave and optical domains, enabling improvements in antenna performance, the design of microwave filters, and the creation of flat optical lenses, among others. This study focuses on utilizing metamaterials to enhance antenna performance by boosting gain, reducing mutual coupling between MIMO antennas, converting polarization, and decreasing radar cross-section (RCS). The primary performance metric considered in this investigation is gain enhancement.

DNA-Engineered Degradable Invisibility Cloaking for Tumor-Targeting Nanoparticles.

Nanoparticle (NP) delivery systems have been actively exploited for cancer therapy and vaccine development. Nevertheless, the major obstacle to targeted delivery lies in the substantial liver sequestration of NPs. Here we report a DNA-engineered approach to circumvent liver phagocytosis for enhanced tumor-targeted delivery of nanoagents in vivo. We find that a monolayer of DNA molecules on the NP can preferentially adsorb a dysopsonin protein in the serum to induce functionally invisibility to livers; whereas the tumor-specific uptake is triggered by the subsequent degradation of the DNA shell in vivo. The degradation rate of DNA shells is readily tunable by the length of coated DNA molecules. This DNA-engineered invisibility cloaking (DEIC) is potentially generic as manifested in both Ag2S quantum dot- and nanoliposome-based tumor-targeted delivery in mice. Near-infrared-II imaging reveals a high tumor-to-liver ratio of up to ∼5.1, approximately 18-fold higher than those with conventional nanomaterials. This approach may provide a universal strategy for high-efficiency targeted delivery of theranostic agents in vivo.

Rayleigh surface waves of extremal elastic materials

Extremal elastic materials here refer to a specific class of elastic materials whose elastic matrices exhibit one or more zero eigenvalues, resulting in soft deformation modes that, in principle, cost no energy. They can be approximated through artificially designed solid microstructures. Extremal elastic materials have exotic bulk wave properties unavailable with conventional solids due to the soft modes, offering unprecedented opportunities for manipulating bulk waves, e.g., acting as phonon polarizers for elastic waves or invisibility cloaks for underwater acoustic waves. Despite their potential, Rayleigh surface waves, crucially linked to bulk wave behaviors of such extremal elastic materials, have largely remained unexplored so far. In this paper, we theoretically investigate the propagation of Rayleigh waves in extremal elastic materials based on continuum theory and verify our findings with designed microstructure metamaterials based on pantographic structures. Dispersion relations and polarizations of Rayleigh waves in extremal elastic materials are derived, and the impact of higher order gradient effects is also investigated by using strain gradient theory. This study provides a continuum model for exploring surface waves in extremal elastic materials and may stimulate applications of extremal elastic materials for controlling surface waves.

Disrupting Dichotomies, Dismantling Hegemonies: Studying Caste Reality in Fiction Through Omprakash Valmiki’s Amma and Other Stories

In the Hindi belt, Dalit literature has taken full shape in the post-Ambedkarite and post-Mandal Commission moments, making this literature layered with the understanding of caste, caste politics and its subtleties, with the central question revolving around the issue of realism. The genre of Dalit fiction transcends the societal and literary boundaries that are guided by hegemonic forces and lifts the cloak of invisibility from the ‘other’. It has the potential to disrupt the dichotomy between the centre and the periphery. By undertaking a critical study of Omprakash Valmiki’s anthology Amma and Other Stories, published in 2008, this article is an attempt to locate the creative and liberating power of Dalit fiction that is seamlessly interwoven with reality. Primarily placed in the rural and urban areas of the state of Uttar Pradesh, Valmiki’s writing situates the Dalit reality directly in front of the readers, all the while challenging the representation and prominent discourse of the Hindi literary tradition. It is in the context of realism that this article focuses on the new kind of realism brought in by Dalit fiction and the revolutionary inversion undertaken by it, which subverts and dismantles the hegemonic traditions of society and literature.

Bilateral Symmetric non-Euclidean multi-frequency invisibility

Light propagation in non-Euclidean geometry has become a hot topic in recent years, while transformation optics theory demonstrates unique advantages in this respect. A notable application of transformation optics in non-Euclidean space is non-Euclidean invisibility cloak which avoids the challenges of negative refraction and anisotropic materials. In this work, we propose another configuration for non-Euclidean invisibility, capable of achieving invisible across a wide spectrum. Using coordinate transformation, we convert this non-Euclidean invisibility into planar gradient medium and validate its effects through full wave simulations. We also discover that the corresponding gradient medium can further relax the material parameters. Our findings suggest diverse strategies for non-Euclidean invisibility and planar gradient media, potentially advancing optical invisibility and transformation optics in non-Euclidean spaces.

Ultrabroadband invisibility cloaking design of a concentric multilayered cylindrical metamaterial

An ultrabroadband invisibility cloak is designed using a genetic algorithm (GA) for a cylindrical target with an infinite length. The cloaking band is 0.4 to 0.7µm, covering almost the entire visible light range. The invisibility cloaking medium consists of concentric multilayered cylindrical metamaterials (CM). When the cloaking target is covered with a well-designed 26+ layer CM, the average scattering cross-section is 6% or less of the uncloaked target’s cross-section. The cloaking performance of the structure designed by the GA was superior to that designed by analytical calculations based on a combination of the transformation optics and the effective medium approximation. The cloaking is confirmed through the time variation of the magnetic fields calculated using the finite-difference time-domain method. In addition, the mechanism of the cloaking is discussed.

Transmissive invisibility cloak using phase gradient metasurfaces

Different from the reflective approach of the carpet cloak designed to conceal surface irregularities on highly reflective surfaces, the transmission invisibility cloak, often capable of achieving perfect invisibility, operates within a transmission geometry. In this configuration, the cloaking mechanism ensures that an object neither reflects nor refracts incoming waves in free space, presenting opportunities for more versatile applications, though with the requirement for intricate designs. This paper introduces a novel methodology for designing a transmissive invisibility cloak, employing a simplified combination structure of two phase-gradient metasurfaces based on the generalized Snell’s law. Initially, we designed a highly transparent metasurface for the millimeter-wave band to yield diverse phase gradients. We confirmed the effectiveness of this metasurface through the observation of abnormal refraction. Then, through a deliberate arrangement of these phase gradients, we construct a transmissive invisibility cloak that guides electromagnetic waves around the cloaked region. Simulations and experimental measurements conducted under plane-wave conditions demonstrate the cloak’s effectiveness and practical applicability. The ensuing comparative investigations of results among free space, uncloaked objects, and cloaked objects validate the expected cloaking effect, offering valuable insights into the design and functionality of transmissive invisibility cloaks.

Military mimicry: the art of concealment, deception, and imitation

ABSTRACT Three dominant thematics emerge from the biological mimicry and camouflage literature, namely, concealment, deception, and imitation. These phenomena are interesting in their own right, but conceptually have similar analogs in the military context that have attracted only minimal intellectual curiosity. Accordingly, the purpose of this paper is to apply biological mimicry and camouflage concepts to the military environment. Concealment in the form of camouflage is traced from its nineteenth century origins to the military's imminent twenty-first century perfection of an “invisibility cloak”. Military deception is the art of duping enemies with fakes and dummies. Finally, imitation is examined from three perspectives: firstly, replacement of military personnel with animals; secondly, exploration of bioengineering, including exploitation of avian aerodynamics, insect biophysical structures, and mammal sonar attributes; and, thirdly, Artificial Intelligence that is driving military mimicry along an evolutionary path towards robots, swarms, and avatars in an emerging and novel military technology revolution.

Ultra-broadband microwave absorber based on disordered metamaterials

Metamaterial absorption technology plays an increasingly important role in military and civilian sectors, serving crucial functions in communication, radar technology, and electromagnetic cloaking. However, traditional metamaterial absorbers are predominantly composed of periodic structures, thus limiting their absorption bandwidth, polarization, and angular flexibility. This study employs disordered structures, utilizing their randomness and diversity, to optimize and enhance the performance of periodic structure metamaterial absorbers. Building upon a well-designed periodic perfect absorption structure, a uniform distribution function is introduced to analyze the effects of positional and size disorder on the absorptive properties of the metamaterial. The mechanisms of the disorder are further investigated through simulation analysis. Subsequently, an innovative approach based on disorder engineering for broadband enhancement of metamaterial absorbers is proposed. Numerical simulation results and experimental validations demonstrate that absorbers constructed using this method significantly broaden the absorption bandwidth while maintaining excellent angular and polarization stability. This research not only offers a new method for the design and performance optimization of metamaterial absorbers but also provides a theoretical foundation for the development of metamaterial self-assembly techniques.

Elastostatics within multi-layer metamaterial structures and an algebraic framework for polariton resonances

Multi-layer structures are ubiquitous in constructing metamaterial devices to realise various frontier applications including super-resolution imaging and invisibility cloaking. In this paper, we develop a general mathematical framework for studying elastostatics within multi-layer material structures in Rd, d = 2, 3. The multi-layer structure is formed by concentric balls and each layer is filled by either a regular elastic material or an elastic metamaterial. The number of layers can be arbitrary and the material parameters in each layer may be different from one another. In practice, the multi-layer structure can serve as the building block for various material devices. Considering the impingement of an incident field on the multi-layer structure, we first derive the exact perturbed field in terms of an elastic momentum matrix, whose dimension is the same as the number of layers. By highly intricate and delicate analysis, we derive a comprehensive study of the spectral properties of the elastic momentum matrix. This enables us to establish a handy algebraic framework for studying polariton resonances associated with multi-layer metamaterial structures, which forms the fundamental basis for many metamaterial applications.

The development of cloaking techniques based on phase-modulated metasurfaces

Abstract More and more attention has been paid to explore the application background of cloaking techniques based on phase-modulated metasurfaces, which play an important role in modern engineering and military applications. Phase-modulated metasurfaces are an emerging field of research that explores how the propagation of electromagnetic waves can be manipulated by precisely controlling the phase on tiny elements. These tiny elements, usually nanoscale structures, are arranged on the metasurface to precisely control the propagation and scattering of electromagnetic waves in a specific frequency range. The thesis first reviews the development of metamaterial cloaking technology, introduces the realization of several mainstream metamaterial cloaking technologies, and points out the benefits of cloaking techniques based on phase-modulated metasurfaces compared with other cloaking technologies. Then the development history of cloaking techniques based on phase-modulated metasurfaces is introduced through four key time nodes. We can find that phase-modulated metasurfaces are not only getting thinner and thinner, but in addition to the stealth effect, i.e., the reduction of the RCS (Radar Cross Section), phase-modulated metasurfaces can also simulate different virtual shapes. Finally, the development and research progress of cloaking techniques based on phase-modulated metasurfaces are summarized, and personal opinions on its future development are put forward. This paper summarizes the advances in the field of phase-modulated metasurfaces over the last decade and presents some representative approaches to phase-modulated metasurfaces at various points in time, which can help readers better understand the field of metasurfaces and metamaterials.

Teachers’ perceptions of challenges in digital multimodal composing with newcomer adolescent students from refugee backgrounds

Research has shown ways in which digital multimodal composing (DMC), defined as the use of digital tools to make meaning with multiple modes (e.g. languages, visuals, sounds, gestures), including video production, can empower adolescent newcomers from refugee backgrounds in school settings. However, few studies have examined teachers’ challenges with these pedagogies, particularly involving ­refugee-background learners, some of whom may have experienced ­significantly interrupted formal education. Comprehensively understanding teachers’ perceived challenges with pedagogies involving DMC to help meet these learners’ needs is a particularly urgent objective in Canada, which is increasingly committing to refugee resettlement. This qualitative case study explored teachers’ perceived challenges in DMC with newcomer adolescent students from refugee backgrounds in a secondary school setting. Guided by a multimodal approach to literacy and an identity investment perspective on participation in learning, reflexive thematic analysis led to the identification of the following teacher challenges: navigating expectations and required scaffolding, mitigating risks associated with difficult knowledge, and students’ ‘cloak of invisibility’. The study contributes an in-depth discussion of these patterns, including possible implications, to better empower educators and teacher educators to address the needs of adolescent newcomer learners from refugee backgrounds in an increasingly complex language and literacy landscape.

Mixing Rules for Left-Handed Disordered Metamaterials: Effective-Medium and Dispersion Properties.

Left-handed materials are known to exhibit exotic properties in controlling electromagnetic fields, with direct applications in negative reflection and refraction, conformal optical mapping, and electromagnetic cloaking. While typical left-handed materials are constructed periodic metal-dielectric structures, the same effect can be obtained in composite guest-host systems with no periodicity or structural order. Such systems are typically described by the effective-medium approach, in which the components of the electric permittivity tensor are determined as a function of individual material properties and doping concentration. In this paper, we extend the discussion on the mixing rules to include left-handed composite systems and highlight the exotic properties arising from the effective-medium approach in this framework in terms of effective values and dispersion properties.

Experimental Realization of a One‐Directional Broadband Transmissive Cloak in Microwaves

AbstractHiding an isolated object in free space using a transmissive invisibility cloak has become a significant research area, propelled by advancements in metamaterials and transformation optics over the past decade. Despite the availability of various simplified methods for implementing transmissive cloaks, issues such as impedance mismatches and narrow working bandwidths often arise, posing challenges. Achieving a broadband transmissive cloak in free space has proven to be particularly arduous. This study presents a near‐perfect one‐directional broadband transmissive cloak constructed from multilayer metasurfaces of arbitrary shapes, showcasing superior performance across a broadband frequency range. The phase distribution of the metasurfaces and the efficacy of the transmissive cloak are assessed using the generalized Snell's law. An experimental near‐perfect broadband transmissive cloak is successfully demonstrated to operate within the frequency range of 8.5 to 11.2 GHz. This study contributes to reducing the density and mass of cloaks, thereby facilitating the expansion of cloaking capabilities in various directions and across different frequency bands.

Ultra-broadband absorber based on algorithmic optimization of MgF2–Fe stacked structure

Broadband absorbers possess a wide array of potential applications, including electromagnetic cloaking, solar energy harvesting, and solar power generation. However, achieving excellent ultra-broadband absorption with plasma-metal materials remains challenging due to their inherently narrow bandwidth. To tackle this issue, we propose an ultra-broadband absorber, free from lithography, composed of a multilayer stack of magnesium-iron fluoride (MgF2–Fe). In this study, we optimized the structural parameters of the absorber using a particle swarm optimization algorithm to achieve theoretically optimal absorptivity. Numerical simulations revealed that the absorber achieved an impressive theoretical average absorption of 98.4% across a broad wavelength range from ultraviolet to near-infrared (321–1800 nm). Importantly, it exhibited stable absorption performance at large incidence angles (0–70°), while maintaining high absorption efficiency. Furthermore, it demonstrated polarization independence at perpendicular incidence angles. Subsequently, experimental preparations and tests were conducted on absorber samples to evaluate their solar energy absorption capacity. Both experimental and theoretical comparisons underscored the promising potential of this structure for applications in solar energy collection, conversion, and solar power generation.

3D Intelligent Cloaked Vehicle Equipped with Thousand-Level Reconfigurable Full-Polarization Metasurfaces.

A crucial aspect in shielding a variety of advanced electronic devices from electromagnetic detection involves controlling the flow of electromagnetic waves, akin to invisibility cloaks. Decades ago, the exploration of transformation optics heralded the dawn of modern invisibility cloaks, which has stimulated immense interest across various physical scenarios. However, most prior research is simplified to low-dimensional and stationary hidden objects, limiting their practical applicability in a dynamically changing world. This study develops a 3D large-scale intelligent cloak capable of remaining undetectable even in non-stationary conditions. By employing thousand-level reconfigurable full-polarization metasurfaces, this work has achieved an exceptionally high degree of freedom in sculpting the scattering waves as desired. Serving as the core computational unit, a hybrid inverse design enables the cloaked vehicle to respond in real-time, with a rapid reaction time of just 70ms. These experiments integrate the cloaked vehicle with a perception-decision-control-execution system and evaluate its performance under random static positions and dynamic travelling trajectories, achieving a background scattering matching degree of up to 93.3%. These findings establish a general paradigm for the next generation of intelligent meta-devices in real-world settings, potentially paving the way for an era of "Electromagnetic Internet of Things."

Dynamically tunable terahertz multi-band perfect absorber based on photosensitive silicon

A tunable narrowband terahertz absorber is proposed based on the photosensitive characteristics of silicon. When silicon is insulating without the pump beam, the absorber realizes three-frequency absorption at 0.731 THz, 1.145 THz, and 1.546 THz with absorptivity of 99.43%, 99.99%, and 99.98%, respectively. When the silicon is excited by the pump beam, it is conducting, and the absorber realizes double-frequency absorption at 0.852 THz, 1.536 THz, with 99.99% and 99.31%. The impedance matching theory explains the perfect absorption, and the electric field and surface current distributions are further discussed to elaborate the physical mechanisms. In addition, the effect of geometric parameters on the absorptivity is discussed. The absorber exhibits wide-angle absorption characteristics when light is polarized along the y-direction, and the absorptivity exhibits weak dependence on the polarization angle. The proposed absorber has promising applications in electromagnetic cloaking, narrow-band thermal radiation, and optoelectronic detection.

Determining the parameters of a dielectric ellipsoid for minimising front echo in Gaussian pulse scattering analysis

AbstractA novel numerical method is introduced for minimising the presence of scattering echoes in the frontal area of a dielectric ellipsoid through the application of the method of auxiliary sources (MAS) method. This approach utilises a Gaussian radio pulse that spans the frequency spectrum from 0 to 6 GHz. The approach involves determining the properties of an ellipsoid, such as its dimensions and a dielectric permittivity, using a specially developed MATLAB software package to make this ellipsoid invisible. This provides a valuable military and defence technology tool, particularly for stealth and invisibility technology applications. It is beneficial for avoiding detection by radar systems utilised for enemy surveillance and identifying concealed objects. This aims to eliminate the need for metamaterials for invisibility cloaking. The suggested methodology has been confirmed through a validation process, which involved a comparison of the outcomes derived from numerical experiments conducted during pulse echo observations using the FDTD method and an analytical solution. MAS offers a notable advantage over other methods and commercial software by significantly reducing computational time for simulating and analysing 3D dielectric objects. This allows for quicker consideration of various parameter scenarios and optimisation for desired outcomes.