Optical Cloaking Research Articles

Electromagnetic Wave Tunneling from Metamaterial Antiparallel Dipole Resonance

Metamaterials In article number 2000186, Zhengli Han, Seigo Ohno, and Hiroaki Minamide describe an electromagnetic wave tunneling effect realized by antiparallel dipole resonance in a multilayer metamaterial. The antiparallel dipole resonance enables the incident electromagnetic wave to pass through barriers, such as resonators or non-resonators, and leads to applications for electrical wire connections in integrated circuits, optical cloaking, and amplitude/phase modulators.

Electromagnetic Wave Tunneling from Metamaterial Antiparallel Dipole Resonance

Metamaterial resonances have demonstrated outstanding abilities to control electromagnetic waves, which brings promises to directly use them to create unique channels for electromagnetic wave tunneling through barriers. Herein, an electromagnetic wave tunneling effect is realized by an antiparallel dipole resonance in a multilayer metamaterial composed of a pair of metallic unsplit ring resonators (USRRs) with a split ring resonator (SRR) sandwiched between them. The sandwiched SRR is known to serve as a barrier to the incident electromagnetic wave at its fundamental resonant frequency. However, with assistance of a “bypass bridge” formed by the magnetic response from the antiparallel dipole resonance excited on the pair of USRRs, the electromagnetic wave tunnels through the barrier. Through field distributions and harmonic model analysis, the universal conditions for the tunneling are found to happen in such a multilayer configuration. As long as the antiparallel dipole resonance is excited on the pair of USRRs, and thus the bypass bridge is formed, no matter what other barrier is sandwiched, the tunneling effect still occurs. Herein, the applications are for electrical wire connections in integrated circuits regarding electromagnetic compatibility, optical cloaking devices, as well as designs for both optical amplitude and phase modulators.

Reconfigurable optical rectangular particle array absorber based on metal–DNA–metal​ structure as a refractive index sensor and optical switch

We have modeled a novel reconfigurable optical absorber using the array of the rectangular nanoparticle array based on the Metal–DNA–Metal (MDM) structure and discussed the effect of the DNA composite on electric field distribution by switching to “On” and “Off” modes based on low conductivity (LC) or high conductivity (HC) attributes of the material. In this structure, we have assumed a model of the optical absorber which contains four rectangular elements and each element has two rectangular metal sections while they are a sandwich of the DNA composite. This structure is over a SiN substrate and a rectangular gold layer under the substrate is utilized for reducing the transmission in the absorber. At last, we have developed a nonlinear model while the unit cells are not similar and we compared both models of absorbers. We show that by variation of the conductivity of the DNA composite section We will be able to control the resonance frequency and the absorbance quality for the structure at the range of 200–300​ THz for wide bandwidth application. We have studied the dimension impacts on reflection and the electric field distribution and clarify how DNA composite material can be used to convey the electric field and control of the phase which is important for the optical cloak.

Successive training of a generative adversarial network for the design of an optical cloak

At the nanoscale level, optical properties of materials depend greatly on their shape. Finding the right geometry for a specific property remains a fastidious and long task, even with the help of modelling tools. In this work, we overcome this challenge by using artificial intelligence to guide a reverse engineering method. We present an optimization algorithm based on a deep convolution generative adversarial network for the design a 2-dimensional optical cloak. The optical cloak consists in a shell of uniform and isotropical dielectric material, and the cloaking is achieved via the geometry of this shell. We use a feedback loop from the solutions of this generative network to successively retrain it and improve its ability to predict and find optimal geometries. This generative method allows to find a global solution to the optimization problem without any prior knowledge of good cloaking geometries.

Recent Progress on Ultrathin Metalenses for Flat Optics.

SummaryAs technology advances, electrical devices such as smartphones have become more and more compact, leading to a demand for the continuous miniaturization of optical components. Metalenses, ultrathin flat optical elements composed of metasurfaces consisting of arrays of subwavelength optical antennas, provide a method of meeting those requirements. Moreover, metalenses have many other distinctive advantages including aberration correction, active tunability, and semi-transparency, compared to their conventional refractive and diffractive counterparts. Therefore, over the last decade, great effort has been focused on developing metalenses to investigate and broaden the capabilities of metalenses for integration into future applications. Here, we discuss recent progress on metalenses including their basic design principles and notable characteristics such as aberration correction, tunability, and multifunctionality.

Robust topology optimization of optical cloaks under uncertainties in wave number and angle of incident wave

SummaryThis article presents a robust topology optimization method for optical cloaks under uncertainties in the wave number and angle in the incident wave. We first discuss the governing equation derived from Maxwell's equation, and extend it to the entire domain including the dielectric material and air, based on the level set‐based topology optimization method. Next, a robust optimization problem is formulated as a minimization problem of the weighted sum of the scattered wave norm and its standard deviation with respect to the wave number and angle of the incident wave. The standard deviation is mathematically expressed by the Taylor series approximation and the use of the adjoint variable method. The design sensitivity of the objective functional is also derived by the adjoint variable method. An optimization algorithm is then constructed, based on the proposed formulation for robust designs of optical cloaks. Several numerical examples are finally provided to demonstrate the validity and utility of the proposed method.

Bio-inspired Photonic Masquerade with Perturbative Metasurfaces

Camouflage is critical for many living organisms to survive in the natural world and has stimulated applications, such as optical cloaking and military affairs. However, most applications adopt crypsis-type camouflage that prevents the organisms from being detected by matching the environment, which is challenging to realize the large angle-of-view and broadband operation at optical frequencies. Here, as inspired by nature's system of masquerade, we demonstrate an optical masquerade, being detected but not recognized, with perturbative metasurfaces that could camouflage an object into another unrelated one under the oblique (±69°) illumination of visible light with an ∼160 nm bandwidth. Predicted by the perturbation theory, the dielectric metastructures encircling a pistol-shaped object have a thin layer of nanodisk array, which can suppress the electromagnetic resonances of nanomodes for mimicking the transmitted intensity and phase of the camouflaged object. We also exhibit that optical masquerade is an invasive, environment-independent, object-unlimited, and material-extendable camouflage, which might benefit optical security, anticounterfeiting, and encoding.

Demonstration of carpet cloaking by an anisotropic zero refractive index medium.

In this Letter, we numerically and experimentally demonstrate the carpet cloaking effect by a rectangular lattice two-dimensional photonic crystal (PC) exhibiting a semi-Dirac cone (SDC) dispersion phenomenon. The proposed SDC PC with an anisotropic zero refractive index medium operates as an optical carpet cloak for a perfect electric conductor surface bump. The experimental verification of the cloak is realized at microwave frequencies at around 12.1 GHz via dielectric rods. A good agreement between experimental measurements and numerical calculations is observed. Finally, features such as rendering larger objects invisible are possible with the proposed idea.

Metasurfaces: Subwavelength nanostructure arrays for ultrathin flat optics and photonics

Abstract

A simple projective setup to study optical cloaking in the classroom

Optical cloaking consists in hiding from sight an object by properly deviating the light that comes from it. An optical cloaking device (OCD) is an artifact that hides the object and, at the same time, its presence is not (or should not be) noticeable for the observer, who will have the impression of looking through it. At the level of paraxial geometrical optics, suitable for undergraduate courses, simple OCDs can be built by combining a series of lenses. With this motivation, here we present an analysis of a simple projective OCD arrangement. First, a simple theoretical account in terms of the transfer matrix method is provided, and then the outcomes from a series of teaching experiments carried out with this device, easy to conduct in the classroom, are discussed. In particular, the performance of such an OCD is investigated by determining the effect of the hidden object, its role here played by the opaque zone of an iris-type diaphragm, on the projected image of an illuminated transparent slide (test object). That is, cloaking is analyzed in terms of the optimal position and opening diameter of a diaphragm that still warrants an almost unaffected projected image. Because the lenses are not high-quality ones, the OCD is not aberration-free, which is advantageously considered to determine acceptable cloaking conditions (i.e. the tolerance of the device).

Eksperimen Gangguan Getaran Pada Fenomena Cloaking Cahaya Tampak Sistem Empat Lensa

Creating optical illusions or invisible objects by using cloaking devices is still being a concern for many researchers in the world. The cloaking devices are working in visible spectrum regime, large dimention, and replicable. Addition with the full-field of light, no new materials, simple infrastructure, and scale easily may be considered an ‘ideal’ cloaking device. Based on four lenses cloaking system by Choi’s work shows an idea for solution of blindspot large vehicle’s problem. The set up for this optical system is regular covex lenses non-aberation based on educational laboratory framework. The experimental confirmation that cloaking area with 4f convex lenses is limited to certain areas only, not 100 % of the object to be observed can disappear from the view. Vibration disturbance generated by rotor module (wheels disturbance analogy on vehicles) applied on the system demonstrate that below on 2000 rotation per minutes (rpm) equal 33.34 hertz, optical illusion is still consistent. The results support the information to create optical cloaking prototype for optimizing large vehicle driver’s visibility as alternative solution blindspot’s problem.

Lattice invisibility effect based on transverse Kerker scattering in 1D metalattices

The present work proposes a novel method to achieve free phase propagation with unitary transmission efficiency, termed as lattice invisibility effects, based on transverse scattering in 1D cylindrical metalattices. Firstly, we extend Kerker conditions parity symmetry of electromagnetic modes, and a conciser scheme to eliminate both forward and backward scattering simultaneously is presented. Compared to sphere-like particles where the first four Mie modes are required, we find that transverse scattering can be realized here if only two resonances with similar parity symmetry have the same amplitude and opposite phase, because of reduced symmetry in cylindrical systems. Further, we generalize this concept to metalattices and propose an explicit principle to realize lattice transparency, as long as all the modes with similar parity profiles interfere destructively. The proof-of-concept demonstrations have been manifested by exciting pure degraded electric dipoles or co-exciting magnetic dipole and electric quadrupoles using SiO2@InSb or Si@InSb cylinders, respectively. Besides, the invisibility performance can be flexibly tuned by changing either the period of metalattices or the magnitude of external magnetic field. The revealed mechanisms will inspire more comprehensive study of directional scattering and free phase propagation, which can also potentially stimulate several advanced applications like optical cloaking, nanophotonic circuits, etc.

Elastodynamic behavior of mechanical cloaks designed by direct lattice transformations

Steering waves in elastic solids is more demanding than steering waves in electromagnetism or acoustics. As a result, designing material distributions which are the counterpart of optical invisibility cloaks in elasticity poses a major challenge. Waves of all polarizations should be guided around an obstacle to emerge on the downstream side as though no obstacle were there. Recently, we have introduced the direct-lattice-transformation approach. This simple and explicit construction procedure led to extremely good cloaking results in the static case. Here, we transfer this approach to the dynamic case, i.e., to elastic waves or phonons. We demonstrate broadband reduction of scattering, with best suppressions exceeding a factor of five when using cubic coordinate transformations instead of linear ones. To reliably and quantitatively test these cloaks efficiency, we use an effective-medium approach.

Beam splitting and unidirectional cloaking using anisotropic zero-index photonic crystals

We propose an anisotropic photonic crystal (PC) structure with close-to-zero effective parameters. The anisotropic zero-index property of PCs provides complex iso-frequency contours, where their shapes can change from ellipses or hyperbolae to a linear crossing. A mechanism of light beam splitting can be achieved by utilizing the linear crossing shaped iso-frequency curve. The manipulation of light beam propagations is verified by both numerical simulations and microwave experiments. By using pure dielectrics and scaling down to optical wavelengths, we also propose a design of nonperiodic PC structures to achieve optical unidirectional cloaking without size or shape limitation. Our design provides a promising platform for cloaking applications in the optical regime.

Ultrasparse Acoustic Absorbers Enabling Fluid Flow and Visible-Light Controls

Although conventional acoustic absorbers work well for noise suppression, they also block fluid (air) flow and visible light, which is inconvenient if $e.g.$ one wants a quiet house near a busy road. This study uses sparsely arranged acoustic resonators that permit fluid flow yet effectively prevent sound propagation. Furthermore, when such a sparse absorber is combined with optical cloaking technology, it can be transparent to the eye. This approach enables innovative applications of multifunctional acoustic metamaterials.

Photonic, Low-Friction and Antimicrobial Applications for an Ancient Icosahedral/Quasicrystalline Nano-composite Bronze Alloy

The recent discovery of an ancient mirror bronze alloy comprising of nano-quasicrystalline features is pursued further in this article. We have reported new evidence that indicates that two or more types of quasicrystalline phases may be present in the alloy along with hints of a glassy phase. The finding opens the possibilities of various new uses for this alloy. The distribution of the various phases and their morphological patterns that result from the solidification sequence of the alloy are suggestive of unique photonic applications such as optical gratings and optical cloaking. Low-friction applications at a high pair velocity are also possible with this alloy based on the modulus and the surface texture. The wear rate of many quasicrystalline phase containing alloys is low because of their minimal strain-rate softening tendencies. Based on the nano-crystal distribution and compared to known alloys, the use of the alloy is also contemplated for contact-surface antimicrobial uses. We have also reported findings of the possible evidence of an icosahedral structure even after some of the tin is “boiled” away leaving behind a copper-tin-zinc variation of the original alloy. New high-resolution electron microscope lattice imaging microstructures and diffraction patterns of the original mirror alloy have been discussed. A solidification sequence that can preserve the icosahedral features across the atomic stacking scale, the nanometer scale of the crystals and the micrometer scale of cells/dendrites has been proposed. Regardless, there is the difficulty of completely characterizing all phases in this complex mirror alloy by electron microscopy methods alone. X-ray diffraction, fracture surface analysis, thermal transformation analysis, ellipsometric and optical imaging methods are also employed for the characterization. The microstructures are compared with patterns of similar Fibonacci analogs in flowers and petals.

Study of Optical Cloaking Lenses Design for Optimizing Large Vehicle Driver's Visibility in Bandar Lampung

Nowadays, invisibility efforts by scientists and engineers have focused on building a large broadband, multidirectional cloaking device. The device is usually a cloak that works in macroscopic dimensions and for the entire visible spectrum. Addition with the full-field of light, no new materials, simple infrastructure, and scale easily may be considered an ‘ideal' cloaking device. This paper report of implementing ray optics cloaking formalism presented previously by Choi and Howell [1] using lenses design for cloaking arbitrarily large, and moving objects such as large vehicle. We apply their formalism using simulation and experiment scheme to general optical systems up to four convex lenses. The results support the information to create optical cloaking prototype for optimizing large vehicle driver's visibility as alternative solution for decreasing traffic accident in Bandar Lampung.

While rotating while cloaking

Invisible optical and thermal cloaking have been explored as the typical demonstrations of the transformation optics and thermotics theory. However, the existing cloaks are realized by only one-coordinate transformation, and the cloaking layout, i.e. the form of electromagnetic wave/heat passing around the invisible region, is single for a long time. Here, we propose a new rotated thermal cloak which can unify the conventional cloaking and rotating together, and realize the while-rotating–while-cloaking effect. The required anisotropic thermal conductivity tensor is deduced from the new geometric mapping. Though rotated, the heat flux can be tuned around the central invisible region perfectly by the proposed rotated thermal cloak. The underlying physics is explored by comprehensive analysis of the distribution of the thermal conductivity tensor, which is further compared with those of the conventional cloak and rotator. The experimental feasibility is also discussed by validating the practical while-rotating–while-cloaking effect through a proof-of-concept design. The proposed rotated thermal cloak is expected to extend the possibility of cloaking scheme, and open avenues for the multiple coordinate transformation in counterpart physical fields, like optics, electrics, acoustics, magnetics, mechanics, etc.

Directional invisibility by genetic optimization.

In this Letter, the design of a directional optical cloaking by a genetic algorithm is proposed and realized experimentally. A three-dimensional finite-difference time-domain method is combined with the genetic optimization approach to generate the cloaking structure to directionally cloak a cylindrical object made of a perfect electrical conductor by suppressing the undesired scattered fields around the object. The optimization algorithm designs the permittivity distribution of the dielectric polylactide material to achieve an optical cloaking effect. Experimental verifications of the designed cloaking structure are performed at microwave frequencies, where the proposed structure is fabricated by 3D printing. The imperfect conformal mapping from a large-scale permittivity distribution and the compensation of the remaining scattering by a small-scale permittivity distribution are the basic physical mechanisms of the proposed optical cloaking.

Direct‐Write Formation and Dissolution of Silver Nanofilaments in Ionic Liquid‐Polymer Electrolyte Composites

Materials with reconfigurable optical properties are candidates for applications such as optical cloaking and wearable sensors. One approach to fabricate these materials is to use external fields to form and dissolve nanoscale conductive channels in well-defined locations within a polymer. In this study, conductive atomic force microscopy is used to electrochemically form and dissolve nanoscale conductive filaments at spatially distinct points in a polyethylene glycol diacrylate (PEGDA)-based electrolyte blended with varying amounts of ionic liquid (IL) and silver salt. The fastest filament formation and dissolution times are detected in a PEGDA/IL composite that has the largest modulus (several GPa) and the highest polymer crystal fraction. This is unexpected because filament formation and dissolution events are controlled by ion transport, which is typically faster within amorphous regions where polymer mobility is high. Filament kinetics in primarily amorphous and crystalline regions are measured, and two different mechanisms are observed. The formation time distributions show a power-law dependence in the crystalline regions, attributable to hopping-based ion transport, while amorphous regions show a normal distribution. The results indicate that the timescale of filament formation/dissolution is determined by local structure, and suggest that structure could be used to tune the optical properties of the film.