{"history":[],"article":{"id":"26ab3348-bdcf-47fb-97fa-3b89748be02c","title":"Metamaterial cloaking","slug":"metamaterial-cloaking","content":":::summary\n:::\n\n# \n**Metamaterial cloaking** is the usage of [metamaterials](https://bliptext.com/articles/metamaterials) in an [invisibility cloak](https://bliptext.com/articles/cloaking-device).\nThis is accomplished by manipulating the paths traversed by light through a novel optical material.\nMetamaterials direct and control the [propagation](https://bliptext.com/articles/wave-propagation) and transmission of specified parts of the [light spectrum](https://bliptext.com/articles/em-spectrum) and demonstrate the potential to render an object seemingly [invisible](https://bliptext.com/articles/invisible).\nMetamaterial cloaking, based on [transformation optics](https://bliptext.com/articles/transformation-optics), describes the process of shielding something from view by controlling [electromagnetic radiation](https://bliptext.com/articles/electromagnetic-radiation).\nObjects in the defined location are still present, but incident waves are guided around them without being affected by the object itself.\n\n# Electromagnetic metamaterials\nElectromagnetic metamaterials respond to chosen parts of radiated light, also known as the [electromagnetic spectrum](https://bliptext.com/articles/electromagnetic-spectrum), in a manner that is difficult or impossible to achieve with [natural materials](https://bliptext.com/articles/natural-material).\nIn other words, these [metamaterials](https://bliptext.com/articles/metamaterials) can be further defined as artificially structured [composite materials](https://bliptext.com/articles/composite-material), which exhibit interaction with light usually not available in nature ([electromagnetic interactions](https://bliptext.com/articles/electromagnetic-interaction)).\nAt the same time, metamaterials have the potential to be engineered and constructed with desirable properties that fit a specific need.\nThat need will be determined by the particular application.\nThe artificial structure for cloaking applications is a [lattice](https://bliptext.com/articles/crystal-structure) design – a sequentially repeating network – of identical elements.\nAdditionally, for [microwave](https://bliptext.com/articles/microwave) frequencies, these materials are analogous to [crystals](https://bliptext.com/articles/crystal-structure) for [optics](https://bliptext.com/articles/optics).\nAlso, a metamaterial is composed of a sequence of elements and spacings, which are much smaller than the selected [wavelength of light](https://bliptext.com/articles/light-spectrum).\nThe selected wavelength could be [radio frequency](https://bliptext.com/articles/radio-frequency), microwave, or other radiations, now just beginning to reach into the [visible frequencies](https://bliptext.com/articles/visible-frequencies).\n[Macroscopic](https://bliptext.com/articles/macroscopic) properties can be directly controlled by adjusting characteristics of the rudimentary [elements](https://bliptext.com/articles/split-ring-resonator), and their arrangement on, or throughout the material.\nMoreover, these metamaterials are a basis for building very small cloaking devices in anticipation of larger devices, adaptable to a broad spectrum of radiated light.\nHence, although light consists of an [electric field](https://bliptext.com/articles/electric-field) and a [magnetic field](https://bliptext.com/articles/magnetic-field), ordinary optical materials, such as [optical microscope](https://bliptext.com/articles/optical-microscope) lenses, have a strong reaction only to the [electric field](https://bliptext.com/articles/electric-field).\nThe corresponding magnetic interaction is essentially nil.\nThis results in only the most common [optical effects](https://bliptext.com/articles/optical-microscopy), such as ordinary [refraction](https://bliptext.com/articles/refraction) with common [diffraction limitations](https://bliptext.com/articles/optical-microscopy) in lenses and [imaging](https://bliptext.com/articles/optical-resolution).\nSince the [beginning of optical sciences](https://bliptext.com/articles/history-of-optics), centuries ago, the ability to control the [light](https://bliptext.com/articles/light) with materials has been limited to these common optical effects.\nMetamaterials, on the other hand, are capable of a very strong interaction, or coupling, with the magnetic component of light.\nTherefore, the range of response to [radiated light](https://bliptext.com/articles/light) is expanded beyond the ordinary optical limitations that are described by the sciences of [physical optics](https://bliptext.com/articles/physical-optics) and [optical physics](https://bliptext.com/articles/optical-physics).\nIn addition, as artificially constructed materials, both the magnetic and electric components of the radiated light can be controlled at will, in any desired fashion as it travels, or more accurately [propagates](https://bliptext.com/articles/wave-propagation), through the material.\nThis is because a metamaterial's behavior is typically formed from individual components, and each component responds independently to a [radiated spectrum](https://bliptext.com/articles/em-radiation) of light.\nAt this time, however, metamaterials are limited.\nCloaking across a broad [spectrum of frequencies](https://bliptext.com/articles/frequency-spectrum) has not been achieved, including the [visible spectrum](https://bliptext.com/articles/visible-spectrum).\n[Dissipation](https://bliptext.com/articles/dissipation), [absorption](https://bliptext.com/articles/absorption-electromagnetic-radiation), and [dispersion](https://bliptext.com/articles/dispersion-relation) are also current drawbacks, but this field is still in its optimistic infancy.\n\n## Metamaterials and transformation optics\nThe field of [transformation optics](https://bliptext.com/articles/transformation-optics) is founded on the effects produced by metamaterials.\nTransformation optics has its beginnings in the conclusions of two research endeavors.\nThey were published on May 25, 2006, in the same issue of [Science](https://bliptext.com/articles/science-journal), a peer-reviewed journal.\nThe two papers are tenable theories on bending or distorting [light](https://bliptext.com/articles/light) to [electromagnetically](https://bliptext.com/articles/electromagnetic-field) conceal an object.\nBoth papers notably map the initial configuration of the [electromagnetic fields](https://bliptext.com/articles/electromagnetic-field) on to a [Cartesian](https://bliptext.com/articles/cartesian-coordinate-system) mesh.\nTwisting the Cartesian mesh, in essence, transforms the coordinates of the electromagnetic fields, which in turn conceal a given object.\nHence, with these two papers, transformation optics is born.\nTransformation optics subscribes to the capability of bending [light](https://bliptext.com/articles/light), or [electromagnetic waves](https://bliptext.com/articles/electromagnetic-waves) and [energy](https://bliptext.com/articles/energy), in any preferred or desired fashion, for a desired application.\n[Maxwell's equations](https://bliptext.com/articles/maxwell-s-equations) do not vary even though [coordinates](https://bliptext.com/articles/coordinate-system) transform.\nInstead it is the values of the chosen parameters of the materials which \"transform\", or alter, during a certain time period.\nSo, transformation optics developed from the capability to choose the parameters for a given material.\nHence, since Maxwell's equations retain the same form, it is the successive values of the parameters, [permittivity](https://bliptext.com/articles/permittivity) and [permeability](https://bliptext.com/articles/permeability-electromagnetism), which change over time.\nFurthermore, permittivity and permeability are in a sense responses to the [electric](https://bliptext.com/articles/electric-field) and [magnetic fields](https://bliptext.com/articles/magnetic-field) of a radiated light source respectively, among other descriptions.\nThe precise degree of electric and magnetic response can be controlled in a metamaterial, point by point.\nSince so much control can be maintained over the responses of the material, this leads to an enhanced and highly flexible [gradient-index](https://bliptext.com/articles/gradient-index-optics) material.\nConventionally predetermined [refractive index](https://bliptext.com/articles/refractive-index) of ordinary materials instead become independent spatial gradients in a metamaterial, which can be controlled at will.\nTherefore, transformation optics is a new method for creating novel and unique [optical devices](https://bliptext.com/articles/optical-device).\n# [Left:The cross section of a PEC cylinder subject to a plane wave (only the electric field component of the wave is shown). The field is scattered. Right: a circular cloak, designed using transformation optics methods, is used to cloak the cylinder. In this case the field remains unchanged outside the cloak and the cylinder is invisible electromagnetically. Note the special distortion pattern of the field inside the cloak. | upload.wikimedia.org/wikipedia/commons/7/73/Circular///EM///cloak///using///transformation///optics.svg]\n\n# Science of cloaking devices\nThe purpose of a cloaking device is to hide something, so that a defined region of space is [invisibly](https://bliptext.com/articles/invisibility) isolated from passing electromagnetic fields (or [sound waves](https://bliptext.com/articles/acoustic-metamaterials)), as with **Metamaterial cloaking**.\nCloaking objects, or making them appear invisible with [metamaterials](https://bliptext.com/articles/metamaterial), is roughly analogous to a magician's sleight of hand, or his tricks with mirrors.\nThe object or subject doesn't really disappear; the vanishing is an illusion.\nWith the same goal, [researchers](https://bliptext.com/articles/researcher) employ metamaterials to create directed blind spots by deflecting certain parts of the [light spectrum](https://bliptext.com/articles/em-spectrum) (electromagnetic spectrum).\nIt is the light spectrum, as the [transmission medium](https://bliptext.com/articles/transmission-medium), that determines what the human eye can see.\nIn other words, light is [refracted](https://bliptext.com/articles/refract) or reflected determining the view, color, or illusion that is seen.\nThe visible extent of light is seen in a [chromatic spectrum](https://bliptext.com/articles/visible-spectrum) such as the [rainbow](https://bliptext.com/articles/rainbow).\nHowever, visible light is only part of a broad spectrum, which extends beyond the sense of sight.\nFor example, there are other parts of the light spectrum which are in common use today.\nThe [microwave spectrum](https://bliptext.com/articles/microwave) is employed by [radar](https://bliptext.com/articles/radar), [cell phones](https://bliptext.com/articles/cell-phone), and [wireless Internet](https://bliptext.com/articles/wireless).\nThe [infrared spectrum](https://bliptext.com/articles/infrared-spectrum) is used for [thermal imaging](https://bliptext.com/articles/thermal-imaging) technologies, which can detect a warm body amidst a cooler night time environment, and infrared illumination is combined with specialized [digital cameras](https://bliptext.com/articles/charge-coupled-device) for [night vision](https://bliptext.com/articles/night-vision).\nAstronomers employ the [terahertz band](https://bliptext.com/articles/terahertz-band) for [submillimeter observations](https://bliptext.com/articles/submillimeter-astronomy) to answer deep [cosmological](https://bliptext.com/articles/astronomy) questions.\nFurthermore, electromagnetic energy is light energy, but only a small part of it is [visible light](https://bliptext.com/articles/visible-light).\nThis energy travels in waves.\nShorter wavelengths, such as visible light and [infrared](https://bliptext.com/articles/infrared), carry more energy per [photon](https://bliptext.com/articles/photon) than longer waves, such as [microwaves](https://bliptext.com/articles/microwave) and [radio waves](https://bliptext.com/articles/radio-waves).\nFor the [sciences](https://bliptext.com/articles/science), the light spectrum is known as the [electromagnetic spectrum](https://bliptext.com/articles/electromagnetic-spectrum).\n\n## The properties of optics and light\n[Prisms](https://bliptext.com/articles/prism-optics), [mirrors](https://bliptext.com/articles/mirror), and [lenses](https://bliptext.com/articles/lens-optics) have a long history of altering the diffracted visible light that surrounds all.\nHowever, the control exhibited by these ordinary materials is limited.\nMoreover, the one material which is common among these three types of directors of light is conventional [glass](https://bliptext.com/articles/glass).\nHence, these familiar [technologies](https://bliptext.com/articles/technologies) are constrained by the fundamental, physical [laws of optics](https://bliptext.com/articles/optics).\nWith metamaterials in general, and the cloaking technology in particular, it appears these barriers disintegrate with advancements in materials and technologies never before realized in the [natural physical sciences](https://bliptext.com/articles/physics).\nThese unique materials became notable because electromagnetic radiation can be bent, reflected, or skewed in new ways.\nThe radiated light could even be slowed or captured before transmission.\nIn other words, new ways to focus and project light and other radiation are being developed.\nFurthermore, the expanded optical powers presented in the science of cloaking objects appear to be technologically beneficial across a wide spectrum of devices already in use.\nThis means that every device with basic functions that rely on interaction with the radiated [electromagnetic spectrum](https://bliptext.com/articles/electromagnetic-spectrum) could technologically advance.\nWith these beginning steps a whole new class of optics has been established.\n\n## Interest in the properties of optics and light\nInterest in the properties of optics, and light, date back to almost 2000 years to [Ptolemy](https://bliptext.com/articles/ptolemy) (AD 85 – 165).\nIn his work entitled Optics, he writes about the properties of [light](https://bliptext.com/articles/light), including [reflection](https://bliptext.com/articles/reflection-physics), [refraction](https://bliptext.com/articles/refraction), and [color](https://bliptext.com/articles/color).\nHe developed a simplified equation for refraction without [trigonometric functions](https://bliptext.com/articles/trigonometric-functions).\nAbout 800 years later, in AD 984, [Ibn Sahl](https://bliptext.com/articles/ibn-sahl-mathematician) discovered a law of refraction mathematically equivalent to [Snell's law](https://bliptext.com/articles/snell-s-law).\nHe was followed by the most notable Islamic scientist, [Ibn Al-Haytham](https://bliptext.com/articles/ibn-al-haytham) (c.965–1039), who is considered to be \"one of the few most outstanding figures in optics in all times\".\nHe made significant advances in the science of [physics](https://bliptext.com/articles/physics) in general, and [optics](https://bliptext.com/articles/optics) in particular.\nHe anticipated the universal laws of light articulated by seventeenth century scientists by hundreds of years.\nIn the seventeenth century both [Willebrord Snellius](https://bliptext.com/articles/willebrord-snellius) and [Descartes](https://bliptext.com/articles/descartes) were credited with discovering the law of refraction.\nIt was Snellius who noted that Ptolemy's equation for refraction was inexact.\nConsequently, these laws have been passed along, unchanged for about 400 years, like the laws of gravity.\n\n## Perfect cloak and theory\n[Electromagnetic radiation](https://bliptext.com/articles/electromagnetic-radiation) and matter have a symbiotic relationship.\nRadiation does not simply act on a material, nor is it simply acted upon by a given material.\nRadiation interacts with [matter](https://bliptext.com/articles/matter).\nCloaking applications which employ [metamaterials](https://bliptext.com/articles/metamaterial) alter how objects interact with the [electromagnetic spectrum](https://bliptext.com/articles/electromagnetic-spectrum).\nThe guiding vision for the metamaterial cloak is a device that directs the flow of light smoothly around an object, like water flowing past a rock in a stream, without [reflection](https://bliptext.com/articles/reflection-physics), rendering the object invisible.\nIn reality, the simple cloaking devices of the present are imperfect, and have limitations.\nOne challenge up to the present date has been the inability of metamaterials, and cloaking devices, to interact at [frequencies](https://bliptext.com/articles/frequencies), or [wavelengths](https://bliptext.com/articles/wavelength), within the visible light spectrum.\n\n## Challenges presented by the first cloaking device\nThe principle of cloaking, with a cloaking device, was first proved (demonstrated) at frequencies in the [microwave radiation band](https://bliptext.com/articles/microwave) on October 19, 2006.\nThis demonstration used a small cloaking device.\nIts height was less than one half inch (< 13 mm) and its diameter five inches (125 mm), and it successfully diverted microwaves around itself.\nThe object to be hidden from view, a small cylinder, was placed in the center of the device.\nThe invisibility cloak deflected [microwave beams](https://bliptext.com/articles/microwave) so they flowed around the cylinder inside with only minor distortion, making it appear almost as if nothing were there at all.\nSuch a device typically involves surrounding the object to be cloaked with a shell which affects the passage of [light](https://bliptext.com/articles/light) near it.\nThere was reduced reflection of electromagnetic waves (microwaves), from the object.\nUnlike a homogeneous natural material with its material properties the same everywhere, the cloak's material properties vary from point to point, with each point designed for specific electromagnetic interactions (inhomogeneity), and are different in different directions (anisotropy).\nThis accomplishes a [gradient](https://bliptext.com/articles/gradient-index-optics) in the material properties.\nThe associated report was published in the journal [Science](https://bliptext.com/articles/science-journal).\nAlthough a successful demonstration, three notable limitations can be shown.\nFirst, since its effectiveness was only in the[ microwave spectrum](https://bliptext.com/articles/microwave) the small object is somewhat invisible only at microwave frequencies.\nThis means invisibility had not been achieved for the [human eye](https://bliptext.com/articles/human-eye), which sees only within the [visible spectrum](https://bliptext.com/articles/visible-spectrum).\nThis is because the [wavelengths](https://bliptext.com/articles/wavelengths) of the visible spectrum are tangibly shorter than microwaves.\nHowever, this was considered the first step toward a cloaking device for visible light, although more advanced [nanotechnology-related](https://bliptext.com/articles/nanotechnology) techniques would be needed due to light's short wavelengths.\nSecond, only small objects can be made to appear as the surrounding air.\nIn the case of the 2006 proof of cloaking demonstration, the hidden from view object, a [copper](https://bliptext.com/articles/copper) cylinder, would have to be less than five inches in diameter, and less than one half inch tall.\nThird, cloaking can only occur over a narrow frequency band, for any given demonstration.\nThis means that a broad band cloak, which works across the [electromagnetic spectrum](https://bliptext.com/articles/electromagnetic-spectrum), from [radio frequencies](https://bliptext.com/articles/radio-frequencies) to microwave to the [visible spectrum](https://bliptext.com/articles/visible-spectrum), and to [x-ray](https://bliptext.com/articles/x-ray), is not available at this time.\nThis is due to the [dispersive nature](https://bliptext.com/articles/dispersion-optics) of present-day metamaterials.\nThe coordinate transformation ([transformation optics](https://bliptext.com/articles/transformation-optics)) requires extraordinary material parameters that are only approachable through the use of [resonant](https://bliptext.com/articles/resonant) elements, which are inherently [narrow band](https://bliptext.com/articles/narrowband), and [dispersive](https://bliptext.com/articles/dispersion-optics) at resonance.\n\n# Usage of metamaterials\nAt the very beginning of the new millennium, metamaterials were established as an extraordinary new medium, which expanded control capabilities over [matter](https://bliptext.com/articles/matter).\nHence, metamaterials are applied to cloaking applications for a few reasons.\nFirst, the parameter known as material response has broader range.\nSecond, the material response can be controlled at will.\nThird, optical components, such as lenses, respond within a certain defined range to [light](https://bliptext.com/articles/light).\nAs stated earlier – the range of response has been known, and studied, going back to [Ptolemy](https://bliptext.com/articles/ptolemy) – eighteen hundred years ago.\nThe range of response could not be effectively exceeded, because natural materials proved incapable of doing so.\nIn scientific studies and research, one way to communicate the range of response is the [refractive index](https://bliptext.com/articles/refractive-index) of a given optical material.\nEvery [natural material](https://bliptext.com/articles/natural-material) so far only allows for a positive refractive index.\nMetamaterials, on the other hand, are an innovation that are able to achieve negative refractive index, zero refractive index, and fractional values in between zero and one.\nHence, metamaterials extend the material response, among other capabilities.\nHowever, negative refraction is not the effect that creates invisibility-cloaking.\nIt is more accurate to say that gradations of refractive index, when combined, create invisibility-cloaking.\nFourth, and finally, metamaterials demonstrate the capability to deliver chosen responses at will.\n\n# Device\nBefore actually building the device, theoretical studies were conducted.\nThe following is one of two studies accepted simultaneously by a scientific journal, as well being distinguished as one of the first published theoretical works for an invisibility cloak.\n\n## Controlling electromagnetic fields\nThe exploitation of \"light\", the [electromagnetic spectrum](https://bliptext.com/articles/electromagnetic-spectrum), is accomplished with common objects and materials which control and direct the [electromagnetic fields](https://bliptext.com/articles/electromagnetic-field).\nFor example, a glass [lens](https://bliptext.com/articles/lens-optics) in a camera is used to produce an image, a metal cage may be used to screen sensitive equipment, and [radio antennas](https://bliptext.com/articles/radio-antenna) are designed to transmit and receive daily FM broadcasts.\n[Homogeneous](https://bliptext.com/articles/homogeneity-physics) materials, which manipulate or modulate [electromagnetic radiation](https://bliptext.com/articles/electromagnetic-radiation), such as glass lenses, are limited in the upper limit of refinements to correct for aberrations.\nCombinations of [inhomogeneous](https://bliptext.com/articles/inhomogeneous) lens materials are able to employ gradient [refractive indices](https://bliptext.com/articles/refractive-indices), but the ranges tend to be limited.\n[Metamaterials](https://bliptext.com/articles/metamaterials) were introduced about a decade ago, and these expand control of parts of the [electromagnetic spectrum](https://bliptext.com/articles/electromagnetic-spectrum); from [microwave](https://bliptext.com/articles/microwave), to [terahertz](https://bliptext.com/articles/terahertz-radiation), to [infrared](https://bliptext.com/articles/infrared).\nTheoretically, metamaterials, as a [transmission medium](https://bliptext.com/articles/transmission-medium), will eventually expand control and direction of electromagnetic fields into the [visible spectrum](https://bliptext.com/articles/visible-spectrum).\nHence, a design strategy was introduced in 2006, to show that a metamaterial can be engineered with arbitrarily assigned positive or negative values of [permittivity](https://bliptext.com/articles/permittivity) and [permeability](https://bliptext.com/articles/magnetic-permeability), which can also be independently varied at will.\nThen direct control of electromagnetic fields becomes possible, which is relevant to novel and unusual lens design, as well as a component of the [scientific theory](https://bliptext.com/articles/scientific-theory) for cloaking of objects from electromagnetic detection.\nEach component responds independently to a radiated [electromagnetic wave](https://bliptext.com/articles/electromagnetic-wave) as it travels through the material, resulting in electromagnetic inhomogeneity for each component.\nEach component has its own response to the external [electric and magnetic fields](https://bliptext.com/articles/electromagnetic-field) of the [radiated source](https://bliptext.com/articles/em-radiation).\nSince these components are smaller than the radiated [wavelength](https://bliptext.com/articles/wavelength) it is understood that a [macroscopic](https://bliptext.com/articles/macroscopic) view includes an [effective value](https://bliptext.com/articles/effective-medium-model) for both permittivity and permeability.\nThese materials obey the [laws of physics](https://bliptext.com/articles/laws-of-physics), but behave differently from normal materials.\nMetamaterials are artificial materials engineered to provide properties which \"may not be readily available in nature\".\nThese materials usually gain their properties from structure rather than composition, using the inclusion of small [inhomogeneities](https://bliptext.com/articles/heterogeneous) to enact [effective macroscopic behavior](https://bliptext.com/articles/effective-medium-model).\nThe [structural units](https://bliptext.com/articles/periodic-function) of metamaterials can be tailored in shape and size.\nTheir composition, and their form or structure, can be finely adjusted.\nInclusions can be designed, and then placed at desired locations in order to vary the function of a given material.\nAs the lattice is constant, the cells are smaller than the radiated light.\nThe design strategy has at its core [inhomogeneous](https://bliptext.com/articles/inhomogeneous) composite [metamaterials](https://bliptext.com/articles/metamaterial) which direct, at will, [conserved quantities](https://bliptext.com/articles/conservation-law-physics) of [electromagnetism](https://bliptext.com/articles/electromagnetism).\nThese quantities are specifically, the [electric displacement field](https://bliptext.com/articles/electric-displacement-field) **D**, the [magnetic field intensity](https://bliptext.com/articles/magnetic-field) **B**, and the [Poynting vector](https://bliptext.com/articles/poynting-vector) **S**.\nTheoretically, when regarding the conserved quantities, or fields, the metamaterial exhibits a twofold capability.\nFirst, the fields can be concentrated in a given direction.\nSecond, they can be made to avoid or surround objects, returning without [perturbation](https://bliptext.com/articles/perturbation) to their original path.\nThese results are consistent with [Maxwell's equations](https://bliptext.com/articles/maxwell-s-equations) and are more than only [ray approximation](https://bliptext.com/articles/ray-optics) found in [geometrical optics](https://bliptext.com/articles/geometrical-optics).\nAccordingly, in principle, these effects can encompass all forms of electromagnetic radiation phenomena on all length scales.\nThe hypothesized design strategy begins with intentionally choosing a configuration of an [arbitrary number](https://bliptext.com/articles/arbitrariness) of embedded sources.\nThese sources become localized responses of [permittivity](https://bliptext.com/articles/permittivity), ε, and [magnetic permeability](https://bliptext.com/articles/magnetic-permeability), μ.\nThe sources are embedded in an [arbitrarily](https://bliptext.com/articles/arbitrariness) selected [transmission medium](https://bliptext.com/articles/transmission-medium) with [dielectric](https://bliptext.com/articles/dielectric) and [magnetic](https://bliptext.com/articles/magnetic) characteristics.\nAs an electromagnetic system the medium can then be schematically represented as a grid.\nThe first requirement might be to move a uniform [electric field](https://bliptext.com/articles/electric-field) through space, but in a definite direction, which avoids an object or obstacle.\nNext remove and embed the system in an elastic medium that can be warped, twisted, pulled or stretched as desired.\nThe initial condition of the fields is recorded on a Cartesian mesh.\nAs the elastic medium is distorted in one, or combination, of the described possibilities, the same pulling and stretching process is recorded by the Cartesian mesh.\nThe same set of contortions can now be recorded, occurring as [coordinate transformation](https://bliptext.com/articles/coordinate-transformation)**:**\nHence, the permittivity, ε, and permeability, μ, is proportionally calibrated by a common factor.\nThis implies that less precisely, the same occurs with the refractive index.\nRenormalized values of permittivity and permeability are applied in the new coordinate system.\nFor the renormalization equations see ref.\n#.\n\n### Application to cloaking devices\nGiven the above parameters of operation, the system, a metamaterial, can now be shown to be able to conceal an object of arbitrary size.\nIts function is to manipulate incoming rays, which are about to strike the object.\nThese incoming rays are instead electromagnetically steered around the object by the metamaterial, which then returns them to their original trajectory.\nAs part of the design it can be assumed that no radiation leaves the concealed volume of space, and no radiation can enter the space.\nAs illustrated by the function of the metamaterial, any radiation attempting to penetrate is steered around the space or the object within the space, returning to the initial direction.\nIt appears to any observer that the concealed volume of space is empty, even with an object present there.\nAn arbitrary object may be hidden because it remains untouched by external radiation.\nA sphere with radius R1 is chosen as the object to be hidden.\nThe cloaking region is to be contained within the annulus R1 < r < R2.\nA simple transformation that achieves the desired result can be found by taking all fields in the region r < R2 and compressing them into the region R1 < r < R2.\nThe coordinate transformations do not alter Maxwell's equations.\nOnly the values of ε and μ change over time.\n\n### Cloaking hurdles\nThere are issues to be dealt with to achieve invisibility cloaking.\nOne issue, related to [ray tracing](https://bliptext.com/articles/ray-tracing-physics), is the [anisotropic](https://bliptext.com/articles/anisotropic) effects of the material on the electromagnetic rays entering the \"system\".\nParallel bundles of rays, (see above image), headed directly for the center are abruptly curved and, along with neighboring rays, are forced into tighter and tighter [arcs](https://bliptext.com/articles/arc-geometry).\nThis is due to rapid changes in the now shifting and transforming [permittivity](https://bliptext.com/articles/permittivity) ε and [permeability](https://bliptext.com/articles/permeability-electromagnetism) μ.\nThe second issue is that, while it has been discovered that the selected metamaterials are capable of working within the parameters of the anisotropic effects and the continual shifting of ε and μ, the values for ε and μ cannot be very large or very small.\nThe third issue is that the selected metamaterials are currently unable to achieve broad, [frequency spectrum](https://bliptext.com/articles/dispersion-optics) capabilities.\nThis is because the rays must curve around the \"concealed\" [sphere](https://bliptext.com/articles/sphere), and therefore have longer trajectories than traversing [free space](https://bliptext.com/articles/free-space), or air.\nHowever, the rays must arrive around the other side of the sphere in [phase](https://bliptext.com/articles/phase-waves) with the beginning radiated [light](https://bliptext.com/articles/light).\nIf this is happening then the [phase velocity](https://bliptext.com/articles/group-velocity) exceeds the [velocity of light](https://bliptext.com/articles/velocity-of-light) in a [vacuum](https://bliptext.com/articles/vacuum), which is the speed limit of the universe.\n(Note, this does not violate the laws of physics).\nAnd, with a required absence of [frequency dispersion](https://bliptext.com/articles/frequency-dispersion), the [group velocity](https://bliptext.com/articles/group-velocity) will be identical with [phase velocity](https://bliptext.com/articles/phase-velocity).\nIn the context of this experiment, group velocity can never exceed the velocity of light, hence the [analytical parameters](https://bliptext.com/articles/numerical-simulation) are effective for only one [frequency](https://bliptext.com/articles/frequency).\n\n## Optical conformal mapping and ray tracing in transformation media\nThe goal then is to create no discernible difference between a concealed volume of space and the propagation of electromagnetic waves through empty space.\nIt would appear that achieving a perfectly concealed (100%) hole, where an object could be placed and hidden from view, is not probable.\nThe problem is the following: in order to carry images, light propagates in a continuous range of directions.\nThe [scattering](https://bliptext.com/articles/scattering) data of electromagnetic waves, after bouncing off an object or hole, is unique compared to light propagating through empty space, and is therefore easily perceived.\nLight propagating through empty space is consistent only with empty space.\nThis includes microwave frequencies.\nAlthough mathematical reasoning shows that perfect concealment is not probable because of the wave nature of light, this problem does not apply to electromagnetic rays, i.e., the domain of [geometrical optics](https://bliptext.com/articles/geometrical-optics).\nImperfections can be made arbitrarily, and [exponentially](https://bliptext.com/articles/exponential-function) small for objects that are much larger than the wavelength of light.\nMathematically, this implies n < 1, because the rays follow the shortest path and hence in theory create a perfect concealment.\nIn practice, a certain amount of acceptable visibility occurs, as noted above.\nThe range of the refractive index of the dielectric (optical material) needs to be across a wide [spectrum](https://bliptext.com/articles/spectrum) to achieve concealment, with the illusion created by wave propagation across empty space.\nThese places where n < 1 would be the shortest path for the ray around the object without phase distortion.\nArtificial propagation of empty space could be reached in the microwave-to-[terahertz](https://bliptext.com/articles/terahertz-radiation) range.\nIn [stealth technology](https://bliptext.com/articles/stealth-technology), impedance matching could result in absorption of beamed electromagnetic waves rather than reflection, hence, evasion of detection by [radar](https://bliptext.com/articles/radar).\nThese general principles can also be applied to [sound waves](https://bliptext.com/articles/sound-waves), where the index n describes the ratio of the local phase velocity of the wave to the bulk value.\nHence, it would be useful to protect a space from any sound sourced detection.\nThis also implies protection from sonar.\nFurthermore, these general principles are applicable in diverse fields such as [electrostatics](https://bliptext.com/articles/electrostatics), [fluid mechanics](https://bliptext.com/articles/fluid-mechanics), [classical mechanics](https://bliptext.com/articles/classical-mechanics), and [quantum chaos](https://bliptext.com/articles/quantum-chaos).\nMathematically, it can be shown that the [wave](https://bliptext.com/articles/wave) propagation is indistinguishable from empty space where [light rays](https://bliptext.com/articles/ray-optics) propagate along straight lines.\nThe medium performs an optical conformal mapping to empty space.\n\n## Microwave frequencies\nThe next step, then, is to actually conceal an object by controlling electromagnetic fields.\nNow, the demonstrated and theoretical ability for controlled electromagnetic fields has opened a new field, transformation optics.\nThis nomenclature is derived from coordinate transformations used to create variable pathways for the propagation of light through a material.\nThis demonstration is based on previous theoretical prescriptions, along with the accomplishment of the prism experiment.\nOne possible application of transformation optics and materials is electromagnetic cloaking for the purpose of rendering a volume or object undetectable to incident radiation, including radiated probing.\nThis demonstration, for the first time, of actually concealing an object with electromagnetic fields, uses the method of purposely designed spatial variation.\nThis is an effect of embedding purposely designed electromagnetic sources in the metamaterial.\nAs discussed earlier, the fields produced by the metamaterial are compressed into a shell (coordinate transformations) surrounding the now concealed volume.\nEarlier this was supported theory; this experiment demonstrated the effect actually occurs.\nMaxwell's equations are scalar when applying transformational coordinates, only the permittivity tensor and permeability tensor are affected, which then become spatially variant, and directionally dependent along different axes.\nThe researchers state**:**\nBy implementing these complex material properties, the concealed volume plus the cloak appear to have the properties of free space when viewed externally.\nThe cloak thus neither scatters waves nor imparts a shadow in the either of which would enable the cloak to be detected.\nOther approaches to invisibility either rely on the reduction of backscatter or make use of a resonance in which the properties of the cloaked object and the must be carefully matched.\n...Advances in the development of [negative index metamaterials], especially with respect to gradient index lenses, have made the physical realization of the specified complex material properties feasible.\nWe implemented a two-dimensional (2D) cloak because its fabrication and measurement requirements were simpler than those of a 3D cloak.\nBefore the actual demonstration, the experimental limits of the transformational fields were computationally determined, in addition to simulations, as both were used to determine the effectiveness of the cloak.\nA month prior to this demonstration, the results of an experiment to spatially map the internal and external electromagnetic fields of negative refractive metamaterial was published in September 2006.\nThis was innovative because prior to this the microwave fields were measured only externally.\nIn this September experiment the permittivity and permeability of the microstructures (instead of external macrostructure) of the metamaterial samples were measured, as well as the scattering by the two-dimensional negative index metamaterials.\nThis gave an average effective refractive index, which results in assuming homogeneous metamaterial.\nEmploying this technique for this experiment, spatial mapping of phases and amplitudes of the microwave radiations interacting with metamaterial samples was conducted.\nThe performance of the cloak was confirmed by comparing the measured field maps to simulations.\nFor this demonstration, the concealed object was a conducting cylinder at the inner radius of the cloak.\nAs the largest possible object designed for this volume of space, it has the most substantial scattering properties.\nThe conducting cylinder was effectively concealed in two dimensions.\n\n## Infrared frequencies\nThe definition optical frequency, in metamaterials literature, ranges from far infrared, to near infrared, through the visible spectrum, and includes at least a portion of ultra-violet.\nTo date when literature refers optical frequencies these are almost always frequencies in the infrared, which is below the visible spectrum.\nIn 2009 a group of researchers announced cloaking at optical frequencies.\nIn this case the cloaking frequency was centered at 1500 nm or 1.5 micrometers – the infrared.\n\n## Sonic frequencies\nA laboratory metamaterial device, applicable to [ultra-sound](https://bliptext.com/articles/ultra-sound) waves was demonstrated in January 2011.\nIt can be applied to [sound wavelengths](https://bliptext.com/articles/wavelength) corresponding to frequencies from 40 to 80 kHz.\nThe metamaterial acoustic cloak is designed to hide objects submerged in water.\nThe metamaterial cloaking mechanism bends and twists sound waves by intentional design.\nThe cloaking mechanism consists of 16 concentric rings in a cylindrical configuration.\nEach ring has acoustic circuits.\nIt is intentionally designed to guide sound waves in two dimensions.\nEach ring has a different [index of refraction](https://bliptext.com/articles/index-of-refraction).\nThis causes sound waves to vary their speed from ring to ring.\n\"The sound waves propagate around the outer ring, guided by the channels in the circuits, which bend the waves to wrap them around the outer layers of the cloak\".\nIt forms an array of cavities that slow the speed of the propagating sound waves.\nAn experimental cylinder was submerged and then disappeared from [sonar](https://bliptext.com/articles/sonar).\nOther objects of various shape and density were also hidden from the sonar.\nThe acoustic cloak demonstrated effectiveness for frequencies of 40 kHz to 80 kHz.\nIn 2014 researchers created a 3D acoustic cloak from stacked plastic sheets dotted with repeating patterns of holes.\nThe [pyramidal](https://bliptext.com/articles/pyramid) geometry of the stack and the hole placement provide the effect.\n\n## Invisibility in diffusive light scattering media\nIn 2014, scientists demonstrated good cloaking performance in murky water, demonstrating that an object shrouded in fog can disappear completely when appropriately coated with metamaterial.\nThis is due to the random scattering of light, such as that which occurs in clouds, fog, milk, frosted glass, etc., combined with the properties of the metatmaterial coating.\nWhen light is diffused, a thin coat of metamaterial around an object can make it essentially invisible under a range of lighting conditions.\n\n# Cloaking attempts\n\n## Broadband ground-plane cloak\nIf a [transformation](https://bliptext.com/articles/transformation-geometry) to quasi-[orthogonal coordinates](https://bliptext.com/articles/orthogonal-coordinates) is applied to [Maxwell's equations](https://bliptext.com/articles/maxwell-s-equations) in order to conceal a perturbation on a flat [conducting plane](https://bliptext.com/articles/permittivity) rather than a singular point, as in the first demonstration of a [transformation](https://bliptext.com/articles/transformation-geometry) optics-based cloak, then an object can be hidden underneath the perturbation.\nThis is sometimes referred to as a \"carpet\" cloak.\nAs noted above, the original cloak demonstrated utilized resonant metamaterial elements to meet the effective material constraints.\nUtilizing a quasi-conformal transformation in this case, rather than the non-conformal original transformation, changed the required material properties.\nUnlike the original (singular expansion) cloak, the \"carpet\" cloak required less extreme material values.\nThe quasi-conformal carpet cloak required anisotropic, inhomogeneous materials which only varied in [permittivity](https://bliptext.com/articles/permittivity).\nMoreover, the permittivity was always positive.\nThis allowed the use of non-resonant metamaterial elements to create the cloak, significantly increasing the bandwidth.\nAn automated process, guided by a set of [algorithms](https://bliptext.com/articles/algorithm), was used to construct a metamaterial consisting of thousands of elements, each with its own [geometry](https://bliptext.com/articles/geometry).\nDeveloping the algorithm allowed the [manufacturing process](https://bliptext.com/articles/manufacturing-process) to be automated, which resulted in fabrication of the metamaterial in nine days.\nThe previous device used in 2006 was rudimentary in comparison, and the manufacturing process required four months in order to create the device.\nThese differences are largely due to the different form of transformation: the original 2006 cloak transformed a singular point, while the ground-plane version transforms a plane, and the transformation in the carpet cloak was quasi-conformal, rather than non-conformal.\n\n# Other theories of cloaking\nOther theories of cloaking discuss various science and research based theories for producing an electromagnetic cloak of invisibility.\nTheories presented employ [transformation optics](https://bliptext.com/articles/transformation-optics), event cloaking, dipolar scattering cancellation, tunneling light transmittance, sensors and active sources, and [acoustic cloaking](https://bliptext.com/articles/acoustic-metamaterials).\n\n# Institutional research\nThe research in the field of [metamaterials](https://bliptext.com/articles/metamaterials) has diffused out into the American government science research departments, including the US [Naval Air Systems Command](https://bliptext.com/articles/naval-air-systems-command), [US Air Force](https://bliptext.com/articles/us-air-force), and [US Army](https://bliptext.com/articles/us-army).\nMany scientific institutions are involved including:\nFunding for research into this technology is provided by the following [American](https://bliptext.com/articles/united-states) agencies:\nThrough this research, it has been realized that developing a method for controlling [electromagnetic fields](https://bliptext.com/articles/electromagnetic-fields) can be applied to escape detection by radiated probing, or [sonar](https://bliptext.com/articles/sonar) technology, and to improve [communications](https://bliptext.com/articles/communications) in the [microwave](https://bliptext.com/articles/microwave) range; that this method is relevant to [superlens](https://bliptext.com/articles/superlens) design and to the cloaking of objects within and from [electromagnetic fields](https://bliptext.com/articles/electromagnetic-fields).\n\n# In the news\nOn October 20, 2006, the day after Duke University achieved enveloping and \"disappearing\" an object in the microwave range, the story was reported by [Associated Press](https://bliptext.com/articles/associated-press).\nMedia outlets covering the story included USA Today, MSNBC's Countdown With Keith Olbermann: Sight Unseen, The New York Times with Cloaking Copper, Scientists Take Step Toward Invisibility, (London) The Times with Don't Look Now—Visible Gains in the Quest for Invisibility, Christian Science Monitor with ''Disappear Into Thin Air?\nScientists Take Step Toward Invisibility, Australian Broadcasting, Reuters with Invisibility Cloak a Step Closer, and the (Raleigh) News & Observer with 'Invisibility Cloak a Step Closer''.\nOn November 6, 2006, the Duke University research and development team was selected as part of the Scientific American best 50 articles of 2006.\nIn the month of November 2009, \"research into designing and building unique 'metamaterials' has received a £4.9 million funding boost. Metamaterials can be used for invisibility 'cloaking' devices, sensitive security sensors that can detect tiny quantities of dangerous substances, and flat lenses that can be used to image tiny objects much smaller than the wavelength of light.\"\nIn November 2010, scientists at the University of St Andrews in Scotland reported the creation of a flexible cloaking material they call \"Metaflex\", which may bring industrial applications significantly closer.\nIn 2014, the world 's first 3D acoustic device was built by Duke engineers.","current_revision":null,"created_by":"kACnuQmH5k9Nr5VNVEpVeg7XpvUmMctD","created_at":"2025-01-26T12:12:47.894Z","updated_at":"2025-01-26T12:12:47.894Z","search_vector":"'cloak':2 'metamateri':1","search_content":" \n \n\n# \n**Metamaterial cloaking** is the usage of [metamaterials](https://bliptext.com/articles/metamaterials) in an [invisibility cloak](https://bliptext.com/articles/cloaking-device).\nThis is accomplished by manipulating the paths traversed by light through a novel optical material.\nMetamaterials direct and control the [propagation](https://bliptext.com/articles/wave-propagation) and transmission of specified parts of the [light spectrum](https://bliptext.com/articles/em-spectrum) and demonstrate the potential to render an object seemingly [invisible](https://bliptext.com/articles/invisible).\nMetamaterial cloaking, based on [transformation optics](https://bliptext.com/articles/transformation-optics), describes the process of shielding something from view by controlling [electromagnetic radiation](https://bliptext.com/articles/electromagnetic-radiation).\nObjects in the defined location are still present, but incident waves are guided around them without being affected by the object itself.\n\n# Electromagnetic metamaterials\nElectromagnetic metamaterials respond to chosen parts of radiated light, also known as the [electromagnetic spectrum](https://bliptext.com/articles/electromagnetic-spectrum), in a manner that is difficult or impossible to achieve with [natural materials](https://bliptext.com/articles/natural-material).\nIn other words, these [metamaterials](https://bliptext.com/articles/metamaterials) can be further defined as artificially structured [composite materials](https://bliptext.com/articles/composite-material), which exhibit interaction with light usually not available in nature ([electromagnetic interactions](https://bliptext.com/articles/electromagnetic-interaction)).\nAt the same time, metamaterials have the potential to be engineered and constructed with desirable properties that fit a specific need.\nThat need will be determined by the particular application.\nThe artificial structure for cloaking applications is a [lattice](https://bliptext.com/articles/crystal-structure) design – a sequentially repeating network – of identical elements.\nAdditionally, for [microwave](https://bliptext.com/articles/microwave) frequencies, these materials are analogous to [crystals](https://bliptext.com/articles/crystal-structure) for [optics](https://bliptext.com/articles/optics).\nAlso, a metamaterial is composed of a sequence of elements and spacings, which are much smaller than the selected [wavelength of light](https://bliptext.com/articles/light-spectrum).\nThe selected wavelength could be [radio frequency](https://bliptext.com/articles/radio-frequency), microwave, or other radiations, now just beginning to reach into the [visible frequencies](https://bliptext.com/articles/visible-frequencies).\n[Macroscopic](https://bliptext.com/articles/macroscopic) properties can be directly controlled by adjusting characteristics of the rudimentary [elements](https://bliptext.com/articles/split-ring-resonator), and their arrangement on, or throughout the material.\nMoreover, these metamaterials are a basis for building very small cloaking devices in anticipation of larger devices, adaptable to a broad spectrum of radiated light.\nHence, although light consists of an [electric field](https://bliptext.com/articles/electric-field) and a [magnetic field](https://bliptext.com/articles/magnetic-field), ordinary optical materials, such as [optical microscope](https://bliptext.com/articles/optical-microscope) lenses, have a strong reaction only to the [electric field](https://bliptext.com/articles/electric-field).\nThe corresponding magnetic interaction is essentially nil.\nThis results in only the most common [optical effects](https://bliptext.com/articles/optical-microscopy), such as ordinary [refraction](https://bliptext.com/articles/refraction) with common [diffraction limitations](https://bliptext.com/articles/optical-microscopy) in lenses and [imaging](https://bliptext.com/articles/optical-resolution).\nSince the [beginning of optical sciences](https://bliptext.com/articles/history-of-optics), centuries ago, the ability to control the [light](https://bliptext.com/articles/light) with materials has been limited to these common optical effects.\nMetamaterials, on the other hand, are capable of a very strong interaction, or coupling, with the magnetic component of light.\nTherefore, the range of response to [radiated light](https://bliptext.com/articles/light) is expanded beyond the ordinary optical limitations that are described by the sciences of [physical optics](https://bliptext.com/articles/physical-optics) and [optical physics](https://bliptext.com/articles/optical-physics).\nIn addition, as artificially constructed materials, both the magnetic and electric components of the radiated light can be controlled at will, in any desired fashion as it travels, or more accurately [propagates](https://bliptext.com/articles/wave-propagation), through the material.\nThis is because a metamaterial's behavior is typically formed from individual components, and each component responds independently to a [radiated spectrum](https://bliptext.com/articles/em-radiation) of light.\nAt this time, however, metamaterials are limited.\nCloaking across a broad [spectrum of frequencies](https://bliptext.com/articles/frequency-spectrum) has not been achieved, including the [visible spectrum](https://bliptext.com/articles/visible-spectrum).\n[Dissipation](https://bliptext.com/articles/dissipation), [absorption](https://bliptext.com/articles/absorption-electromagnetic-radiation), and [dispersion](https://bliptext.com/articles/dispersion-relation) are also current drawbacks, but this field is still in its optimistic infancy.\n\n## Metamaterials and transformation optics\nThe field of [transformation optics](https://bliptext.com/articles/transformation-optics) is founded on the effects produced by metamaterials.\nTransformation optics has its beginnings in the conclusions of two research endeavors.\nThey were published on May 25, 2006, in the same issue of [Science](https://bliptext.com/articles/science-journal), a peer-reviewed journal.\nThe two papers are tenable theories on bending or distorting [light](https://bliptext.com/articles/light) to [electromagnetically](https://bliptext.com/articles/electromagnetic-field) conceal an object.\nBoth papers notably map the initial configuration of the [electromagnetic fields](https://bliptext.com/articles/electromagnetic-field) on to a [Cartesian](https://bliptext.com/articles/cartesian-coordinate-system) mesh.\nTwisting the Cartesian mesh, in essence, transforms the coordinates of the electromagnetic fields, which in turn conceal a given object.\nHence, with these two papers, transformation optics is born.\nTransformation optics subscribes to the capability of bending [light](https://bliptext.com/articles/light), or [electromagnetic waves](https://bliptext.com/articles/electromagnetic-waves) and [energy](https://bliptext.com/articles/energy), in any preferred or desired fashion, for a desired application.\n[Maxwell's equations](https://bliptext.com/articles/maxwell-s-equations) do not vary even though [coordinates](https://bliptext.com/articles/coordinate-system) transform.\nInstead it is the values of the chosen parameters of the materials which \"transform\", or alter, during a certain time period.\nSo, transformation optics developed from the capability to choose the parameters for a given material.\nHence, since Maxwell's equations retain the same form, it is the successive values of the parameters, [permittivity](https://bliptext.com/articles/permittivity) and [permeability](https://bliptext.com/articles/permeability-electromagnetism), which change over time.\nFurthermore, permittivity and permeability are in a sense responses to the [electric](https://bliptext.com/articles/electric-field) and [magnetic fields](https://bliptext.com/articles/magnetic-field) of a radiated light source respectively, among other descriptions.\nThe precise degree of electric and magnetic response can be controlled in a metamaterial, point by point.\nSince so much control can be maintained over the responses of the material, this leads to an enhanced and highly flexible [gradient-index](https://bliptext.com/articles/gradient-index-optics) material.\nConventionally predetermined [refractive index](https://bliptext.com/articles/refractive-index) of ordinary materials instead become independent spatial gradients in a metamaterial, which can be controlled at will.\nTherefore, transformation optics is a new method for creating novel and unique [optical devices](https://bliptext.com/articles/optical-device).\n# [Left:The cross section of a PEC cylinder subject to a plane wave (only the electric field component of the wave is shown). The field is scattered. Right: a circular cloak, designed using transformation optics methods, is used to cloak the cylinder. In this case the field remains unchanged outside the cloak and the cylinder is invisible electromagnetically. Note the special distortion pattern of the field inside the cloak. | upload.wikimedia.org/wikipedia/commons/7/73/Circular///EM///cloak///using///transformation///optics.svg]\n\n# Science of cloaking devices\nThe purpose of a cloaking device is to hide something, so that a defined region of space is [invisibly](https://bliptext.com/articles/invisibility) isolated from passing electromagnetic fields (or [sound waves](https://bliptext.com/articles/acoustic-metamaterials)), as with **Metamaterial cloaking**.\nCloaking objects, or making them appear invisible with [metamaterials](https://bliptext.com/articles/metamaterial), is roughly analogous to a magician's sleight of hand, or his tricks with mirrors.\nThe object or subject doesn't really disappear; the vanishing is an illusion.\nWith the same goal, [researchers](https://bliptext.com/articles/researcher) employ metamaterials to create directed blind spots by deflecting certain parts of the [light spectrum](https://bliptext.com/articles/em-spectrum) (electromagnetic spectrum).\nIt is the light spectrum, as the [transmission medium](https://bliptext.com/articles/transmission-medium), that determines what the human eye can see.\nIn other words, light is [refracted](https://bliptext.com/articles/refract) or reflected determining the view, color, or illusion that is seen.\nThe visible extent of light is seen in a [chromatic spectrum](https://bliptext.com/articles/visible-spectrum) such as the [rainbow](https://bliptext.com/articles/rainbow).\nHowever, visible light is only part of a broad spectrum, which extends beyond the sense of sight.\nFor example, there are other parts of the light spectrum which are in common use today.\nThe [microwave spectrum](https://bliptext.com/articles/microwave) is employed by [radar](https://bliptext.com/articles/radar), [cell phones](https://bliptext.com/articles/cell-phone), and [wireless Internet](https://bliptext.com/articles/wireless).\nThe [infrared spectrum](https://bliptext.com/articles/infrared-spectrum) is used for [thermal imaging](https://bliptext.com/articles/thermal-imaging) technologies, which can detect a warm body amidst a cooler night time environment, and infrared illumination is combined with specialized [digital cameras](https://bliptext.com/articles/charge-coupled-device) for [night vision](https://bliptext.com/articles/night-vision).\nAstronomers employ the [terahertz band](https://bliptext.com/articles/terahertz-band) for [submillimeter observations](https://bliptext.com/articles/submillimeter-astronomy) to answer deep [cosmological](https://bliptext.com/articles/astronomy) questions.\nFurthermore, electromagnetic energy is light energy, but only a small part of it is [visible light](https://bliptext.com/articles/visible-light).\nThis energy travels in waves.\nShorter wavelengths, such as visible light and [infrared](https://bliptext.com/articles/infrared), carry more energy per [photon](https://bliptext.com/articles/photon) than longer waves, such as [microwaves](https://bliptext.com/articles/microwave) and [radio waves](https://bliptext.com/articles/radio-waves).\nFor the [sciences](https://bliptext.com/articles/science), the light spectrum is known as the [electromagnetic spectrum](https://bliptext.com/articles/electromagnetic-spectrum).\n\n## The properties of optics and light\n[Prisms](https://bliptext.com/articles/prism-optics), [mirrors](https://bliptext.com/articles/mirror), and [lenses](https://bliptext.com/articles/lens-optics) have a long history of altering the diffracted visible light that surrounds all.\nHowever, the control exhibited by these ordinary materials is limited.\nMoreover, the one material which is common among these three types of directors of light is conventional [glass](https://bliptext.com/articles/glass).\nHence, these familiar [technologies](https://bliptext.com/articles/technologies) are constrained by the fundamental, physical [laws of optics](https://bliptext.com/articles/optics).\nWith metamaterials in general, and the cloaking technology in particular, it appears these barriers disintegrate with advancements in materials and technologies never before realized in the [natural physical sciences](https://bliptext.com/articles/physics).\nThese unique materials became notable because electromagnetic radiation can be bent, reflected, or skewed in new ways.\nThe radiated light could even be slowed or captured before transmission.\nIn other words, new ways to focus and project light and other radiation are being developed.\nFurthermore, the expanded optical powers presented in the science of cloaking objects appear to be technologically beneficial across a wide spectrum of devices already in use.\nThis means that every device with basic functions that rely on interaction with the radiated [electromagnetic spectrum](https://bliptext.com/articles/electromagnetic-spectrum) could technologically advance.\nWith these beginning steps a whole new class of optics has been established.\n\n## Interest in the properties of optics and light\nInterest in the properties of optics, and light, date back to almost 2000 years to [Ptolemy](https://bliptext.com/articles/ptolemy) (AD 85 – 165).\nIn his work entitled Optics, he writes about the properties of [light](https://bliptext.com/articles/light), including [reflection](https://bliptext.com/articles/reflection-physics), [refraction](https://bliptext.com/articles/refraction), and [color](https://bliptext.com/articles/color).\nHe developed a simplified equation for refraction without [trigonometric functions](https://bliptext.com/articles/trigonometric-functions).\nAbout 800 years later, in AD 984, [Ibn Sahl](https://bliptext.com/articles/ibn-sahl-mathematician) discovered a law of refraction mathematically equivalent to [Snell's law](https://bliptext.com/articles/snell-s-law).\nHe was followed by the most notable Islamic scientist, [Ibn Al-Haytham](https://bliptext.com/articles/ibn-al-haytham) (c.965–1039), who is considered to be \"one of the few most outstanding figures in optics in all times\".\nHe made significant advances in the science of [physics](https://bliptext.com/articles/physics) in general, and [optics](https://bliptext.com/articles/optics) in particular.\nHe anticipated the universal laws of light articulated by seventeenth century scientists by hundreds of years.\nIn the seventeenth century both [Willebrord Snellius](https://bliptext.com/articles/willebrord-snellius) and [Descartes](https://bliptext.com/articles/descartes) were credited with discovering the law of refraction.\nIt was Snellius who noted that Ptolemy's equation for refraction was inexact.\nConsequently, these laws have been passed along, unchanged for about 400 years, like the laws of gravity.\n\n## Perfect cloak and theory\n[Electromagnetic radiation](https://bliptext.com/articles/electromagnetic-radiation) and matter have a symbiotic relationship.\nRadiation does not simply act on a material, nor is it simply acted upon by a given material.\nRadiation interacts with [matter](https://bliptext.com/articles/matter).\nCloaking applications which employ [metamaterials](https://bliptext.com/articles/metamaterial) alter how objects interact with the [electromagnetic spectrum](https://bliptext.com/articles/electromagnetic-spectrum).\nThe guiding vision for the metamaterial cloak is a device that directs the flow of light smoothly around an object, like water flowing past a rock in a stream, without [reflection](https://bliptext.com/articles/reflection-physics), rendering the object invisible.\nIn reality, the simple cloaking devices of the present are imperfect, and have limitations.\nOne challenge up to the present date has been the inability of metamaterials, and cloaking devices, to interact at [frequencies](https://bliptext.com/articles/frequencies), or [wavelengths](https://bliptext.com/articles/wavelength), within the visible light spectrum.\n\n## Challenges presented by the first cloaking device\nThe principle of cloaking, with a cloaking device, was first proved (demonstrated) at frequencies in the [microwave radiation band](https://bliptext.com/articles/microwave) on October 19, 2006.\nThis demonstration used a small cloaking device.\nIts height was less than one half inch (< 13 mm) and its diameter five inches (125 mm), and it successfully diverted microwaves around itself.\nThe object to be hidden from view, a small cylinder, was placed in the center of the device.\nThe invisibility cloak deflected [microwave beams](https://bliptext.com/articles/microwave) so they flowed around the cylinder inside with only minor distortion, making it appear almost as if nothing were there at all.\nSuch a device typically involves surrounding the object to be cloaked with a shell which affects the passage of [light](https://bliptext.com/articles/light) near it.\nThere was reduced reflection of electromagnetic waves (microwaves), from the object.\nUnlike a homogeneous natural material with its material properties the same everywhere, the cloak's material properties vary from point to point, with each point designed for specific electromagnetic interactions (inhomogeneity), and are different in different directions (anisotropy).\nThis accomplishes a [gradient](https://bliptext.com/articles/gradient-index-optics) in the material properties.\nThe associated report was published in the journal [Science](https://bliptext.com/articles/science-journal).\nAlthough a successful demonstration, three notable limitations can be shown.\nFirst, since its effectiveness was only in the[ microwave spectrum](https://bliptext.com/articles/microwave) the small object is somewhat invisible only at microwave frequencies.\nThis means invisibility had not been achieved for the [human eye](https://bliptext.com/articles/human-eye), which sees only within the [visible spectrum](https://bliptext.com/articles/visible-spectrum).\nThis is because the [wavelengths](https://bliptext.com/articles/wavelengths) of the visible spectrum are tangibly shorter than microwaves.\nHowever, this was considered the first step toward a cloaking device for visible light, although more advanced [nanotechnology-related](https://bliptext.com/articles/nanotechnology) techniques would be needed due to light's short wavelengths.\nSecond, only small objects can be made to appear as the surrounding air.\nIn the case of the 2006 proof of cloaking demonstration, the hidden from view object, a [copper](https://bliptext.com/articles/copper) cylinder, would have to be less than five inches in diameter, and less than one half inch tall.\nThird, cloaking can only occur over a narrow frequency band, for any given demonstration.\nThis means that a broad band cloak, which works across the [electromagnetic spectrum](https://bliptext.com/articles/electromagnetic-spectrum), from [radio frequencies](https://bliptext.com/articles/radio-frequencies) to microwave to the [visible spectrum](https://bliptext.com/articles/visible-spectrum), and to [x-ray](https://bliptext.com/articles/x-ray), is not available at this time.\nThis is due to the [dispersive nature](https://bliptext.com/articles/dispersion-optics) of present-day metamaterials.\nThe coordinate transformation ([transformation optics](https://bliptext.com/articles/transformation-optics)) requires extraordinary material parameters that are only approachable through the use of [resonant](https://bliptext.com/articles/resonant) elements, which are inherently [narrow band](https://bliptext.com/articles/narrowband), and [dispersive](https://bliptext.com/articles/dispersion-optics) at resonance.\n\n# Usage of metamaterials\nAt the very beginning of the new millennium, metamaterials were established as an extraordinary new medium, which expanded control capabilities over [matter](https://bliptext.com/articles/matter).\nHence, metamaterials are applied to cloaking applications for a few reasons.\nFirst, the parameter known as material response has broader range.\nSecond, the material response can be controlled at will.\nThird, optical components, such as lenses, respond within a certain defined range to [light](https://bliptext.com/articles/light).\nAs stated earlier – the range of response has been known, and studied, going back to [Ptolemy](https://bliptext.com/articles/ptolemy) – eighteen hundred years ago.\nThe range of response could not be effectively exceeded, because natural materials proved incapable of doing so.\nIn scientific studies and research, one way to communicate the range of response is the [refractive index](https://bliptext.com/articles/refractive-index) of a given optical material.\nEvery [natural material](https://bliptext.com/articles/natural-material) so far only allows for a positive refractive index.\nMetamaterials, on the other hand, are an innovation that are able to achieve negative refractive index, zero refractive index, and fractional values in between zero and one.\nHence, metamaterials extend the material response, among other capabilities.\nHowever, negative refraction is not the effect that creates invisibility-cloaking.\nIt is more accurate to say that gradations of refractive index, when combined, create invisibility-cloaking.\nFourth, and finally, metamaterials demonstrate the capability to deliver chosen responses at will.\n\n# Device\nBefore actually building the device, theoretical studies were conducted.\nThe following is one of two studies accepted simultaneously by a scientific journal, as well being distinguished as one of the first published theoretical works for an invisibility cloak.\n\n## Controlling electromagnetic fields\nThe exploitation of \"light\", the [electromagnetic spectrum](https://bliptext.com/articles/electromagnetic-spectrum), is accomplished with common objects and materials which control and direct the [electromagnetic fields](https://bliptext.com/articles/electromagnetic-field).\nFor example, a glass [lens](https://bliptext.com/articles/lens-optics) in a camera is used to produce an image, a metal cage may be used to screen sensitive equipment, and [radio antennas](https://bliptext.com/articles/radio-antenna) are designed to transmit and receive daily FM broadcasts.\n[Homogeneous](https://bliptext.com/articles/homogeneity-physics) materials, which manipulate or modulate [electromagnetic radiation](https://bliptext.com/articles/electromagnetic-radiation), such as glass lenses, are limited in the upper limit of refinements to correct for aberrations.\nCombinations of [inhomogeneous](https://bliptext.com/articles/inhomogeneous) lens materials are able to employ gradient [refractive indices](https://bliptext.com/articles/refractive-indices), but the ranges tend to be limited.\n[Metamaterials](https://bliptext.com/articles/metamaterials) were introduced about a decade ago, and these expand control of parts of the [electromagnetic spectrum](https://bliptext.com/articles/electromagnetic-spectrum); from [microwave](https://bliptext.com/articles/microwave), to [terahertz](https://bliptext.com/articles/terahertz-radiation), to [infrared](https://bliptext.com/articles/infrared).\nTheoretically, metamaterials, as a [transmission medium](https://bliptext.com/articles/transmission-medium), will eventually expand control and direction of electromagnetic fields into the [visible spectrum](https://bliptext.com/articles/visible-spectrum).\nHence, a design strategy was introduced in 2006, to show that a metamaterial can be engineered with arbitrarily assigned positive or negative values of [permittivity](https://bliptext.com/articles/permittivity) and [permeability](https://bliptext.com/articles/magnetic-permeability), which can also be independently varied at will.\nThen direct control of electromagnetic fields becomes possible, which is relevant to novel and unusual lens design, as well as a component of the [scientific theory](https://bliptext.com/articles/scientific-theory) for cloaking of objects from electromagnetic detection.\nEach component responds independently to a radiated [electromagnetic wave](https://bliptext.com/articles/electromagnetic-wave) as it travels through the material, resulting in electromagnetic inhomogeneity for each component.\nEach component has its own response to the external [electric and magnetic fields](https://bliptext.com/articles/electromagnetic-field) of the [radiated source](https://bliptext.com/articles/em-radiation).\nSince these components are smaller than the radiated [wavelength](https://bliptext.com/articles/wavelength) it is understood that a [macroscopic](https://bliptext.com/articles/macroscopic) view includes an [effective value](https://bliptext.com/articles/effective-medium-model) for both permittivity and permeability.\nThese materials obey the [laws of physics](https://bliptext.com/articles/laws-of-physics), but behave differently from normal materials.\nMetamaterials are artificial materials engineered to provide properties which \"may not be readily available in nature\".\nThese materials usually gain their properties from structure rather than composition, using the inclusion of small [inhomogeneities](https://bliptext.com/articles/heterogeneous) to enact [effective macroscopic behavior](https://bliptext.com/articles/effective-medium-model).\nThe [structural units](https://bliptext.com/articles/periodic-function) of metamaterials can be tailored in shape and size.\nTheir composition, and their form or structure, can be finely adjusted.\nInclusions can be designed, and then placed at desired locations in order to vary the function of a given material.\nAs the lattice is constant, the cells are smaller than the radiated light.\nThe design strategy has at its core [inhomogeneous](https://bliptext.com/articles/inhomogeneous) composite [metamaterials](https://bliptext.com/articles/metamaterial) which direct, at will, [conserved quantities](https://bliptext.com/articles/conservation-law-physics) of [electromagnetism](https://bliptext.com/articles/electromagnetism).\nThese quantities are specifically, the [electric displacement field](https://bliptext.com/articles/electric-displacement-field) **D**, the [magnetic field intensity](https://bliptext.com/articles/magnetic-field) **B**, and the [Poynting vector](https://bliptext.com/articles/poynting-vector) **S**.\nTheoretically, when regarding the conserved quantities, or fields, the metamaterial exhibits a twofold capability.\nFirst, the fields can be concentrated in a given direction.\nSecond, they can be made to avoid or surround objects, returning without [perturbation](https://bliptext.com/articles/perturbation) to their original path.\nThese results are consistent with [Maxwell's equations](https://bliptext.com/articles/maxwell-s-equations) and are more than only [ray approximation](https://bliptext.com/articles/ray-optics) found in [geometrical optics](https://bliptext.com/articles/geometrical-optics).\nAccordingly, in principle, these effects can encompass all forms of electromagnetic radiation phenomena on all length scales.\nThe hypothesized design strategy begins with intentionally choosing a configuration of an [arbitrary number](https://bliptext.com/articles/arbitrariness) of embedded sources.\nThese sources become localized responses of [permittivity](https://bliptext.com/articles/permittivity), ε, and [magnetic permeability](https://bliptext.com/articles/magnetic-permeability), μ.\nThe sources are embedded in an [arbitrarily](https://bliptext.com/articles/arbitrariness) selected [transmission medium](https://bliptext.com/articles/transmission-medium) with [dielectric](https://bliptext.com/articles/dielectric) and [magnetic](https://bliptext.com/articles/magnetic) characteristics.\nAs an electromagnetic system the medium can then be schematically represented as a grid.\nThe first requirement might be to move a uniform [electric field](https://bliptext.com/articles/electric-field) through space, but in a definite direction, which avoids an object or obstacle.\nNext remove and embed the system in an elastic medium that can be warped, twisted, pulled or stretched as desired.\nThe initial condition of the fields is recorded on a Cartesian mesh.\nAs the elastic medium is distorted in one, or combination, of the described possibilities, the same pulling and stretching process is recorded by the Cartesian mesh.\nThe same set of contortions can now be recorded, occurring as [coordinate transformation](https://bliptext.com/articles/coordinate-transformation)**:**\nHence, the permittivity, ε, and permeability, μ, is proportionally calibrated by a common factor.\nThis implies that less precisely, the same occurs with the refractive index.\nRenormalized values of permittivity and permeability are applied in the new coordinate system.\nFor the renormalization equations see ref.\n#.\n\n### Application to cloaking devices\nGiven the above parameters of operation, the system, a metamaterial, can now be shown to be able to conceal an object of arbitrary size.\nIts function is to manipulate incoming rays, which are about to strike the object.\nThese incoming rays are instead electromagnetically steered around the object by the metamaterial, which then returns them to their original trajectory.\nAs part of the design it can be assumed that no radiation leaves the concealed volume of space, and no radiation can enter the space.\nAs illustrated by the function of the metamaterial, any radiation attempting to penetrate is steered around the space or the object within the space, returning to the initial direction.\nIt appears to any observer that the concealed volume of space is empty, even with an object present there.\nAn arbitrary object may be hidden because it remains untouched by external radiation.\nA sphere with radius R1 is chosen as the object to be hidden.\nThe cloaking region is to be contained within the annulus R1 < r < R2.\nA simple transformation that achieves the desired result can be found by taking all fields in the region r < R2 and compressing them into the region R1 < r < R2.\nThe coordinate transformations do not alter Maxwell's equations.\nOnly the values of ε and μ change over time.\n\n### Cloaking hurdles\nThere are issues to be dealt with to achieve invisibility cloaking.\nOne issue, related to [ray tracing](https://bliptext.com/articles/ray-tracing-physics), is the [anisotropic](https://bliptext.com/articles/anisotropic) effects of the material on the electromagnetic rays entering the \"system\".\nParallel bundles of rays, (see above image), headed directly for the center are abruptly curved and, along with neighboring rays, are forced into tighter and tighter [arcs](https://bliptext.com/articles/arc-geometry).\nThis is due to rapid changes in the now shifting and transforming [permittivity](https://bliptext.com/articles/permittivity) ε and [permeability](https://bliptext.com/articles/permeability-electromagnetism) μ.\nThe second issue is that, while it has been discovered that the selected metamaterials are capable of working within the parameters of the anisotropic effects and the continual shifting of ε and μ, the values for ε and μ cannot be very large or very small.\nThe third issue is that the selected metamaterials are currently unable to achieve broad, [frequency spectrum](https://bliptext.com/articles/dispersion-optics) capabilities.\nThis is because the rays must curve around the \"concealed\" [sphere](https://bliptext.com/articles/sphere), and therefore have longer trajectories than traversing [free space](https://bliptext.com/articles/free-space), or air.\nHowever, the rays must arrive around the other side of the sphere in [phase](https://bliptext.com/articles/phase-waves) with the beginning radiated [light](https://bliptext.com/articles/light).\nIf this is happening then the [phase velocity](https://bliptext.com/articles/group-velocity) exceeds the [velocity of light](https://bliptext.com/articles/velocity-of-light) in a [vacuum](https://bliptext.com/articles/vacuum), which is the speed limit of the universe.\n(Note, this does not violate the laws of physics).\nAnd, with a required absence of [frequency dispersion](https://bliptext.com/articles/frequency-dispersion), the [group velocity](https://bliptext.com/articles/group-velocity) will be identical with [phase velocity](https://bliptext.com/articles/phase-velocity).\nIn the context of this experiment, group velocity can never exceed the velocity of light, hence the [analytical parameters](https://bliptext.com/articles/numerical-simulation) are effective for only one [frequency](https://bliptext.com/articles/frequency).\n\n## Optical conformal mapping and ray tracing in transformation media\nThe goal then is to create no discernible difference between a concealed volume of space and the propagation of electromagnetic waves through empty space.\nIt would appear that achieving a perfectly concealed (100%) hole, where an object could be placed and hidden from view, is not probable.\nThe problem is the following: in order to carry images, light propagates in a continuous range of directions.\nThe [scattering](https://bliptext.com/articles/scattering) data of electromagnetic waves, after bouncing off an object or hole, is unique compared to light propagating through empty space, and is therefore easily perceived.\nLight propagating through empty space is consistent only with empty space.\nThis includes microwave frequencies.\nAlthough mathematical reasoning shows that perfect concealment is not probable because of the wave nature of light, this problem does not apply to electromagnetic rays, i.e., the domain of [geometrical optics](https://bliptext.com/articles/geometrical-optics).\nImperfections can be made arbitrarily, and [exponentially](https://bliptext.com/articles/exponential-function) small for objects that are much larger than the wavelength of light.\nMathematically, this implies n < 1, because the rays follow the shortest path and hence in theory create a perfect concealment.\nIn practice, a certain amount of acceptable visibility occurs, as noted above.\nThe range of the refractive index of the dielectric (optical material) needs to be across a wide [spectrum](https://bliptext.com/articles/spectrum) to achieve concealment, with the illusion created by wave propagation across empty space.\nThese places where n < 1 would be the shortest path for the ray around the object without phase distortion.\nArtificial propagation of empty space could be reached in the microwave-to-[terahertz](https://bliptext.com/articles/terahertz-radiation) range.\nIn [stealth technology](https://bliptext.com/articles/stealth-technology), impedance matching could result in absorption of beamed electromagnetic waves rather than reflection, hence, evasion of detection by [radar](https://bliptext.com/articles/radar).\nThese general principles can also be applied to [sound waves](https://bliptext.com/articles/sound-waves), where the index n describes the ratio of the local phase velocity of the wave to the bulk value.\nHence, it would be useful to protect a space from any sound sourced detection.\nThis also implies protection from sonar.\nFurthermore, these general principles are applicable in diverse fields such as [electrostatics](https://bliptext.com/articles/electrostatics), [fluid mechanics](https://bliptext.com/articles/fluid-mechanics), [classical mechanics](https://bliptext.com/articles/classical-mechanics), and [quantum chaos](https://bliptext.com/articles/quantum-chaos).\nMathematically, it can be shown that the [wave](https://bliptext.com/articles/wave) propagation is indistinguishable from empty space where [light rays](https://bliptext.com/articles/ray-optics) propagate along straight lines.\nThe medium performs an optical conformal mapping to empty space.\n\n## Microwave frequencies\nThe next step, then, is to actually conceal an object by controlling electromagnetic fields.\nNow, the demonstrated and theoretical ability for controlled electromagnetic fields has opened a new field, transformation optics.\nThis nomenclature is derived from coordinate transformations used to create variable pathways for the propagation of light through a material.\nThis demonstration is based on previous theoretical prescriptions, along with the accomplishment of the prism experiment.\nOne possible application of transformation optics and materials is electromagnetic cloaking for the purpose of rendering a volume or object undetectable to incident radiation, including radiated probing.\nThis demonstration, for the first time, of actually concealing an object with electromagnetic fields, uses the method of purposely designed spatial variation.\nThis is an effect of embedding purposely designed electromagnetic sources in the metamaterial.\nAs discussed earlier, the fields produced by the metamaterial are compressed into a shell (coordinate transformations) surrounding the now concealed volume.\nEarlier this was supported theory; this experiment demonstrated the effect actually occurs.\nMaxwell's equations are scalar when applying transformational coordinates, only the permittivity tensor and permeability tensor are affected, which then become spatially variant, and directionally dependent along different axes.\nThe researchers state**:**\nBy implementing these complex material properties, the concealed volume plus the cloak appear to have the properties of free space when viewed externally.\nThe cloak thus neither scatters waves nor imparts a shadow in the either of which would enable the cloak to be detected.\nOther approaches to invisibility either rely on the reduction of backscatter or make use of a resonance in which the properties of the cloaked object and the must be carefully matched.\n...Advances in the development of [negative index metamaterials], especially with respect to gradient index lenses, have made the physical realization of the specified complex material properties feasible.\nWe implemented a two-dimensional (2D) cloak because its fabrication and measurement requirements were simpler than those of a 3D cloak.\nBefore the actual demonstration, the experimental limits of the transformational fields were computationally determined, in addition to simulations, as both were used to determine the effectiveness of the cloak.\nA month prior to this demonstration, the results of an experiment to spatially map the internal and external electromagnetic fields of negative refractive metamaterial was published in September 2006.\nThis was innovative because prior to this the microwave fields were measured only externally.\nIn this September experiment the permittivity and permeability of the microstructures (instead of external macrostructure) of the metamaterial samples were measured, as well as the scattering by the two-dimensional negative index metamaterials.\nThis gave an average effective refractive index, which results in assuming homogeneous metamaterial.\nEmploying this technique for this experiment, spatial mapping of phases and amplitudes of the microwave radiations interacting with metamaterial samples was conducted.\nThe performance of the cloak was confirmed by comparing the measured field maps to simulations.\nFor this demonstration, the concealed object was a conducting cylinder at the inner radius of the cloak.\nAs the largest possible object designed for this volume of space, it has the most substantial scattering properties.\nThe conducting cylinder was effectively concealed in two dimensions.\n\n## Infrared frequencies\nThe definition optical frequency, in metamaterials literature, ranges from far infrared, to near infrared, through the visible spectrum, and includes at least a portion of ultra-violet.\nTo date when literature refers optical frequencies these are almost always frequencies in the infrared, which is below the visible spectrum.\nIn 2009 a group of researchers announced cloaking at optical frequencies.\nIn this case the cloaking frequency was centered at 1500 nm or 1.5 micrometers – the infrared.\n\n## Sonic frequencies\nA laboratory metamaterial device, applicable to [ultra-sound](https://bliptext.com/articles/ultra-sound) waves was demonstrated in January 2011.\nIt can be applied to [sound wavelengths](https://bliptext.com/articles/wavelength) corresponding to frequencies from 40 to 80 kHz.\nThe metamaterial acoustic cloak is designed to hide objects submerged in water.\nThe metamaterial cloaking mechanism bends and twists sound waves by intentional design.\nThe cloaking mechanism consists of 16 concentric rings in a cylindrical configuration.\nEach ring has acoustic circuits.\nIt is intentionally designed to guide sound waves in two dimensions.\nEach ring has a different [index of refraction](https://bliptext.com/articles/index-of-refraction).\nThis causes sound waves to vary their speed from ring to ring.\n\"The sound waves propagate around the outer ring, guided by the channels in the circuits, which bend the waves to wrap them around the outer layers of the cloak\".\nIt forms an array of cavities that slow the speed of the propagating sound waves.\nAn experimental cylinder was submerged and then disappeared from [sonar](https://bliptext.com/articles/sonar).\nOther objects of various shape and density were also hidden from the sonar.\nThe acoustic cloak demonstrated effectiveness for frequencies of 40 kHz to 80 kHz.\nIn 2014 researchers created a 3D acoustic cloak from stacked plastic sheets dotted with repeating patterns of holes.\nThe [pyramidal](https://bliptext.com/articles/pyramid) geometry of the stack and the hole placement provide the effect.\n\n## Invisibility in diffusive light scattering media\nIn 2014, scientists demonstrated good cloaking performance in murky water, demonstrating that an object shrouded in fog can disappear completely when appropriately coated with metamaterial.\nThis is due to the random scattering of light, such as that which occurs in clouds, fog, milk, frosted glass, etc., combined with the properties of the metatmaterial coating.\nWhen light is diffused, a thin coat of metamaterial around an object can make it essentially invisible under a range of lighting conditions.\n\n# Cloaking attempts\n\n## Broadband ground-plane cloak\nIf a [transformation](https://bliptext.com/articles/transformation-geometry) to quasi-[orthogonal coordinates](https://bliptext.com/articles/orthogonal-coordinates) is applied to [Maxwell's equations](https://bliptext.com/articles/maxwell-s-equations) in order to conceal a perturbation on a flat [conducting plane](https://bliptext.com/articles/permittivity) rather than a singular point, as in the first demonstration of a [transformation](https://bliptext.com/articles/transformation-geometry) optics-based cloak, then an object can be hidden underneath the perturbation.\nThis is sometimes referred to as a \"carpet\" cloak.\nAs noted above, the original cloak demonstrated utilized resonant metamaterial elements to meet the effective material constraints.\nUtilizing a quasi-conformal transformation in this case, rather than the non-conformal original transformation, changed the required material properties.\nUnlike the original (singular expansion) cloak, the \"carpet\" cloak required less extreme material values.\nThe quasi-conformal carpet cloak required anisotropic, inhomogeneous materials which only varied in [permittivity](https://bliptext.com/articles/permittivity).\nMoreover, the permittivity was always positive.\nThis allowed the use of non-resonant metamaterial elements to create the cloak, significantly increasing the bandwidth.\nAn automated process, guided by a set of [algorithms](https://bliptext.com/articles/algorithm), was used to construct a metamaterial consisting of thousands of elements, each with its own [geometry](https://bliptext.com/articles/geometry).\nDeveloping the algorithm allowed the [manufacturing process](https://bliptext.com/articles/manufacturing-process) to be automated, which resulted in fabrication of the metamaterial in nine days.\nThe previous device used in 2006 was rudimentary in comparison, and the manufacturing process required four months in order to create the device.\nThese differences are largely due to the different form of transformation: the original 2006 cloak transformed a singular point, while the ground-plane version transforms a plane, and the transformation in the carpet cloak was quasi-conformal, rather than non-conformal.\n\n# Other theories of cloaking\nOther theories of cloaking discuss various science and research based theories for producing an electromagnetic cloak of invisibility.\nTheories presented employ [transformation optics](https://bliptext.com/articles/transformation-optics), event cloaking, dipolar scattering cancellation, tunneling light transmittance, sensors and active sources, and [acoustic cloaking](https://bliptext.com/articles/acoustic-metamaterials).\n\n# Institutional research\nThe research in the field of [metamaterials](https://bliptext.com/articles/metamaterials) has diffused out into the American government science research departments, including the US [Naval Air Systems Command](https://bliptext.com/articles/naval-air-systems-command), [US Air Force](https://bliptext.com/articles/us-air-force), and [US Army](https://bliptext.com/articles/us-army).\nMany scientific institutions are involved including:\nFunding for research into this technology is provided by the following [American](https://bliptext.com/articles/united-states) agencies:\nThrough this research, it has been realized that developing a method for controlling [electromagnetic fields](https://bliptext.com/articles/electromagnetic-fields) can be applied to escape detection by radiated probing, or [sonar](https://bliptext.com/articles/sonar) technology, and to improve [communications](https://bliptext.com/articles/communications) in the [microwave](https://bliptext.com/articles/microwave) range; that this method is relevant to [superlens](https://bliptext.com/articles/superlens) design and to the cloaking of objects within and from [electromagnetic fields](https://bliptext.com/articles/electromagnetic-fields).\n\n# In the news\nOn October 20, 2006, the day after Duke University achieved enveloping and \"disappearing\" an object in the microwave range, the story was reported by [Associated Press](https://bliptext.com/articles/associated-press).\nMedia outlets covering the story included USA Today, MSNBC's Countdown With Keith Olbermann: Sight Unseen, The New York Times with Cloaking Copper, Scientists Take Step Toward Invisibility, (London) The Times with Don't Look Now—Visible Gains in the Quest for Invisibility, Christian Science Monitor with ''Disappear Into Thin Air?\nScientists Take Step Toward Invisibility, Australian Broadcasting, Reuters with Invisibility Cloak a Step Closer, and the (Raleigh) News & Observer with 'Invisibility Cloak a Step Closer''.\nOn November 6, 2006, the Duke University research and development team was selected as part of the Scientific American best 50 articles of 2006.\nIn the month of November 2009, \"research into designing and building unique 'metamaterials' has received a £4.9 million funding boost. Metamaterials can be used for invisibility 'cloaking' devices, sensitive security sensors that can detect tiny quantities of dangerous substances, and flat lenses that can be used to image tiny objects much smaller than the wavelength of light.\"\nIn November 2010, scientists at the University of St Andrews in Scotland reported the creation of a flexible cloaking material they call \"Metaflex\", which may bring industrial applications significantly closer.\nIn 2014, the world 's first 3D acoustic device was built by Duke engineers.","revisionCount":0},"hasMore":false,"nextCursor":null}

History