Metamaterial Cloaking - Invisibility Cloak



Metamaterial cloaking is the scientific application of metamaterials in order to achieve invisibility-cloaking. This is accomplished by manipulating the paths traversed by light through a novel optical material. Metamaterials direct and control the propagation and transmission of specified parts of the light spectrum and demonstrate the potential to render an object seemingly invisible. Metamaterial cloaking, based on transformation optics, describes the process of shielding something from view by controlling electromagnetic radiation. Objects in the defined location are still present, but incident waves are guided around them without being affected by the object itself. Read more ...




Invisibility in Mythology, Fiction, and Pseudoscience

Have you ever thought about being invisible at will - using a cloak or other device to get there - the magic of Harry Potter? Whether you reason it would take you to the world of scientific discovery, espionage, pseudoscience, most people at some point dream about the power of invisibility. In physical reality our powers are stripped away ... but somehow you know that's all about to magically change along with what we know about the Illusion of Time.

In science fiction cloaking, there is generally presented an assumed quasi-scientific, in-universe basis for the concept of achieving invisibility. Conversely, invisibility and cloaking is commonly presented in the science fantasy genre as a magical phenomenon, rather than in forms that rely on pure science.

Camouflaging cloaks form a central plot element in Samuel R. Delany's 1975 novel Dhalgren and the Harry Potter series of novels by J.K. Rowling. Harry uses the cloak to sneak into forbidden areas of the school.

The Cap of Invisibility has appeared in Greek myth: Hades was ascribed possession of a cap or helmet that made the wearer invisible. In some versions of the Perseus myth, Perseus borrows this cap from the goddess Athena and uses it to sneak up on the sleeping Medusa when he kills her.

A similar helmet, the Tarnhelm, is found in Norse mythology. In the Second Branch of the Mabinogi, one of the important texts of Welsh mythology, Caswallawn (the historical Cassivellaunus) murders Caradog ap Bran and other chieftains left in charge of Britain while wearing a cloak of invisibility.

The cloaking devices appearing in Star Wars, Star Trek and Stargate, present a similar notion in a science fiction form, but are generally used to hide larger scale objects, such as space ships.

Cloaks of invisibility are relatively rare in folklore; although they do occur in some fairy tales, such as The Twelve Dancing Princesses, a more common trope is the cap of invisibility.

Edgar Rice Burroughs uses the idea of an invisibility cloak in his 1931 novel A Fighting Man of Mars.

The movie, Erik the Viking humorously depicts the title character using a cloak of invisibility, which he does not realize apparently works only on elderly men.

In the Lord of the Rings, Frodo's elven cloak camouflaged him so that the enemy could see "nothing more than a boulder where the Hobbits were"; this magical item was used in the Dungeons & Dragons game.

Cloaking devices have been used in video games, for example Battlefield Heroes Team Fortress 2 and the Halo series, where they aid stealth-based characters. Also, Crossfire has a game mode (appropriately called Ghost Mode) with the Black List Terrorists cloaked, and using stealth to detonate specific targets.




In the News ...





New invisibility cloak to conceal objects in diffusive atmospheres not just in direct light   PhysOrg - January 22, 2017




Researchers determine fundamental limits of invisibility cloaks   PhysOrg - July 5, 2016

Researchers have been able to quantify fundamental physical limitations on the performance of cloaking devices, a technology that allows objects to become invisible or undetectable to electromagnetic waves including radio waves, microwaves, infrared and visible light. The researchers' theory confirms that it is possible to use cloaks to perfectly hide an object for a specific wavelength, but hiding an object from an illumination containing different wavelengths becomes more challenging as the size of the object increases.




Ultrathin 'Invisibility Cloak' Can Match Any Background   Live Science - September 17, 2015

Researchers have built an ultrathin "invisibility cloak" that gets around this problem, by turning objects into perfect, flat mirrors. Invisibility cloaks are designed to bend light around an object, but materials that do this are typically hard to shape and only work from narrow angles - if you walk around the cloaked object, for instance, it's visible. But a new cloak avoids that problem, and is thin and flexible enough to be wrapped around an object of any shape, the researchers said. It can also be "tuned" to match whatever background is behind it - or can even create illusions of what's there. The group constructed a thin film consisting of a 50-nanometer-thick layer of magnesium fluoride topped by a varying pattern of tiny, brick-shaped gold antennas, each 30 nanometers thick. The "bricks" were built in six different sizes, ranging from about 30 to 220 nanometers long and 90 to 175 nanometers wide.




Mathematicians formulate equations, bend light and figure out how to hide things   PhysOrg - June 22, 2015

Today, between illusionists who make the Statue of Liberty disappear to Harry Potter's invisibility cloak that not only hides him from view but also protects him from spells, pop culture has embraced the idea of hiding behind force fields and magical materials. And not too surprisingly, National Science Foundation (NSF)-funded mathematicians, scientists and engineers are equally fascinated and looking at how and if they can transform science fiction into, well, just science.




The invisibility cloak for TIME: Experts create a cloaking device capable of hiding entire events in 'bubbles'   Daily Mail - December 2, 2014
The quest for a true invisibility cloak, seen in films such as Harry Potter, is the holy grail for physicists. But while many have been able to briefly conceal objects from view, and even shield sounds, one team has developed a way to cloak entire events. By concealing these events behind strands of laser light, the researchers could make it appear as if a moment - such as an object moving from one position to another - had spontaneously occurred of travelled inside a 'bubble of time.'




Elastic Cloaking Material Makes Objects Unfeelable   Scientific American - July 3, 2014
Invisibility cloaks, once thought of as the province only of Harry Potter tales and Star Trek, have become reality in the past decade or so. Now scientists have developed an "unfeelability cloak," a material that hides objects within it from being felt or touched. The researchers suggest that in the future such cloaks might find help protect objects from bumps and pokes that might otherwise harm them. Invisibility cloaks work by smoothly guiding light waves around objects so the waves ripple along their original trajectories as if nothing were there to block them. Scientists have designed cloaking materials that work against other kinds of waves as well—for example, inaudibility cloaks hide objects from the acoustic waves used in sonar.

The unfeelability cloak is a so-called pentamode metamaterial, an artificial structure that, despite being a solid, can behave like a fluid; although difficult to compress, its shape is otherwise easy to shift. The specific material the researchers devised is a three-dimensional hexagonal lattice reminiscent of a honeycomb, with the rods making up this lattice wider at their middles than at their ends. "This is the first experimental example of an elastomechanical unfeelability cloak, and one of the first applications demonstrated for pentamode materials.3




Invisibility 'time cloak' developed   BBC - June 6, 2013

An "invisibility" time cloak which is able to hide events in a continuous stream of light has been developed by scientists. The cloak works by manipulating the speed of light in optical fibres and means any interaction which takes place during this "hole in time" is not detected. That is, a beam of light can be manipulated along its path. An important part of the present paper exploits the principle of space-time duality, which means that like in the original concept of a temporal cloak, one of the directions of spatial cloaking had been replaced by time. It shows how beautiful the space-time principles are that can be used in optics. While previous cloaks are interesting as well, in the sense that they change optics in space, now we can change the way light, and thus information, behaves in space and time.




Drone Couture: Designing Invisibility   Smithsonian - February 2, 2013

Harvey, in collaboration with fashion designer Johanna Bloomfield, has developed Stealth Wear, a new fashion line designed for counter-surveillance. The Anti-Drone garments are a response to a very real concern about the growing loss of privacy and the increased use of new surveillance technologies and autonomous drones. The Stealth Wear garments are made with a light-weight, metalized (half-silver, half-nickel) fabric that reflects heat, making it capable of blocking IR and thermal imaging scans.




Invisibility cloaking in 'perfect' demonstration   BBC - November 12, 2012
Scientists have succeeded in "cloaking" an object perfectly for the first time, rendering a centimetre-scale cylinder invisible to microwaves. Many "invisibility cloak" efforts have been demonstrated, but all have reflected some of the incident light, making the illusion incomplete.




Scientists create first free-standing 3-D cloak   PhysOrg - January 26, 2012
Researchers in the US have, for the first time, cloaked a three-dimensional object standing in free space, bringing the much-talked-about invisibility cloak one step closer to reality. While previous studies have either been theoretical in nature or limited to the cloaking of two-dimensional objects, this study shows how ordinary objects can be cloaked in their natural environment in all directions and from all of an observer's positions.




'Cloaking' a 3-D object from all angles demonstrated   BBC - January 26, 2012
Researchers have "cloaked" a three-dimensional object, making it invisible from all angles, for the first time. However, the demonstration works only for waves in the microwave region of the electromagnetic spectrum. It uses a shell of what are known as plasmonic materials; they present a "photo negative" of the object being cloaked, effectively canceling it out.




"Time Cloak" Created; Can Make Events Disappear   National Geographic - January 6, 2012
Einstein's theories of relativity suggest that gravity can cause time to slow down. Now scientists have demonstrated a way to stop time altogether - or at least, to give the appearance of time stopping by bending light to create a hole in time. The new research builds on recent demonstrations of "invisibility cloaks" that can make objects seem to disappear by bending waves of visible light. The idea is that, if light moves around an object instead of striking it, that light doesn't get scattered and reflected back to an observer, making the object essentially invisible. Now Cornell University scientists have used a similar concept to create a hole in time, albeit a very short one: The effect lasts around 40 trillionths of a second. To conduct their time-stopping experiment, Gaeta and colleagues aimed a laser beam at a probe and passed the beam through a device called a time lens. While a conventional optical lens bends a beam of light in space, the time lens modifies the light's temporal - not spatial - distribution.




Pentagon-backed 'time cloak' stops the clock   PhysOrg - January 5, 2012
Pentagon-supported physicists on Wednesday said they had devised a "time cloak" that briefly makes an event undetectable. Think of it as an art heist that takes place before your eyes and surveillance cameras. You don't see the thief strolling into the museum, taking the painting down or walking away, but he did. It's not just that the thief is invisible - his whole activity is.




  Invisibility cloaking benefits from crystal-clear idea   BBC - February 1, 2011
Researchers have demonstrated an idea for an invisibility cloak using calcite, a common crystalline material. Cloaking relies on guiding light waves such that waves from a hidden object do not reach the eye. Calcite accomplishes this by sending the two "polarizations" of light - directions in which the light waves oscillate - in different directions.




'Invisibility Cloak' Hides 3-D Objects From Naked Eye   Live Science - February 1, 2011
An "invisibility cloak" that's able to hide items thousands of times larger than before now exists, scientists say. The first hints that cloaking devices might one day become more than just a "Star Trek" fantasy began emerging five or so years ago, and since then researchers have made such cloaks a reality by warping light.




Scientists have created an "invisibility cloak" - able to bend light around solid objects - out of silk   Telegraph.co.uk - August 12, 2010
At the moment the cloak only works for light outside the visible spectrum, in the terahertz band between radio and infrared. But its developers, at Boston University and Tufts University, believe that it could be made to work at far smaller wavelengths, possibly even including visible light. The researchers hope it will have applications in medical science, as well as opening the possibility of making people or objects invisible. The "metamaterial" is made of silk covered in tiny gold structures, each a tiny spiral known as a "split ring resonator" or SSR. SSRs have fascinating effects on light - they can absorb, or reflect, all the light at a given wavelength, or bend that light around an object. The silk metamaterial has 10,000 SSRs per square centimetre. Normally, terahertz waves would pass through silk unaffected. But the new meta-silk resonated when the light struck it. Since silk is "biocompatible" - it doesn't spark an immune reaction when implanted in the human body - the meta-silk can be used widely in medicine. Fiorenzo Omenetto, one of the Tufts researchers, told Discovery: "This is an unusual angle for a metamaterial because of silk's ability to interface with the human body."




Researchers design more reliable invisibility cloak   PhysOrg - June 24, 2010


Researchers have proposed a new design for an invisibility cloak - a device that could make objects invisible by guiding light around anything placed inside the cloak. Currently, most metamaterial cloak designs require that the metamaterial response be homogeneous. However, the new design relies on simulations of a true multi-element cloak structure and takes into account the inhomogeneity of a real metamaterial response.




Invisibility cloak that generates virtual images gets closer to realization   PhysOrg - April 2, 2010

In a twist on the concept of an invisibility cloak, researchers have designed a material that not only makes an object invisible, but also generates one or more virtual images in its place. Because it doesn't simply display the background environment to a viewer, this kind of optical device could have applications that go beyond a normal invisibility cloak. Plus, unlike previously proposed illusion devices, the design proposed here could be realized with artificial metamaterials.




Invisibility cloak created in 3-D   BBC - March 18, 2010

Scientists have created the first device to render an object invisible in three dimensions. The "cloak", described in the journal Science, hid an object from detection using light of wavelengths close to those that are visible to humans. Previous devices have been able to hide objects from light traveling in only one direction; viewed from any other angle, the object would remain visible. This is a very early but significant step towards true invisibility cloaks.




Tiny nano-electromagnets turn a cloak of invisibility into a possibility   PhysOrg - December 22, 2009
A team of researchers at the FOM institute AMOLF (The Netherlands) has succeeded for the first time in powering an energy transfer between nano-electromagnets with the magnetic field of light. This breakthrough is of major importance in the quest for magnetic 'meta-materials' with which light rays can be deflected in every possible direction. This could make it possible to produce perfect lenses and, in the fullness of time, even 'invisibility cloaks'.




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