Author Topic: Metamaterials: a real invisibility cloak  (Read 54566 times)

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Offline Thucydides

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Re: Metamaterials: a real invisibility cloak
« Reply #50 on: January 26, 2012, 16:51:46 »
Further advances in cloaking technologies:

http://iopscience.iop.org/1367-2630/14/1/013054/pdf/1367-2630_14_1_013054.pdf

http://nextbigfuture.com/2012/01/experimental-verification-of-three.html

Quote
Three-dimensional plasmonic cloak hides a cylinder from microwaves
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 New Journal of Physics - Experimental verification of three-dimensional plasmonic cloaking in free-space (14 pages) They optimized the cloak design for the 3 GHz range. They have hidden a cylinder from microwaves, demonstrating cloaking of an object in free space, rather than a two-dimensional image. The group has not been able to scatter visible light, but it expects that cloaking small objects is possible. The results pave the way to realistic, practical applications of 3D stand-alone cloaks for radar evasion and non-invasive radio frequency probing.

BBC News - The approach used is unlikely to work at the visible light part of the spectrum. Prof Alu explained that the approach could be applied to the tips of scanning microscopes - the most high-resolution microscopes science has - to yield an improved view of even smaller wavelengths of light.

In future applications, plasmonic materials could be combined with the structured metamaterials idea already in development elsewhere. Light can be channelled where it needs to go, or its effects undone, as need be.

Prof Apu said that if he had to bet in five years what kind of cloaking technique might be used for applications, for practical purposes, then he would say plasmonic cloaking is a good bet.

We report the experimental verification of metamaterial cloaking for a 3D object in free space. We apply the plasmonic cloaking technique, based on scattering cancellation, to suppress microwave scattering from a finite-length dielectric cylinder. We verify that scattering suppression is obtained all around the object in the near- and far-field and for different incidence angles, validating our measurements with analytical results and full-wave simulations. Our nearfield and far-field measurements confirm that realistic and robust plasmonic metamaterial cloaks may be realized for elongated 3D objects with moderate transverse cross-section at microwave frequencies.
Dagny, this is not a battle over material goods. It's a moral crisis, the greatest the world has ever faced and the last. Our age is the climax of centuries of evil. We must put an end to it, once and for all, or perish - we, the men of the mind. It was our own guilt. We produced the wealth of the world - but we let our enemies write its moral code.

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Re: Metamaterials: a real invisibility cloak
« Reply #51 on: January 26, 2012, 16:58:31 »
All the invisibility posts are now here, merged.

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Re: Could this have serious military aplication... Invisibility?
« Reply #52 on: January 27, 2012, 07:21:05 »
Bah, that's nothing, I have known several Gunners and Bombadiers that perfected vanishing to an art form, You would see them in the morning and then at the end of the day.
Same for some officers.
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Re: Metamaterials: a real invisibility cloak
« Reply #53 on: January 27, 2012, 19:54:07 »
All the invisibility posts are now here, merged.

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If they are invisible, how are we going to read them? ;)
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Offline Thucydides

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Re: Metamaterials: a real invisibility cloak
« Reply #54 on: February 23, 2012, 19:25:49 »
Low cost metamaterials are within reach. Once it becomes practical to make this in bulk many exciting possibilities open up:

http://oregonstate.edu/ua/ncs/archives/2012/feb/discovery-opens-avenue-“negative-refraction”-new-products-and-industries

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“NEGATIVE REFRACTION” OPENS AVENUE TO NEW PRODUCTS AND INDUSTRIES

CORVALLIS, Ore. – Researchers at Oregon State University have discovered a way to make a low-cost material that might accomplish negative refraction of light and other radiation – a goal first theorized in 1861 by a giant of science, Scottish physicist James Maxwell, that has still eluded wide practical use.

Other materials can do this but they are based on costly, complex crystalline materials. A low-cost way that yields the same result will have extraordinary possibilities, experts say – ranging from a “super lens” to energy harvesting, machine vision or “stealth” coatings for seeming invisibility.

Entire new products and industries could be possible. The findings have just been published and a patent has been applied for on the technology.

The new approach uses ultra-thin, ultra-smooth, all-amorphous laminates, essentially a layered glass that has no crystal structure. It is, the researchers say, a “very high-tech sandwich.”

The goal is to make radiation bend opposite to the way it does when passing through any naturally occurring material. This is possible in theory, as Maxwell penciled out during the American Civil War. In reality, it’s been pretty difficult to do.

“To accomplish the task of negative refraction, these metamaterials have to be absolutely perfect, just flawless,” said Bill Cowell, a doctoral candidate in the OSU School of Electrical Engineering and Computer Science. “Everyone thought the only way to do that was with perfectly crystalline materials, which are quite expensive to produce and aren’t very practical for large-area commercial application.

“We now know these materials may not need to be that exotic.”

The new study has explained how easy-to-produce laminate materials, created with technology similar to that used to produce a flat panel television, should work for this purpose. The findings outline the component materials and the theoretical behavior of the laminates, Cowell said. They were just published in Physica Status Solidi A, in work supported by the National Science Foundation.

“We haven’t yet used this approach to achieve negative refraction, but the findings suggest it should work for that,” he said. “That will be one goal of continuing research. No one had thought of using amorphous metals for this purpose. They didn’t think it could be that simple.”

Negative refraction, Cowell said, is a brilliant idea. It is based on the equations developed by the young physicist and mathematician Maxwell more than 150 years ago – work for which he is revered, along with Isaac Newton and Albert Einstein, as one of the greatest physicists who ever lived. Einstein kept a photograph of Maxwell on his office wall.

But for generations, theory is about all that it was. Just in the past decade have researchers finally figured out how to create materials of any type that can achieve negative refraction. A way to accomplish that at low cost for the commercial marketplace could be of considerable importance, scientists say.

One application of particular interest is a “super lens,” a device that might provide light magnification at levels that dwarf any existing technology. Many applications are possible in electronics manufacturing, lithography, biomedicine, insulating coatings, heat transfer, space applications, and perhaps new approaches to optical computing and energy harvesting.

The discovery of amorphous metamaterials is an outgrowth of recent findings at OSU about ways to create a metal-insulator-metal, or MIM diode, also of commercial significance. The OSU research is one of the latest advances in “dispersion engineering,” or the control of electromagnetic radiation.

About the OSU College of Engineering: The OSU College of Engineering is among the nation’s largest and most productive engineering programs. In the past six years, the College has more than doubled its research expenditures to $27.5 million by emphasizing highly collaborative research that solves global problems, spins out new companies, and produces opportunity for students through hands-on learning.

The "other" applications of metamaterials such as super lenses and energy harvesting have applications on the military side as well; high power optics that are much smaller and lighter than existing ones, and the ability to concentrate solar energy to make small man portable collectors that can generate useful amounts of energy when the sun is shining come to mind.
Dagny, this is not a battle over material goods. It's a moral crisis, the greatest the world has ever faced and the last. Our age is the climax of centuries of evil. We must put an end to it, once and for all, or perish - we, the men of the mind. It was our own guilt. We produced the wealth of the world - but we let our enemies write its moral code.

Offline Thucydides

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Re: Metamaterials: a real invisibility cloak
« Reply #55 on: September 13, 2013, 23:47:44 »
Another group of scientists discovers a simpler method of achievig the effects of metamaterials. Since both metamaterials and this technique uses material science properties to refract the incoming waves in the direction the designers choose, this can be literally "bolted on" to existing vehicles, buildings etc:

http://nextbigfuture.com/2013/09/shaped-teflon-can-hide-objects-from.html

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Shaped teflon can hide objects from microwaves from one direction

Look out for mass-produced invisibility cloaks thanks to an entirely new way of designing and manufacturing them out of materials such as Teflon. The new approach is to create a computer model of the cloak in the form of a conventional material with fixed light bending properties. The model simulates how this conventional material distorts light as it passes by. The computer then changes the shape and topology of the material to reduce this distortion. By repeating this process many times, it is possible to find a topology that minimises the distortion of light so that it remains more or less unchanged as it passes by. The result is an invisibility cloak; not a perfect one but one that can hold its own against many of those made of metamaterials.

Today, Lu Lan at Zhejiang University in China and a few pals have actually created the first invisibility cloak designed using topology optimisation. They carved it out of Teflon and it took them all of 15 minutes using a computer-controlled engraving machine. “The fabrication process of a sample is substantially simplified,” they say.

The resulting “Teflon eyelid” invisibility cloak hides a cylindrical disc of metal the size of poker chip from microwaves. But crucially, its performance closely matches the prediction of the computer simulation

The same approach can work in optical wavelengths. “Such a cloaking setup won’t be a big problem to replicate in the THz or even optical spectrum,” they say.

Next the researchers want to develop the technique to create cloaks that work over a range of frequencies and at a range of angles.
Dagny, this is not a battle over material goods. It's a moral crisis, the greatest the world has ever faced and the last. Our age is the climax of centuries of evil. We must put an end to it, once and for all, or perish - we, the men of the mind. It was our own guilt. We produced the wealth of the world - but we let our enemies write its moral code.

Offline Colin P

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Re: Metamaterials: a real invisibility cloak
« Reply #56 on: September 24, 2013, 10:13:49 »
Interesting,  can see the application of this on the upper works of ships, to reduce both the radar and visual signature. As Radar is technical LOS, cloaking the upper works would reduce the "visible" range that you would be spotted, combined with low signature construction, you may have a very hard to detect ship.

Offline Thucydides

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Re: Metamaterials: a real invisibility cloak
« Reply #57 on: November 16, 2013, 00:46:48 »
U of T discovers yet another way to bend radiiation around an object. there are multiple means to achieve invisibility now, the real question is becoming which one is most robust and cost effective for a particular application?

http://media.utoronto.ca/media-releases/thin-active-invisibility-cloak-demonstrated-for-first-time/

Quote
Thin, active invisibility cloak demonstrated for first time
Posted on November 12, 2013
TORONTO, ON — Invisibility cloaking is no longer the stuff of science fiction: two researchers in The Edward S. Rogers Sr. Department of Electrical & Computer Engineering have demonstrated an effective invisibility cloak that is thin, scalable and adaptive to different types and sizes of objects.

Professor George Eleftheriades and PhD student Michael Selvanayagam have designed and tested a new approach to cloaking—by surrounding an object with small antennas that collectively radiate an electromagnetic field. The radiated field cancels out any waves scattering off the cloaked object. Their paper ‘Experimental demonstration of active electromagnetic cloaking’ appears today in the journal Physical Review X.

“We’ve taken an electrical engineering approach, but that’s what we are excited about,” says Eleftheriades. “It’s very practical.”

Picture a mailbox sitting on the street. When light hits the mailbox and bounces back into your eyes, you see the mailbox. When radio waves hit the mailbox and bounce back to your radar detector, you detect the mailbox. Eleftheriades and Selvanyagam’s system wraps the mailbox in a layer of tiny antennas that radiate a field away from the box, cancelling out any waves that would bounce back. In this way, the mailbox becomes undetectable to radar.

“We’ve demonstrated a different way of doing it,” says Eleftheriades. “It’s very simple: instead of surrounding what you’re trying to cloak with a thick metamaterial shell, we surround it with one layer of tiny antennas, and this layer radiates back a field that cancels the reflections from the object.”

Their experimental demonstration effectively cloaked a metal cylinder from radio waves using one layer of loop antennas. The system can be scaled up to cloak larger objects using more loops, and Eleftheriades says the loops could become printed and flat, like a blanket or skin. Currently the antenna loops must be manually attuned to the electromagnetic frequency they need to cancel, but in future they could function both as sensors and active antennas, adjusting to different waves in real time, much like the technology behind noise-cancelling headphones.

Work on developing a functional invisibility cloak began around 2006, but early systems were necessarily large and clunky—if you wanted to cloak a car, for example, in practice you would have to completely envelop the vehicle in many layers of metamaterials in order to effectively “shield” it from electromagnetic radiation. The sheer size and inflexibility of the approach makes it impractical for real-world uses.  Earlier attempts to make thin cloaks were not adaptive and active, and could work only for specific small objects.

Beyond obvious applications, such as hiding military vehicles or conducting surveillance operations, this cloaking technology could eliminate obstacles—for example, structures interrupting signals from cellular base stations could be cloaked to allow signals to pass by freely. The system can also alter the signature of a cloaked object, making it appear bigger, smaller, or even shifting it in space. And though their tests showed the cloaking system works with radio waves, re-tuning it to work with Terahertz (T-rays) or light waves could use the same principle as the necessary antenna technology matures.

“There are more applications for radio than for light,” says Eleftheriades. “It’s just a matter of technology—you can use the same principle for light, and the corresponding antenna technology is a very hot area of research.”

-30-

For more information, contact:

Marit Mitchell
Senior Communications Officer
The Edward S. Rogers Sr. Department of Electrical & Computer Engineering
Tel: 416-978-7997
marit.mitchell@utoronto.ca
Dagny, this is not a battle over material goods. It's a moral crisis, the greatest the world has ever faced and the last. Our age is the climax of centuries of evil. We must put an end to it, once and for all, or perish - we, the men of the mind. It was our own guilt. We produced the wealth of the world - but we let our enemies write its moral code.

Offline Thucydides

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Re: Metamaterials: a real invisibility cloak
« Reply #58 on: March 15, 2014, 22:22:50 »
More on using these techniques to bend sound. Soon these techniques may have advanced to the point that they can be applied to ships and submarines as well:

http://www.pratt.duke.edu/news/acoustic-cloaking-device-hides-objects-sound

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Acoustic Cloaking Device Hides Objects from Sound
March 11, 2014
Duke engineers build world’s first 3-D acoustic cloaking device
By Ken Kingery

Using little more than a few perforated sheets of plastic and a staggering amount of number crunching, Duke engineers have demonstrated the world’s first three-dimensional acoustic cloak. The new device reroutes sound waves to create the impression that both the cloak and anything beneath it are not there.

The acoustic cloaking device works in all three dimensions, no matter which direction the sound is coming from or where the observer is located, and holds potential for future applications such as sonar avoidance and architectural acoustics.

The study appears online in Nature Materials.

Bogdan Popa, a research scientist in electrical and computer engineering, shows off the 3D acoustic cloak he helped design and build as a member of Steven Cummer’s laboratory.

“The particular trick we’re performing is hiding an object from sound waves,” said Steven Cummer, professor of electrical and computer engineering at Duke University. “By placing this cloak around an object, the sound waves behave like there is nothing more than a flat surface in their path.”

To achieve this new trick, Cummer and his colleagues turned to the developing field of metamaterials—the combination of natural materials in repeating patterns to achieve unnatural properties. In the case of the new acoustic cloak, the materials manipulating the behavior of sound waves are simply plastic and air. Once constructed, the device looks like several plastic plates with a repeating pattern of holes poked through them stacked on top of one another to form a sort of pyramid.

A close up view of the 3D acoustic cloak. The geometry of the plastic sheets and placement of the holes interacts with sound waves to make it appear as if it isn’t there.

To give the illusion that it isn’t there, the cloak must alter the waves’ trajectory to match what they would look like had they had reflected off a flat surface. Because the sound is not reaching the surface beneath, it is traveling a shorter distance and its speed must be slowed to compensate.
“The structure that we built might look really simple,” said Cummer. “But I promise you that it’s a lot more difficult and interesting than it looks. We put a lot of energy into calculating how sound waves would interact with it. We didn’t come up with this overnight.”

To test the cloaking device, researchers covered a small sphere with the cloak and “pinged” it with short bursts of sound from various angles. Using a microphone, they mapped how the waves responded and produced videos of them traveling through the air.

Cummer and his team then compared the videos to those created with both an unobstructed flat surface and an uncloaked sphere blocking the way. The results clearly show that the cloaking device makes it appear as though the sound waves reflected off an empty surface.

Although the experiment is a simple demonstration showing that the technology is possible and concealing an evil super-genius’ underwater lair is a long ways away, Cummer believes that the technique has several potential commercial applications.

“We conducted our tests in the air, but sound waves behave similarly underwater, so one obvious potential use is sonar avoidance,” said Cummer. “But there’s also the design of auditoriums or concert halls—any space where you need to control the acoustics. If you had to put a beam somewhere for structural reasons that was going to mess up the sound, perhaps you could fix the acoustics by cloaking it.”

This research was supported by Multidisciplinary University Research Initiative grants from the Office of Naval Research (N00014-13-1-0631) and from the Army Research Office (W911NF-09-1-00539).

###

“Three-dimensional broadband omnidirectional acoustic ground cloak,” Zigoneanu L., Popa, B., Cummer, S.A. Nature Materials, March 9, 2014. DOI: 10.1038/NMAT3901
Dagny, this is not a battle over material goods. It's a moral crisis, the greatest the world has ever faced and the last. Our age is the climax of centuries of evil. We must put an end to it, once and for all, or perish - we, the men of the mind. It was our own guilt. We produced the wealth of the world - but we let our enemies write its moral code.

Offline Thucydides

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Re: Metamaterials: a real invisibility cloak
« Reply #59 on: April 04, 2014, 22:56:20 »
More advances in metamaterial technologies:

http://today.ucf.edu/nanotech-leads-break-stealth-technology/

Quote
NanoTech Leads to Break-Through in Stealth Technology

Controlling and bending light around an object so it appears invisible to the naked eye is the theory behind fictional invisibility cloaks.

It may seem easy in Hollywood movies, but is hard to create in real life because no material in nature has the properties necessary to bend light in such a way. Scientists have managed to create artificial nanostructures that can do the job, called metamaterials. But the challenge has been making enough of the material to turn science fiction into a practical reality.

The work of Debashis Chanda at the University of Central Florida, however, may have just cracked that barrier. The cover story in the March edition of the journal Advanced Optical Materials, explains how Chanda and fellow optical and nanotech experts were able to develop a larger swath of multilayer 3-D metamaterial operating in the visible spectral range. They accomplished this feat by using nanotransfer printing, which can potentially be engineered to modify surrounding refractive index needed for controlling propagation of light.

“Such large-area fabrication of metamaterials following a simple printing technique will enable realization of novel devices based on engineered optical responses at the nanoscale,” said Chanda, an assistant professor at UCF.

The nanotransfer printing technique creates metal/dielectric composite films, which are stacked together in a 3-D architecture with nanoscale patterns for operation in the visible spectral range. Control of electromagnetic resonances over the 3-D space by structural manipulation allows precise control over propagation of light. Following this technique, larger pieces of this special material can be created, which were previously limited to micron-scale size.

By improving the technique, the team hopes to be able to create larger pieces of the material with engineered optical properties, which would make it practical to produce for real-life device applications. For example, the team could develop large-area metamaterial absorbers, which would enable fighter jets to remain invisible from detection systems.

Other members of the research team include: Li Gao, Youngmin Kim, Kazuki Shigeta, Steven Hartanto and John Rogers from the University of Illinois at Urbana-Champaign; Abraham Vasquez-Guardado and Daniel Franklin from UCF: Christopher J. Progler from Photronics Inc. and Gregory R. Bogart from the Sandia National Laboratories.

Chanda joined UCF in Fall 2012 from University of Illinois at Urbana-Champaign with joint appointment with the Nanoscience Technology Center and the College of Optics and Photonics (CREOL). He has published multiple articles on light-matter interactions and metamaterials and is a reviewer for multiple journals in his field. For some of his pioneering works, Debashis was awarded a Department of Energy solar innovation award and a Natural Sciences and Engineering Research Council award among others. He also earned a National Science Foundation Summer Institute Fellowship in 2013.
Dagny, this is not a battle over material goods. It's a moral crisis, the greatest the world has ever faced and the last. Our age is the climax of centuries of evil. We must put an end to it, once and for all, or perish - we, the men of the mind. It was our own guilt. We produced the wealth of the world - but we let our enemies write its moral code.

Offline Thucydides

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Re: Metamaterials: a real invisibility cloak
« Reply #60 on: September 19, 2014, 11:11:27 »
Digital metamaterials promise to open up yet another avenue for the production of "invisibility" cloaks and coatings for equipment and facilities. I have now counted four different means in this thread to create an "invisibility" shield, which means the next huge frontier in military technology will be to discover ways to detect "cloaked" men, equipment and facilities:

http://theconversation.com/invisibility-cloaks-closer-thanks-to-digital-metamaterials-31562

Quote
Invisibility cloaks closer thanks to ‘digital metamaterials’
The concept of “digital metamaterials” – a simple way of designing metamaterials with bizarre optical properties that could…

Author
Penny Orbell
 
Editor at The Conversation
 
Interviewed
Min Gu
Professor of Optoelectronics at Swinburne University of Technology

Boris Kuhlmey
Associate Professor of Photonics and Optics at University of Sydney

Tiffany Walsh
Professor of Bionanotechnology at Deakin University

The concept of “digital metamaterials” – a simple way of designing metamaterials with bizarre optical properties that could hasten the development of devices such as invisibility cloaks and superlenses – is reported in a paper published today in Nature Materials.

Metamaterials are artificially engineered out of microscopic subunits – such as glass, metal or plastic – arranged in a repeating fashion. Once assembled, these metamaterials possess unique properties, such as interacting with light in unusual ways, which aren’t often seen in natural materials.

“The idea behind metamaterials is to mimic the way atoms interact with light, but with artificial structures much smaller than the wavelength of light itself,” said Boris Kuhlmey, associate professor of photonics and optics at the University of Sydney.

“This way, optical properties are no longer restricted to those of the constituent materials, and can be designed almost arbitrarily.”

The material world goes digital

The researchers of the Nature Materials paper, from the University of Pennsylvania, were inspired to develop digital metamaterials by the binary numeral system of Boolean algebra.

The binary system is used internally by most digital electronic devices, such as computers and smartphones. Complex digital devices have their digital information simply encoded as a string of 1s and 0s called “bits”.

The proposed method for digital metamaterials is a simplified way of building metamaterials, yet still allows for complex and diverse properties to be achieved.

“The beauty of the new method is its simplicity,” said Min Gu, professor of optoelectronics at Swinburne University of Technology.

Through the use of simulations in two-dimensional space, the researchers explored the possibility of creating metamaterials with only two specially chosen component parts, called metamaterial bits – analogous to the 1 and 0 “bits” of binary computer code. The arrangement of metamaterial bits represents the “digitising” of metamaterials.

In their study, the researchers chose to use nano-sized pieces of silver and silica (glass) as their repeating metamaterial bits. These are materials that interact with light in very different ways on an individual level. Once they were “digitised”, the resulting metamaterial had its own unique properties, very different to those of its constituent parts.

“The components of the material work together to generate effects or give rise to phenomena that you wouldn’t observe if they weren’t arranged together in 3D (or in this case, 2D) space as an ordered assembly,” said Tiffany Walsh, professor of bionanotechnology at Deakin University.

Sourcing material parts in order to achieve unusual properties of a metamaterial can be time consuming and expensive. This new way of thinking about the design of metamaterials may allow researchers to produce the optical properties they want from the metamaterial using only two component parts.

“What this [research] really does is put a new spin on the idea that with only two set materials arranged with the right portions – one metal, one insulator, here silver and silica – almost any optical property can be achieved,” said Associate Professor Kuhlmey.

Professor Walsh said: “This is like the concept of turning sound waves from analog into digital – and they’ve pushed it into a new realm of physics.

“They’ve been able to take the permittivity – the response of the material when it’s exposed to radiation – and digitised this. They’ve turned it into something that is more readily manipulated.”

Waves and matter collide

One of the key applications for metamaterials lies in their ability to manipulate light.

“We already have knowledge about how to manipulate radiation (such as light) – we can use lenses, like a magnifying glass, for example, which focus light down on a spot; we can use mirrors to reflect light and change its direction,” Professor Walsh said.

“But what these [metamaterials] can do is something more sophisticated: they’re able to bend light, to scatter it, to manipulate it in unusual ways.”

Using their digital method, the researchers showed that it is possible to create certain metamaterials with very low permittivity, which are rarely found in nature. Having control over these properties may open doors to more advanced technological applications, such as invisibility cloaking devices.

“It would be interesting in future to see if such a digital design method can facilitate the construction of optical, or invisibility, cloaks,“ said Professor Gu.

“With varying changes of silver/glass ratios (structured at the nanoscale) it is then in principle possible to make flat lenses and other tiny optical elements,” Associate Professor Kuhlmey said.

“The authors […] showed in simulations that nano-patterned glass/silver structures can then bend light, which is also the principle behind invisibility cloaking.”

He added that fabricating the proposed structures would be challenging but not impossible.

“[It would] require structuring glass and metal with a precision of a few atoms in thickness only – but thinking of metamaterials as binary structures may help devise new nano-patterning lithography (printing) techniques that take advantage of this,” he said.
Dagny, this is not a battle over material goods. It's a moral crisis, the greatest the world has ever faced and the last. Our age is the climax of centuries of evil. We must put an end to it, once and for all, or perish - we, the men of the mind. It was our own guilt. We produced the wealth of the world - but we let our enemies write its moral code.

Offline Thucydides

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Re: Metamaterials: a real invisibility cloak
« Reply #61 on: October 12, 2014, 22:22:34 »
While very short on details, this is yet another way to make an "invisibility cloak". Metamaterials use precise tailoring of materials, this apparently only needs four lenses:

http://www.wired.co.uk/news/archive/2014-09/29/off-the-shelf-invisibility-cloak

Quote
Invisibility cloaks built from 'off-the-shelf materials'
SCIENCE 29 SEPTEMBER 14  by KATIE COLLINS   

The Rochester CloakUniversityRochester

The creation of invisibility cloaks has always been considered very much on the cutting edge of physics and technology, treading the line between science, science fiction and pure magic.

But while many researchers beaver away in labs playing around with complex nanotechnology and light refraction techniques, physicists Joseph Choi and John Howell from the University of Rochester in the US have figured out a way to build an invisibility cloak using off-the-shelf materials. And the good news is that they're not even that expensive.

"There have been many high-tech approaches to cloaking and the basic idea behind these is to take light and have it pass around something as if it isn't there, often using high-tech or exotic materials," explains Howell in a University of Rochester video. "We just figured a very simple of doing that would be using standard lenses and things we would normally find in the lab."

So what would you need if you decided to attempt to make an invisibility cloak yourself? According to a paper by Choi and Howell, all you would need to invest in some isotropic, off-the-shelf optics. To be precise, you would need four lenses called achromatic doublets. These can be purchased from specialist optics retailers are commonly used in imaging and physics for applications including colour correction and focussing laser beams. They don't even need to be too large, as the model is scalable and the lenses are capable of bending light no matter what their size.

To design a working invisibility cloak, there are several things you have to keep in mind. Firstly you need to make sure that it can cope with the fact that the world works in three dimensions, meaning that if the cloak or the viewer moves, the illusion must be maintained. It's not enough for the cloak to work if the viewer is only standing straight-on to it. Secondly, it is all very well to hide the object under the cloak, but to maintain the illusion, the background cannot be distorted by it.

Choi and Howell claim to overcome both of these problems with their combination of lenses in what Choi describes as "simplified version of a perfect cloaking device". His definition of "perfect" is that it can do three-dimensional, continuous, multi-directional cloaking, no matter what direction the rays of light are coming from. "Perfect" does not yet mean a Harry Potter-style sheet of material that one could slip over oneself at any given moment, but at least we know the technology now exists to make the magic happen.
Dagny, this is not a battle over material goods. It's a moral crisis, the greatest the world has ever faced and the last. Our age is the climax of centuries of evil. We must put an end to it, once and for all, or perish - we, the men of the mind. It was our own guilt. We produced the wealth of the world - but we let our enemies write its moral code.

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Re: Metamaterials: a real invisibility cloak
« Reply #62 on: April 07, 2015, 10:11:55 »
Coating submarines and naval vessels in "bubble wrap" to abosorb sound waves. This is much lighter and more efficient than current sound absorbing tiles, and I suspect that is can be much less expensive as well:

http://army.ca/forums/index.php?action=post;topic=50470.50;last_msg=1332344

Quote
New industrial bubble wrap material and metamaterial for manipulating or absorbing sound for stealthier submarines

1. Current subs use 1-inch-thick rubber foam to reduce sonar detection. Materials scientists at Université Paris-Diderot, the University of Manitoba, and PSL Research University are working on a technique to create a much thinner sheet of rubber populated by thousands of bubbles that work to deflect sonar while saving on weight over the bulky foam. Lab tests have shown that the material cuts down on radar wave detection by 10,000 times.

In practice, a 4-millimeter film of bubble material could dampen a sonar signal by as much as 99 percent. A 0.16-inch-thick (4 millimeters) film with 0.08-inch (2 millimeters) bubbles could absorb more than 99 percent of the energy from sonar, cutting down reflected sound waves by more than 10,000-fold, or about 100 times better than was previously assumed possible.
 
In underwater experiments, the scientists bombarded a meta-screen placed on a slab of steel with ultrasonic frequencies of sound. They found that the meta-screen dissipated more than 91 percent of the incoming sound energy and reflected less than 3 percent of the sound energy. For comparison, the bare steel block reflected 88 percent of the sound energy.
 
There is the challenge of being able to economically produce durable bubble material to cover an entire submarine.
 
Physical Review B - Superabsorption of acoustic waves with bubble metascreens
 


A bubble metascreen, i.e., a single layer of gas inclusions in a soft solid, can be modeled as an acoustic open resonator, whose behavior is well captured by a simple analytical expression. We show that by tuning the parameters of the metascreen, acoustic superabsorption can be achieved over a broad frequency range, which is confirmed by finite element simulations and experiments. Bubble metascreens can thus be used as ultrathin coatings for turning acoustic reflectors into perfect absorbers.

 2. Imagine a material that wicks sound across its surface like water droplets sliding over a windowpane. For submarines, such a coating would mean an entirely new way to slip past sonar without detection as sound waves pass harmlessly around the vessel.
 
Physicist Baile Zhang and his colleagues at Nanyang Technological University in Singapore think they may have found a way to design such a coating, which could work for any 3D shape—sharp corners included.
 
Sound waves like sonar hit his proposed coating, they strike an acoustically tuned material called a phononic crystal. That crystal bends the waves so that when they bounce off the hull, they loops around—smacking right back onto the surface to bounce over and over again. Zhang likens the process to a professional soccer player curving the ball.
 





Physical Review Letters- Topological Acoustics
 

The manipulation of acoustic wave propagation in fluids has numerous applications, including some in everyday life. Acoustic technologies frequently develop in tandem with optics, using shared concepts such as waveguiding and metamedia. It is thus noteworthy that an entirely novel class of electromagnetic waves, known as “topological edge states,” has recently been demonstrated. These are inspired by the electronic edge states occurring in topological insulators, and possess a striking and technologically promising property: the ability to travel in a single direction along a surface without backscattering, regardless of the existence of defects or disorder. Here, we develop an analogous theory of topological fluid acoustics, and propose a scheme for realizing topological edge states in an acoustic structure containing circulating fluids. The phenomenon of disorder-free one-way sound propagation, which does not occur in ordinary acoustic devices, may have novel applications for acoustic isolators, modulators, and transducers.

 


A two-dimensional acoustic topological insulator and its band structure. (a) Triangular acoustic lattice with lattice constant a. a=0.2  m in the following calculation. Inset: unit cell containing a central metal rod of radius r1=0.2a, surrounded by an anticlockwise circulating fluid flow (flow direction indicated by red arrows) in a cylinder region of radius r2=0.4a. (b) Band structures of the acoustic lattice without the circulating fluid flow
Dagny, this is not a battle over material goods. It's a moral crisis, the greatest the world has ever faced and the last. Our age is the climax of centuries of evil. We must put an end to it, once and for all, or perish - we, the men of the mind. It was our own guilt. We produced the wealth of the world - but we let our enemies write its moral code.

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Re: Metamaterials: a real invisibility cloak
« Reply #63 on: July 17, 2015, 10:14:59 »
NBF on a 3D cloak that works across a wide range of frequencies. If I am reading this right it is prooff against microwaves and sonar, so would make a good all purpose coating for a submarne (invisible to radar when surfaced and invisible to sonar when submerged). The wide frequency range also answers the problem with first generation metamaterials, they were very effective over a very narrow frequency range.:

http://nextbigfuture.com/2015/07/broadband-surface-wave-transformation.html

Quote
Broadband surface-wave transformation cloak - Molding acoustic, electromagnetic and water waves with a single cloak

Two experiments demonstrating that a cylindrical cloak formerly introduced for linear surface liquid waves works equally well for sound and electromagnetic waves. This structured cloak behaves like an acoustic cloak with an effective anisotropic density and an electromagnetic cloak with an effective anisotropic permittivity, respectively. Measured forward scattering for pressure and magnetic fields are in good agreement and provide first evidence of broadband cloaking. Microwave experiments and 3D electromagnetic wave simulations further confirm reduced forward and backscattering when a rectangular metallic obstacle is surrounded by the structured cloak for cloaking frequencies between 2.6 and 7.0 GHz. This suggests, as supported by 2D finite element simulations, sound waves are cloaked between 3 and 8 KHz and linear surface liquid waves between 5 and 16 Hz. Moreover, microwave experiments show the field is reduced by 10 to 30 dB inside the invisibility region, which suggests the multi-wave cloak could be used as a protection against water, sonic or microwaves.
 
The cloak could have potential applications in telecommunications (protection against mobile phone radiations) and soundproof devices (humans are most sensitive to sound waves of frequencies between 2 and 5 kHz). The design could also be scaled down in order to achieve cloaking at optical wavelengths, what would require an analysis of cloak’s dispersion.
 
Since the cloak works both in acoustic and microwave domains, it might offer new opportunities to drive and tune one field with the other one: for instance if the fluid within which pressure waves propagate is no longer air but some gas plasma or liquid electrolyte, we might be able to tune the density profile by microwave signals. On larger scales, control of sound and elastic waves could be used in anti-earthquake designs if protection is achieved via cloaking with seismic metamaterials34, that is for frequencies below 50 Hz. Finally, we note that preliminary numerical simulations suggest our cloak should also work in the context of management of thermal flux.

Guiding surface electromagnetic waves around disorder without disturbing the wave amplitude or phase is in great demand for modern photonic and plasmonic devices. In this work, we introduce a class of cloaks capable of remarkable broadband surface electromagnetic waves guidance around ultrasharp corners and bumps with no perceptible changes in amplitude and phase. This work provides strong support for the application of transformation optics to plasmonic circuits and could pave the way for high-performance, large-scale integrated photonic circuits.
 
Abstract - Broadband surface-wave transformation cloak
 Guiding surface electromagnetic waves around disorder without disturbing the wave amplitude or phase is in great demand for modern photonic and plasmonic devices, but is fundamentally difficult to realize because light momentum must be conserved in a scattering event. A partial realization has been achieved by exploiting topological electromagnetic surface states, but this approach is limited to narrow-band light transmission and subject to phase disturbances in the presence of disorder. Recent advances in transformation optics apply principles of general relativity to curve the space for light, allowing one to match the momentum and phase of light around any disorder as if that disorder were not there. This feature has been exploited in the development of invisibility cloaks. An ideal invisibility cloak, however, would require the phase velocity of light being guided around the cloaked object to exceed the vacuum speed of light—a feat potentially achievable only over an extremely narrow band. In this work, we theoretically and experimentally show that the bottlenecks encountered in previous studies can be overcome. We introduce a class of cloaks capable of remarkable broadband surface electromagnetic waves guidance around ultrasharp corners and bumps with no perceptible changes in amplitude and phase. These cloaks consist of specifically designed nonmagnetic metamaterials and achieve nearly ideal transmission efficiency over a broadband frequency range from 0+ to 6 GHz. This work provides strong support for the application of transformation optics to plasmonic circuits and could pave the way toward high-performance, large-scale integrated photonic circuits.
 
Broadband surface-wave transformation cloak

scientists at Zhejiang University in Hangzhou, China, Nanyang Technological University, Singapore, and Massachusetts Institute of Technology created (so-called invisibility) cloaks based on specifically-designed nonmagnetic anisotropic, or directionally dependent, metamaterials that achieve nearly ideal transmission efficiency over a broadband frequency range.
 
● Overcome the challenge of momentum mismatch by adopting strict transformation optics with anisotropy in the design – a strategy that can also work for ultrasharp corners and bumps
 
● Realized broadband performance by employing an area-preserving coordinate transformation, which can produce non-magnetic constitutive parameters for a surface plasmon polariton (SPP –an electromagnetic excitation existing on the surface of an appropriate metal) wave cloak
 
● Experimentally demonstrated SPP cloak performance by designing a layered metamaterial composed of microwave ceramic plates and low-permittivity foam with subwavelength periodicity (in which the periodicity of the metamaterial is much smaller than the wavelength of the electromagnetic wave being cloaked)
 
The key result being reported is that unlike topological electromagnetic surface states, in the new approach phase is preserved when surface waves are perfectly guided by the cloaks. "Sharp bending of surface waves was previously achieved only in topological electromagnetic edge states," Zhang tells Phys.org. "Because the required materials generally are magnetic, it suffers from narrow bandwidth. In our work, however, the use of anisotropic non-magnetic materials and transformation optics ensure the phase preservation of surface waves."
 
Chen says that the scientists plan to extend their experimental demonstration from microwaves to higher frequencies, including infrared and visible light. "This may push our work much closer to practical application. Moreover, scientists have extended the concept of transformation beyond electromagnetic fields to other types of physical fields, such as heat, diffusive light, acoustics, and static fields. "No matter which kind of physical field it is, the fundamental point is to control the propagation of the waves and the distribution of the field," Chen concludes. "Our work can therefore be extended to many other areas of research."
 
SOURCES - PNAS, Nature, Phys.org
Dagny, this is not a battle over material goods. It's a moral crisis, the greatest the world has ever faced and the last. Our age is the climax of centuries of evil. We must put an end to it, once and for all, or perish - we, the men of the mind. It was our own guilt. We produced the wealth of the world - but we let our enemies write its moral code.

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Re: Metamaterials: a real invisibility cloak
« Reply #64 on: July 18, 2015, 12:41:38 »
More advances in metamaterials. This article describes a one layer "cloak", which is much lighter and simpler than other designs.
Advances in this sort of design could lead to coverings over vehicles and structures, and eventually uniforms (I want one for parades!)

http://nextbigfuture.com/2015/07/engineers-give-invisibility-cloaks.html

Quote
Engineers give invisibility cloaks a slimmer design
 
Researchers have developed a new design for a cloaking device that overcomes some of the limitations of existing “invisibility cloaks.” In a new study, electrical engineers at the University of California, San Diego have designed a cloaking device that is both thin and does not alter the brightness of light around a hidden object. The technology behind this cloak will have more applications than invisibility, such as concentrating solar energy and increasing signal speed in optical communications.

“Invisibility may seem like magic at first, but its underlying concepts are familiar to everyone. All it requires is a clever manipulation of our perception,” said Boubacar Kanté, a professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering and the senior author of the study. “Full invisibility still seems beyond reach today, but it might become a reality in the near future thanks to recent progress in cloaking devices.”

An extremely thin cloaking device is designed using dielectric materials. The cloak is a thin Teflon sheet (light blue) embedded with many small, cylindrical ceramic particles (dark blue). Credit: Li-Yi Hsu/UC San Diego.

The reflection pattern from an uncloaked object on a flat surface (top) compared to the reflection pattern of the same object covered with the cloaking device (bottom), which effectively mimics the reflection from a completely flat surface. Credit: Li-Yi Hsu/UC San Diego

Extremely thin Dielectric Metasurface for carpet cloaking

As their name implies, cloaks are devices that cover objects to make them appear invisible. The idea behind cloaking is to change the scattering of electromagnetic waves — such as light and radar — off an object to make it less detectable to these wave frequencies.

One of the drawbacks of cloaking devices is that they are typically bulky.

“Previous cloaking studies needed many layers of materials to hide an object, the cloak ended up being much thicker than the size of the object being covered,” said Li-Yi Hsu, electrical engineering Ph.D. student at UC San Diego and the first author of the study, which was recently published in the journal Progress In Electromagnetics Research. “In this study, we show that we can use a thin single-layer sheet for cloaking.”

The researchers say that their cloak also overcomes another fundamental drawback of existing cloaking devices: being “lossy.” Cloaks that are lossy reflect light at a lower intensity than what hits their surface.

“Imagine if you saw a sharp drop in brightness around the hidden object, it would be an obvious telltale. This is what happens when you use a lossy cloaking device,” said Kanté. “What we have achieved in this study is a ‘lossless’ cloak. It won’t lose any intensity of the light that it reflects.”

Many cloaks are lossy because they are made with metal particles, which absorb light. The researchers report that one of the keys to their cloak’s design is the use of non-conductive materials called dielectrics, which unlike metals do not absorb light. This cloak includes two dielectrics, a proprietary ceramic and Teflon, which are structurally tailored on a very fine scale to change the way light waves reflect off of the cloak.

In their experiments, the researchers specifically designed a “carpet” cloak, which works by cloaking an object sitting on top of a flat surface. The cloak makes the whole system — object and surface — appear flat by mimicking the reflection of light off the flat surface. Any object reflects light differently from a flat surface, but when the object is covered by the cloak, light from different points is reflected out of sync, effectively cancelling the overall distortion of light caused by the object’s shape.

“This cloaking device basically fools the observer into thinking that there’s a flat surface,” said Kanté.

The researchers used Computer-Aided Design software with electromagnetic simulation to design and optimize the cloak. The cloak was modeled as a thin matrix of Teflon in which many small cylindrical ceramic particles were embedded, each with a different height depending on its position on the cloak.

“By changing the height of each dielectric particle, we were able to control the reflection of light at each point on the cloak,” explained Hsu. “Our computer simulations show how our cloaking device would behave in reality. We were able to demonstrate that a thin cloak designed with cylinder-shaped dielectric particles can help us significantly reduce the object’s shadow.”

“Doing whatever we want with light waves is really exciting,” said Kanté. “Using this technology, we can do more than make things invisible. We can change the way light waves are being reflected at will and ultimately focus a large area of sunlight onto a solar power tower, like what a solar concentrator does. We also expect this technology to have applications in optics, interior design and art.”

Abstract:
We demonstrate a novel and simple geometrical approach to cloaking a scatterer on a ground plane. We use an extremely thin dielectric metasurface to reshape the wavefronts distorted by a scatterer in order to mimic the reflection pattern of a flat ground plane. To achieve such carpet cloaking, the reflection angle has to be equal to the incident angle everywhere on the scatterer. We use a graded metasurface and calculate the required phase gradient to achieve cloaking. Our metasurface locally provides additional phase to the wavefronts to compensate for the phase difference amongst light paths induced by the geometrical distortion. We design our metasurface in the microwave range using highly sub-wavelength dielectric resonators. We verify our design by full-wave time-domain simulations using micro-structured resonators and show that results match theory very well. This approach can be applied to hide any scatterer under a metasurface of class C1 (first derivative continuous) on a ground plane not only in the microwave regime, but also at higher frequencies up to the visible.

SOURCES - UCSD, research paper Progress In Electromagnetics Research - Extremely thin dielectric metasurface for carpet cloaking
Dagny, this is not a battle over material goods. It's a moral crisis, the greatest the world has ever faced and the last. Our age is the climax of centuries of evil. We must put an end to it, once and for all, or perish - we, the men of the mind. It was our own guilt. We produced the wealth of the world - but we let our enemies write its moral code.

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Re: Metamaterials: a real invisibility cloak
« Reply #65 on: September 07, 2015, 17:03:51 »
This seems to be a development that is practical for both manufacturing and deployment. Invisible vehicles and systems coming soon...

https://www.hdiac.org/node/2071

Quote
Bringing Invisibility Cloaks to Reality     

Researchers are working to create a new design for the Harry Potter-esque invisibility cloak, which will conceal objects, making them more difficult for adversaries to detect.

Scientists are working on creating a new design for a technology that redefines what the public views as imaginary. Inspired by the well-known Invisibility Cloak from Harry Potter, electrical engineers at the University of California, San Diego have created a new design for their cloaking device, using a Teflon substrate, studded with cylinders of ceramic, that is thinner than any prior development and does not alter the brightness of light around concealed objects. The Teflon has a low refractive index, while the ceramic’s refractive index is higher, which allows light to be dispersed through the sheet without any absorption. [1] Compared to an invisibility cloak, this technology has not only the ability to conceal, but the ability to increase optical communication signal speed and to collect solar energy. [2]

The goal of this design is to create devices that make any object appear invisible by scattering the electromagnetic waves, such as light and radar, off an object making it less detectable to these wave frequencies. Metamaterial that surrounds the target is able to force light to bypass a region of space, which effectively “cloaks” the object, making it isolated from incoming electromagnetic waves. [3]

Prior developments to this technology needed many layers in order to cover an object, resulting in a very thick layer that enclosed the object. With this new, super-thin design, this technology has the capability to better hide the three-dimensionality and shadow of an object. Additionally, this new cloaking device addresses the issue with the brightness of the space behind them. The University of California has achieved a cloak that won’t reduce any intensity when light is reflected so the concealed object will remain undetectable and will appear completely flat to an observer’s eyes. [2]

"Invisibility may seem like magic at first, but its underlying concepts are familiar to everyone. All it requires is a clever manipulation of our perception," said Boubacar Kanté, a professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering and the senior author of the study. "Full invisibility still seems beyond reach today, but it might become a reality in the near future thanks to recent progress in cloaking devices." [2]

An extremely thin cloaking devise is designed using dielectric materials. The cloak is a thin Teflon sheet (light blue) embedded with many small, cylindrical ceramic particles (dark blue). (Photo courtesy of Li-Yi Hsu/University of California, San Diego)

An extremely thin cloaking devise is designed using dielectric materials. The cloak is a thin Teflon sheet (light blue) embedded with many small, cylindrical ceramic particles (dark blue). (Photo courtesy of Li-Yi Hsu/University of California, San Diego)
Having the ability to create ultimate stealth protection for anything over a battlefield or warzone provides enormous military advantage over the adversary. In theory, creating a cloaking device would be used to conceal larger objects. This cloaking device would be valuable to many technologies, including unmanned air vehicles (UAVs) due to the capability to disappear from view and leaving no visual, electronic or infrared signature for an enemy to detect. [4] Creating the effect of an invisibility cloak offers a real-world solution to concealment, which can provide the military with air superiority. While this cloak has numerous applications for the military, this technology will create a ripple effect beyond the battlefield that will improve the performance of other diverse applications.

"Doing whatever we want with light waves is really exciting," said Kanté. "Using this technology, we can do more than make things invisible. We can change the way light waves are being reflected at will and ultimately focus a large area of sunlight onto a solar power tower, like what a solar concentrator does. We also expect this technology to have applications in optics, interior design and art." [2]

References:

Sklar, P. (2015). Scientists make a breakthrough in invisibility cloak technology. Escapist Magazine.
Engineers give invisibility cloaks a slimmer design. (2015). ScienceDaily.
Hsu, L.Y., Lepetit, T., & Kanté, B. (2015). Extremely thin dielectric metasurface for carpet cloaking. Progress in Electromagnetics Research, 152, 33-40.
Toensmeier, P. (2012). Technologies evolving to cloak battlefield vehicles from sensors.Aviation Week Network.
- See more at: https://www.hdiac.org/node/2071#sthash.rtG0NFGg.q8oZQTGP.dpuf
Dagny, this is not a battle over material goods. It's a moral crisis, the greatest the world has ever faced and the last. Our age is the climax of centuries of evil. We must put an end to it, once and for all, or perish - we, the men of the mind. It was our own guilt. We produced the wealth of the world - but we let our enemies write its moral code.

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Re: Metamaterials: a real invisibility cloak
« Reply #66 on: October 03, 2016, 21:57:37 »
Another approach to metamaterial cloaking in radar frequencies.

http://www.nextbigfuture.com/2016/10/metamaterials-for-reduces-reflection-of.html

Quote
Metamaterials for reduces the reflection of radar might make stealth aircraft invisible to microwaves

Iowa State researchers Liang Dong and Jiming Song are working on technology that could someday make a microwave invisibility cloak for stealth aircraft a reality.

Along with assistance from Iowa State students, they've developed a flexible, stretchable, and tunable "meta-skin" that uses rows of small liquid-metal devices to cloak objects from radar by reducing the reflection of a wide range of radar frequencies. This makes it different from traditional stealth technologies that only reduce the power reflected back to a probing radar. It also makes the meta-skin one step closer to helping conceal aircraft entirely.

Dong, associate professor of electrical and computer engineering, and Song, professor of electrical and computer engineering, are continuing to develop meta-skin materials that could continue to change the world of stealth technology. "The long-term goal is to shrink the size of these devices," Dong said. "Then, hopefully, we can do this with higher frequency electromagnetic waves." Which could help produce a form of meta-skin that might someday coat the surface of stealth aircraft to make them invisible to radar devices of all kinds.
Dagny, this is not a battle over material goods. It's a moral crisis, the greatest the world has ever faced and the last. Our age is the climax of centuries of evil. We must put an end to it, once and for all, or perish - we, the men of the mind. It was our own guilt. We produced the wealth of the world - but we let our enemies write its moral code.