Tag Archives: invisibility cloak

Invisibility cloak effective in fog and milk

It’s an intriguing notion, an invisibility cloak that’s effective in milk and I suspect that I’ve never entirely understood the implications of the research featured in a June 6, 2014 news item on Nanowerk,

Real invisibility cloaks are rather complex and work in certain situations only. The laws of physics prevent an optical invisibility cloak from making objects in air invisible for any directions, colors, and polarizations. If the medium is changed, however, it becomes much easier to hide objects. KIT physicists have now succeeded in manufacturing with relatively simple means and testing an ideal invisibility cloak for diffusive light-scattering media, such as fog or milk. …

A June 6, 2014 Karlsruhe Institute of Technology (KIT) press release, which originated the news item, provides more details,

In diffusive media, light does no longer propagate linearly, but is scattered permanently by the particles in the medium. Examples are fog, clouds, or frosted glass panes that let the light in, but hide the light source. “This property of light-scattering media can be used to hide objects inside,” says Robert Schittny, first author of the study. “The new invisibility cloaks have a rather simple structure.”

In the experiment, Schittny used an extended light source to illuminate a Plexiglas tank of a few centimeters in width from the back. The tank was filled with a white, turbid liquid. Objects inside cast a visible shadow onto the tank wall. Simple metal cylinders or spheres of a few centimeters in diameter were used as test objects. To hide them, they were first coated with a white dispersion paint, such that the light was reflected in a diffusive manner. To pass the light around the object, the researchers applied a thin shell made of the transparent silicon material PDMS, to which a certain concentration of light-scattering melamine microparticles was added. The silicon/melamine shell caused a quicker diffusion than in the environment and, thus, passed the light around the objects. Hence, they did no longer cast a shadow. “Disappearance of the shadow indicates successful cloaking.”

“Ideal optical invisibility cloaks in air have a drawback,” Martin Wegener points out. He conducts research at the KIT Institute of Applied Physics and the KIT Institute of Nanotechnology. “They violate Albert Einstein’s theory of relativity that prescribes an upper limit for the speed of light. “In diffuse media, in which light is scattered several times, however, the effective speed of light is reduced. Here, ideal invisibility cloaks can be realized.”

The researchers have provided this image to illustrate their work,

In a diffusive light-scattering medium, light moves on random paths (see magnifying glass). A normal object (left) casts a shadow, an object with an invisibility cloak (right) does not. (Image: R. Schittny / KIT)

In a diffusive light-scattering medium, light moves on random paths (see magnifying glass). A normal object (left) casts a shadow, an object with an invisibility cloak (right) does not. (Image: R. Schittny / KIT)

I’m not convinced that there’s actually a cloaked object in that image but I appreciate the call to use my imagination.

Here’s a link to and a citation for the paper,

Invisibility cloaking in a diffusive light scattering medium by Robert Schittny, Muamer Kadic, Tiemo Bückmann, and Martin Wegener. Science DOI: 10.1126/science.1254524 Published Online June 5 2014

This article is behind a paywall.

Battery-powered invisibiity cloak

You’d never guess it from the title of their paper but researchers at the University of Texas at Austin have conceptualized and designed a battery-operated invisibility cloak, according to a Dec. 18, 2013 news item on Nanowerk,

Researchers at The University of Texas at Austin have proposed the first design of a cloaking device that uses an external source of energy to significantly broaden its bandwidth of operation.

Andrea Alù, associate professor in the Department of Electrical and Computer Engineering at the Cockrell School of Engineering, and his team have proposed a design for an active cloak that draws energy from a battery, allowing objects to become undetectable to radio sensors over a greater range of frequencies.

The Dec. 18, 2013 University of Texas at Austin news release (also on EurekAlert), which originated the news item, describes the current state of cloaking technology,

Cloaks have so far been realized with so-called passive technology, which means that they are not designed to draw energy from an external source. They are typically based on metamaterials (advanced artificial materials) or metasurfaces (a flexible, ultrathin metamaterial) that can suppress the scattering of light that bounces off an object, making an object less visible. When the scattered fields from the cloak and the object interfere, they cancel each other out, and the overall effect is transparency to radio-wave detectors. They can suppress 100 times or more the detectability at specific design frequencies. Although the proposed design works for radio waves, active cloaks could one day be designed to make detection by the human eye more difficult.

“Many cloaking designs are good at suppressing the visibility under certain conditions, but they are inherently limited to work for specific colors of light or specific frequencies of operation,” said Alù, David & Doris Lybarger Endowed Faculty Fellow in the Department of Electrical and Computer Engineering. In this paper, on the contrary, “we prove that cloaks can become broadband, pushing this technology far beyond current limits of passive cloaks. I believe that our design helps us understand the fundamental challenges of suppressing the scattering of various objects at multiple wavelengths and shows a realistic path to overcome them.”

The news release details the new battery-powered design,

The proposed active cloak uses a battery, circuits and amplifiers to boost signals, which makes possible the reduction of scattering over a greater range of frequencies. This design, which covers a very broad frequency range, will provide the most broadband and robust performance of a cloak to date. Additionally, the proposed active technology can be thinner and less conspicuous than conventional cloaks.

In a related paper, published in Physical Review X in October, Alù and his graduate student Francesco Monticone proved that existing passive cloaking solutions are fundamentally limited in the bandwidth of operation and cannot provide broadband cloaking. When viewed at certain frequencies, passively cloaked objects may indeed become transparent, but if illuminated with white light, which is composed of many colors, they are bound to become more visible with the cloak than without. The October paper proves that all available cloaking techniques based on passive cloaks are constrained by Foster’s theorem, which limits their overall ability to cancel the scattering across a broad frequency spectrum.

In contrast, an active cloak based on active metasurfaces, such as the one designed by Alù’s team, can break Foster’s theorem limitations. The team started with a passive metasurface made from an array of metal square patches and loaded it with properly positioned operational amplifiers that use the energy drawn from a battery to broaden the bandwidth.

“In our case, by introducing these suitable amplifiers along the cloaking surface, we can break the fundamental limits of passive cloaks and realize a ‘non-Foster’ surface reactance that decreases, rather than increases, with frequency, significantly broadening the bandwidth of operation,” Alù said.

The researchers are continuing to work both on the theory and design behind their non-Foster active cloak, and they plan to build a prototype.

Alù and his team are working to use active cloaks to improve wireless communications by suppressing the disturbance that neighboring antennas produce on transmitting and receiving antennas. They have also proposed to use these cloaks to improve biomedical sensing, near-field imaging and energy harvesting devices.

Here’s a link to and a citation for the team’s paper about active cloaking,

Broadening the Cloaking Bandwidth with Non-Foster Metasurfaces by Pai-Yen Chen, Christos Argyropoulos, and Andrea Alù. Phys. Rev. Lett. 111, 233001 (2013) [5 pages] DOI: 10.1103/PhysRevLett.111.233001

This paper is behind a paywall.

Here’s a link to and a citation for the related and previously published paper (authors: Alù and Monticone),

Do Cloaked Objects Really Scatter Less? by Francesco Monticone and Andrea Alù. Phys. Rev. X 3, 041005 (2013) [10 pages] DOI: 10.1103/PhysRevX.3.041005

The authors have included both an abstract and a popular summary. I’ve excerpted the popular summary,

From ancient times, humanity has been fascinated by the concept of invisibility, and recently, scientists have moved a step closer to bringing this idea to reality by exploiting engineered artificial materials, or metamaterials. Several recent studies have indeed shown that a properly tailored metamaterial cover can, in principle, render an object invisible when illuminated by an electromagnetic wave oscillating at the specific frequency of interest. Yet, experimental realizations and theoretical investigations have consistently shown that reducing the visibility of an object with a passive cloak in a specific window of the electromagnetic spectrum is generally accompanied by a drastic increase of its visibility in other frequency ranges. Making an object invisible to red light, for instance, may actually make it bright blue, increasing its overall visibility.

In this paper, we quantitatively assess the potentials and limitations of passive cloaks in terms of overall visibility, integrated over the entire frequency spectrum. Quite surprisingly, our results show that any linear, causal, and passive invisibility cloak, without special superconducting features, is deemed to increase the scattering and visibility of the original uncloaked object, when integrated over all frequencies. This result confirms that the most popular cloaking devices actually scatter more, not less, when considered over a sufficiently broad frequency range, allowing easy detection using, e.g., pulsed excitation.

Our general theorem holds a relevant exception if specific covers with a strong static diamagnetism are considered, and, based on this principle, we propose a technique to reduce the global scattering, as well as the local response around a frequency of interest, using diamagnetic and superconducting thin cloaking layers. More generally, our results provide a quantitative measure to compare the overall performance of different cloaking devices and generally assess their detectability. These findings may open important research directions in the quest for invisibility, not only in the electromagnetic domain but also for acoustic, mechanical, and matter waves.

This is an open access paper.

University of Toronto’s (Canada) invisibility cloak

University of Toronto researchers, Michael Selvanayagam and George V. Eleftheriades, have offered a popular summary of their work. from the popular summary (on the website where they’ve published their academic paper),

We “see” a physical object by detecting electromagnetic waves scattered from the object. A device that can “correct” or cancel that scattering would take the notion of a magic invisibility cloak from the realm of science fiction to reality. In fact, such physical devices already exist, accomplishing their feat based on metamaterials that bend light around the object to be cloaked, “correcting” the scattering. Designing metamaterials with the right light-bending properties for this purpose is, however, quite challenging, and the designs often require a thick “cloak.” An alternative approach to this problem is “active cloaking”: surrounding the object to be cloaked with electromagnetic sources that are carefully tuned to cancel the electromagnetic field scattered by the object. In this work, we demonstrate the first experimental realization of such a thin active cloak for microwaves.

The sources we have used are specially designed antennas and phase shifters, which can be configured into thin layers with flexibility in shape. We have succeeded in cloaking a sizable metallic cylinder by properly tuning the phase of the radiation from the antennas so that the radiation cancels the field scattered by the cylinder. We have gone a step further than cloaking and have also demonstrated how the object can be disguised as another object by tuning the antennas in a controlled way. The catch with active cloaking, however, is that knowledge of the incident field is required to tune the antennas. To tackle this issue, we have discussed some potential solutions that also utilize the antennas as sensors to detect the incident field.

Future work along this line will aim to extend the bandwidth of the cloak (with respect to pulsed incident fields) as well as design active cloaks that can adaptively respond to an incident field.

A Nov. 12, 2013 news item on ScienceDaily, offers information augmenting the popular summary,

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.

The Nov. 13, 2013 University of Toronto news release, which originated the news item and was posted a day later, provides more specific details about the research,

“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.

For now, the antenna loops must be manually attuned to the electromagnetic frequency they need to cancel. But in future, researchers say, 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 that 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.

The cloaking technology holds possiblities that go beyond obvious applications such as hiding military vehicles or conducting surveillance operations. For example, structures that interrupt 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.

For those who feel inclined to explore this work further,

Experimental Demonstration of Active Electromagnetic Cloaking by Michael Selvanayagam and George V. Eleftheriades. Phys. Rev. X (Volume 3 Issue 4) or Phys. Rev. X 3, 041011 (2013) [13 pages]  DOI: 10.1103/PhysRevX.3.041011

Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

This article is open access.

Fish gets invisibility cloak first, cat waits patiently

An invisibility cloak devised by researchers in Singapore and China is receiving a high degree of interest online with a June 14, 2013 news item on Nanowerk, a June 11, 2013 article by Philip Ball for Nature, and a June 13, 2013 article by Sarah Gates for Huffington Post.

The research paper, Natural Light Cloaking for Aquatic and Terrestrial Creatures by Hongsheng Chen, Bin Zheng, Lian Shen, Huaping Wang, Xianmin Zhang, Nikolay Zheludev, Baile Zhang was submitted June 7, 2013 to arXiv.org (arXiv is an e-print service in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance and statistics. Submissions to arXiv must conform to Cornell University academic standards. arXiv is owned and operated by Cornell University, a private not-for-profit educational institution),

A cloak that can hide living creatures from sight is a common feature of mythology but still remains unrealized as a practical device. To preserve the phase of wave, the previous cloaking solution proposed by Pendry \emph{et al.} required transforming electromagnetic space around the hidden object in such a way that the rays bending around it have to travel much faster than those passing it by. The difficult phase preservation requirement is the main obstacle for building a broadband polarization insensitive cloak for large objects. Here, we suggest a simplifying version of Pendry’s cloak by abolishing the requirement for phase preservation as irrelevant for observation in incoherent natural light with human eyes that are phase and polarization insensitive. This allows the cloak design to be made in large scale using commonly available materials and we successfully report cloaking living creatures, a cat and a fish, in front of human eyes.

What they seem to be saying is that it’s possible to create an invisibility cloak perceptible to the human eye that is made of everyday materials.

I’ll show the fish video first. Pay attention as that fish darts behind its invisibility cloak almost as soon as the video starts (from the Nanowerk Youbube channel; June 14, 2013 Nanowerk news item),

Then, there’s the cat (also from the Nanowerk Youtube channel),


The June 11, 2013 article by Philip Ball for Nature describes the device which provides invisibility,

… This latest addition to the science of invisibility cloaks is one of the simplest implementations so far, but there’s no denying its striking impact.

The ‘box of invisibility’ has been designed by a team of researchers at Zhejiang University in Hangzhou, China, led by Hongsheng Chen, and their coworkers. The box is basically a set of prisms made from high-quality optical glass that bend light around any object in the enclosure around which the prisms are arrayed, the researchers describe in a paper posted on the online repository arXiv.

Ball suggests that this latest invisibility cloak is very similar to a Victorian era music hall trick,

As such, the trick is arguably closer to ‘disappearances’ staged in Victorian music hall using arrangements of slanted mirrors than to the modern use of substances called metamaterials to achieve invisibility by guiding light rays in unnatural ways.

As far as I know, the ‘metamaterial’ invisibility cloaks require very sophisticated equipment for their production, are incredibly expensive, and aren’t all that practical.

Gates’s June 13, 2013 article for the Huffington Post provides an overview of some of the recent work on invisibility cloaks and metamaterials, as well as, previous work done by Dr. Hongsheng Chen, an electromagnetics professor at Zhejiang University (China), and Baile Zhang, an assistant physics professor at Singapore’s Nanyang Technological University before they unveiled this latest invisibility cloak.

My most recent posting on the topic was a June 6, 2013 piece on a temporal invisibility cloak.

An invisibility cloak close to home courtesy of HyperStealth Biotechnology Corp.

H/T (hat tip) to My Science Academy and its early Jan. 2013 article titled 27 Science Fictions That Became Science Facts in 2012 for the information about an invisibility cloak that has been developed by HyperStealth Biotechnology Corp. based in Maple Ridge, BC, Canada.  Here’s more from the company’s Oct. 19, 2012 news release,

Once thought to be only a Science Fiction/Fantasy technology, Guy Cramer, President/CEO of Hyperstealth Biotechnology Corp., discusses the implications of militaries which can now become invisible with his light bending technology called “Quantum Stealth”.

Hyperstealth is a successful Canadian camouflage design company with over two million military issued uniforms and over 3000 vehicles and fighter jets using their patterns around the world.

Quantum Stealth is a material that renders the target completely invisible by bending light waves around the target. The material removes not only your visual, infrared (night vision) and thermal signatures but also the target’s shadow.

Two separate command groups within the U.S. Military and two separate Canadian Military groups as well as Federal Emergency Response Team (Counter Terrorism) have seen the actual material so they could verify that I was not just manipulating video or photo results; These groups now know that it works and does so without cameras, batteries, lights or mirrors…It is lightweight and quite inexpensive. Both the U.S. and Canadian military have confirmed that it also works against military IR scopes and Thermal Optics.

This  brief video interview of Guy Cramer highlights various company products including Quantum Stealth ,

Cramer has included a number of updates, corrections, and additions to his  company’s Oct. 19, 2012 news release (which you will see if you keep scrolling down past the original release) ,

Isn’t there a risk that someone else may figure this out or copy what you’ve done? Yes, but I’ve already developed a countermeasure for Quantum Stealth so we would be able to detect anyone else with something identical or similar to Quantum Stealth.

Do I care that people remain skeptical? Nope, the people that need to know that it works have seen it and verified it and their opinions are the only ones that matter.

Will Quantum Stealth be available for the general public or commercial market? Not in the near future unless the Military decided to release the technology and I don’t anticipate that will happen anytime soon.

Is there anything planned for the commercial market? I am working on a number of non-powered color changing camouflage materials for the commercial market which utilize different technologies than either Quantum Stealth or Smartcamo. Colors change with climate, seasons cause environmental colors to change and even the 24 hour day can cause a large color discrepancy between camouflage and the background as day becomes night. People want camouflage which can change with these variables.

Have you made camouflage obsolete? Not necessarily, standard camouflage should continue to have its place, however, on the front lines it might become your second choice behind Quantum Stealth if you’re Canadian, American or British and your group is authorized to use it.

Update: December 18, 2012

I, Guy Cramer, have conducted about 20 interviews on this subject, however, there are now over 5,000 worldwide news stories that have come out in the past 8 days. With the internet, one news group just plagiarizes a previous story and fact checking is out the window. Some of you may remember the game as a child where you whisper a phrase in the ear of the first person and they whisper the same phase to the next person…10 people later you ask the last person to say the phase that they just were whispered and the phase is often completely different.

Some inaccuracies have come up which need clarification – if you have read this whole page you know the photos are mock-ups to show the concept, we have never told the media that these are photos of the real technology and in fact we’ve asked them to mark the photos as mock-ups or explain it on T.V.- but doing so doesn’t sell the story, so most of the articles leave this key point out. I started the process of correcting the reporters with the first reports I could find that did not mention this correctly such as http://www.businessinsider.com/cnn-new-camoflague-technology-makes-troops-invisible-2012-12 (these two reporters never did correct it after I asked them to) but the story went viral. There is no way for me to get 5,000 news sources to mark the images as mockups that didn’t even bother to interview me in the first place. I have not been dishonest about this, the reporters have.

I’ve never been to the Pentagon and the Pentagon is not backing our Light Bending technology at this time, this inaccuracy came about from the Daily Mail article where they combined the information on this page with the CNN interview where the reported [sic] is a Pentagon correspondent – but the Daily Mail never interviewed me, their online story is one key article that caused the story to go viral. I have only claimed that two separate U.S. Military command groups have seen demonstrations of the material, this doesn’t mean the Pentagon was one of them. The Canadian Government only provided authorization 2 weeks ago to be able to move forward with the U.S. Military regarding this technology.

Canadian Foreign Affairs is involved in determining the potential restrictions and clearance in moving forward with the British Military and will not make a decision until at least the Spring of 2013. At their request, I did a presentation of the technology at British Military Headquarters in Bristol in 2011 as well as the SBS (Special Boat Service) in Poole accompanied by two former U.S. Navy SEALs, one being Bill Jarvis mentioned in an earlier update.

Some critics are claiming that our technology (if it works) must only work in one direction at one angle. I can tell you that we have demonstrated that it can work in 360 degrees meaning you will see what is on the other side of the target and some one at a different angle will see what is immediately behind the target from their vantage point.

This story is not new, the “Atlantic” magazine article published in July 2011 discussed the technology from a video I showed him and the same reporter was shown our actual Smartcamo (color changing material) and confirmed that technology really works. It is only recently that the media has become focused on light bending technology due to the CNN interview which was not our intended story with them but when a three hour interview with CNN is cut into 2 minutes, the focus of their story was on this and not on the ADS Inc./Guy Cramer US4CES Family of camouflage finalist with the U.S. Army camouflage improvement program, which is the initial story they were there for.

There was no biographical information (that I could find) for Cramer on the company website but there is this in an April 22, 2011 posting on the Tactical Gear Military Clothing News site,

Guy Cramer is the President/CEO of HyperStealth Biotechnology Corp. CCD (Camouflage, Concealment & Deception) Specialist, IT Analyst: Level 4, Inventor of the Passive Negative Ion Generator, World Expert on Air Ions as per NASA JPL. He has worked on projects with, NASA Headquarters, U.S. Marine Corps, U.S. Army, U.S. Navy, and Senator John Warner’s office while he was Chairman of the Armed Services Committee. Cramer also worked under the direct commission of King Abdullah II of Jordan. He is the former research assistant and grandson to Donald L. Hings, P.Eng, M.B.E. (Member of British Empire), C.M. (Order of Canada) inventor of the Walkie-Talkie prior to WWII. Cramer has designed over 9,000 camouflage patterns for over 34 countries and recently developed Smartcamo, a textile which can change color to match the surrounding environment.

It’s a shame there aren’t more technical details about Quantum Stealth as it would be interesting for someone to compare and contrast this technology with other light bending technologies (invisibility cloaks).

Huge jump forward to invisibility cloaking device

This is pretty remarkable. Usually when researchers talk about invisibility cloaks, they’ve managed to cloak something that is measured at the nanoscale. Here they’ve (Fractal Antenna Systems Inc.) managed to cloak a person within a certain bandwidth,

The Nov. 16, 2012 news item on Azosensors provides more detail,

Fractal Antenna Systems, Inc. today disclosed that it has successfully rendered a man invisible. The firm’s new invisibility cloak hid a subject named Peter at microwaves over a wide bandwidth at high fidelity. This is the first time any large object has been rendered invisible and the first time a person has disappeared from view using invisibility cloak technology.

Invisibility cloaks differ from other types of stealth or camouflage by diverting waves around an object. The opposing side is visible but the object is not. They are unpowered and passively convey that view.

The heart of the invisibility cloak is a close-spaced resonator arrangement made from almost 10,000 fractal shapes. Such shapes are built from simple patterns that are scaled. The human-sized cloak was configured as a large hollow cylinder with the cloak surrounding it as a thin shell. Subject Peter hunkered down inside to become hidden from microwave view, and thus realized a dream of countless generations–the quest to be invisible.

There’s more at Azosensors including an image and a history of invisibility cloaks. You can also visit the Fractal Antenna Systems Inc. website.

University of Texas at Dallas lab demos cloaking device visible to naked eye

Invisibility cloaks have been everywhere lately and I’ve been getting a little blasé about them but then I saw this Oct. 4, 2011 news item on physorg.com,

Scientists have created a working cloaking device that not only takes advantage of one of nature’s most bizarre phenomenon, but also boasts unique features; it has an ‘on and off’ switch and is best used underwater.

For the first time, I was able to see an invisibility cloak in action, here’s the video,

For the curious here’s how it works (from the Oct. 4, 2011 news release on the Institute of Physics website),

This novel design, presented today, Tuesday 4 September [Tuesday 4 October?], in IOP [Institute of Physics] Publishing’s journal Nanotechnology, makes use of sheets of carbon nanotubes (CNT) – one-molecule-thick sheets of carbon wrapped up into cylindrical tubes.

CNTs have such unique properties, such as having the density of air but the strength of steel, that they have been extensively studied and put forward for numerous applications; however it is their exceptional ability to conduct heat and transfer it to surrounding areas that makes them an ideal material to exploit the so-called “mirage effect”.

The most common example of a mirage is when an observer appears to see pools of water on the ground. This occurs because the air near the ground is a lot warmer than the air higher up, causing lights rays to bend upward towards the viewer’s eye rather than bounce off the surface.

This results in an image of the sky appearing on the ground which the viewer perceives as water actually reflecting the sky; the brain sees this as a more likely occurrence.

Through electrical stimulation, the transparent sheet of highly aligned CNTs can be easily heated to high temperatures. They then have the ability to transfer that heat to its surrounding areas, causing a steep temperature gradient. Just like a mirage, this steep temperature gradient causes the light rays to bend away from the object concealed behind the device, making it appear invisible.

With this method, it is more practical to demonstrate cloaking underwater as all of the apparatus can be contained in a petri dish. It is the ease with which the CNTs can be heated that gives the device its unique ‘on and off’ feature.

Congratulations to Dr. Ali Aliev (lead author) and the rest of the University of Texas at Dallas team!

ETA Oct. 5, 2011: I added the preposition ‘of’ to the title and I’m adding a comment about invisibility cloaks.

Comment: Most of the invisibility cloaks I’ve read about are at the nanoscale which means none of us outside a laboratory could possibly observe the cloak in action. Seeing this video demonstrating an invisibility cloak in the range of visible light and at a macroscale was a dream come true, so to speak.

Splitting light to make events invisible

It’s always about bending light so that an object becomes invisible when you hear about scientists working on invisibility cloaks. Dexter Johnson (Nanoclast blog on the IEEE [Institute of Electrical and Electronics Engineers] website) recently featured some of the newest work in this area in his July 7, 2011 posting about a graphene cloaking device (based on the concept of ‘mantle cloaking’) proposed by researchers at the University of Texas at Austin.

Ian Sample in his July 13, 2011 posting on The Guardian Science blogs describes an entirely different approach, one that focusses on cloaking events rather than objects. From Samples’s posting,

The theoretical prospect of a “space-time” cloak – or “history editor” – was raised by Martin McCall and Paul Kinsler at Imperial College in a paper published earlier this year. The physicists explained that when light passes through a material, such as a lens, the light waves slow down. But it is possible to make a lens that splits the light in two, so that half – say the shorter wavelengths – speed up, while the other half, the longer wavelengths, slow down. This opens a gap in the light in which an event can be hidden, because half the light arrives before it has happened, and the other half arrives after the event.

In their paper, McCall and Kinsler outline a scenario whereby a video camera would be unable to record a crime being committed because there was a means of splitting the light such that 1/2 of it reached the camera before the crime occurred and the other 1/2  reached the camera afterwards. Fascinating, non?

It seems researchers at Cornell University have developed a device that can in a rudimentary fashion cloak events (from Samples’s posting),

The latest device, which has been shown to work for the first time by Moti Fridman and Alexander Gaeta at Cornell University, goes beyond the more familiar invisibility cloak, which aims to hide objects from view, by making entire events invisible.

Fridman’s and Gaeta’s research is to be published in Nature magazine at some time in the future and I look forward to hearing more about how this ‘space/time invisibility cloak’ works and whether or not other scientists can replicate the effect.

One final comment, Samples mentioned a special July 2011 issue (freeish download)  of Physics World devoted to invisibility. Excerpted from Matin Durrani’s July 8, 2011 posting on the Physics World blog,

It is perhaps a little-known fact that Griffin – the main character in H G Wells’ classic novel The Invisible Man – was a physicist. In the 1897 book, Griffin explains how he quit medicine for physics and developed a technique that made himself invisible by reducing his body’s refractive index to match that of air.

While Wells’ novel is obviously a work of fiction, the quest for invisibility has made real progress in recent years – and is the inspiration for this month’s special issue of Physics World, which you can download for free via this link [they do  want your contact details].

Kicking off the issue is Sidney Perkowitz, who takes us on a whistle-stop tour of invisibility through the ages – from its appearance in Greek mythology to camouflaging tanks on the battlefield – before bringing us up to date with recent scientific developments.

While it’s not yet possible to hear more Fridman’s and Gaeta’s device until Nature publishes their research, Sample offers more details based on materials, Demonstration of temporal cloaking, the researchers submitted to the arvix database on Monday, July 11, 2011.

I wonder what would happen if you had both kinds of invisibility cloaks at work. It brings to mind a Zen koan (I’ve paraphrased it), If a tree falls in the forest and no one is there, does it make a sound?

Or in this case: If you can’t see the object (light bending cloak), and you never saw the event (temporal cloak), did it exist and did it happen?

http://physicsworld.com/cws/download/jul2011

Hiding a peppercorn with your invisibility cloak

It’s the first time I’ve heard of an invisibility cloak that can hide something visible to the naked eye.  A peppercorn may not sound like much but compared to cloaked objects that are usually measured at the nanoscale (nano means one billionth of a metre), this is a huge step forward. What makes this discovery even more interesting is that it’s simple and inexpensive compared to the other systems used to achieve invisibility. From the Jan. 25, 2011 news item on Nanowerk,

Unlike the other attempts to produce invisibility by constructing synthetic layered materials, the new method uses an ordinary, common mineral called calcite — a crystalline form of calcium carbonate, the main ingredient in seashells. “Very often, the obvious solution is just sitting there,” says MIT mechanical-engineering professor George Barbastathis, one of the new report’s co-authors.

The work is being done by a team of researchers from the Singapore-MIT Alliance for Research & Technology (SMART). From the SMART website,

Established in 2007, the SMART Centre is MIT’s first research centre outside of Cambridge, MA and its largest international research endeavor. The Centre is also the first entity in the Campus for Research Excellence and Technological Enterprise (CREATE) currently being developed by NRF.

Here’s how they created an invisibility cloak,

In the experiment reported in this paper, the system works in a very carefully controlled setting: The object to be hidden (a metal wedge in the experiment, or anything smaller than it) is placed on a flat, horizontal mirror, and a layer of calcite crystal — made up of two pieces with opposite crystal orientations, glued together — is placed on top of it. When illuminated by visible light and viewed from a certain direction, the object under the calcite layer “disappears,” and the observer sees the scene as if there was nothing at all on top of the mirror.