Invisibility cloaks shield the large and visible

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Jan 1, 2011

Two independent groups of physicists have built invisibility cloaks that can shield large objects lying on a plane. These "carpet cloaks" are far closer to the intuitive idea of an invisibility cloak than devices previously built, they argue, because they hide objects that can be seen with the naked eye and do so at visible wavelengths. The cloaks are also relatively cheap and easy to make, being constructed from the natural material calcite.

Carpet cloaks were proposed in 2008 by John Pendry of Imperial College, London as a way of extending the operating range of invisibility cloaks, which were mostly limited to microwave wavelengths. These devices are placed over an object sitting on a reflective plane and alter the path of light bouncing off the object in such a way that the light appears to have bounced straight off the plane.

However, all visible-light carpet cloaks built so far were demonstrated under a microscope, hiding objects no larger than 100 wavelengths across (about 50 µm). In addition, these cloaks were difficult to make, since they consisted of complex, artificially engineered materials. And they were not portable because the cloak, object and surrounding medium all tended to be made from a single structure.

The latest devices follow on from the work of Yu Luo of Zhejiang University in China and colleagues who realized last year that carpet cloaks can be built from homogeneous – rather than more complex inhomogeneous – materials, as long as those materials are anisotropic. Both devices in fact are built from the naturally occurring crystalline material calcite, the refractive index of which depends on the relative orientation of an incoming light wave’s polarization axis and the calcite’s optical axis.

George Barbastathis and co-workers at the Singapore-MIT Alliance for Research and Technology (SMART) in Singapore made their cloak by gluing together two pieces of calcite with differently oriented optical axes. These orientations were fixed such that light waves with a given polarization that bounce off a wedge-shaped object placed underneath the cloak emerge travelling in the same direction and at the same height that they would have done had they bounced straight off the mirror beneath the object. The wedge, having a base length, width and height of 38 mm, 10 mm and 2 mm respectively, can easily be seen with the naked eye.

The team used a technique known as transformation optics to design their cloak. They calculated the optical parameters that were needed to transform the space between a small and a large triangle, such that light passing through this space would do so as if it were passing through all of the larger triangle, thereby effectively rendering the smaller triangle – the wedge – invisible. Having calculated these parameters the researchers were able to construct their cloak using conventional lens fabrication, with the cross section of the cloak being equal to the space between the two triangles, minus the top of the larger triangle.

The researchers then tested the cloak by directing a polarized laser beam so that the beam passed through a stencil, and then part of the emerging beam entered the cloak and bounced off the wedge while the remainder of the beam bounced directly off a reflective surface beneath the wedge. By detecting the different parts of the beam with a CCD camera they were able to show that the cloak worked. The team optimized the cloak so that it worked best at green wavelengths, which is where the eye is most sensitive, but showed that, some aberration aside, it also worked at red and blue wavelengths (arXiv:1012.2238).

Both the device and the experiment used to test it bear a striking resemblance to those of the second group, which includes John Pendry. This team published its research at arXiv:1012.2783 but is unable to discuss its work while the paper is being reviewed for acceptance in a scientific journal.

Pendry’s group tested its device in air, while Barbastathis and team immersed their cloak and wedge in a tank of colourless laser oil. They did this in order to simulate the environment in which the device would be used – in the sea around Singapore. Light tends to have a particular polarization in water and so the device can be tuned to that polarization. According to SMART’s Baile Zhang, the cloak could be used by engineers to hide cables along the seabed or to help biologists image the behaviour of fish and other sea creatures unobtrusively.

Tomas Tyc of Masaryk University in the Czech Republic, who was not a member of either group, thinks that the papers "describe important achievements in the area of experimental cloaking." But he maintains that a carpet cloak is quite different to a fully fledged Harry Potter-style invisibility cloak. He points out that a carpet cloak only really works when viewing an object – be it a rucksack or a sword on someone's back, for example – side on. Otherwise the object will appear flat but still be visible.

Zhang acknowledges this. Making free-standing cloaks, he says, "is the direction we need to move in". But doing so, he adds, will be very difficult since it will require the fabrication of materials with "extreme parameters".

Edwin Cartlidge is a science writer based in Rome

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Space–time invisibility cloak could 'edit history'

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Nov 16, 2010

Physicists in the UK have proposed a "space–time" invisibility cloak that, if built, could be used to prevent signal interference or give the illusion of a Star Trek teleportation device.

The idea comes after four years of research by different groups that are creating devices to make objects invisible. In 2006 researchers at Duke University in the US created the first device that could cloak a small object in two dimensions in the microwave region of the electromagnetic spectrum. Last year groups at Cornell University and the University of California at Berkeley, US, independently created 2D cloaks that operated at optical wavelengths. Then, earlier this year, a team at the Karlsruhe Institute of Technology in Germany went one step further to produce a 3D optical cloak.

The latest development, by Martin McCall and colleagues of Imperial College, London, and the University of Salford, might see cloaks add yet another dimension to their capability: time. The idea is to create a tunnel through which an object could perform an action – move or change shape, for example – while appearing as though it is doing nothing at all.

"It means that you can allow an object to do something for a short period of time in such a way that it can't be detected," McCall told physicsworld.com. "A good way to think about it is a small piece of an object’s history just being cut out, so you would see the object suddenly jump from one place to another." In principle, says McCall, such a system would enable a thief to enter a room, steal the contents of a safe and leave the scene as it was before, while security personnel watching CCTV are none the wiser.

In practice, the device would need two transparent walls to act as the tunnel, or space–time cloak. As an object enters the cloak to perform its action, the rear wall would compress light waves passing through from a source behind. Once the object completes its action and leaves the cloak, however, the front wall would stretch the light waves passing through so that they would merge seamlessly with those outside, whose profile had not been altered.

An analogy, says McCall, is a chicken crossing a busy road. Once the chicken steps onto the road cars must stop to let it pass, but as soon as it leaves the other side the cars would accelerate to catch up with the traffic ahead. To an observer farther down the road, the stream of passing cars would display no evidence of having slowed down.

Although McCall gives safecracking as a potential for the space–time cloak, his group does have ideas for more savoury applications. In the basic set-up it might appear as similar to a transporter from Star Trek, with a person entering the cloak on one side appearing at the other side moments later, apparently having skipped the journey. But the cloak could also find uses in signal processing: a detector placed inside the cloak would be able to "pause" a signal travelling through the wall while it first deals with a signal passing through the tunnel.

All of this, however, relies on someone being able to make the device – and in particular the walls that compress or stretch light waves. McCall admits a perfect implementation "is certainly beyond current technology", but points to advances in so-called nonlinear systems: materials that change their refractive index – a property that governs light propagation speed – given illumination with strong lasers. One of the problems with this route is that changes in refractive index introduce reflections, which means the cloak, while hiding the object within, would nonetheless reveal its presence with a telling glow.

But, explains McCall, "Provided we're prepared to throw away some aspects of the cloak, we can point towards more practical, proof-of-concept experiments that are currently accessible with current technology."

The research is published today in the Journal of Optics.

Jon Cartwright is a freelance journalist based in Bristol, UK

View the original article here

Space–time invisibility cloak could 'edit history'

To enjoy free access to all high-quality "In depth" content, including topical features, reviews and opinion sign up

Nov 16, 2010

Physicists in the UK have proposed a "space–time" invisibility cloak that, if built, could be used to prevent signal interference or give the illusion of a Star Trek teleportation device.

The idea comes after four years of research by different groups that are creating devices to make objects invisible. In 2006 researchers at Duke University in the US created the first device that could cloak a small object in two dimensions in the microwave region of the electromagnetic spectrum. Last year groups at Cornell University and the University of California at Berkeley, US, independently created 2D cloaks that operated at optical wavelengths. Then, earlier this year, a team at the Karlsruhe Institute of Technology in Germany went one step further to produce a 3D optical cloak.

The latest development, by Martin McCall and colleagues of Imperial College, London, and the University of Salford, might see cloaks add yet another dimension to their capability: time. The idea is to create a tunnel through which an object could perform an action – move or change shape, for example – while appearing as though it is doing nothing at all.

"It means that you can allow an object to do something for a short period of time in such a way that it can't be detected," McCall told physicsworld.com. "A good way to think about it is a small piece of an object’s history just being cut out, so you would see the object suddenly jump from one place to another." In principle, says McCall, such a system would enable a thief to enter a room, steal the contents of a safe and leave the scene as it was before, while security personnel watching CCTV are none the wiser.

In practice, the device would need two transparent walls to act as the tunnel, or space–time cloak. As an object enters the cloak to perform its action, the rear wall would compress light waves passing through from a source behind. Once the object completes its action and leaves the cloak, however, the front wall would stretch the light waves passing through so that they would merge seamlessly with those outside, whose profile had not been altered.

An analogy, says McCall, is a chicken crossing a busy road. Once the chicken steps onto the road cars must stop to let it pass, but as soon as it leaves the other side the cars would accelerate to catch up with the traffic ahead. To an observer farther down the road, the stream of passing cars would display no evidence of having slowed down.

Although McCall gives safecracking as a potential for the space–time cloak, his group does have ideas for more savoury applications. In the basic set-up it might appear as similar to a transporter from Star Trek, with a person entering the cloak on one side appearing at the other side moments later, apparently having skipped the journey. But the cloak could also find uses in signal processing: a detector placed inside the cloak would be able to "pause" a signal travelling through the wall while it first deals with a signal passing through the tunnel.

All of this, however, relies on someone being able to make the device – and in particular the walls that compress or stretch light waves. McCall admits a perfect implementation "is certainly beyond current technology", but points to advances in so-called nonlinear systems: materials that change their refractive index – a property that governs light propagation speed – given illumination with strong lasers. One of the problems with this route is that changes in refractive index introduce reflections, which means the cloak, while hiding the object within, would nonetheless reveal its presence with a telling glow.

But, explains McCall, "Provided we're prepared to throw away some aspects of the cloak, we can point towards more practical, proof-of-concept experiments that are currently accessible with current technology."

The research is published today in the Journal of Optics.

Jon Cartwright is a freelance journalist based in Bristol, UK

View the original article here