tomahawk6 said:All we need for our space ship is a warp drive. ;D
Friday, January 16, 2009
Invisibility-Cloak Breakthrough
New software has enabled metamaterials to work with a broad band of frequencies.
By Katherine Bourzac
Metamaterials interact with light in ways that appear to violate the laws of physics. They can bend light around an object as if it weren't there, or narrow the resolution of light microscopes down to a few nanometers. But metamaterials must be painstakingly structured at the nano- and microscales in order to achieve these exotic effects. Now the Duke University researcher who built the first invisibility cloak in 2006 has created software that speeds up the design of metamaterials. He and his colleagues have used the program to build a complex light cloak that's invisible to a broad band of microwave light--and they did it in only about 10 days.
David R. Smith of Duke and Tai Jun Cui of Southeast University, in Nanjing, China, led the work, which is a landmark in the field of metamaterials. The cloak that the researchers built works with wavelengths of light ranging from about 1 to 18 gigahertz--a swath as broad as the visible spectrum. No one has yet made a cloaking device that works in the visible spectrum, and those metamaterials that have been fabricated tend to work only with narrow bands of light. But a cloak that made an object invisible to light of only one color would not be of much use. Similarly, a cloaking device can't afford to be lossy: if it lets just a little bit of light reflect off the object it's supposed to cloak, it's no longer effective. The cloak that Smith built is very low loss, successfully rerouting almost all the light that hits it.
"Their cloak . . . answers the naysayers who predicted that cloaks would always be narrowband and lossy," says John Pendry, chair in theoretical solid-state physics at Imperial College London. Pendry did the theoretical work upon which both the first invisibility cloak and its new successor are based. "Needless to say, I am delighted with this development," says Pendry. He and his Imperial College colleague Jensen Li proposed a theoretical version of a broadband cloak just last year, and at that time, he says, he "did not expect such rapid experimental progress."
The broadband cloak is a rectangular structure measuring about 50 by 10 centimeters, with a height of about 1 centimeter. It's made up of roughly 600 I-shaped copper structures. Making each structure is a simple matter, says Smith. "They're copper patterns on a circuit board, cut up and arranged. It's a well-known, inexpensive technology." The hard part is determining the dimensions of each of these 600 structures and how to arrange them. With the first light cloak, which had only 10 such pieces, "we had to design each element by numerical simulations," Smith says. Applying the same approach to the more complicated cloak would have eaten up months.
Even for physicists and engineers, the math involved in the theoretical design of cloaking devices is very difficult, says Nicholas Fang, a professor of mechanical science and engineering at the University of Illinois at Urbana-Champaign. The way that a material interacts with light's magnetic and electric components is taken into account in determining the size, shape, and orientation of each structure in a metamaterial. Pendry and Li's theoretical work described how to make a broadband cloak by using materials structured so that they have an electrical response to light, but not a magnetic one. But it wasn't clear how to put this idea into practice. The Southeast University researchers developed new algorithms to greatly speed up the process, says Smith. These algorithms make it possible to quickly predict how a structure with a particular shape will interact with light.
The cloak itself, described this week in Science, is indeed impressive, says Fang, who's working on metamaterials for super-resolution biological imaging. But what's more exciting is that the new approach to design will accelerate the development of other metamaterials. Smith says that he and his group have already moved beyond the cloak reported in Science, but because their latest work is unpublished, he can't specify what they've made. "Now [that] this is becoming a more feasible technology," he says, "we will start to see a lot more of it."
Other applications of metamaterials, says Smith, include optical devices that take light energy and concentrate it, instead of turning it away--conceptually, the opposite of a cloak. "You could improve solar cells by making structures to increase the field strength of the light," he says. The new work suggests that this could be done over the whole spectrum of wavelengths found in sunlight. Similarly, broadband "hyperlenses" that gather up light missed by normal lenses could revolutionize biological imaging. Fang and others have developed narrowband hyperlenses with resolutions of only a few nanometers, which make the molecular workings of cells visible. A broadband hyperlens could work with all colors of visible and infrared light.
The ultimate goal, says Pendry, is cloaking in the visible-light spectrum, and Smith's latest work points the way forward. "There are no insuperable obstacles to making a cloak work at optical frequencies," Pendry says. "The Duke paper brings this goal a step closer."
Copyright Technology Review 2009.
Acoustic superlens could cloak objects from sonar
First experimental demonstration of a technology that could trigger a new game of cat and mouse beneath the waves
Wednesday, April 01, 2009
Researchers have been messing about with optical metamaterials and invisibility cloaks for a few years now. And while progress has been rapid, nobody's going to be fooling Voldemort any time soon.
But the same exotic tricks that apply to light can equally be applied to sound. And potentially more easily too because sound has a longer wavelength. The business parts of acoustic metamaterials should therefore be significantly easier to build than their optical counterparts.
And that's just what Nicholas Fang and buddies from the University of Illinois at Urbana- Champaign have done: create a flat slab of acoustic metamaterial that focuses sound with a negative refractive index. They've even fashioned a design that works as a "superlens" that focuses the so-called evanescent sound waves that form within a single wavelength of the source-- a world first apparently.
Fang and co have created an acoustic metamaterial by carving an array of holes into an aluminium sheet and filling the holes with water. The holes then resonate when water moves over them, like wind over the mouth of a bottle.
In theory, superlenses can far outperform the resolution of conventional lenses but Fang's lens isn't super just yet: its resolution is only about half the length of the incident waves. But that's pretty good and among the best that has ever been possible with purely passive focusing elements. And while conventional optics can never beat this kind of resolution, Fang's superlenses can almost certainly be improved.
Another big advantage is the shape of the lens which is entirely flat and just a few centimetres square. That makes it much easier to make than the spherical optics that have been necessary in the past.
Obviously, the new technique will be handy for medical imaging and nondestructive testing but the authors hint at a more exotic application. They say:
"This design approach may lead to novel strategies of acoustic cloak for camouflage under sonar.
Tantalising! What on Earth could they mean?
Numerically Shown Multiple Frequency Active Cloaking of Any Shape Object by Devices that Generate EM
These images are from animated computer simulations of a new method -- developed by University of Utah mathematicians -- for cloaking objects from waves of all sorts. While the new method is unlikely to lead to invisibility cloaking like that in 'Star Trek' or 'Harry Potter' movies, it may eventually help shield submarines from sonar, planes from radar, buildings from earthquake waves, and oil rigs and coastal structures from tsunamis. The top three images show a wave front passing the kite-shaped object in the middle and hitting the object as it does. In the bottom three images, the kite-shaped object if surrounded by three cloaking devices and the waves they emit. So when the wave front passes, it moves by the object without touching it. Photo Credit: Fernando Guevara Vasquez
University of Utah mathematicians developed a new cloaking method which someday might shield submarines from sonar, planes from radar, buildings from earthquakes, and oil rigs and coastal structures from tsunamis.
We have shown that it is numerically possible to cloak objects of any shape that lie outside the cloaking devices, not just from single-frequency waves, but from actual pulses generated by a multi-frequency source," says Graeme Milton, senior author of the research and a distinguished professor of mathematics at the University of Utah.
It's called active cloaking, which means it uses devices that actively generate electromagnetic fields rather than being composed of 'metamaterials' [exotic metallic substances] that passively shield objects from passing electromagnetic waves."
Radar microwaves have wavelengths of about four inches, so Milton says the study shows it is possible to use the method to cloak from radar something 10 times wider, or 40 inches. That raises hope for cloaking larger objects. So far, the largest object cloaked from microwaves in actual experiments was an inch-wide copper cylinder.
Most previous research used interior cloaking, where the cloaking device envelops the cloaked object. Milton says the new method "is the first active, exterior cloaking" technique: cloaking devices emit signals and sit outside the cloaked object.
The new studies are numerical and theoretical, and show how the cloaking method can work. "The research simulates on a computer what you should see in an experiment," Milton says. "We just do the math and hope other people do the experiments."
The Physical Review Letters study demonstrates the new cloaking method at a single frequency of electromagnetic waves, while the Optics Express paper demonstrates how it can work broadband, or at a wide range of frequencies.
In Optics Express, the mathematicians demonstrate that three cloaking devices together create a "quiet zone" so that "objects placed within this region are virtually invisible" to incoming waves.
Three-Dimensional Invisibility Cloak at Optical Wavelengths
A three-dimensional invisibility-cloaking structure operating at optical wavelengths based on transformation optics has been designed and realized. Our blueprint uses a woodpile photonic crystal with tailored polymer filling fraction to hide a bump in a gold reflector. Structures and controls are fabricated by direct laser writing and characterized by simultaneous high-numerical-aperture far-field optical microscopy and spectroscopy. Cloaking operation with large bandwidth of unpolarized light from 1.4- to 2.7-µm wavelength is demonstrated for viewing angles up to 60 degrees
Beyond military applications, cloaking devices are drawing interest from telecommunications companies, who see them as a way to send information by light more efficiently. One idea is to use the new materials to build "superantennas" that can concentrate light and other electromagnetic waves to make laser-like beams.
From physorg,
the cloak is a structure of crystals with air spaces in between, sort of like a woodpile, that bends light, hiding the bump in the gold later beneath, the researchers reported in Thursday's online edition of the journal Science.
In this case, the bump was tiny, a mere 0.00004 inch high and 0.0005 inch across (100 microns x 30 microns), so that a magnifying lens was needed to see it.
"In principle, the cloak design is completely scalable; there is no limit to it," Ergin said. But, he added, developing a cloak to hide something takes a long time, "so cloaking larger items with that technology is not really feasible."
"Other fabrication techniques, though, might lead to larger cloaks," he added in an interview via e-mail.
The value of the finding, Ergin said, "is that we learn more about the concepts of transformation optics, and that we have made a first step in producing 3-D structures in that field."
Guardian UK - Tolga Ergin and Nicolas Stenger at the Karlsruhe Institute of Technology in Germany used a technique called direct laser writing lithography to create a sheet of cloaking material from tiny plastic rods. The spacing of the rods, each of which measured one thousandth of a millimetre wide, alters a property of the material known as the refractive index, which changes the speed of light inside it.
The researchers placed a piece of the material over a dimple in a gold sheet and used infrared cameras to see what happened. When the cloak was in place, it altered the speed of light around the bump in such a way that the gold sheet appeared to be flat. The experiment was equivalent to hiding something under a carpet and having the carpet disappear too.
It is the first time researchers have demonstrated a cloak that works in three dimensions. Previous devices have hidden objects when looked at head-on, but did not work if viewed from the side. "We were surprised that the cloaking effect was still so good, Ergin told the US journal, Science.
Inside the material, the plastic rods are arranged like planks of wood piled up on each other. The high precision of the structure means it is possible to control the refractive index so it varies in just the right way to bend light around whatever object is hidden beneath it.
"The material has a higher refractive index on top of the bump, so light hitting that part is slowed down a little bit compared with light impinging on the rest of the surface," said Stenger. "That compensates for the shape of the bump, and in the end, it is exactly as if there was no bump."
Research into cloaking devices has attracted funding from military organisations, such as the US Defence Advanced Research Projects Agency, which backs high-risk science research for the Pentagon. In the near term, cloaking materials are expected to be used to hide aircraft from radar more effectively.
Sound Bullets Generated by Metamaterials for Killing Cancer Tumors or Submarines
PNAS - Generation and control of sound bullets with a nonlinear acoustic lens
Acoustic lenses are employed in a variety of applications, from biomedical imaging and surgery to defense systems and damage detection in materials. Focused acoustic signals, for example, enable ultrasonic transducers to image the interior of the human body. Currently however the performance of acoustic devices is limited by their linear operational envelope, which implies relatively inaccurate focusing and low focal power. Here we show a dramatic focusing effect and the generation of compact acoustic pulses (sound bullets) in solid and fluid media, with energies orders of magnitude greater than previously achievable. This focusing is made possible by a tunable, nonlinear acoustic lens, which consists of ordered arrays of granular chains. The amplitude, size, and location of the sound bullets can be controlled by varying the static precompression of the chains. Theory and numerical simulations demonstrate the focusing effect, and photoelasticity experiments corroborate it. Our nonlinear lens permits a qualitatively new way of generating high-energy acoustic pulses, which may improve imaging capabilities through increased accuracy and signal-to-noise ratios and may lead to more effective nonintrusive scalpels, for example, for cancer treatment.
Discovery News has coverage
The simple set up belies the power of the new metamaterial. Not only did the scientists focus all of the sound waves onto one specific area; they also amplified those waves more than 100 times than what any other metamaterial had previously produced. Those numbers could easily go higher, said Daraio.
The sound waves Daraio and Spadonia manipulated were too high pitched for human ears to detect. Properly adapted to audible sound, the new metamaterial could turn a normal sentence into a split second ear drum rupturing explosion.
If these sound bullets were actual bullets, the metamaterial would be like transforming hot lead projectiles into rocket propelled grenades, all converging on one place at one time. The damage such concentrated waves of pressure could create would be devastating.
Like normal bullets, sound bullets can travel through air. Unlike normal bullets, sound bullets can also easily travel through liquids and solids. Sound bullets could be used by the military to create submarine melting waves of pressure or shock waves powerful enough to destroy caves otherwise untouchable by conventional weapons.
The beauty of this system is that it’s just a bunch of ball bearings that we control with weights,” said Chiara Daraio, a member of the research team. Caltech’s acoustic lens relies on the same principle as Newton’s cradle-that toy your high school science teacher probably kept on his or her desk with metal balls on strings that demonstrated the conservation of energy.
In this design, 21 parallel chains each contain 21 bearings. When the team strikes one end, it starts a compression wave that carries through the system. But instead of having the last ball swing out like a pendulum and bring the momentum back into the system, like the toy does, the acoustic lens focuses all the energy at the end of the system onto one spot, just a few inches away from the metamaterial.
The paper also hints at use in defense systems, though it leaves the implications of that to the imaginations of others. Sound bullets could be used by the military to create submarine melting waves of pressure or shock waves powerful enough to destroy caves otherwise untouchable by conventional weapons.
If they ARE truly invisible, how would we know?GAP said:Invisible Tanks, Planes and Armor Could Hit Battlefields in 5 Years
Article Link
Published January 18, 2011
Invisible tanks -- and maybe invisible soldiers -- may soon be charging onto battlefields ....
Invisibility cloaking for the whole spectrum
A light trajectory is shown against the distribution of the εσ values. The light ray enters the device, completes a loop, bounces off the mirror twice and leaves the cloak with its original direction restored (A). Panel (B) gives a closer view of the vicinity of the inner branch of the cloak. Objects placed within the white region are invisible.
An undergraduate student has overcome a major hurdle in the development of invisibility cloaks by adding an optical device into their design that not only remains invisible itself, but also has the ability to slow down light.
The optical device, known as an 'invisible sphere', would slow down all of the light that approaches a potential cloak, meaning that the light rays would not need to be accelerated around the cloaked objects at great speeds ― a requirement that has limited invisibility cloaks to work only in a specified region of the visible spectrum.
This new research, published today, Tuesday 9 August, in the Institute of Physics and German Physical Society's New Journal of Physics, could open up the possibility for a potential invisibility cloak wearer to move around amongst ever-changing backgrounds of a variety of colours.
New Journal of Physics - Invisibility cloaking without superluminal propagation
Conventional cloaking based on Euclidean transformation optics requires that the speed of light should tend to infinity on the inner surface of the cloak. Non-Euclidean cloaking still needs media with superluminal propagation. Here we show by giving an example that this is no longer necessary.
In this paper, we give an example of a device that achieves complete electromagnetic cloaking—not just `carpet cloaking'—while all light velocities within the cloak are finite and less than the speed of light. Through this example, we demonstrate that invisibility cloaking is possible without superluminal propagation and anomalous material requirements.
The usual approach to designing an invisibility cloak works on the basis of bending light ― using highly specific materials ― around an object that you wish to conceal, thereby preventing the light from hitting the object and revealing its presence to the eye of the observer.
When the light is bent, it engulfs the object, much like water covering a rock sitting in a river bed, and carries on its path making it seem as if nothing is there.
Light, however, can only be accelerated to a speed faster than it would travel in space under certain conditions, and this restricts invisibility cloaks to work in a limited part of the spectrum ― essentially just one colour.
This would be ideal if somebody was planning to stand still in camouflage; however, the moment that they start to move the scenery will begin to distort, revealing the person under the cloak.
By slowing all of the light down with an invisible sphere, it does not need to be accelerated to such high speeds and can therefore work in all parts of the spectrum.
Perczel said, "I started to work on the problem of superluminal propagation as Professor Leonhardt's summer student with an EPSRC grant. Once the idea was present, I worked for over eight months to overcome the technical barriers and to make the proposal practicable."
An Institute of Physics spokesperson said, "This new development opens up further possibilities for the design of a practical invisibility cloak ― overcoming the problem of light speed that other advances have struggled to address and, very impressively, this significant advance was achieved by an undergraduate student."