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Monday, November 08. 2010
Via MIT Technology Review
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A flexible metamaterial that manipulates visible light could lead to better camouflage.
By Stephen Cass
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Now you see it: A sheet of Metaflex, a new metamaterial that could be used in devices and fabrics that can manipulate visible light.
Credit: University of St. Andrews |
Researchers at the University of St. Andrews have created sheets of a flexible metamaterial that can manipulate visible light. "It's a pretty significant step forward," says Stephen Cummer, professor of electrical and computer engineering at Duke University and the inventor of the first metamaterial-based invisibility cloak. "At radio frequencies we know how to make a lot of these things. But at optical wavelengths, things have been very fabrication-limited."
Metamaterials allow researchers to manipulate electromagnetic waves beyond the boundaries of what physics allows in natural materials. As well as promising better solar cells and high-resolution microscope lenses, metamaterials have also been used to create so-called invisibility cloaks, in which electromagnetic waves are bent around an object as if it simply weren't there.
However, metamaterials must be constructed out of elements smaller than the wavelength of the electromagnetic radiation being manipulated. This means that invisibility cloaks (and most metamaterial devices in general) only work with wavelengths longer than those found in visible light, such as radio and microwave frequencies. Metamaterials designed to work with optical wavelengths are built on rigid and fragile substrates, and as a result they've been confined to the lab.
The new metamaterial, dubbed "Metaflex" by its creators, is manufactured on top of a rigid substrate. An initial, sacrificial layer of the material is deposited on this substrate to stop the subsequent layers from sticking to this substrate. A sheet of a flexible, transparent, plastic polymer is then laid down. Next, a lithographic process, similar to that used to make silicon chips, creates a lattice of gold bars, each 100 to 200 nanometers long and 40 nanometers thick, on top of the polymer. (These bars act as "nanoantennas" that interact with incoming electromagnetic waves.) The Metaflex material is then bathed in a chemical that releases the polymer from the layer below and from the rigid substrate.
By varying the length and spacing of the nanoantennas, Metaflex can be tuned to interact with different wavelengths of light. The simple sheets tested by the researchers simply blocked a portion of an incoming beam of light at specific wavelengths, but this is enough to demonstrate that Metaflex is a working metamaterial. The St. Andrew's researchers tested wavelengths as short as 620 nanometers (corresponding to a red color).
So far, the researchers have produced flexible sheets as large as five by eight millimeters and as thin as four micrometers. While a fingernail-sized sample may seem small, it's a big step up from the microscopic dimensions of other optical metamaterials. The St. Andrew's scientists are confident that Metaflex can be produced in even larger sizes and at high volumes. "It's absolutely scalable to industrial levels," says Andrea Di Falco, the lead author of a paper published in the New Journal of Physics yesterday that describes the material.
Even at small sizes, the flexibility of the material is likely to confer some big advantages. "You really would like to be able to shape optical metamaterials into cylinders or spherical sections." This could allow, for example, the creation of curved superlenses that could magnify objects so small that they currently can't be seen with optical lenses due to diffraction effects. "On rigid substrates, it's just next to impossible to fabricate that kind of thing," says Duke University's Cummer, but with a flexible material, "you could fabricate flat and easily bend it into shape."
Di Falco believes it should be possible to stack sheets of Metaflex together to create thick layers and blocks of the material, creating the first optical metamaterial with a significant three-dimensional bulk. Such a development would open the door to new properties, including, perhaps, the ability to work with more than a single wavelength at a time. Other researchers have been able to create metamaterials that can be tuned to respond to different single wavelengths after fabrication, but ideally, they'd like a material that can work across a wide band of wavelengths simultaneously. This might be achieved through stacking sheets of MetaFlex, each tuned to a different wavelength.
The researchers' next step is to create these stacks and study how the properties of Metaflex change when sheets are twisted, stretched, or bent.
Ultimately, Di Falco says, Metaflex could have applications such as manipulating light from an LED built into a contact lens for augmented reality, so that computer-generated images are projected onto the wearer's retina. And of course, there's invisibility. "If you have something flexible, you could embed it into a fabric. Then you could think of tuning the properties of each individual layer to change the response of the fabric, giving something similar to camouflage. So, yes—there's some grounds for [an invisibility cloak]. Not tomorrow. But that's what I'll be working on," says Di Falco.
Copyright Technology Review 2010.
Tuesday, June 15. 2010
Via The Guardian
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Alok Jha
Scientists have developed a metamaterial that reflects almost no light - making it very black indeed
To the probable disappointment of fashionistas everywhere, scientists have taken it on themselves to decide on the new black. And it is (drumroll please): black. But it's a black that's blacker than any black before it. How much more black could you get? As Spinal Tap guitarist Nigel Tuffnell said of the cover of the band's last album, Smell the Glove: "None more black."
The "blacker than black" substance developed by scientists does not occur in nature; nor is it some sort of paint. Rather, it is a "metamaterial": an intricately constructed array of tiny silver wires embedded in aluminium oxide, which does weird things to the light waves that hit it, bending them in odd ways and sending them in unnatural directions.
Made by a team of scientists led by Evgenii Narimanov of Purdue University in Indiana, the result of this metamaterial is something that reflects almost no light, meaning it looks very, very black. Why would you want such a material? Narimanov tells New Scientist that the primary application of his type of material is likely to be military, specifically in building equipment invisible to radar.
But the next stage – creating metamaterials that can manipulate visible light to the point that objects become invisible to the naked eye – is much harder, as the wavelength of visible light is thousands of times smaller than that of radio waves. So, sadly for Harry Potter fans, it will be a long time before scientists can weave a cloak of invisibility.
Tuesday, May 11. 2010
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Metamaterials allow the creation of adjacent spaces with their own laws of physics, just like the multiverse.
Metamaterials are substances in which physicists have fiddled with a material's ability to support electric and magnetic fields. They can be designed to steer electromagnetic waves around, over and behind objects to create invisibility cloaks that hide objects.
If that sounds a little like the way gravitational fields can bend light, then you won't be surprised to learn that there is a formal mathematical analogy between optical metamaterials and general relativity.
The idea that anything Einstein can do, metamaterials can do too has fueled an explosion of interest in "electromagnetic space". Physicists have already investigated black holes that suck light in but won't let it out and wormholes that connect different regions of electromagnetic space.
Today, Igor Smolyaninov at the University of Maryland in College Park says that the analogy with spacetime can be taken much further. He says it is possible to create metamaterials that are analogous to various kinds of spaces dreamt up by cosmologists to explain aspects of the Universe.
In these theories, space can have different numbers of dimensions that become compactified early in the Universe's history, leaving the three dimensions of space and one of time (3+1) that we see today. In symmetries of these spaces depend on the dimensions and the way they are compactified and this in turn determines the laws of physics in these regions.
It turns out, says Smolyaninov, that it is possible to create metamaterials with electromagnetic spaces in which some dimensions are compactified. He says it is even possible to create substances in which the spaces vary from region to region, so a space with 2 ordinary and 2 compactified dimensions, could be adjacent to a space with just 2 ordinary dimensions and also connected to a 2d space with 1 compactified dimension and so on.
The wormholes that make transitions between these regions would be especially interesting. It ought to be possible to observe the birth of photons in these regions and there is even a sense in which the transition could represent the birth of a new universe."A similar topological transition may have given birth to our own Universe," says Smolyaninov.
He goes on to show that these materials can be used to create a multiverse in which different universes have different properties. In fact it ought to be possible create universes in which different laws of physics arise.
That opens up a new area for optical devices. Smolyaninov gives the example of electromagnetic universes in which photons behave as if they are massive, massless or charged depending on the topology of space and the laws of physics this gives rise to.
Just what kind of devices could exploit this behaviour isn't clear yet. If you think of any, post them here. This is clearly a field that for the moment appears to be limited only by the mind of the designer.
Ref: arxiv.org/abs/1005.1002: Metamaterial "Multiverse"
Tuesday, August 12. 2008
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Invisible net: A new material that can bend near-infrared light in a unique way has a fishnet structure. These images of a prism made from the material were taken with a scanning electron microscope. The holes in the net enable the material to interact with the magnetic component of the light, which enables the unusual bending and demonstrates its promise for use in future invisibility cloaks. In the inset, the layers of metal and insulating material that make up the metamaterial are visible.
Credit: Jason Valentine et al. |
The fabrication of two new materials for manipulating light is a key step toward realizing cloaking.
By Katherine Bourzac
In an important step toward the development of practical invisibility cloaks, researchers have engineered two new materials that bend light in entirely new ways. These materials are the first that work in the optical band of the spectrum, which encompasses visible and infrared light; existing cloaking materials only work with microwaves. Such cloaks, long depicted in science fiction, would allow objects, from warplanes to people, to hide in plain sight.
Both materials, described separately in the journals Science and Nature this week, exhibit a property called negative refraction that no natural material possesses. As light passes through the materials, it bends backward. One material works with visible light; the other has been demonstrated with near-infrared light.
The materials, created in the lab of University of California, Berkeley, engineer Xiang Zhang, could show the way toward invisibility cloaks that shield objects from visible light. But Steven Cummer, a Duke University engineer involved in the development of the microwave cloak, cautions that there is a long way to go before the new materials can be used for cloaking. Cloaking materials must guide light in a very precisely controlled way so that it flows around an object, re-forming on the other side with no distortion. The Berkeley materials can bend light in the fundamental way necessary for cloaking, but they will require further engineering to manipulate light so that it is carefully directed.
One of the new Berkeley materials is made up of alternating layers of metal and an insulating material, both of which are punched with a grid of square holes. The total thickness of the device is about 800 nanometers; the holes are even smaller. "These stacked layers form electrical-current loops that respond to the magnetic field of light," enabling its unique bending properties, says Jason Valentine, a graduate student in Zhang's lab. Naturally occurring materials, by contrast, don't interact with the magnetic component of electromagnetic waves. By changing the size of the holes, the researchers can tune the material to different frequencies of light. So far, they've demonstrated negative refraction of near-infrared light using a prism made from the material.
Researchers have been trying to create such materials for nearly 10 years, ever since it occurred to them that negative refraction might actually be possible. Other researchers have only been able to make single layers that are too thin--and much too inefficient--for device applications. The Berkeley material is about 10 times thicker than previous designs, which helps increase how much light it transmits while also making it robust enough to be the basis for real devices. "This is getting close to actual nanoscale devices," Cummer says of the Berkeley prism.
The second material is made up of silver nanowires embedded in aluminum. "The nanowire medium works like optical-fiber bundles, so in principle, it's quite different," says Nicholas Fang, mechanical-science and -engineering professor at the University of Illinois at Urbana-Champagne, who was not involved in the research. The layered grid structure not only bends light in the negative direction; it also causes it to travel backward. Light transmitted through the nanowire structure also bends in the negative direction, but without traveling backward. Because the work is still in the early stages, it's unclear which optical metamaterial will work best, and for what applications. "Maybe future solutions will blend these two approaches," says Fang.
Making an invisibility cloak will pose great engineering challenges. For one thing, the researchers will need to scale up the material even to cloak a small object: existing microwave cloaking devices, and theoretical designs for optical cloaks, must be many layers thick in order to guide light around objects without distortion. Making materials for microwave cloaking was easier because these wavelengths can be controlled by relatively large structural features. To guide visible light around an object will require a material whose structure is controlled at the nanoscale, like the ones made at Berkeley.
Developing cloaking devices may take some time. In the short term, the Berkeley materials are likely to be useful in telecommunications and microscopy. Nanoscale waveguides and other devices made from the materials might overcome one of the major challenges of scaling down optical communications to chip level: allowing fine control of parallel streams of information-rich light on the same chip so that they do not interfere with one another. And the new materials could also eventually be developed into lenses for light microscopes. So-called superlenses for getting around fundamental resolution limitations on light microscopes have been developed by Fang and others, revealing the workings of biological molecules with nanoscale resolution using ultraviolet light, which is damaging to living cells in large doses. But it hasn't been possible to make superlenses that work in the information-rich and cell-friendly visible and near-infrared parts of the spectrum.
Copyright Technology Review 2008.
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