Invisibility cloak made from simple mirrors can stop time indefinitely
By Sebastian Anthony on August 14, 2013 at 11:16 am
Researchers at Northwestern University have designed an invisibility cloak that can temporally hide objects for an indefinite period of time. Objects covered by this invisibility cloak wouldn’t disappear from sight, but rather it would appear that time has completely stopped for the cloaked object. A clock, for example, would continue to tick — but to the human observer, the hands would never move. One use for such a cloaking device is security, where a thief might set up a temporal cloak in front of an object that he intends to steal — so that to any outside observers or security cameras, the theft — the event, in physics terminology — hasn’t yet occurred. There are, of course, lots of military uses as well.
The best bit, though, is that the Northwestern invisibility cloak is fashioned out of plain old mirrors, which means that the device should cloak the entire visible light spectrum. Most of the recent research into invisibility cloaks (both spatial and temporal) has focused on the use of metamaterials — man-made materials that can bend light in weird and wonderful ways. So far, though, we have only been able to engineer metamaterials that operate in narrow electromagnetic bands, such as infrared light or microwaves — which isn’t very useful if you’re trying to hide something from humans.
The process is somewhat complex, but by changing the reflectivity of the four electrically switched mirrors, events that occur at the object (O) can be hidden from the observer. The duration of the temporal cloak is twice the time it takes light to travel between A and B — and so if you place B a very, very long way away — such as on another planet — you could theoretically cloak an object for minutes, or hours… or light years. I guess you could even do it here on Earth, if you created a massive (and I mean millions of miles massive) network of mirrors capable of bouncing light around for a few seconds or minutes.
For now, it sounds like Lerma hasn’t yet built the device — but there’s no reason his design couldn’t be implemented today, using existing and readily available materials. In fact, given the time machine nature of this device, it’s possible that someone — perhaps the military — has already built it. We just can’t see it yet…
New invisibility cloak combines metamaterials and fancy electronics to be thinner, lighter, more invisible
By Sebastian Anthony on November 11, 2013 at 12:00 pm
A researcher at the University of Texas at Austin has devised an invisibility cloak that could work over a broad range of frequencies, including visible light and microwaves. This is a significant upgrade from current invisibility cloaks that only cloak a very specific frequency — say, a few hertz in the microwave band — and, more importantly, actually make cloaked objects more visible to other frequencies. The UT Austin cloak would achieve this goal by being active and electrically powered, rather than dumb and passive like existing invisibility cloaks.
As you probably know, the last few years have seen a lot of research into invisibility cloaks. These cloaks are mostly based on metamaterials — special, man-made materials that bend radiation in ways that shouldn’t technically be possible, allowing for cloaking devices that bend radiation around an object, hiding it from view. (Check out our featured story, The wonderful world of wonder materials.) The problem with these cloaks is that metamaterials are tuned to a very specific frequency — so, while that specific frequency (say, a thin band of microwaves) passes around the object, every other frequency scatters off the cloak. In a beautiful twist of irony, most invisibility cloaks actually create more scattered light, making the cloaked object stand out more than if it was just standing there uncloaked.
According to Andrea Alù at UT Austin, this is a fundamental issue of passive invisibility cloaks, and the only way to get around it is to use cloaks fashioned out of active, electrically active materials. It might change in the future with more advanced passive metamaterials, but for now active designs are the way forward. Research into active invisibility cloaks is currently being carried out by multiple groups, but none have yet been built.
Alù’s proposed design consists of a conventional metamaterial base, but with CMOS negative impedance converters (NICs) placed at the corner of each metamaterial square (top image). A NIC is an interesting electronic component that adds negative resistance to a circuit, injecting energy rather than consuming it. NICs are not widely used as we’re not entirely sure how to use them. Alù seems to propose that by interspersing NICs (which must be powered) with the metamaterial, multiple frequencies can be cloaked. In the image above, you can see a standard metamaterial cloak (blue), vs Alù’s metamaterial-and-NIC cloak (green). Alù’s proposed cloak is invisible over a large range of frequencies, while a standard passive cloak is only invisible for a small range, and more visible than non-cloaked devices in other ranges.
From our own experience with writing about invisibility cloaks on ExtremeTech, we’d have to agree that active designs make more sense. Where passive cloaks have all been incredibly bulky and not all that effective, an active cloak can be thinner, more flexible, and capable of cloaking a much wider range of frequencies. Given our mastery of CMOS, and the utterly insane things that we can do with computer chips, it seems foolhardy to not pursue active, electronic invisibility cloaks.
In hindsight, maybe this is the approach that the Canadian camouflage kook is using to achieve his Quantum Stealth tech. But I doubt it somehow.
Stanford engineers' new metamaterial doubles up on invisibility
The new material's artificial "atoms" are designed to work with a broad range of light frequencies. With adjustments, the researchers believe it could lead to perfect microscope lenses or invisibility cloaks.
BY BJORN CAREY
Stanford Report, May 6, 2013
All natural materials have a positive index of refraction – the degree to which they refract light. The nanoscale artificial "atoms" that constitute the metamaterial prism shown here, however, were designed to exhibit a negative index of refraction, and skew the light to the left. Technology that manipulates light in such unnatural ways could one day lead to invisibility cloaks.
One of the exciting possibilities of metamaterials – engineered materials that exhibit properties not found in the natural world – is the potential to control light in ways never before possible. The novel optical properties of such materials could lead to a "perfect lens" that allows direct observation of an individual protein in a light microscope or, conversely, invisibility cloaks that completely hide objects from sight.
Although metamaterials have revolutionized optics in the past decade, their performance so far has been inhibited by their inability to function over broad bandwidths of light. Designing a metamaterial that works across the entire visible spectrum remains a considerable challenge.
Now, Stanford engineers have taken an important step toward this future, by designing a broadband metamaterial that more than doubles the range of wavelengths of light that can be manipulated.
The new material can exhibit a refractive index – the degree to which a material skews light's path – well below anything found in nature.
"The library of refractive indexes that nature gives us is limited," said Jennifer Dionne, an assistant professor of materials science and engineering and an affiliate member of the Stanford Institute for Materials and Energy Sciences at SLAC National Accelerator Laboratory. "All natural materials have a positive refractive index."
For example, air at standard conditions has the lowest refractive index in nature, hovering just a tick above 1. The refractive index of water is 1.33. That of diamond is about 2.4. The higher a material's refractive index, the more it distorts light from its original path.
Really interesting physical phenomena can occur, however, if the refractive index is near-zero or negative.
Picture a drinking straw leaning in a glass of water. If the water's refractive index were negative, the straw would appear inverted – a straw leaning left to right above the water would appear to slant right to left below the water line.
In order for invisibility cloak technology to obscure an object or for a perfect lens to inhibit refraction, the material must be able to precisely control the path of light in a similar manner. Metamaterials offer this potential.
Unlike a natural material whose optical properties depend on the chemistry of the constituent atoms, a metamaterial derives its optical properties from the geometry of its nanoscale unit cells, or "artificial atoms." By altering the geometry of these unit cells, one can tune the refractive index of the metamaterial to positive, near-zero or negative values.
One hitch is that any such material needs to interact with both the electric and magnetic fields of light. Most natural materials are blind to the magnetic field of light at visible and infrared wavelengths. Previous metamaterial efforts have created artificial atoms composed of two constituents – one that interacts with the electric field, and one for the magnetic. A drawback to this combination approach is that the individual constituents interact with different colors of light, and it is typically difficult to make them overlap over a broad range of wavelengths.
As detailed in the cover story of the current issue of Advanced Optical Materials, Dionne's group – which included graduate students Hadiseh Alaeian and Ashwin Atre, and postdoctoral fellow Aitzol Garcia – set about designing a single metamaterial "atom" with characteristics that would allow it to efficiently interact with both the electric and magnetic components of light.
The group arrived at the new shape using complex mathematics known as transformation optics. They began with a two-dimensional, planar structure that had the desired optical properties, but was infinitely extended (and so would not be a good "atom" for a metamaterial).
Then, much like a cartographer transforms a sphere into a flat plane when creating a map, the group "folded" the two-dimensional infinite structure into a three-dimensional nanoscale object, preserving the original optical properties.
The transformed object is shaped like a crescent moon, narrow at the tips and thick in the center; the metamaterial consists of these nanocrescent "atoms" arranged in a periodic array. As currently designed, the metamaterial exhibits a negative refractive index over a wavelength range of roughly 250 nanometers in multiple regions of the visible and near-infrared spectrum. The researchers said that a few tweaks to its structure would make this metamaterial useful across the entire visible spectrum.
"We could tune the geometry of the crescent, or shrink the atom's size, so that the metamaterial would cover the full visible light range, from 400 to 700 nanometers," Atre said.
That composite material probably won't resemble an invisibility cloak like Harry Potter's anytime soon; while it could be flexible, manufacturing the metamaterial over extremely large areas could be tricky. Nonetheless, the authors are excited about the research opportunities the new material will open.
"Metamaterials will potentially allow us to do many new things with light, things we don't even know about yet. I can't even imagine what all the applications might be," Garcia said. "This is a new tool kit to do things that have never been done before."
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