Forget about fancy metamaterials that can make microscopic objects invisible–researchers at two different universities have independently shown that larger objects can be rendered invisible using a mineral that’s both naturally occurring and common: calcite.
This latest step in physicists’ ongoing quest to create an invisibility cloak come from an MIT lab, with a paper published in Physical Review Letters, and a University of Birmingham lab, whose paper just came out in Nature Communications. Both teams explained that they used calcite to make objects that are large enough to be seen with the naked eye invisible.
“By using natural crystals for the first time, rather than artificial metamaterials, we have been able to scale up the size of the cloak and can hide larger objects, thousands of times bigger than the wavelength of the light,” said Shuang Zhang, the University of Birmingham physicist who led the research…. “This is a huge step forward as, for the first time, the cloaking area is rendered at a size that is big enough for the observer to ‘see’ the invisible object with the naked eye.” [BBC]
The researchers constructed their cloaks from two glued-together calcite crystals, which have a convenient optical property called birefringence–that means they can bend a ray of light in two different directions. Then they placed the objects to be concealed in a notch beneath the crystals.
First came the formulation of an invisibility cloak that could bend light around an object. Then, this spring, German scientists took that idea and made it three-dimensional. Is the invisibility cloak now ready to go 4D? For a study in the Journal of Optics, British researcher Martin McCall’s team adds the dimension of time to the invisibility cloak idea, creating a theoretical “space-time cloak.”
The key feature of the proposed space–time cloak is that its refractive index — the optical property that governs the speed of light within a material — is continually changed, pulling light rays apart in time. When the leading edge of a light wave hits the cloak, the material is manipulated to speed up the light, but when the trailing edge hits, the light is slowed down and delayed. “Between these two parts of the light, there will be a temporal void — a space in which there will be no illuminating light for a brief period of time,” explains McCall. [Nature]
Taking advantage of these differences, he says, it is theoretically possible to imagine a cloak that allows you—at least from my point of view—to transport instantaneously across space.
It’s become one of our favorite rituals: Researchers come out with a paper pushing the science of invisibility cloaks a little further, inspiring everyone to go giddy with visions of Harry Potter and Romulan Warbirds. This week’s study in Science is another small step, but it’s a crucial one. Scientists in Germany have created the first rudimentary “invisibility cloak” in 3D.
Invisibility cloak mania started in 2006, when a Duke University team created the technology to bend light waves around an object; since the tiny object neither absorbed nor reflected the experiment’s microwaves, it was essentially “cloaked.” (The researchers used microwaves instead of visible light because microwaves have longer wavelengths, and are therefore easier to control.) The invisibility excitement struck again two years later when researchers refined their technique to hide a nanoscale object from visible light waves.
Now, researchers have created a cloak that not only works in infrared light wavelengths that are close to humans’ visual range, but also in 3D, too. Previous devices have been able to hide objects from light travelling in only one direction; viewed from any other angle, the object would remain visible [BBC News].
The team from Karlsruhe Institute of Technology didn’t exactly make the Statue of Liberty disappear. The “bump” made invisible is a spot in a layer of gold that’s 0.00004 inches high by 0.00005 inches wide. That hasn’t dampened lead researcher Tolga Ergin’s excitement, though. “In principle, the cloak design is completely scalable; there is no limit to it,” Ergin said. Developing the fabrication technology so that the crystals were smaller could “lead to much larger cloaks” [The Independent].
The sci-fi kind of cloaking will be harder to achieve, since visible wavelengths of light are shorter than infrared and thus harder to control. But Ergin’s 3D cloak is another step toward humanity’s ultimate dream: not being bothered by other humans.
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Researchers who created the first so-called invisibility cloak in 2006, have made significant advances that could lead to an invisibility cloak for visible light in as little as six months. “A large number of folks are looking at it, and I think it’s a matter of coupling the right material to the right device,” [Discovery News] said researcher David Smith. His team has developed an algorithm that speeds up the design of materials that can bend light around an object. Using the new algorithm, they were able to create an invisibility cloak that can bend much wider spectrum of microwaves than previous versions.
Invisibility cloaks rely on metamaterials, ones with unique properties that derive from [their] physical structure, not [their] chemical make up [Discovery News]. Smith compares the effect of metamaterials on light to mirages that appear over a road on sweltering days. “You see what looks like water hovering over the road, but it is in reality a reflection from the sky,” Smith said. “In that example, the mirage you see is cloaking the road below. In effect, we are creating an engineered mirage with this latest cloak design” [AFP].
Researchers at the University of California, Berkeley, announced yesterday that they were able to construct a prism that bent light “the wrong way” and so would make an object appear to vanish [Times UK]. Details of the two different experiments will be published separately later this week in Nature and Science.
To bend the light, the scientists used “metamaterials,” mixtures of metal and circuit board materials such as ceramic, Teflon or fiber composite [AP]. One group built what they called a metal “fishnet” of alternating silver and magnesium fluoride; the other used tiny silver nanowires. Both created negative refraction: Light is neither absorbed nor reflected by the objects, passing “like water flowing around a rock,” according to the researchers. As a result, only the light from behind the objects can be seen [BBC].