New approach can conceal objects big enough to see
Professor Snape beware — invisibility cloaks aren’t just for the microscopic anymore.
Using natural crystals, two independent research teams have designed “carpet cloaks” that can abracadabra 3-D objects as big as an ant or a grain of sand seemingly into nothing. Up to now, making things invisible has relied on tiny structures called metamaterials. These fabrications are often a mix of stacks and crisscrosses of nano-sized metals and other materials that can guide electromagnetic rays, such as microwaves or infrared and visible light, around objects. If researchers tweak metamaterials just right, they can make tiny things disappear — at certain light wavelengths and from certain angles, at least.
But now two teams, including an MIT group that published its results online in Physical Review Letters January 18 and another from England and Denmark that published online February 1 in Nature Communications, didn’t bother with metamaterials. They adopted calcite prisms, a type of naturally occurring crystal, to build carpet cloaks. Carpet cloaks aren’t true now-you-see-them-now-you-don’t apparatuses. The bottom of the cloaking device is notched with a small triangle that looks like a bent mirror. Thanks to the optical properties of metamaterials or, in this case, calcite, the bent mirror can look like a flat plane when viewed from some angles. Anything hiding in the notch vanishes.
This low-tech design sidesteps some of the limitations of metamaterial invisibility cloaks, says Ulf Leonhardt, a physicist at the University of St. Andrews in Scotland who was not involved in either study. His landmark 2006 paper in Science helped to launch invisibility research. Because metamaterials require intricate sculpting by lasers or other tools, scientists can make them only so big. Harry Potter would need to be more than paper-thin to hide under early carpet cloaks. The calcite shields, on the other hand, can disappear objects 1 to 2 millimeters tall. Metamaterial designs “liberated the imaginations,” he says. “Now, it’s time to come back to reality.”
But with such tricky optical sleight of hand, reality may seem like a misnomer. With the right type of light, calcite prisms can bend laser beams in different directions based on the crystal’s orientation. Light enters the cloak — a triangle or trapezoid made of two prisms glued together — and bounces off the bent mirror at the bottom into the second prism, then out. By the time the beams leave the cloak, they look like they changed direction only once, says George Barbastathis of MIT, coauthor of the Physical Review Letters article. His team used the cloak to hide a small metal wedge. “Putting calcite on top of the wedge, the light goes back into the same direction that it would have with a flat mirror,” he says. But it’s not just the same direction — the light looks exactly like it bounced off a flat mirror. The metal wedge vanishes.
“It’s not a Harry Potter cloak,” says Shuang Zhang, a physicist at the University of Birmingham in England and one of the Nature Communications study coauthors. The cloak works only under one light polarization. And while it works at all angles, it’s not three-dimensional. It only cloaks when Zhang aims the light source dead-on at the crystals. But, he says, scaling up to 3-D isn’t too big of a leap from 2-D. Zhang imagines similar technology one day concealing submarines on the sea floor.
Leonhardt says the future of optical legerdemain lies not in hiding things, but in revealing them. He uses the same geometric tools to design better microscopes. “We use similar ideas not to make things disappear but to make them visible,” he says.
Now that’s something Professor Snape could get behind.
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