seeing double A calcite crystal laid on a paper causes double refraction. The large calcite crystal is at the National Museum of Natural History.
From H. G. Wells to Harry Potter, the idea of making things invisible has been a staple of science fiction and fantasy literature. In recent years several teams have shown that an “invisibility cloak” could be possible in the real world, at least in limited applications. But most such concepts have relied on exotic nanostructured, layered synthetics that could keep only microscopic objects from being detected—and only by certain specific frequencies of radiation, such as microwaves.
Now, a team of researchers has made a big leap toward realizing the sci-fi dream. George Barbastathis and others at the Singapore-MIT Alliance for Research and Technology (SMART) Centre reported in Physical Review Letters a relatively simple system that can hide an object as thick as a peppercorn from view under ordinary visible light.
The method uses calcite—a crystalline form of calcium carbonate, the main ingredient in seashells. “Very often, the obvious solution is just sitting there,” says Barbastathis. Calcite has unique optical properties, including the ability to bend (or refract) a ray of light differently depending on the light’s polarization (the orientation of its electric field). These properties can cause double refraction—a viewer sees “doubles” when looking through calcite under regular unpolarized light, which contains a random mixture of differently polarized rays.
In the experiment, the researchers hid a two-millimeter-thick, 38-millimeter-wide metal wedge by placing it on a flat horizontal mirror beneath two pieces of calcite with opposite orientations, glued together along their adjoining edges like tiles on a floor. Under visible light, from a certain direction, it looked as if there was nothing at all on top of the mirror—the light emerging from top of the calcite appeared to be coming directly from the base of the setup. The angle and path length of the emerging light beam were also the same as they would have been if reflected from the mirror itself, which means that no optical instrument could have revealed the cloaked wedge. The researchers had to submerge the apparatus in liquid with a carefully chosen degree of refraction to preserve the illusion. Otherwise, the light’s transition into air would have caused some blurring that made the effect less convincing.
Barbastathis says the same method could someday be used in real-life situations to conceal an object up to the size of the calcite crystal used; the largest known natural calcite crystal measures more than 21 feet across. For now, it’s just two-dimensional, limiting the cloaking effect to only a very narrow range of viewing angles. But the work has just begun. “We do have some ideas for how to make it fully three-dimensional,” he says.