There’s been a lot of excitement, both in the scientific community and in the popular media, about the possibility of creating cloaking materials that make people or military vehicles appear to vanish. But that goal seemed nearly impossible until Baile Zhang came up with a simple, promising solution. While Zhang’s technique has serious limitations—for one thing, it works only in an exotic medium called laser oil—it does suggest a possible path to making practical invisibility cloaks.
Most previously developed invisibility cloaks were made with materials painstakingly fabricated in the lab to have micro- or nanoscale patterns that bend light waves. But labs couldn’t turn out more than tiny amounts of these materials. What’s more, most existing examples of cloaking materials work only with microwaves and other nonvisible forms of light.
Reading about these exotic materials, Zhang, a professor at the Nanyang Technological University in Singapore, remembered a high-school physics demonstration of how calcite, an inexpensive natural mineral, bends light in strange ways. That, in turn, led him to come up with a simpler way to make a large cloak: gluing two pieces of calcite together.
Zhang demonstrated that his calcite sandwich could hide the middle section of a Post-it note rolled into a tube and placed on a mirror submerged in a liquid. The calcite cloak on top of the tube guides light from the space behind the tube to a point directly over it, so that the eye is, in effect, seeing right “through” the rolled-up paper. It turns out calcite’s crystal structure already resembles the sorts of artificial nanoscale patterns that other labs have been struggling to fabricate with electron beams.
“This shows better than any other experiment that the basic concept of cloaking can work,” says Steven Cummer, an engineering professor at Duke University, who was on the team that made the first cloaking device. But Cummer cautions that Zhang has a lot of work ahead to make this simple cloak more practical.
Right now the calcite trick works only if the medium around it helps to bend the light, which means the medium has to have just the right refractive index. The bath of laser oil used for the initial demonstration did the trick, but water or air won’t work.
Zhang is hoping, however, that some new tricks he has in mind will allow the cloak to work in air. That’s a project worth keeping an eye on.
—Katherine Bourzac
