The day when an invisibility cloak hangs in the closet of every home may not be far off. A small, $1,000-device that can render even large objects invisible was created by scientists at the Singapore-MIT Alliance for Research and Technology (SMART) Centre.
The invisibility cloak is made from calcite crystal and uses optical anisotropy to give the illusion of seeing through it.
“By ‘deforming’ the space, we can make the light ray emerge after reflection from the wedge at exactly the same orientation and position as if it had been reflected from a flat mirror,” said George Barbastathis, professor of optics and mechanical engineering who is working on the project, in an e-mail interview.
Barbastathis and his team adhered two pieces of calcite together in a “mirror-like configuration” to create the illusion. “One could use different materials, but calcite offers the best combination of exhibiting this high anisotropy and at the same time being widely available and relatively cheap,” he said.
An image shows the cloak in action. It covers a bright pink piece of paper immersed in “laser oil,” which is rendered invisible in the spot covered by the calcite, including its color.
The technology is still not perfect, however.
“The main limitation at the moment is that it is two-dimensional; i.e. the cloaking effect is noticeable on one place only, whereas from the side the wedge is quite clearly visible,” Barbastathis said.
The cloak is also less effective in nonliquid environments. According to Barbastathis, using laser oil or water “brings the cloaking operation closer to perfect,” while the effect is less obvious when exposed to air.
“But again, all this is based on what we know at the moment, and this is an active field of research so we can be surprised,” he said.
Although the technology may not yet be mature enough to have practical use, it is a leap forward for researchers. Scientists have delved into invisibility technology for years, yet this is the first natural invisibility cloak that works on objects outside the micro-scale.
German scientists were able to render a piece of gold invisible from all angles using porous crystals in 2010, yet were only able to accomplish the illusion with microscopic objects. There is also a Japanese scientist who is developing “optical camouflage,” yet it relies on cameras to project an image of what is behind it.
“At the moment, we (and other research groups around the world) are still figuring out the basic physics of cloaking,” Barbastathis said.
His current interest comes from the elegance and challenge of the math and physics involved in trying to bend light around an object. He adds that while researchers have learned quite a bit about the physics of light by working on invisibility cloaks, for himself as a scientist and engineer, “it is very exciting to push the limits of how we can make light behave in strange ways, ways that before had been thought impossible.”
The practical uses of an invisibility cloak are varied. Aside from the military and security uses, Barbastathis suggests they could be used for anything from toys to interior design, yet is quick to add that it’s all just speculation, as once the technology hits the market there’s no telling what companies could come up with.
“History has shown that, when science gets as exciting as this, ample applications follow to benefit people in their every day lives, even if right now we are not sure exactly what these applications might be,” Barbastathis said.
“Commercial applications have a way of surprising us when the real industry gets involved,” he said.