White Light Black Light

by Lisa Grossman

AN ILLUSION device that makes one object look like another could one day be used to camouflage military planes or create "holes" in solid walls.

The idea builds on the optical properties of so-called metamaterials, which can bend light in almost any direction. In 2006, researchers used this idea to create an "invisibility cloak" that bent microwaves around a central cavity, like water flowing around a stone. Any object in this cavity is effectively invisible.

Now a group of researchers has gone a step further. "Invisibility is just an illusion of free space, of air," says Che Ting Chan, a physicist at the Hong Kong University of Science and Technology and a co-author of the study. "We are extending that concept. We can make it look like not just air but anything we want."

Instead of bending light around a central cavity, the team has worked out the mathematical rules for bending light in other ways. For example, a material could be designed to bend light in the same way as a spoon would. So the light hitting the material would be distorted to make it look as if a spoon were there.

It is also possible to design a complementary material that has the opposite effect - to exactly cancel out the effect that an object has on light. So light distorted by a spoon could be passed through a complementary material to eliminate these distortions.

The new illusion device uses these two ideas together. To make a cup look like a spoon, for example, light first strikes the cup and is distorted. It then passes through a complementary metamaterial which cancels out the distortions to make the cup seem invisible. The light then moves into a region of the metamaterial that creates a distortion as if a spoon were present. The result is that an observer looking at the cup through the metamaterial would see a spoon (Physical Review Letters, vol 102, p 253902).

The idea has some surprising applications. Chan says the technique could be used to change the optical properties of an opaque material, allowing light to tunnel from one side to the other. That could lead to a device that when attached to a wall, creates a "hole" through which viewers could see the other side.

But although illusion cloaks are theoretically sound, there are numerous engineering challenges to overcome before they might become practical.

Metamaterials are difficult to build. Their components need to be much smaller than the wavelength of the light they distort, which for visible light is less than a micrometre. It would also be challenging to design both parts of the device so that they don't interfere with each other.

John Pendry, the physicist at Imperial College London who devised the theory behind invisibility cloaking in the 1990s, thinks these difficulties can be overcome. "I don't see any obstacles to this device being built," he says.