Researchers have taken the next step on the road to constructing a cloak of invisibility or a powerful "superlens" capable of capturing fine details undetectable to current lenses. A group from the University of California, Berkeley, this week is publishing the first demonstrations of materials capable of bending visible or near-visible light the "wrong" way in three dimensions.
Both are examples of metamaterials—specially designed structures that cause light to do things it normally wouldn't—in this case, bending backward, an effect called negative refraction. Researchers have built metamaterials capable of negatively refracting microwaves, but despite some successes bending visible light in two dimensions, they've had a harder time making three-dimensional versions.
In a study to be published in Nature, the Berkeley group led by Xiang Zhang, bent red light using a fishnet-shaped stack of 21 layers of silver and magnesium fluoride, each a few tens of nanometers thick (see diagram). (One nanometer is a billionth of a meter.) The group will also report in Science that it bent near-infrared light using a thinner sheet of aluminum oxide containing silver nanowires. The researchers believe the second material ought to work on red light as well.
Both devices absorbed relatively little of the incoming light—a problem in earlier metamaterials, the group says.
In school we learned that a beam of light passing from air to water or glass at a shallow angle will slow down and bend away from the surface of the denser medium it passed through. On the way out, that angle shrinks again. The result: A straw in a glass of water takes on a zig-zag shape as seen from outside.
But this only holds true for materials that have a positive index of refraction—a measure of the speed of light in a material. The new metamaterials both exhibit a negative index of refraction. A straw placed in a glass of negative-index material would look like a ">".
One potential application of negative refraction is a superlens capable of picking up fine details in reflected light and magnifying them—another area where Zhang's group has had some success.
For invisibility, researchers need their metamaterials to have an index less than one (the index of air). This makes it possible to channel light around a region like air around an airplane wing. No light inside means there is no reflection to reveal the contents of the space, hence, invisibility.
In 2006 a group at Duke University demonstrated partial cloaking in two dimensions with a pizza-size disk of copper rings. Look for researchers to try that soon with visible light.
Image credit: J.Valentine et al.