According to the Duke team, this experiment shows it should be possible to make an object invisible to the human eye as well, but there are major technical hurdles. For cloaking to work, the metal shapes stamped on the metamaterial must be smaller than the wavelength of the electromagnetic radiation that is aimed at them. The wavelength of the microwaves is a little over 3 centimeters (just over an inch), and the shapes on the surface of the metamaterials are closer to 3 millimeters long. Green light, by contrast, has a wavelength of 500 nanometers—60,000 times smaller—so the shapes that could cloak it would have to be around 50 nanometers long. Theoretically, you could pattern metamaterials at that tiny scale using specialized methods like focused beams of charged atoms, but such materials would be difficult to mass-produce.
At this point, then, cloaking objects from visible light is still pie in the sky. In the meantime, the far more accessible applications of microwave cloaking have already garnered intense interest—mainly from the military. Smith is up-front as he rattles off their funding sources: DARPA (the Defense Advanced Research Projects Agency); the Air Force, the Army, the Navy, the intelligence community. One of the technique's most practical and immediate uses would be to hide obstructions that block wireless communication. But since Smith and Schurig's technique bends electromagnetic radiation in a controlled manner, it could someday also be co-opted to focus or concentrate energy in highly efficient ways. For example, it could be used to create supersensitive solar cells or even to power a Mars rover that would gather energy from a microwave beam sent by a satellite orbiting the Red Planet.
The Duke researchers are not the only ones scrambling to create cloaking devices. When their theory first appeared in the May 26 edition of Science Express, it was published alongside an independent article that outlined a similar proposal. The author of that paper, theoretical physicist Ulf Leonhardt of the University of St. Andrews in Scotland, proposed using slightly different types of engineered materials to accomplish the trick. A few weeks before that, a pair of math-loving physicists, Graeme Milton of the University of Utah and Nicolae Nicorovici of the University of Sydney in Australia, came up with yet another, drastically different scheme for making objects the size of dust specks invisible.
The Milton-Nicorovici hypothesis, which is based on rigorously proved mathematical calculations, relies on the use of a superlens, a thin transparent film that can resolve light finer than its wavelength (long considered a theoretical impossibility), producing extremely sharp images. A superlens made from a thin film of silver could have a negative index of refraction, bending light outside of its normal path. "What we found was that if you put a speck of dust near the superlens and shine light on the dust, then part of the scattered light gets trapped at the front surface of the superlens," Milton explains. "That trapped light builds up in intensity until it almost exactly cancels the incoming light," in the same way that two colliding sound waves can zero each other out. It is as if there is no light there at all, and the dust particle becomes invisible. (For an action-packed movie of this phenomenon, see Milton's Web site at www.physics.usyd.edu.au/cudos/research/plasmon.html.)
