The first studies calculated the optical properties needed for an invisibility cloak to work for a single color of light.
One involved "plasmonic shielding"—a coating that would prevent light from bouncing off a small object, making the object appear so small it all but disappears.
In 2006 more studies—including one by Leonhardt—showed how to make light bend around an object so that the object appears not to be there. This scheme, however, could only work for a single wavelength or color of light.
Later that year a team including David Schurig of North Carolina State University built a cloak that rendered an object nearly invisible to microwaves.
In 2007 a team from Purdue University published a study showing how to make an object invisible with visible light, and in 2008, Duke University's Steven Cummer's team calculated how it could be done with sound waves.
(Related: "Invisibility-Cloak Materials Bend Light 'Backward' [August 12, 2008].)
Invisibility cloaks are "feasible now," new study author Leonhardt said. "That's the bottom line."
Steven Cummer of Duke University is an engineering professor who did not participate in the new study.
Leonhardt and colleagues made an important advance, he said, because prior cloaking designs worked only for one single color of light at a time.
But it's still an imperfect cloak, he added.
One problem, he said, is that the phase of the light changes as it passes thought the cloak.
"This means that the light has been scrambled a bit in traversing the cloak. This isn't perfect invisibility, but it may work for some applications."
North Carolina State's Schurig sees the same problem.
"Nothing that remains in our universe can be completely undiscoverable," he said by email.
"Every cloaking scheme has some vulnerability."
For one, the cloak can't just be a garment. It has to be larger than what it hides—a problem if you're trying to hide an airplane.
Nor can the cloaks called for by Leonhardt's calculations simply be spheres or cylinders.
"The shapes are pretty crazy," Duke University's Cummer said.
"This probably makes them harder to build. Frankly, I think that we will see a single-color 'traditional' cloak before a multicolor cloak based on this approach.
"But it is definitely valuable to have a recipe for an imperfect multicolor cloak."
Schurig added that these new designs might be hard to implement because in different parts of the cloak, the material must either speed up or slow down light significantly.
From an engineering perspective, he said, "the extreme parts of the material specification may be very difficult or impossible to achieve over the entire visible spectrum. In principle this cloak could be broadband, but it's material requirements might preclude it from being so."
(Explore an interactive on the power of light.)
More Visible, Not Less
Ironically, the first applications of the invisibility technology might make objects more visible, rather than less visible.
That's because the same light-bending properties that would create an invisibility shield could also be used in reverse.
Thus, scientists could use the method to create anything from better reflectors for nighttime cyclists to radar beacons for orbital satellites.
Though a bit farther off, the method may also sharpen transmission of communications from wireless devices such as cell phones and computer wi-fi servers.
SOURCES AND RELATED WEB SITES