A universal mirror, an object that reflects all light waves back at their source, has been created by scientists in Europe and Asia.
Imagine a tennis player hitting a ball against a wall. The ball would bounce right back to the player no matter what angle he or she directed the shot. A universal mirror has the same effect, except with light waves.
Unlike an ordinary mirror, which only reflects objects at 90 degrees, a universal mirror reflects objects back at any angle. In other words, a person positioned in front of a large, optical universal mirror would see his or her own reflection perfectly no matter where the person stands.
"(A universal mirror) makes things become very visible," said Ulf Leonhardt, a professor at the University of St. Andrews and co-author of a paper in the current issue of Nature Materials. "It's the exact opposite of an invisibility cloak."
Unlike a universal mirror, an invisibility cloak guides light waves around an object in order to conceal it. Although universal mirrors and invisibility cloaks might perform opposite functions, they each employ the same technology: metamaterials.
While the properties of normal materials are predominantly determined by their chemical composition, metamaterials are artificial materials that derive their properties from their physical structures.
Manipulating light waves
Universal mirrors and invisibility cloaks both manipulate light waves by using tiny structures much smaller than the wavelength of light itself. This capability is engineered using the metamaterials from which universal mirrors and invisibility cloaks are created.
Creating structures as small as metamaterials is a difficult task, which is why both invisibility cloaks and universal mirrors can only handle relatively long microwaves. Structures that perfectly manipulate shorter wavelengths of light — wavelengths the human eye can actually see — have yet to be produced.
The universal mirror, or omnidirectional retroreflector as it's called in the Nature Materials paper, is about one centimeter high (0.4 inches), 10 centimeters (four inches) in diameter, and made of copper circuit boards covered in circles three millimeters (0.1 inches) across.
When microwaves three centimeters (1.2 inches) long hit the small circles, they are forced backward at the same angle from which they came.
When microwaves hit most other materials, they bounce forward at the same angle.
"This is really only the second time that somebody has successfully attempted and built a device based on transformation optics," the field of research that works on metamaterials, said Steve Cummer, a professor at Duke University who helped developed the first transformation optics device, a microwave invisibility cloak, back in 2006.
"How they designed the device is pretty interesting; they used basically the same approach we used for an invisibility shell, but to do something entirely different than a cloaking shell," said Cummer.
A universal mirror would serve a variety of purposes. Installed on aircraft, boats or satellites, a universal mirror would makes these objects easier to track with radar. When radio waves ordinarily hit these objects, they scatter in many different directions, and only a few radio waves bounce back to the original source of the radar.
With a radio universal mirror, all the radio waves would bounce back to their original source, making them much easier to detect and giving the object a much larger radio profile.
The universal mirror would also have military applications. Many munitions use laser beams to lock onto a target. But if the laser beam hit, say, a tank covered in this material, the laser beam wouldn't be able to lock on to the target, because the beam would simply bounce off it.
The metamaterial could also act like a aggressive shield, protecting objects from airplane-based, high-energy laser systems, which are being developed by Boeing, by bouncing the lasers beam back at their source.
Those capabilities are still years away, but one of Leonhardt's collaborators, Aaron Danner at the National University of Singapore, is working with photonic crystals to extend the range of the universal mirror down into the visible range.