Perfect mirror debuts

Material that reflects light without letting any escape could improve lasers

A new type of mirror consists of a slab of silicon nitride drilled with tiny holes (illustrated). The holes and the gaps between them are smaller than the wavelength of visible light.

Bo Zhen

A new type of mirror that reflects light perfectly has been constructed, a feat many scientists thought wasn’t possible. The mirror could find its way into powerful lasers and other devices.

Modern technology relies on trapping light and shuttling it around, whether to carry data over the Internet or to play a DVD. Engineers prevent light from escaping by directing it to bounce off reflective materials, but there are drawbacks to that process. Many mirrors absorb, rather than reflect, some of the light that strikes them, and the glass in fiber-optic cables works only if light grazes it at a very low angle.

Physicist Chia Wei Hsu and colleagues at MIT weren’t looking to invent a mirror when they were studying the behavior of light interacting with a photonic crystal, a slab of material with a network of drilled holes, each so small that it can manipulate individual light waves. Most of the time, light penetrated at least partially into the team’s crystal, a block of perforated silicon nitride. But when the researchers shined a specific frequency of red light at a 35-degree angle to the surface of the slab, they were surprised to find that it bounced back completely — none of it leaked away or got absorbed.

The cause of this quirky phenomenon, Hsu’s team reports in the July 11 Nature, is light interfering with itself. A bunch of combined factors — including the light’s wavelength, the angle at which it hits the surface and the pattern of drilled holes — orients the light so that waves trying to enter the slab cancel each other out. The waves’ only option is to get reflected.

Computer scientist and physicist John von Neumann proposed a similar phenomenon of trapped waves in 1929, but all previous attempts to demonstrate it experimentally had failed. “No one had thought of a practical method to do this,” says Douglas Stone, a physicist at Yale University.

Now that physicists know this mirror is very real, Stone says it may prove useful in lasers, which are concentrated beams of light at a single wavelength. Specially designed photonic crystals tuned to specific wavelengths could enable engineers to amp up the energy of lasers without sacrificing efficiency, he says.

In addition, Hsu notes that this mirror may have uses beyond optics. At least in theory, he says, crystals with distinct patterns of drilled holes should be able to perfectly reflect sound waves and even water waves.

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