Flooding light through tiny holes.

A silver coating serves as an excellent reflector of visible light. A thin, perforated silver film deposited on quartz, however, can be remarkably transparent, even when the openings are considerably narrower than the light's wavelength.

Researchers have observed that a regular array of microscopic holes penetrating a thin metal layer lets through a surprisingly large amount of light in certain wavelength ranges. Photons appear to traverse the apertures much more easily than theorists would have expected.

"We weren't looking for this," says Thomas W. Ebbesen of Louis Pasteur University in Strasbourg, France. "It's a surprisingly strong effect." Ebbesen and his collaborators report their findings in the Feb. 12 Nature.

The researchers used metal films, from 100 to 500 nanometers thick, with holes 150 nm in diameter and from 0.6 to 1.8 micrometers apart. The holes were arranged in square or triangular grids. BaThed in light ranging from 200 to 1,200 nm in wavelength, the perforated metal films selectively transmitted light with wavelengths up to five times the aperture diameter. "What really matters is the spacing between the holes," Ebbesen says.

The phenomenon apparently involves plasmons--mobile pockets of oscillating electromagnetic fields associated with quantities of electric charge sloshing around on a metal's surface. The presence of a regular array of holes disturbs the motion of these plasmons, causing them to interact strongly with the electromagnetic fields of photons striking the metal surface.

In effect, incoming light is absorbed via surface plasmons, then reradiated out the holes. "What's surprising is that about 1,000 times more light gets through than you would expect, at least according to [conventional] theory," Ebbesen says.

In addition, there's a lensing effect, he notes. Some of the light that hits the metal around the holes gets channeled by the plasmons through the holes instead of being reflected.

By controlling such factors as hole spacing, comments Roy Sambles of the University of Exeter in England, it may be possible to create optical filters that selectively transmit certain wavelengths with no diffractive effects.