Researchers have found that light can slip through holes up to 10 times narrower than its wavelength. The finding, reported in tomorrow's issue of Nature, contradicts the traditional view that light can efficiently pass through holes only if they are at least as wide as its wavelength; in smaller holes the waves should collide with the walls and get absorbed. This unexpected effect could open the way to new kinds of optical filters, the researchers say.
The research team had a different purpose in mind when they deposited thin films of silver on quartz and pierced them with ordered arrays of holes just 150 nanometers--about 1000 atom widths--in diameter. "We were looking for quantum optic effects," such as how these tiny holes might trap or alter shorter wavelength light, says team leader Thomas Ebbesen of the NEC Research Institute in Princeton, New Jersey, and Louis Pasteur University in Strasbourg, France. "After making lots of these holes, we just checked transmission properties." As expected, wavelengths shorter than the diameter of the holes passed through easily--but so did light at much longer wavelengths, such as about 1500 nanometers. "We got very excited, of course," says Ebbesen.
A series of further experiments pointed to an explanation. The light apparently rides through the holes on an electromagnetic shuttle: oscillating magnetic fields in the silver film called surface plasmons. One clue was the group's discovery that light does not pass through similar holes in nonconducting materials, such as germanium. In metals, however, electrons are mobile, so light striking the surface near a hole can set the electrons oscillating. This dance of electrons creates plasmons, which travel through the hole and somehow reconstitute the light on the far side of the film.
Because the effect works only for certain wavelengths, depending on the size and spacing of the holes, these tiny sieves could serve as light filters, says Ebbesen. Roy Sambles of the University of Exeter in the U.K., who is so impressed by the discovery that he plans to pursue it himself, adds that it should be possible to create a tunable filter by adding a coating of liquid crystals to the metal film. When an electric field causes the liquid crystals to line up, they should affect the plasmons and hence the wavelength they transmit--an effect that might be harnessed in technologies for communicating or computing with light. "Having the voltage controllability would be quite exciting," says Sambles.