Photons Squeeze Through Tiny Holes

Aaron J. Hand

Ask most chip-makers today what they think about using photolithography for submicron components, and they'll say it has reached its limits. The small mask openings required to make equally small features do not allow the necessary light to pass through, since everyone knows light cannot transmit effectively through a hole smaller than its wavelength.
They might be surprised to know that physicists have watched light transmitted perfectly through a hole as much as 10 times as small as the light's wavelength.
Researchers led by Thomas Ebbesen of the Louis Pasteur University in Strasbourg, France, and the NEC Research Institute in Princeton, N.J., stumbled upon this phenomenon while testing the optical properties of submicron holes. Essentially, the light squeezes through the holes when it couples with plasmons (electronic excitations) on the surface of a patterned metal film. When the energy and momentum of the photons match the energy and momentum of the plasmons, the photons are absorbed and radiated again on the other side.
Although plasmons are present in any given surface, without the holes they could not couple with the photons. In turn, changing the grating pattern -- hole size and spacing, as well as the lattice shape -- produces different spectra. Ebbesen and his team tested all variables, including hole diameter, space between holes, and the thickness and type of metal. Hole diameter was varied from 150 nm to 1 µm, and hole spacing from 0.6 to 1.8 µm. The plasmon coupling was not effective if the surface was not metallic, so experiments included the use of silver, gold and chromium films.
The researchers came up with some surprising results. The transmission efficiency was 1000 times as high as expected for a single aperture -- as much as 200 percent when normalized to the hole area. In other words, even some of the light that did not impinge on the holes was transmitted.
The photonics industry will no doubt be able to put this discovery to good use, but it is too soon to know the complete range of possibilities. "In a couple of years, we should know how many potential applications there are," Ebbesen said.
Ultimately, the discovery may enable new technologies in such instruments as subwavelength photolithography, or near-field scanning optical microscopes, where developers could make use of enhanced light transmission through tiny holes.