LIGHT can squeeze through what seem like impossibly small holes, physicists
have found. They say that the frequency of light transmitted through a grid of
holes can be changed by altering the pattern, and that the effect could
eventually be used in devices that control light, such as optical computers.
A team led by Thomas Ebbesen of the Louis Pasteur University in Paris and the
NEC Research Institute in Princeton, New Jersey, made the discovery by chance
when they set out to make 鈥渜uantum cavities鈥� in a glass-backed metal film. To
check the quality of the cavities鈥攈oles bored through the metal to the
glass鈥攖hey illuminated them with a range of wavelengths.
鈥淭o our astonishment, we saw light transmitted with 100 per cent efficiency
at a wavelength 10 times bigger than the diameter of the hole,鈥� says Ebbesen.
Theory has suggested that the photon has an effective size roughly equal to its
wavelength. 鈥淪o it should not be able to squeeze through a smaller hole,鈥� says
Ebbesen.
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The researchers have now carried out more experiments using a metal film
riddled with millions of holes about 150 nanometres across and 0.5 micrometres
apart. They say in this week鈥檚 Nature (vol 391, p 667) that there are
peaks of transmission at various wavelengths, and these change if the hole
spacing and arrangement changes.
Ebbesen suggests that the light manages to squeeze through the holes thanks
to 鈥減lasmons鈥濃�攖he natural sloshing motion of electrons in the surface of
the metal. If the frequency of incident photons matches that of a plasmon, the
photons are absorbed by the electrons on one side of the film and reradiated on
the other, yielding perfect transmission. But depending on their positions, the
holes hinder plasmons at certain energies. 鈥淭his is why we get different
transmission peaks for different spacings and arrangements of holes,鈥� says
Ebbesen.
Oddly, the team have even observed 200 per cent transmission鈥攖wice as
much light emerging on the far side of the film as impinges on the holes. 鈥淭his
occurs because even the light which falls between the holes can excite plasmons,
producing light on the far side of the film,鈥� says Ebbesen.
The team says that the phenomenon might one day find a use in 鈥減hotonic鈥�
devices that control light. It could also overcome a fundamental limitation of
photolithography, the technique used to make silicon chips. Light shining
through a mask casts the shadows of electronic components on a substrate.
Unfortunately, making components smaller than the wavelength of the light
requires correspondingly small openings in the mask and light will not pass
through them. 鈥淗owever, we have shown we can get 1000-nanometre light though
150-nanometre holes,鈥� says Ebbesen.
