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Sharper than expected

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11:15am, December 5, 2007
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A new technique beats the resolution limits of ordinary microscopes in a way that seems to defy conventional optical theory, its discoverers say.

Peter Stark of Harvard Medical School in Boston and his collaborators exposed a photosensitive film to light coming from microscopic holes in a thin silver plate. Ordinarily, those images would record no details smaller than half the wavelength of light, a fact that limits the resolution of optical microscopes to about 200 nanometers. Instead, the researchers' 60-nm-wide holes were in perfect focus.

That's because the team shined light from the holes' rims rather than through the holes, Stark says. The researchers pointed a violet laser straight at the silver plate, where the electric field of the laser's photons interacted with the metal's electrons and formed perturbations called surface plasmons. The plasmons traveled on the silver's surface. At the holes, some of their energy converted back into photons, which then created an image. Stark speculates that these photons were acting like particles rather than waves—a behavior that would be expected only over a longer range of distances. The team's findings appear in the Nov. 27 Proceedings of the National Academy of Sciences.

Researchers have circumvented the diffraction limit before. So called near-field probes pick up light within 5 nm from the sample, a range where diffraction effects are negligible, Stark explains. However, that process is very slow, since a probe must scan a sample pixel by pixel.

Stark says the new technique might allow scientists to quickly image living cells or other samples in great detail. He also says that it might lead to less-expensive alternatives to microchip fabrication technologies that use ultraviolet light to etch features that are only a few tens of nanometers wide.

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