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Metamaterials make super lenses

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Scientists working in the US have developed a lens that could one day allow people to take optical pictures of things as small as a molecule of DNA.

Two groups have come up with different solutions to a problem that was first posed in 2000. Each has built a so called "magnifying super lens" using metamaterials, that is materials that have been carefully constructed on a nanoscale by a person.

Unlike any naturally occurring material, a metamaterial can have a negative refractive index, bending light in the opposite way from a normal ground glass lens. It is this characteristic that might allow optical pictures of the utterly tiny.

The size of an object that a glass lens can image is set by the diffraction limit - the minimum angular separation of two light sources that can be resolved by the lens. In practical terms, this means it is hard for a lens to resolve anything smaller than the wavelength of light being used to create the image.

The super lens goes beyond this point by gathering what are known as "evanescent waves". These are quickly decaying light waves that normal lenses can't capture.

In 2000, Imperial College's John Pendry proposed that their decay could be offset using a material with a negative refractive index. Such a material, he suggested, would allow the evanescent waves to be amplified and turned into normal propagating waves that could be captured by a microscope.

While several metamaterials have been developed since 2000 that can transmit these waves, until now none has been able to take the final step of converting them into propagating waves.

This final piece of the puzzle has been found independently, and using slightly different techniques, by two groups of researchers.

Nature.com reports that one group, from the University of Maryland, has built a flat lens using concentric polymer rings deposited onto a thin film of gold. The team has used it to take a picture of a series of 70nm dots etched into the inner ring.

The other, from the University of California, Berkeley, has constructed a three dimensional stack of silver and aluminium oxide on a quartz substrate. Again, the team has taken a picture of something built into the lens: the word ON was etched into the lens and imaged at a resolution of 130nm.

Both techniques have been published in the journal Science (reference 315 1699, and 315 1686, respectively).

There is, however, a trade off: to gain the increased focus, the lenses sacrifice vast amounts of field depth. According to Igor Smolyanivov from the University of Maryland, this means the biggest problem researchers will have is finding the sample to take a picture of it.

He told PhysicsWeb: "You won't be able to see it if it's out of focus." ®

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