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23 Mar 2007
Two US research teams have independently created the first truly magnifying "superlenses" using metamaterials with a negative index of refraction.
Unlike conventional lenses, which are prevented by the diffraction limit from resolving features much smaller than the wavelength of light, superlenses can provide images of almost limitless resolution. As a consequence, researchers believe that superlenses could one day enable the optical imaging of proteins, viruses and DNA.
The key to breaking the diffraction limit lies in finding a way to collect the idle "evanescent" waves that exist close to the surface of an object. These waves can resolve surface features much smaller than normal propagating waves, but they decay too quickly to be captured by conventional lenses.
In 2000, John Pendry of Imperial College, London, predicted that the decay of evanescent waves could be offset by amplifying them in a material with a negative refractive index. In theory, he said, such a negative-index "superlens" could capture evanescent waves from a surface and convert them into propagating waves that travel far enough to be picked up by a conventional microscope.
Since Pendry's prediction, several superlenses have been built that have successfully transmitted evanescent waves. However, none have been able to make the crucial conversion to propagating waves – which has left the evanescent waves with the same fast decay rate as when they started.
Now, two groups have created superlenses that can convert evanescent waves into propagating waves. At the University of Maryland, a team led by Igor Smolyanivov has created a flat superlens consisting of concentric polymer rings deposited onto a thin film of gold (Science 315 1699). Meanwhile, a team led by Xiang Zhang at the University of California, Berkeley, has opted for a 3D stack of curved silver and aluminium-oxide layers on a quartz substrate (Science 315 1686).
Both of these designs are classified as "metamaterials" – artificial nanostructures that a negative index of refraction. Superlenses have been made from metamaterials before, but the cylindrical geometry of these new designs enables evanescent waves emitted from illuminated objects to be guided outward. Because momentum must be conserved, this separation forces the "tangential" or side-to-side momentum of the waves to be compressed, resulting in a magnified image beyond the diffraction limit – and one that a conventional microscope can register.
Smolyanivov used his flat superlens to image rows of polymer dots deposited near the inner ring with a resolution of 70 nm, which is seven times smaller than the diffraction limit of the illuminating laser. Zhang, however, took his 3D superlens (or "hyperlens" as he prefers because of the hyperbolic shape) one step further by imaging the word "ON" inscribed on the surface, albeit with a slightly lower resolution of 130 nm.
Although both of these superlenses imaged objects that were "built-in" to the material, in practice the object would be kept separate but still close enough so that the evanescent waves can be captured. Even so, Smolyanivov told physicsweb.org that widespread applications may be some way off. This is because a side effect of the increased resolution is the vastly reduced depth of field, which means that focusing must be much more accurate.
"The real challenge will be to locate a sample," he said. "You won't be able to see it if it's out of focus."
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