A new kind of 鈥渟uperlens鈥�, capable of focusing light to a spot far smaller than its own wavelength, could be far easier to build than other proposed designs, researchers say.
Traditional lenses are hampered by the 鈥渄iffraction limit鈥� 鈥� an effect that prevents them from focusing on anything that鈥檚 much smaller than the wavelength of light. For visible light, this limit is around 600 nanometres.
It turns out, however, that special materials can defeat the diffraction limit using a strange property called 鈥渘egative index of refraction.鈥� However, 鈥渘ature does not give us negative-index materials,鈥� says Roberto Merlin of the University of Michigan in Ann Arbor, US.
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Instead, researchers have to make 鈥溾��, for example using grid-like arrangements of tiny metal coils. Such metamaterials can bend light in the opposite direction to a normal lens, and can therefore focus light to a point much smaller than its wavelength. These arrangements are complex to create, and cannot yet focus visible light much better than a conventional lens.
Concentric rings
Merlin has devised a different way of making a superlens that promises to focus light more efficiently, and to an even smaller spot 鈥� perhaps 500 times smaller than light鈥檚 wavelength. Also, compared with conventional metamaterial lenses, this new kind 鈥渨ould definitely be easier to make,鈥� Merlin says.
His theoretical study shows that a more effective superlens could be made from a thin plate containing concentric rings made of two different materials. This would be reminiscent of a tree trunk and its annual growth rings. The rings would alternate between material that blocks light 鈥� such as metal 鈥� and rings that let light through, like silicon or glass.
Overall, the set of rings should 鈥渟culpt鈥� light emerging from the plate to creating a focused point of light. And using this method ought to be much more flexible than other ways of making superlenses, Merlin says. In addition to focusing light into a point, 鈥淚 could make a line,鈥� he adds. 鈥淚 could probably write 鈥楿niversity of Michigan鈥�.鈥�
Evanescent waves
As with existing metamaterial superlenses, Merlin鈥檚 proposed lens would exploit so-called evanescent waves. These exist right next to a lens, but quickly fade away with distance from the lens鈥檚 surface. Such lenses can only beat the diffraction limit by focusing less than a few wavelengths away.
So, these technologies could not make a 鈥渟upertelescope鈥� for seeing over long distances. But superlenses could have many applications, Merlin says, such as higher density data storage on optical discs, and more precise lithography 鈥� the process used to make computer chips.
In metamaterial lenses, 鈥渓oss [of light] is unavoidable鈥�, says Willie Padilla of Boston College in Massachusetts. 鈥淭he approach that Merlin is proposing can avoid these losses.鈥�
