'Superlens' has its reach extended


By Tom Simonite (Image: University of California, Berkeley/ACS) A lens capable of resolving objects too small for conventional optics has been dramatically improved by US researchers. It can now transmit ultra high-resolution images over much longer distances. Previously, the sensor used to capture an image had to be placed within nanometres of the lens. The researchers involved say it has potential for imaging manmade or biological structures so-far seen only using electron microscopes. The team from the University of California, Berkeley, in the US say the advance should make it easier to build so-called “superlenses”, which can image nanostructures never before resolved using visible light. Conventional lenses can only see details roughly down to the size of half the wavelength of light. This limit is due to interference and diffraction that occurs as light bounces off an object. A superlens gets around this limit by collecting light waves that only occur very close to an illuminated object. These “evanescent waves” contain information about at finer resolution but are hard to use because these waves decay rapidly. The nanometre-scale region in which they exist is known as the “near-field”. The University of California team, led by Xiang Zhang, developed the first working superlens in 2005. They used a very thin layer of silver, placed in the near-field to collect and focus evanescent light waves. However, the original superlens could only transmit near-field waves to a sensor positioned within its own near-field. Now Zhang and colleagues have developed a lens that coverts near-field waves into normal light waves, which do not decay over distance. This means a light sensing device like a microscope could be placed further away from the superlens itself, although the lens would still need to be within it target’s near-field. “It represents a significant step towards a practical imaging application,” Zhang told new Scientist. University of California team improved their silver-film superlens by adding 35-nanometre-wide corrugations to its surface. These diffract light waves from an object’s near-field, turning them into normal light waves. To test the superlens, they used a UV laser to illuminate pairs of nanowires separated by small gaps. The superlens was placed 35 nm from the wires while a normal optical microscope was positioned further away. The superlens was able to distinguish two wires positioned just 70 nm apart – a resolution nearly three times better than that of conventional optics. The team are currently using it to examine more complex nanoscale objects. “This long-awaited paper is an essential step forward for the field of superlens research,” photonic physicist Nikolay Zheludev, at Southampton University, UK, told New Scientist. Being able to project the super-resolution image beyond the near-field could make the superlens much easier to use, Zheludev says. “But the lens still has to be positioned close to the object to be in its near-field,” he points out. A superlens that could focus on objects from beyond the near-field would be truly revolutionary, he adds. “There are suggestions that it’s possible,
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