In an effort that might affect the future of nanomedicine, scientists at UC Berkley have developed a novel “hyperlens” which they claim brings them a step closer to nanoscale optical imaging. Here’s a scoop from UC Berkeley:
Propagating waves can travel far and be collected by an optical lens, including the human eye, to form an image. Evanescent waves contain far greater detail and resolution of an object, but they decay too quickly for conventional lenses to capture them.
“Capturing the information carried by the evanescent waves is the Holy Grail of optical imaging,” said Xiang Zhang, UC Berkeley professor of mechanical engineering and principal investigator of the study. “The hyperlens shows a new way to beat the diffraction limit, which would allow biologists to not only see a cell’s nucleus and other smaller components, but to study the movement and behavior of individual molecules in living cells in real time. In technology, this could eventually lead to higher density integrated circuits and DVDs.”
“In this experiment, the loss of the evanescent waves results in a diffraction limit of 260 nanometers,” said Zhaowei Liu, UC Berkeley postdoctoral researcher in mechanical engineering and co-lead author of the paper. “The hyperlens breaks this limit by capturing an image of objects smaller than 150 nanometers.”
The researchers used nanowires that were 35 nanometers wide and inscribed onto an inner layer of chrome that sits atop the hyperlens. In the first experiment, two nanowires were placed parallel to each other 150 nanometers apart. The researchers also shaped nanowires into the letters O and N for another demonstration.
The hyperlens consists of multiple layers of silver and aluminum oxide placed along the cavity of half a cylinder carved out of quartz. When an object is illuminated, its evanescent waves travel through the lens. As the wave vectors move outward, they are progressively compressed. This compression allows the image of the objects to be magnified by the time it reaches the outer layers of the hyperlens. At this point, it can be captured by a conventional optical lens and projected outward onto a far-field plane a meter away…
The researchers point out that, unlike the hyperlens, the superlens does not alter the nature of the evanescent waves, so once the waves leave the lens, they decay quickly. This exponential loss of the evanescent waves required the image plane to sit close to the lens.
“The superlens we showed before is near-sighted – the projected image only exists near the surface of the lens with no magnification,” explained Liu. “That limits the practical applications for the superlens, since the camera needs to be within an object’s ‘near-field’ range. To make a lens useful for far-field imaging below a diffraction limit, you must convert evanescent waves to propagation waves, which is what the hyperlens does.”
Zhang noted that while the hyperlens can project a magnified image of a sub-diffraction object up to a meter away, the object that is being imaged still needs to be placed in the near-field zone of the lens. “We have not yet reached the goal of making a far-field optical nanoscope,” he said. “But we are one major step closer.”
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