Hyperlens Achieves 130-nm Resolution
David L. Shenkenberg
Because metamaterial-based lenses can overcome the diffraction limit of light waves, they may enable optical imaging of objects much smaller than typical lenses can resolve. Researchers from the University of California, Berkeley, achieved 130-nm resolution with a metamaterial-based “hyperlens” that they made.
Researchers created a hyperlens that can magnify and project subdiffraction-limited objects onto a far-field plane. The hyperlens and objects are enlarged to show details, but they are actually much smaller than a conventional lens. Images reprinted with permission of Science.
“A hyperlens can be used for any fields where real-time high-resolution optical imaging is required,” said postdoctoral fellow Zhaowei Liu, one of the primary contributors to the work. The lens can be used to image moving molecules. Principal investigator Xiang Zhang said that it also will be useful for making nanometer-size objects with lithography. “If you can see beyond the diffraction limit, you can make things much smaller,” he said.
To fashion the hyperlens, the researchers created a cavity in quartz, then deposited multiple periodic layers of silver and aluminum oxide. Zhang said that the deposition can be done with either electron beam evaporation or sputtering. The curvature, the thickness of each layer, the total number of layers and the materials that comprise the layers all influence the behavior of the hyperlens.
The hyperlens can magnify subdiffraction-limited objects because its radial and tangential permittivities have different signs. Because of this difference, the dispersion is hyperbolic, a property that gives the hyperlens its name. Zhang said that this medium causes waves to propagate and to travel outward because the tangential momentum must be conserved. Thus, the real image is magnified and projected into 3-D space.
The left scanning electron microscopy image shows nanowires shaped in the word “ON.” The right image was captured using the hyperlens, which shows that it can produce the same information.
In their experiment, the investigators placed the hyperlens between a conventional optical microscope and an object with two 35-nm-wide lines spaced 150 or 130 nm apart. The hyperlens could image the latter pair of lines, achieving 130-nm resolution — below the diffraction limit. They also made an object with the subdiffraction-limited word “ON.” Using the hyperlens, they could see the word. “Our preliminary result already shows a superior performance for practical use,” Liu said.
Zhang said that the scientists plan several improvements to the hyperlens. They would like to design it so that the object can be farther away from the lens, and they would like to correct for spherical distortion. He said that their cylindrical lens magnifies in X, but they want to design a spherical lens because it could magnify in both X and Y. Liu said that it also would be useful to develop a hyperlens that works in the visible region, because currently the lens requires UV excitation.
Science, March 23, 2007, p. 1686.
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