Silver Superlens Enables Subdiffraction-Limit Imaging
Lens resolution can never be too flat or too thin. At least, that's one conclusion that can be drawn from recent research at the University of Canterbury in Christchurch, New Zealand. Electrical and computer engineering associate professor Richard J. Blaikie and graduate student David O.S. Melville have demonstrated resolution below the conventional diffraction limit for far-field imaging using a 50-nm-thick silver layer with a roughness of approximately 1 nm.
Using a 50-nm-thick silver slab, researchers imaged a 145-nm-period grating with 365-nm radiation. ©OSA.
The limit to conventional far-field resolution is a function of the illumination wavelength and the refractive index of the lens. Somewhat controversial theoretical calculations suggest that a thin layer of silver could act as a "superlens" as a consequence of the action of surface plasmons: charge oscillations on the surface of the silver. Through a series of incremental steps (see "Silver Slab Promises Subwavelength Imaging," Photonics Spectra, July 2004, page 24) -- and a lot of hard work -- the researchers created a silver layer thin and flat enough to test the prediction.
They used this silver layer as part of a lens stack that consisted of a layer of poly(methylmethacrylate) (PMMA), the silver planar lens and a surface dielectric of either silicon dioxide or PMMA. Because the initial PMMA layer was smooth, the silver deposited atop it also was very smooth and flat.
To test the resolving power of the silver lens, the researchers fabricated the lens stack on a tungsten mask -- patterned via electron-beam lithography with the features to be imaged -- and glass support, creating a conformal mask. They used the mask and a photolithographic tool to transfer the features to a resist on a silicon substrate. With 365-nm radiation from a mercury lamp, they imaged grating patterns down to 145 nm, well below the conventional resolution limit of 243 nm.
Future research could involve thinner silver layers and other lens materials. As for applications, Blaikie pointed to the possibility of higher-density optical disks, better near-field microscopy and innovations in lithography.
"If better image quality can be achieved, then there is no reason why this shouldn't be used as an improvement for regular contact lithography -- which is widely used -- to give this simple technique nanofabrication capability," he said.
Blaikie cautioned, however, that these applications are speculative and that considerable work will be required to make any of them a reality.
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