Silver Superlens Achieves 60-nm Resolution
Nancy D. Lamontagne
Scientists at the University of California, Berkeley, have realized a resolution of 60 nm using a film of silver as a superlens, and they demonstrated it by imaging the word "NANO" and an array of nanowires.
Researchers imaged the word "NANO" using a silver superlens. The word contains lines approximately 40 nm wide. The red line shows the enhancement of the evanescent waves. Image by Cheng Sun of the University of California, Berkeley.
Instead of imaging the propagating light from an object, this technique images the evanescent fields coming from the object. This type of imaging isn't easy because the fields decay exponentially and thus are normally lost before they can reach the imaging plane.
To image the fields requires a material that can enhance the evanescent waves as they travel through it. Silver film is one such material because it can resonate with a band of evanescent fields of the near-field object. After the waves have left the material, they decay again until they hit the imaging plane.
The distance between the object and the imaging plane (determined by the thickness of the material and the slab surface) is important because, ideally, the imaging plane would be placed at the point at which the evanescent waves return to their original amplitude. In addition, the thickness and defects in the materials between the object and the image must be at a minimum to decrease losses as the waves travel through them.
Several groups have been working with silver superlenses (see May Photonics Spectra, p. 112). The California researchers' previous study showed that the optimized layer thickness for the silver is about 35 nm, said principal investigator Xiang Zhang. Thus, in this experimental work, they had to figure out how to fabricate smooth silver film with a precisely controlled thickness of around that size.
They deposited a 35-nm-thick film of silver on 40-nm-thick PMMA using electron-beam evaporation. The smooth PMMA surface improves the surface quality of the silver, and they found that depositing it faster (>5 nm/s) made the surface less rough than doing so at a slower rate of ~1 Å/s. Atomic force microscopy showed the silver's roughness was <2 nm. A photoresist on the side of the superlens opposite from the PMMA recorded an image when the object received 365-nm from a mercury lamp.
The researchers recorded an image of the word "NANO" -- made with 40-nm-thick lines -- onto an organic polymer. Atomic force microscopy of the developed superlens image revealed that the lines could be resolved to 89 nm. Without the superlens, the resolution was only 321 nm. They also resolved 60-nm-wide nanowires in an array to a resolution of one-sixth the illumination wavelength.
Zhang said the superlens could have far-reaching impact on the development of detailed biomedical imaging, higher-density electronic circuitry and data storage with larger capacity.
The researchers are working to overcome several constraints in the engineering design and to develop applications for this silver superlens.
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