Smoothing the Way for Light

A technique makes smooth metal films for optical computing and imaging.

Researchers at the University of Minnesota have developed a cheap way to repeatedly make very smooth nanopatterned thin films. The advance could have implications for making devices--such as more efficient solar cells, higher-resolution microscopes, and optical computers--that use light in an unconventional way.

Guiding light: Silver films patterned with structures like this pyramid guide light along their surface and concentrate it at the tips. This structure’s surface is very smooth, which prevents scattering. Credit: Science/American Association for the Advancement of Science

Surface waves of light called plasmons can do things that ordinary light waves can't--squeezing into much smaller spaces for high-resolution imaging or miniaturized optical circuits, for example. These surface waves can be generated and controlled by shining light on thin, smooth, patterned metal films. But plasmons scatter easily, so the nanopatterned metal films that guide plasmons must be very smooth. And such smooth metal patterns are difficult to make.

"People have shown useful effects with plasmons, but the problem is doing it on a substrate you could cheaply and reproducibly make," says David Norris, professor of chemistry at the University of Minnesota. Up till now, researchers have been making plasmonic devices one at a time using techniques such as blasting out metal patterns using beams of high-energy ions or electrons. Because each of these devices is "handmade," says Norris, each is different, making standardization difficult. And while these methods are good for carving out nanoscale features in metal, they have the unintended consequence of making the surface rougher. As a result, harnessing plasmons has remained largely a laboratory curiosity and not a practical technology.

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The way plasmons move through a metal film can be controlled by patterning the film. Plasmons travel along the surface of metal films just like a wave travels on the surface of a pond. Surface roughness in the metal is like a leaf on the pond's surface, causing the waves to scatter. Today in the journal Science, Norris's group describes a way of making very smooth metal patterns using silicon molds. These surfaces are incredibly smooth--if they were pond surfaces, the leaves would be only four-tenths of a nanometer thick.

The Minnesota researchers use the lithography techniques honed by the semiconducting industry for patterning silicon to make a very smooth mold, which they cover with a metal film. "The top surface of the metal is now rough, but the bottom surface in contact with the silicon is quite smooth," says Norris. He then covers the film with a strong adhesive and peels off the patterned metal so that the smooth side is now exposed. The silicon molds can be used again and again. The Minnesota researchers have used the technique to make bull's-eyes, arrays of bumps and pyramids, and long ridges.