Smoothing the Way for Light

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.

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.

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.

There are many competitive processes for making smooth films, says Nicholas Fang, assistant professor of mechanical science and engineering at the University of Illinois at Urbana-Champaign. Norris’s method for smoothing metal surfaces is “quite unique,” says Fang, and should prove useful for making plasmonic structures, particularly if the molds prove to be durable over the long term. However, surface roughness is only one source of problems with plasmonics, says Fang. Now what are needed are methods for making the edges of the features in these patterned metal films smooth.

Harry Atwater, professor of applied physics and materials science at Caltech, agrees. “When you’re making waveguides, the edges are just as important as the surfaces.” Atwater is developing plasmonic concentrators for solar cells. Silicon solar cells are usually about 100 micrometers thick; thinner cells would be cheaper, but their performance suffers. Atwater has found that adding a patterned layer of metal that can interact with plasmons makes it possible to collect and concentrate light from wider angles and improves performance in thin silicon solar cells. Techniques for printing plasmonics for solar cells will have to be cheap and scalable because cost per unit area is such an important consideration for photovoltaics. Norris’s technique is “a useful idea,” says Atwater, but only time will tell whether it can work repeatedly over the large areas required for solar cells.

The future of plasmonics, says Atwater, will probably be in new materials besides metal. Metals like gold and silver, which have been used in plasmonics for about a decade, have an intrinsic electrical resistance that causes plasmons to scatter, no matter how smooth the surface and edges. The carbon nanomaterial graphene, which has a low resistance, might fit the bill. Atwater says scientists will also have to “pull out the metallurgy textbooks” to look for other materials.