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  • Smooth flakes of gold build better plasmonic devices

Photonics Spectra
Mar 2011
Marie Freebody, Contributing Editor,

WÜRZBURG, Germany – Size is everything when it comes to fabricating plasmonic structures. For optical nanoantennas, for example, we’re talking about tens of nanometers in length, with a gap of less than 10 nm. At this scale, even slight defects can become a massive problem.

A new nanofabrication method developed by researchers at the University of Würzburg could provide solutions for building high-definition complex plasmonic nanodevices and extended optical nanocircuits.

One such device is the optical nano-antenna, which can collect and concentrate light for more efficient photovoltaic power generation. But the applications of nanoantennas do not stop there, researcher Bert Hecht said.

Bert Hecht examines an optical nanoantenna developed using a new fabrication method developed at the University of Würzburg. Courtesy of Bert Hecht.

“The ability to concentrate light is of interest in photovoltaics,” he said. “The ability to concentrate light to subwavelength dimensions into the nanometric antenna gap is of importance for optical sensors with single-molecule sensitivity and for high-resolution microscopy. Furthermore, single molecules or quantum dots coupled to antennas can lead to highly efficient single-photon emitters for quantum communication.”

Today’s optical nanoantennas often are sculpted from multi-crystalline evaporated thin films; more precisely, a thin layer of metal is applied to a carrier substrate by means of vapor deposition. Then, just as in sculpting, desired patterns can be worked into this “blank.”

The problem is that the base layer consists of many small individual crystals, resulting in a grainy structure. When trying to cut smooth structures with any kind of precision, the graininess can result in irregular shapes that can render the device useless. This is mostly because various crystal facets etch with different rates during the focused ion beam milling process.

Hecht’s team worked with researchers from Swiss Federal Laboratories for Materials Testing and Research in Dübendorf, Switzerland, and from Polytechnic University of Milan in Italy to come up with an alternative way of preparing the gold substrate.

They used chemically synthesized gold flakes, which consist of a single gold crystal, to avoid the problems caused by any internal granular structure. By means of ion beam milling, they fabricated nano-structures from these flakes that exhibit greater precision than ever.

An artist’s rendition of a single-crystalline gold flake from which optical nanoantennas, among other nanostructures, were fabricated with great precision. The optical nanoantennas, which appear as cubes arranged in pairs, can collect and concentrate light 10 times more efficiently than can multicrystalline antennas. Illustration by Thorsten Feichtner.

“We have found a simple method which allows everybody to use single-crystalline gold to fabricate plasmonic structures of much higher quality and much higher precision,” Hecht said. “This will allow us to realize structures that were not possible before, and it will raise the overall quality standards for plasmonic structures.”

The method is described in the Dec. 21, 2010, issue of Nature Communications. In the approach, large and thin single-crystalline gold flakes are chemically grown and deposited onto a glass/indium tin oxide surface.

From this more homogeneous structure, the group fabricated nanostructures that exhibit ultrasmooth surfaces and small gaps over extended distances, not easily achieved using other approaches.

“The use of single-crystalline gold allows us to produce reproducible optical nanoantennas with very small tolerances,” Hecht said. “Their optical properties – the light enhancement in the feedgap – are roughly 10 times better than for multi-crystalline antennas.”

Although photovoltaic systems could stand to profit from easier and cheaper ways to produce nanoantennas, Hecht admits that rooftop solar cells probably will benefit from cheaper and simpler methods before nanoantennas become sufficiently cheap and are able to be produced in large enough arrays.

Nevertheless, the investigators plan to follow up the potential of antennas for optoelectronic applications.

“We will further push the limits of fabrication to produce ultrasmall gaps, which should yield even higher field enhancement,” Hecht said. “The reproducibility of the structures and their high precision are instrumental for systematic studies of such effects. It will allow us to gain a deeper understanding of the underlying physics.”

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