BEAST Technology Allows Scientists to Study Metal-Surface Hotspots

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Claire Perlman explains the importance of BEAST, also known as the Brownian Emitter Adsorption Super-resolution Technique.

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Researchers at the Lawrence Berkeley National Laboratory have created a BEAST - a device that probes into the electromagnetic field created when light strikes metal, entering an area of research previously off-limits to scientists that could increase the efficiency of solar-powered energy devices.

Research of the technique, also known as the Brownian Emitter Adsorption Super-resolution Technique, was published in the Jan. 20 issue of the journal Nature. The method has enabled scientists for the first time to measure the electromagnetic field inside a hotspot - an area of metal often only a few nanometers in size where the light's rays cause electrons to oscillate, creating an electromagnetic field.

"Because the surface of most metal, although it looks smooth from eyes, is actually very rough on the nanoscale ... the random roughness significantly affects how the electrons oscillate," said Hu Cang, a scientist in the Materials Science Division of Berkeley Lab and co-author of the paper, in an e-mail. "As a result, the distribution of light near the surface of the metal is no longer homogeneous; rather, it is concentrated at a few spots - hotspots. This is like a lens, concentrating light to a small focus."

According to Cang, scientists have been trying to look at hotspots on the nanoscale for 30 years but have been unable to do so because of the imprecision of available instruments and the difficulty of locating random hotspots on metal's surface.

But with the single-molecule technology of the BEAST, focusing in on the tiny hotspots has become possible in what MIT associate professor of mechanical engineering Nicholas Fang, an expert in the field, said in an e-mail is "indeed an exciting breakthrough."

Cang and his team first placed a metal sample in a solution of fluorescent dye, allowing the dye to become absorbed into the sample. By illuminating the sample with a laser beam, the researchers can see the hotspots and use the fluorescence of the dye molecules to figure out the intensity of the light in the hotspot.

"To look inside a hotspot requires a very tiny probe," Cang said in the e-mail. "A molecule is smaller than one nanometer, therefore it can reach the interior of a hotspot that is hard to access by other methods. In this way, the Brownian motion of the dye molecules not only helps us find the hotspots easily but also reconstruct the profile of the local field of the hotspots."

One of the most distinctive features of hotspots is their ability to act as lenses, which may help scientists in their quest to develop a method of harvesting solar energy with greater efficiency.

"Its focusing power is well beyond any conventional optics," Cang said in the e-mail. "While a conventional lens can only focus light to a spot about half the wavelength of light, about 300 nanometers, it has been speculated for a long time and we are the first to confirm that a hotspot can focus light to a nanometer-sized spot. Therefore hotspots can maximize the light harvesting efficiency in solar devices."

In addition, the hotspots' focusing power could enhance scientists' ability to detect and read chemical signals. Cang said in the case of DNA sequencing, the hotspot could turn its lens on a single molecule of interest, thereby minimizing background noise and improving the signal.

"(The BEAST) is a perfect probe to study the nano world that has been forbidden for previous experiments," Cang said in the e-mail. "It offers an unprecedented opportunity to measure interactions between light and matter at the nanoscale."


Claire Perlman is the lead research and ideas reporter. Contact her at [email protected]

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