World's Smallest Nano-Spirals May Guard Against Identity Theft

The world's smallest spirals may do something amazing: guard against identify theft. If you take gold spirals about the size of a dime and shrink them down about six million times, the resulting nano-spirals have unique optical properties that would be almost impossible to counterfeit if they were added to identity cards, currency, and other important objects.

"They are certainly smaller than any of the spirals we've found reported in the scientific literature," said Roderick Davidson II, one of the researchers, in a news release.

Most other researchers who have studied the properties of microscopic spirals have done so by arranging discrete nanoparticles in a spiral pattern, similar to spirals drawn with a series of drops of ink on a piece of paper. These new nano-spirals, though, have solid arms and are much smaller. In fact, a square array with 100 nano-spirals on a side is less than a hundredth of a millimeter wide.

These spirals have unusual properties. When they're illuminated with infrared laser light, they emit visible blue light. A number of crystals produce this effect, called frequency doubling or harmonic generation, to various degrees. The strongest frequency doubler previously known is the synthetic crystal beta barium borate, but the nano-spirals produce four times more blue light per unit volume.

When infrared laser light strikes the spirals, it's absorbed by electrons in the gold arms. The arms are so thin that the electrons are forced to move along the spiral. Electrons that are driven toward the center absorb enough energy so that some of them emit blue light at double the frequency of the incoming infrared light.

"This is similar to what happens with a violin string when it is bowed vigorously," said Richard Haglund, who directed the research. "If you bow a violin string very lightly it produces a single tone. But, if you bow it vigorously, it also beings producing higher harmonics, or overtones. The electrons at the center of the spirals are driven pretty vigorously by the laser's electric field. The blue light is exactly an octave higher than the infrared-the second harmonic."

The nano-spiral, with their unique properties, could be used to prevent fraud. If they were embedded in a credit card or identification card, they could be detected by a device comparable to a barcode reader.

The findings are published in the journal Nanophotonics.

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