World’s smallest spirals could guard against identity theft

World’s smallest spirals could guard against identity theft

5:16 PM, 9th June 2015
World’s smallest spirals could guard against identity theft
Scanning electron microscope image of an individual nano-spiral. (Haglund Lab / Vanderbilt)

NASHVILLE, US: Take gold spirals about the size of a dime and shrink them down about six million times. The result is the world’s smallest continuous spirals: “nano-spirals” with unique optical properties that would be almost impossible to counterfeit if they were added to identity cards, currency and other important objects.

Students and faculty at Vanderbilt University fabricated these tiny Archimedes' spirals and then used ultrafast lasers at Vanderbilt and the Pacific Northwest National Laboratory in Richland, Washington, to characterize their optical properties.

“They are certainly smaller than any of the spirals we’ve found reported in the scientific literature,” said Roderick Davidson II, the Vanderbilt doctoral student who figured out how to study their optical behaviour. The spirals were designed and made at Vanderbilt by Jed Ziegler, doctoral student.

Most other investigators who have studied the remarkable properties of microscopic spirals have done so by arranging discrete nanoparticles in a spiral pattern. By contrast, the new nano-spirals have solid arms and are much smaller: A square array with 100 nano-spirals on a side is less than a hundredth of a millimetre wide.

When these spirals are shrunk to sizes smaller than the wavelength of visible light, they develop unusual optical properties. For example, when they are 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 tiny spirals, it is 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 centre absorb enough energy so that some of them emit blue light at double the frequency of the incoming infrared light. The nano-spirals also have a distinctive response to polarized laser light.

The combination of the unique characteristics of their frequency doubling and response to polarized light provide the nano-spirals with a unique, customizable signature that would be extremely difficult to counterfeit, the researchers said.

So far, Davidson has experimented with small arrays of gold nano-spirals on a glass substrate made using scanning electron-beam lithography. Silver and platinum nano-spirals could be made in the same way. Because of the tiny quantities of metal actually used, they can be made inexpensively out of precious metals, which resist chemical degradation. They can also be made on plastic, paper and a number of other substrates.

“If nano-spirals were embedded in a credit card or identification card, they could be detected by a device comparable to a barcode reader,” said Richard Haglund, Stevenson professor of physics, who directed the research.

The researchers also argue that coded nano-spiral arrays could be encapsulated and placed in explosives, chemicals and drugs – any substance that someone wants to track closely – and then detected using an optical readout device.

© Vanderbilt University News

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