Nano-spirals to guard against identity theft

Researchers have created the world's smallest spirals with unique optical properties that are nearly impossible to counterfeit if added to identity cards, currency and other objects.
Vanderbilt University researchers fabricated these tiny Archimedes' spirals and then used ultrafast lasers to characterise their optical properties.
"They are certainly smaller than any of the spirals we've found reported in the scientific literature," said Roderick Davidson II, who figured out how to study their optical behaviour.
The spirals were designed and made by doctoral student, Jed Ziegler, now at the Naval Research Laboratory.
Most other investigators who have studied the remarkable properties of microscopic spirals have done so by arranging discrete nanoparticles in a spiral pattern: similar to spirals drawn with a series of dots of ink on a piece of paper.

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.

The combination of the unique characteristics of their frequency doubling and response to polarised light provide the nano-spirals with a unique, customisable 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 Professor of Physics Richard Haglund, who directed the research.
The study was published in the Journal of Nanophotonics.