Tiny swirls of gold, so small they're invisible to the human eye, could be a new weapon in the war against identity theft, researchers say.
Such "nano-spirals," if they were printed onto objects like currency or identity cards, could make those objects almost impossible to counterfeit, they say.
Known as Archimedes spirals, they were created at Vanderbilt University by students and faculty, who then utilized ultrafast lasers to examine their optical properties, a study in the Journal of Nanophotonics reports.
"They are certainly smaller than any of the spirals we've found reported in the scientific literature," says doctoral student Roderick Davidson II, who developed a method for studying the spiral's optical behavior.
Shrunk down to sizes smaller than wavelengths of visible light, they glow blue only when exposed to infrared light.
The also have a unique response to polarized laser light, the researchers explain, giving off varying amounts of blue light as the polarization plane's angle is rotated.
The unique characteristics of the spirals, when exposed to such varied wavelengths and types of light, give them customizable, unique "signatures" that would be extremely difficult to counterfeit, their developers say.
The spirals are so small than even a square array of 100 would still be invisible to the naked eye, the researchers note, while their "signature" responses are so strong they can be easily detected with the appropriate sensors.
That means the nano-spiral arrays could be placed in a secret place on a card or currency, creating yet another barrier to counterfeiters.
Inexpensive to produce because they use only minuscule quantities of precious metals — in addition to gold, silver and platinum can be used in the same way — they can be printed onto paper, plastic and a number of other substrates.
So far, the researchers have created arrays of the spirals on glass substrates using scanning electron-beam lithography.
Because they are made with precious metals, the nano-spirals would resist any chemical degradation once printed on a substrate.
"If nano-spirals were embedded in a credit card or identification card, they could be detected by a device comparable to a barcode reader," says physics professor Richard Haglund.
In addition to providing a barrier to identity theft or counterfeiting, the nano-spirals could be encapsulated, then placed in substances that need to be closely tracked, such as drugs, chemicals or explosives, the researchers suggest.