Researchers from Vanderbilt University in the United States have developed a new way to guard against identify theft. The team succeeded in producing the smallest known nano-spirals, and then used ultrafast lasers to create unique optical properties that would be almost impossible to counterfeit if they were added to identity cards, currency, or dangerous objects such as explosives.
The research was described in paper published by the Journal of Nanophotonics at the end of May.
Archimedes' spirals − spirals which start at the origin and form a curve of three rounds, with the distance between each spiral branch remaining the same − are used in applications such as digital light processing, microbiology and for the coils of watch balance springs.
The new nano-spirals designed and made by Vanderbilt by doctoral student, Jed Ziegler, have solid arms and are much smaller than other spirals that exist today. A square array with 100 nano-spirals on a slide is less than a hundredth of a millimetre wide. 'They are certainly smaller than any of the spirals we've found reported in the scientific literature,' said Roderick Davidson II, a doctoral studentat Vanderbilt University.
The Vanderbilt team has so far experimented with gold nano-spirals on a glass substrate made using scanning electron-beam lithography, but it was found that 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. The spirals can also be produced on plastic, paper and a number of other substrates.
The frequency doubling effect is strong enough so that arrays that are too small to see with the naked eye can be detected easily. That means they could be placed in a secret location on a card, which would provide an additional barrier to counterfeiters.
‘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 Stevenson Professor of Physics Richard Haglund, who directed the research.
According to the researchers, the coded nano-spiral arrays could also be encapsulated and placed in explosives, chemicals and drugs, and then detected using an optical readout device.
This is not the first time optical technologies have been used to fight counterfeit goods. Colour imaging systems that offer extended sensitivity into the infrared and ultraviolet are being developed to inspect printed money. In addition, Cambridge University researcher, Damien Gardiner, has been awarded grants to further his work with lasers developing unique optical signatures to provide security and brand authentication on features and devices.
The spirals created by the Vanderbilt University team emit visible blue light when illuminated with infrared laser light.
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.
‘This is similar to what happens with a violin string when it is bowed vigorously,’ explained Haglund. ‘If you bow a violin string very lightly it produces a single tone. But, if you bow it vigorously, it also begins producing higher harmonics, or overtones. The electrons at the centre 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-spirals also have a distinctive response to polarised laser light. Linearly polarised light, like that produced by a Polaroid filter, vibrates in a single plane. When struck by such a light beam, the amount of blue light the nano-spirals emit varies as the angle of the plane of polarisation is rotated through 360°.
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, according to the researchers.