Washington, June 4 (IANS): A team of researchers has created the world's smallest continuous spirals or "nano-spirals" that can guard against theft if they were added to identity cards or currency.

When these spirals are shrunk to sizes smaller than the wavelength of visible light, they develop unusual optical properties.

"They are certainly smaller than any of the spirals we've found reported in the scientific literature," said Roderick Davidson II from Vanderbilt University, US.

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," said Richard Haglund from Vanderbilt University, who directed the research.

The nano-spirals also have a distinctive response to polarised laser light.

When struck by such a light beam, the amount of blue light the nano-spirals emit varies as the angle of the plane of polarization is rotated through 360 degrees.

"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," researchers said.

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," Haglund pointed out.

The study appeared in the Journal of Nanophotonics.