The nano spirals emit a very specific optical signature that could be recognized by a barcode reader-like device Vanderbilt University

Nano spirals, many million times smaller than a coin, could be employed in currencies and cards to tackle counterfeits, thanks to their unique optical properties.

Fabricated by students and faculty at Vanderbilt University, the nano spirals with customisable signatures are easy to detect but extremely difficult to counterfeit.

Very small arrays that cannot be seen by the eye can be placed secretly on the object as the optical effect is very strong and can be detected easily.

The spiral arrays can also be encapsulated and placed in explosives and chemicals which can be detected using an optical barcode-like reader.

By virtue of their nanoscale optical properties, the spirals interact with light to produce a frequency doubling, similar to the harmonics produced when a violin string is bowed vigorously.

When infrared laser light strikes the tiny spirals, it drives the toward the centre of the spiral where they absorb more of the incoming laser energy to emit blue light at double the frequency of the infrared light.

The small structure with thin arms enables the movement of the electrons to the centre.

"This is similar to what happens with a violin string when it is bowed vigorously," said Stevenson Professor of Physics Richard Haglund, who directed the research by his students. "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."

Unique response

The nano spirals also have a unique response to polarised laser light which vibrates in a single plane. The amount of blue light emitted varies with the angle of the plane of polarisation.

With circularly polarised light, where the polarisation plane rotates clockwise or counter clockwise, the nano spirals react dramatically.

The left-handed spirals on encountering clockwise polarised light will maximise blue light by pushing more electrons toward the centre of the spiral.

Counter clockwise polarised light, on the other hand, pushes the electrons outward.

The strongest frequency doubler known so far is the synthetic crystal beta barium borate, but the latest nano spirals produce four times more blue light per unit volume.

The team experimented with gold nano spirals on glass substrate but silver and platinum can also be used similarly.

Requiring tiny quantities of metal, the spirals 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.