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Archimedian spirals secure microchips

11 Jun 2015  | R. Colin Johnson

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With counterfeiting of currency, credit cards, and microchips becoming widespread, researchers have taken it upon themselves to ensure that fakes are easily detected with the likes of holograms on credit cards and currency. Researchers from U.S.-based Vanderbilt University believe they have found a technique that may work even better—embedding microscopic Archimedean spirals that returns a signature signal when pulsed by an infrared laser.

Named after the 3rd century BC Greek mathematician, Archimedean spirals consist of arms moving away from a central point with a constant speed and along a line that rotates with constant angular velocity. Their unique anti-counterfeiting property is that they can be constructed at microscopic sizes and yet still return a distinctive signature of visible light that is the second harmonic of the infrared laser that pulses them.

"Our Archimedean spirals' longest dimension is typically less than 500nm. The arms are 60 to 70nm wide, and the inter-arm spacing is of order 40 to 50nm. For optical studies, these are typically fabricated in arrays by electron-beam lithography with mechanical perfection," Vanderbilt University professor Richard Haglund said. "They were created by doctoral candidate Jed Ziegler (now at the Naval Research Laboratory) with the gifts he developed as he became a genuine virtuoso in electron-beam lithography."

Archimedian spirals

Ziegler's innovation was to fabricate well-formed sub-wavelength gold spirals with more intense near-field interactions between the arms—four times as intense as their competitor, synthetic crystal beta barium borate crystals.(Source: Vanderbilt)

Others have tried to create Archimedean spirals using arrangements of centro-symmetric nanodisks formed into spiral arrays, but Ziegler's innovation was to fabricate well-formed sub-wavelength gold spirals with more intense near-field interactions between the arms—four times as intense as their nearest competitor, synthetic crystal beta barium borate crystals, according to Vanderbilt.

The mechanism that explains their behaviour is that infrared laser light is absorbed by the electrons in the gold arms, driving them along the spiral towards the centre where so much energy is accumulated that it is released by emitting blue light at double the frequency of the incoming infrared light.

And when the incoming light is polarised in a plane that is rotated through 360 degrees, the outgoing light intensity varies in a distinctive repeatable pattern. The effect is maximised when left-handed nanospirals are illuminated with clockwise polarised light, because the intensity of blue light produced increases as the polarisation pushes the electrons towards the centre of the spiral. Likewise the emitted blue light is minimised when the circularly polarised light is rotated counter-clockwise, thus destructively pushing the electrons away from the centre, making its response unique enough to serve as a strong authentication mechanism that would be extremely difficult for counterfeiters to reproduce.


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