Counterfeiting today has become epidemic for all things valuable, from currency to credit cards to microchips. Making sure that fakes can be detected is an on-going process that researchers worldwide are developing, with many successes like the holograms on credit cards and currency. Now researchers at Vanderbilt University (Nashville, Tennessee) believe they have found another technique that may work even better — embedding microscopic Archimedean spirals that return a unique 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 500 nanometers. The arms are 60-to-70 nanometers wide, and the inter-arm spacing is of order 40-to-50 nanometers. For optical studies, these are typically fabricated in arrays by electron-beam lithography with mechanical perfection," Vanderbilt University professor Richard Haglund told EE Times. "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."

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 behavior is that infrared laser light is absorbed by the electrons in the gold arms, driving them along the spiral toward the center 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 polarized in a plane that is rotated through 360 degrees, the outgoing light intensity varies in a distinctive repeatable pattern. The effect is maximized when left-handed nano-spirals are illuminated with clockwise polarized light, because the intensity of blue light produced increases as the polarization pushes the electrons toward the center of the spiral. Likewise the emitted blue light is minimized when the circularly polarized light is rotated counter-clockwise, thus destructively pushing the electrons away from the center, making its response unique enough to serve as a strong authentication mechanism that would be extremely difficult for counterfeiters to reproduce.

"Jed realized these spirals would have distinctive properties related to the 'self-interactions' and the fundamental asymmetry of the spiral," Haglund told us. "That conviction sustained him through a challenging and steep learning curve especially when it came to developing lift-off protocols that would remove the resist while not ripping the nano-spirals to shreds."

The proof was in the pudding, as Haglund and Ziegler's microscopic Archimedean spirals return a strong second-harmonic signal — easily detectible visible blue light when pulsed with an infrared laser — giving them their unique signature characteristic that enables them to be used as an anti-counterfeiting tool.

"The idea that the nanospirals would be efficient sources of nonlinear optical effects because of their inherent asymmetry — such as second-harmonic generation (SHG) — was part of Jed's insight as he began his graduate program, and fortunately he was able to stay at Vanderbilt long enough after receiving his Ph.D. for Rod Davidson, the principal author of the Nanophotonics article to lead the completion of the first SHG experiments."

To achieve their anti-counterfeiting goal, Haglund, Ziegler and Davidson's Archimedean spirals can be very accurately placed in hidden locations on currency, credit card, microchips or anything else the user wants to protect from counterfeiting, then pulsed with an infrared laser to make sure they return a strong second-harmonic signal, thus revealing the authenticity of the item.

"Once deposited en masse on a suitable substrate (and electron-bean lithography can be made to work on most any substrate, including flexible ones), these could be transferred in much the same way that holograms are presently transferred to everything from currency to credit cards to microchips," Haglund told us.

The ultrafast lasers used to verify that the team's Archimedean spirals performed properly were provided by the Pacific Northwest National Laboratory (PNNL, Richland, Washington) where their optical properties were carefully characterized.

For mass production they could be made from silver or platinum using extremely small amounts of material, and deposited on inexpensive plastic or even paper substrates for easy transfer. Readers would be constructed in a manner similar to bar-code readers. Besides currency, credit cards and microchips, coded patterns of nano-spirals could be embedded in explosives, chemicals, drugs and other items that manufacturers want to closely track.

Funding was provided by the Department of Energy Office of Science and the National Science Foundation (NSF). Other contributors included research professor Sergey Avanesyan and doctoral candidate Guillermo Vargas at Vanderbilt along with Yu Gong and Wayne Hess, scientific staff members at PNNL.

By: DocMemory
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