Tiny nano-spirals with unusual optical properties that could be virtually impossible to counterfeit when added to identity cards, currency and other valuables have bben developed by Vanderbilt University students and faculty.
Roderick Davidson II, the Vanderbilt doctoral student who worked out how to study the spiral’s optical behavior, says:
“They are certainly smaller than any of the spirals we’ve found reported in the scientific literature.”
The spirals were designed and fabricated at Vanderbilt by another doctoral student, Jed Ziegler, now at the Naval Research Laboratory. Then, ultrafast lasers at Vanderbilt and the Pacific Northwest National Laboratory in Richland, Washington, were utilized to investigate their optical properties.
Most previous researched into the unique properties of microscopic spirals has worked with arranging discrete nanoparticles in a spiral pattern, similar to spirals drawn with a series of dots of ink on a piece of paper.
By comparison, the new nano-spirals have solid arms and are much smaller. Less than a hundredth of a millimeter wide for a square array with 100 nano-spirals per side.
The spirals are shrunk to sizes smaller than the wavelength of visible light. They then display exceptional optical properties.
When lit with infrared laser light, for example, they emit visible blue light. A number of crystals produce this effect, called frequency doubling or harmonic generation, to various degrees.
The most powerful frequency doubler previously known is the synthetic crystal beta barium borate, but the nano-spirals produce four times more blue light per unit volume.
When infrared laser light hits the microsopic 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 pushed toward the center 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 Stevenson Professor of Physics Richard Haglund, research director for the project. “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 center 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 effect of the frequency doubling effect is strong enough that arrays 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 Haglund.
The researchers also envision that coded nano-spiral arrays could be encapsulated and located in explosives, chemicals and drugs, in fact, any substance that someone wants to track closely, and then detected only using an optical readout device.
Roderick B. Davidson, Jed I. Ziegler, Guillermo Vargas, Sergey M. Avanesyan, Yu Gong, Wayne Hess, Richard F. Haglund Jr.
Eflcient forward second-harmonic generation from planar archimedean nanospirals
Nanophotonics. Volume 4, Issue 1, ISSN 2192-8614, DOI: 10.1515/nanoph-2015-0002
Illustration: Computer simulation of the harmonic emissions produced by a nano-spiral when it is being illuminated by infrared light. Credit: Haglund Lab / Vanderbilt