Take gold spirals about the size of a dime...and shrink them down about six million times. The result is the world's smallest continuous spirals: "nano-spirals" with unique optical properties that would be almost impossible to counterfeit if they were added to identity cards, currency and other important objects. Students and faculty at Vanderbilt University fabricated these tiny Archimedes' spirals and then used ultrafast lasers at Vanderbilt and the Pacific Northwest National Laboratory in Richland, Washington to characterize their optical properties. The results are reported in a paper published online by the Journal of Nanophotonics on May 21.

"They are certainly smaller than any of the spirals we've found reported in the scientific literature," said Roderick Davidson II, the Vanderbilt doctoral student who figured out how to study their optical behavior. The spirals were designed and made at Vanderbilt by another doctoral student, Jed Ziegler, now at the Naval Research Laboratory.

Most other investigators who have studied the remarkable properties of microscopic spirals have done so by arranging discrete nanoparticles in a spiral pattern: similar to spirals drawn with a series of dots of ink on a piece of paper. By contrast, the new nano-spirals have solid arms and are much smaller: A square array with 100 nano-spirals on a side is less than a hundredth of a millimeter wide.

When these spirals are shrunk to sizes smaller than the wavelength of visible light, they develop unusual optical properties. For example, when they are illuminated with infrared laser light, they emit visible blue light. A number of crystals produce this effect, called frequency doubling or harmonic generation, to various degrees. The strongest frequency doubler previously known is the synthetic crystal beta barium borate, but the nano-spirals produce four times more blue light per unit volume.