The world's smallest gold spirals with unique optical properties could make identity theft almost impossible.
These nano-spirals are about six million times smaller than a dime, and could be incorporated into identity cards, currency, and other objects for close to foolproof protection, Vanderbilt University reported.
"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.
When the spirals are shrunk down to this miniscule size, which is smaller than the wavelength of visible light, they develop key optical properties. When illuminated with infrared lasers, they emit blue visible light. The infrared light is absorbed the electrons in the gold, which are forced to move along the spiral. The electrons at the center absorb enough energy to emit blue light at double the frequency of the incoming laser.
"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. "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 spirals also have distinct responses to polarized leaser light, the amount of blue light emitted varies as the angle of the polarization's vibration plane is rotated. These unique characteristics of frequency doubling and polarized light responses allow for the nano-spirals to be given customizable signatures that would be extremely difficult to counterfeit.
"If nano-spirals were embedded in a credit card or identification card, they could be detected by a device comparable to a barcode reader," Haglund said.
The findings were published in a recent edition of the Journal of Nanophotonics.