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Researchers 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.
When these spirals are shrunk to sizes smaller than the wavelength of visible light, they develop unusual optical properties. 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 combination of the unique characteristics of their frequency doubling and response to polarized light provide the nano-spirals with a unique, customizable signature that would be extremely difficult to counterfeit.Image credit: Roderick B Davidson II, Department of Physics and Astronomy, Vanderbilt University
