A team of researchers led by Dr Roderick Davidson II of Vanderbilt University has created nano-spirals with unique optical properties that would be almost impossible to counterfeit if they were added to credit cards and other important objects.
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 nano-spirals created by Dr Davidson and co-authors have solid arms and are much smaller: a square array with 100 nano-spirals on a side is less than 0.01 mm wide.
“They are certainly smaller than any of the spirals we’ve found reported in the scientific literature,” said Dr Davidson, first author of the paper reporting the results in the journal Nanophotonics.
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.
When infrared laser light strikes the tiny spirals, it is absorbed by electrons in the gold arms. These arms are so thin that the electrons are forced to move along the spiral. Electrons that are driven toward the center absorb enough energy so that some of them emit blue light at double the frequency of the incoming infrared light.
The spirals also have a distinctive response to polarized laser light. Linearly polarized light, like that produced by a Polaroid filter, vibrates in a single plane. When struck by such a light beam, the amount of blue light the nano-spirals emit varies as the angle of the plane of polarization is rotated through 360 degrees.
The effect is even more dramatic when circularly polarized laser light is used. In circularly polarized light, the polarization plane rotates either clockwise or counterclockwise. When left-handed nano-spirals are illuminated with clockwise polarized light, the amount of blue light produced is maximized because the polarization pushes the electrons toward the center of the spiral.
Counterclockwise polarized light, on the other hand, produces a minimal amount of blue light because the polarization tends to push the electrons outward so that the waves from all around the nano-spiral interfere destructively.
“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,” the scientists said.
Roderick B. Davidson II et al. 2015. Efficient forward second-harmonic generation from planar Archimedean nanospirals. Nanophotonics, vol. 4, no. 1, pp. 108-113; doi: 10.1515/nanoph-2015-0002