In a recently published open access paper ‘Optical micro-resonator arrays of fluorescence-switchable diarylethenes with unreplicable spectral fingerprints’, researchers from the University of Tsukuba in Japan offer the prospect of a pattern that, they say, is impossible to duplicate or forge; a feat that could help to defeat counterfeiters.
The key to these patterns lies in a two-step verification system that incorporates both micropatterns and resonance. Resonance is a wave phenomenon that allows energy to be stored as a standing wave rather than transmitted as a progressive wave.
Those of us old enough to remember rewritable CDs (CD-RW) will appreciate the technical breakthrough in having a compact disc format that could be written, read, erased, and re-written several times. To apply this principle to anti-counterfeiting data storage and rewritable memory devices, the research group used chromic materials, where the dichromatic colours can be switched by external stimuli. By embedding additional individual information in each pixel, a much higher-level security system than the binary zero/one data array was realised.
To achieve this, the group proposed that photo-switchable optical micro-resonators made of a fluorescent photochromic organic material could be adapted as anti-counterfeiting, rewritable optical memories.
The micro-resonator structures developed for the research are similar in construction to a traditional whispering gallery, where two large, concave dishes are placed at opposite ends of a long hall. A whisper into one of these plastic dishes can be heard clearly by someone standing in the other one down the hall.
Recognising this similarity, the team from Tsukuba named their technique whispering gallery mode (WGM). Photoluminescence of the micro-resonators can be switched on and off repeatedly by irradiation alternatively with ultraviolet and visible light.
The shape of the micro-resonator varies from a sphere to an oblate ellipsoid and hemisphere, depending on the self-assembly process, and the WGM spectral pattern depends sensitively on the form and structure of the resonators. By assembling the resonators onto a micro-patterned substrate that alternates between areas that tend to attract or repel water (hydrophobic/hydrophilic) allows a highly integrated array of micro-resonators as dense as millions of pixels per square centimetre to be built.
The anti-counterfeit micropatterns created by the Tsukuba researchers are designed to be used for authentication purposes. Security measures such as these often embed physical functions into the product during the manufacturing process. In this case, researchers were able to embed the resonance produced by standing light waves into a microscopic image.
To create the patterned microscopic image, described in the journal Materials Horizons, the researchers embedded a lightwave fingerprint under a 1mm-wide drawing of Mona Lisa containing millions of evenly spaced pixels per square centimetre.
Within each of the millions of pixels sits a WGM fingerprint, a unique colour signature created in a microscopic, rounded cavity with reflective surfaces. Each of these reflective cavities has a unique shape. Shapes can vary from elliptical, spherical, oblate-spherical, to hemispherical, but they are always rounded and always have a reflective surface that will facilitate the whisper chamber effect.
Like human fingerprints, no two of these cavities are identical. Into each uniquely shaped cavity, the Tsukuba researchers placed a microscopic droplet of fluorescent dye that is chemically sensitive to light.
Once the dye was in place, the researchers shone visible and ultraviolet (UV) light onto the dye, in a random, unpredictable manner. The lightwaves reflected inside each cavity, similar to the way sound reflects in a whisper gallery, which caused the dye molecules to react. The variety of cavity shapes combined with the exposure of unique patterns of light applied to the fluorescent dye, resulted in a unique colour signature within each pixel.
The final step taken by the researchers was to cover the entire array of millions of WGM fingerprints with another material. This is the material they used to paint the tiny, visible image of Mona Lisa.
The research group believes that in the future, governments and businesses could utilise patterns, created through this process, to combat counterfeiting and that patterns containing WGM fingerprints, might be used to draw your picture on your credit card and driving licence.