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Counterfeit Protection

March 15, 2021

As a child in the BC years (Before Computers), most of my education and entertainment came from reading books and magazines. Some of these books, such as The Golden Book of Science (1956), The Giant Golden Book of Mathematics (1960), and zoologist, Lancelot Hogben's, The Wonderful World of Mathematics (1955), are still on my bookshelves. Other supposedly educational children's books were merely collections of trivia designed to capture the interest of young readers.

Children's science and math books of the mid-20th century.

Some children's books about science and math from the mid-20th century. From left to right, cover and frontpiece of the The Golden Book of Science (1956), cover and frontpiece of The Wonderful World of Mathematics (1955), and frontpiece of The Golden Encyclopedia (1946). (Scans of my copies. Click for larger image.)

One unforgettable piece of trivia from one of those books was an illustration and explanation of the Yap Island Rai stones, also known as Yapese stone money. These are huge disks with a central hole carved from limestone. The largest known of these is about 12 feet (3.6 meters) in diameter, 20 inches (0.5 meter) thick, weighing 8,800 pounds (4,000 kilograms). These are known as a primitive form of money and an example that anything can be money if it's valued by many people. A modern example would be cryptocurrency. The value of one such cryptocurrency, bitcoin, is about $30,000 at the time of this writing.

Yapese Stone Money with Bitcoin

Is there any real difference between Yap Island Rai stones, also known as Yapese stone money, and bitcoin?

Each of these illustrates the economic principle that something has value when many people believe that it does.

An interesting example of this is the Dutch tulip mania of the early 17th century.

(Created using the GNU Image Manipulation Program (GIMP) from Wikimedia Commons source images of a Yap Stone Coin in the British Museum by Ken Eckert and a supposed bitcoin logo.)

The world is presently transitioning to digital currency, and all money transferred between the Federal Reserve and US banks is electronic. However, in the past, the paper dollar was king, and it was the sole method of commerce. This led to the problem of counterfeit money in which criminals would produce dollar imitations. After all, what could be simpler than printing money? This led to escalating efforts by the US Bureau of Engraving and Printing to make its dollar distinguishable from a counterfeit dollar.

The first effort was to produce fine detail in printing that was impossible for a counterfeiter to do. This led to the plots of many B-movies in which US Treasury agents searched for some superbly engraved printing plates created by counterfeiters. Security was also in the paper, being made from special materials with embedded colored threads. Over time, more technologically advanced anti-counterfeiting measures have been taken. These include the use of holograms, embedded strips, raised printing and microprinting, watermarking, inks whose colors change depending on the angle of the light, and measuring the fluorescence lifetime of embedded phosphors.

As you can surmise, the physics disciplines of optics and optical materials have contributed to anti-counterfeiting measures in currency. These, and others, are also being used for consumer goods in creation of anti-counterfeiting packaging and anti-theft systems. I participated in research in both of these areas. For anti-theft, my magnetism background was applied in the area of electro-magnetic security tags. My experience in phosphor materials was used in anti-counterfeiting.

Consumer packaging is adaptable to many more anti-theft and anti-counterfeiting technologies than currency. The most prominent of these for preventing theft of more expensive goods is the use of radio-frequency identification (RFID) tags. There's an arms race between manufacturers and counterfeiters in this area, just as for currency; so, there's always a need for innovation in low-cost measures. As the Wikipedia article on anti-counterfeiting packaging states,
With the increasing sophistication of counterfeiters techniques, there is an increasing need for designers and technologists to develop even more creative solutions to distinguish genuine products from frauds, incorporating unique and less obvious aspects of identification into the design of goods.

A research team at the Pohang University of Science & Technology (POSTECH, Pohang, South Korea) has gone beyond the simple hologram as an anti-counterfeiting measure to an optical tag that changes its image as a function of light polarization.[1-2] The research team was led by Junsuk Rho, a professor of mechanical engineering and chemical engineering, and it included graduate students from both disciplines.[2] This research is published as an open access paper in Nanophotonics.[1]

This security tag is enabled by a polarizer constructed as a metamaterial. Metamaterial are materials whose unique properties are based on their geometrical structure, rather than their specific composition. In order for these to manipulate light, their features must be of the order of a wavelength of light, about 500 nanometers, so they are enabled by nanoscale technology. I wrote about such optical metamaterials in two previous articles, Harvard Metamaterial Flat lens (July 4, 2016) and Metamaterial Lenses (November 18, 2019). One such metamaterial is illustrated below.

Titanium dioxide pillar metamaterial lens

A metamaterial lens developed at Harvard University.[3-4] This lens is built from many microstructures, each of which bend light to a focus. Left image, the process parameters for titanium dioxide pillars on silica. The phase shift of light through the lens at each point can be adjusted through rotation of the pillars. Right image, a scanning electron micrograph of the metamaterial lens. The titanium dioxide pillars are formed by atomic layer deposition on a silica glass substrate. (Left image, created using Inkscape. Right image, Capasso Lab image via Harvard University.)

The security tag surface metamaterials, metasurfaces, allow display of full-color images during an on state and show no images in an off state; alternatively, they can switch between different images.[2] The technologically advanced processing makes it difficult to counterfeit. Says Chunghwan Jung, the first author of the paper describing this research, "This new device is practically impossible to forge because it requires an electron microscope with magnification capacity of several thousand and a nanometer-scale production equipment."[2] This research was funded by the Samsung Research Funding & Incubation Center for Future Technology.[2]

Principle of the metamaterial polarization security tag

Principle of the metamaterial polarization security tag. The colors appear for horizontally polarized incident light (a), but are concealed for vertically polarized light (b). The metamaterial structure is shown at the right. (Fig. 1 of ref. 1,[1] also appearing here.[2] POSTECH illustration licensed under the Creative Commons Attribution 4.0 International License. Click for larger image.)

Overlapped images, portraits of Albert Einstein and Ludwig Boltzmann, are selected by different polarizations.

Overlapped images, portraits of Albert Einstein and Ludwig Boltzmann, are selected by different polarizations. The Boltzmann image is not as distinct, but it illustrates the principle. (Portion of Fig. 5 of ref. 1.[1] POSTECH illustration licensed under the Creative Commons Attribution 4.0 International License. Click for larger image.)


  1. Chunghwan Jung, Younghwan Yang, Jaehyuck Jang, Trevon Badloe. Taejun Lee, Jungho Mun, Seong-Won Moon, and and Junsuk Rho, "Near-zero reflection of all-dielectric structural coloration enabling polarization-sensitive optical encryption with enhanced switchability," Nanophotonics, vol. 10, no. 2 (online, November 26, 2020), DOI:https://doi.org/10.1515/nanoph-2020-0440. This is an open access article, published under the Creative Commons Attribution 4.0International License. A PDF file is available at the article location.
  2. A display that completely blocks off counterfeits, Pohang University of Science & Technology (POSTECH) Press Release, January 21, 2021. Also available here.
  3. Mohammadreza Khorasaninejad, Wei Ting Chen, Robert C. Devlin, Jaewon Oh, Alexander Y. Zhu, and Federico Capasso, "Metalenses at visible wavelengths: Diffraction-limited focusing and subwavelength resolution imaging," Science, vol. 352, no. 6290 (June 3, 2016), pp. 1190-1194, DOI: 10.1126/science.aaf6644.
  4. Leah Burrows, "Metalens works in the visible spectrum, sees smaller than a wavelength of light," Harvard University Press Release, June 2, 2016.

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