Researchers use metamaterials to create two-part optical security functions

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Researchers developed advanced optical security features using a two-piece metamaterial system to create an optical phenomenon that is difficult to reproduce. Metamaterials are designed to have a property that is not found in natural materials. The new security features could offer improved protection against falsification of valuable products or banknotes, as well as encryption of information such as personal identification numbers physically sent to recipients.

Optical security features are often used nowadays to check the authenticity of currency, identification cards and valuable products such as electronic products. These features include holograms that change color at different viewing angles or patterns that only appear under ultraviolet light. Security features with many features that are difficult to replicate are the safest because they are difficult to replicate.

"An ideal optical security feature should be difficult to copy by unlicensed people, easy to mass produce and be conveniently interrogated," said head of the research team, Gokhan Bakan, of the Manchester University in the United Kingdom. "Our approach meets all these requirements and could offer all transactions of goods and information more secure."

In the journal of the Optical Society (OSA) Optical Letters, the researchers describe a system in which two thin optical pieces must be placed together to form a metamaterial revealing a hidden message or a readable QR code to the naked eye. The re-encryption is performed by detaching the upper part, which contains no information, from the lower part which encodes a message.

To protect sensitive information such as credit cards, a customer can get the top of the security function (the key) of the bank at the time of the request. When the encrypted pin arrives by mail, the customer uses the key to reveal the pin. If the print with the encrypted personal identification number is stolen, it would be impossible to decrypt it without the key.

Reveal the secret message

The new two – part security features have a lower part formed by applying a thin insulating material, or dielectric, to a silver film of a thickness of about 120 nanometers. This lower part is essentially a mirror to the extent that it reflects most of the incoming light. Because its optical properties are defined by the silver film and unaffected by the thickness of the dielectric layer, a message can be masked on the dielectric by simply adding more dielectric in the form of the message.

The top of the security element is a transparent elastic substrate covered with a metal layer of about 10 nanometers in thickness. This part, which plays the key role, contains no information and seems semi-transparent. When the two parts are assembled, the thin metal of the upper part facing the lower part, it constitutes an optical cavity whose properties, such as the color of the surface, strongly depend on the thickness of the dielectric. Thus, when the two pieces are placed together, a striking color contrast appears and reveals the hidden message, which is readable to the naked eye. This approach can be used to create specific keys for each message or to create a master key that would work for any message.

"When the two elements of the security function are glued together, it creates an optical phenomenon called an optical cavity effect enhanced by plasmonics," said Bakan. "Although this effect is commonly used for various applications, such as optical filters, we have only separated the optical cavity in two, which allows information to be hidden in one part in a way that can only be revealed when it is needed. With the right key. "

Flexible security

The researchers demonstrated their new approach by coding QR codes on rigid substrates as well as flexible substrates that can be used on almost any surface, including banknotes. The QR codes were invisible to the naked eye until an adhesive patch of the transparent elastic substrate was applied. They also used the approach to encode various motifs and words.

Although researchers have demonstrated a specific application to optical safety, this approach could also be used for optical detection of chemical or biological applications. For example, if some proteins are attached to a thin film, the modular metamaterial can produce a reading visible to the naked eye or readable by a camera.

The researchers plan to further develop the new optical security feature by using it with other optical phenomena. They also want to communicate the technology to security label developers and banks so that the technology can be tested and developed for real-world applications.

"Our research shows that converting static optical features into modular optics can open up new applications," said Bakan. "This offers a new perspective that scientists could use to develop other established optical methods."