From Costa Rican Butterflies to Meta’s Latest Plasmonic Banknote Features

Meta Materials Inc (previously Nanotech Security Corp) has been a pioneer in optically variable device technology for the past decade, with innovations in micro, nano, and now plasmonic structures. The company has also developed in-house secure foil production so that it can offer banknote stripes direct to customers.

The physics of surface plasmons deals with phenomena that occur at an interface, or boundary, between a dielectric (an insulator whose electric charge can be shifted without flowing) and a metal (a conductor whose electric charge can flow).

Under certain controlled conditions, energy, in the form of electromagnetic excitations, moves along the interface between the dielectric and the metal in the form of a wave. These so-called evanescent waves are trapped and can only move in particular directions. The plasmons can be controlled through careful modelling and application of, amongst other things, the dielectric and metal layer thickness in a ‘sandwich’ structure.

When the sandwich structure is applied as a device to a security document, it allows light that falls on the surface of the device to be controlled in terms of its wavelength (allowing control over the colour) and amplitude (giving rise to the intensity, or brightness of the colour generated).

In the recent webinar ‘Producing the World’s First Fully Plasmonic 3D Animated Banknote Security Feature’, hosted by Reconnaissance International, Meta’s VP of Product Management, Neal Skura, and VP of Technology & Innovation, Clint Landrock, explained how they were inspired to create Meta’s KolourOptik® Stripe and its toolkit of customisable authentication effects with 3D parallax.

Inspired by a butterfly

Everyone’s route into science is different and Clint’s portion of the webinar took us on his personal journey, exploring the jungles and volcanoes of Costa Rica as a young grad student: ‘I was inspired by how the blue morpho butterfly’s brilliant colours never faded or dulled under the dark canopy. This curiosity drove me into looking deeper into how these beautiful butterflies managed this feat of evolutionary engineering.’ 

What’s interesting about this particular butterfly is that its wings are actually colourless, meaning that there are no pigments or dyes creating the colour. If you look at them through a light source, they are actually transparent. The colour is generated by a combination of macro, micro and nanoscale structures on the wings that combine to filter the light, except for the light in the blue part of the spectrum. The blue light is captured and reflected back, and this is what makes the wings appear as a very bright blue colour even in dark, challenging environments.

Nanostructures in nature (© Meta).

Clint explained that Meta has followed nature’s multifaceted structural colour approach. It has combined macro size, unit cells of microscale structures to create the foundations of 3D display pixels, and then combined those with nanoscale plasmonic meta structures that, like the blue morpho, filter out light they don’t want. This allows them to capture and select the colour of light they want to use for a given image subpixel.

These nanostructures are impressively small, more than five times smaller than the wavelength of visible light, and so densely packed that the distance between neighbouring structures is much smaller than the wavelength of visible light.

Being able to manipulate light at the wavelength level is one thing but, as Clint went on to explain, to achieve visual authenticity in a feature requires full colour, movement, and 3D parallax – and to achieve the best levels of authentication requires a combination of all three. By combining plasmonic colour subpixels onto microscale pixels, Meta can produce a visual authentication feature that combines these three vital elements in a single origination.

This fits well with Meta’s sustainability ethos of doing more with less, as mass replication of the feature requires a single embossing layer which in turn reduces the amount of base materials needed to produce KolourOptik Stripe.

Simple, compatible, consistent

During the webinar, Neal Skura took over the theme of banknote authenticity from Clint, summarising that optical features need to be immediately obvious and engaging but at the same time complicated enough to prevent counterfeiters from reproducing them, and impossible to simulate with other technologies.

Many believe banknotes to be the best protected and most viewed pieces of artwork in the world, but with the added requirement that the artwork is produced by the millions, and carries optical effects of an identical quality and appearance.

Neal went on to explain that KolourOptik’s ultra-thin surface-applied form factor makes it compatible with all banknote substrates in production equipment, removing the risk of banknote distortion and without requiring specialised application machinery or changes to existing processes. The ultra-thin form factor derives from its single layer of embossed structures, reducing the number of production variables and simplifying both the manufacturing and application processes.

‘There’s no need for laminated layers of lens arrays, focal spacers or inks and dyes. Importantly, because we’re not using inks or dyes, multiple colours can easily be included in the design without additional processes or costs,’ said Neal.

Added to the cost saving aspect of structural colour, designers can be confident that the colours they select remain constant across the entire viewing angle to deliver a consistent and repeatable authentication feature.

Meta has been innovating not only the nanostructures and plasmonic effects, but also the production processes and tools required to bring their technology into high volume manufacturing.

Manufacturing the KolourOptik Stripe requires industrialised nanofabrication processes that ensure that the fidelity of the nanostructure is preserved at every step, so that the visual effects remain consistent and true to the intended design.

It should be made clear, though, that while the nanostructures and production processes are innovative and proprietary, the resulting finished trade product is in a standard surface-applied format. Neal emphasised that ‘it was important throughout the development process that our innovations in nano optics were delivered in a format that was accessible to all with compatibility across all existing banknote industry materials and production processes’.

In the past year, Meta has successfully qualified its in-house volume production capabilities, demonstrated industrial application of over 100,000 stripes onto banknote paper and completed durability tests with multiple test partners with good results.

Extension to ID and other documents

As the webinar moved into the Q&A session, one questioner pressed Clint and Neal on whether there are any issues with integrating the product onto cotton banknotes. For example, do the materials react differently as the temperature varies or in general usage such as folding and crumpling? The pair assured the participants that the material had performed well during the extensive testing that had been conducted and they were confident of its durability and performance.

Another questioner from the virtual floor of the webinar asking whether KolourOptik Stripe is only available for banknotes, drew the response that ‘yes’ KolourOptik Stripe technology is currently only available for banknotes, but that the company is working on other colour optic technologies for ID and other government-issued secure documents.

And with the team at Meta’s ingenuity in bringing KolourOptik Stripe technology from the rainforest canopy to banknotes of the world, who would doubt them?