Light steering approach holds promise for biosensors, optical security apps

Light steering approach holds promise for biosensors, optical security apps

Technology News |
By Rich Pell

Today’s diffractive laminates such as Surys’ DID optical security markers found across millions of banknotes and ID documents are based on nanostructured resonant waveguide gratings (RWGs). So far, the markers’ colour changes (when seen at different angles) can’t be reproduced by counterfeiters, but being visible, fine imitations of these visual elements will trick the unwary consumer.

Stepping up the game in mass-producible anti-counterfeit measures, Swiss researchers from CSEM SA and EPFL Lausanne have come up with a novel way to use RWGs, pairing them to yield new optical properties. Moving away from specular reflection that gives a visible image under daylight, from any angle, the researchers coupled RWGs of different periods in such a way that the direction of the out-coupled light differs from the direction of the incoming light and can be tuned so as to be invisible except under a narrow viewpoint under a strong white light.

A single unit cell showing the steering in reflection. (a) Corrugated ultrathin waveguide coats a first and a second adjacent gratings, schematized respectively in pink and blue. A specific wavelength range is in-coupled inside the waveguide by the first grating from a white incident light beam, and out-coupled from the second grating. (b) By changing the period of the second grating, it is possible to out-couple the light at a different in-plane angle. (c) and (d) Corresponding amplitude of the transversal near-field obtained with FDTD simulations.
The resonant waveguide gratings can be engineered
on a surface so as to redirect the light coming from
one point to another using constructive interference.

Publishing their findings in ACS Photonics under the titled “Color-Selective and Versatile Light Steering with up-scalable Subwavelength Planar Optics“, the researchers put their theory in practice, crafting patterns of nanostructures that remain essentially transparent (invisible to the eye) but that can be revealed on a smartphone’s screen when flashed by the camera’s white LED.

The team demonstrated large (10x10mm) multi-colour security markers that would not be seen under natural light (due to a low resonance efficiency) and engineered so that the out-coupled light from the patterns would narrowly be steered to the smartphone’s camera (remaining invisible from looking aside).

Customized patterns (red, blue, and green squares
each 10x10mm) revealed on a smartphone’s screen
(the light source is the flash, the camera is the only
observer as the devices are not visible outside the
designed observation point.

Talking to eeNews Europe, “If you use a traditional hologram as a security feature, then it is always visible and it can be more easily copied or faked”, said Guillaume Basset, Project Manager at CSEM and one of the authors of the paper. “Here we can choose to make it visible only at specific angles under white light illumination so it remains hidden except when examined with a smartphone camera. This allows the general public to perform the authentication, without specific training”.

Admitting some similarities with Surys’ DID technology (at least for the use of RWGs), Basset clarified: “We are working with Surys on various technologies but this development has nothing to do with them. The DID is also an example of technology we transferred to SURYS quite some time ago”.

Talking about the colour-selective light steering RWGs, Basset continued “we patented almost everything on this technology, from building blocks to integration and variations (hidden, animation, MRF etc…)”.

Discussing alternatives, such as meta-surfaces that are trending now in literature, or other hidden anti-counterfeit tags revealed through UV or IR light and dedicated readers, the researcher dismisses them as too costly to implement on a wide scale.

“A lot of startups propose unique fingerprinting solutions using invisible encoded tags and dedicated readers, but it is more of an academic technology push than what industry wants. Nobody uses them because they require a dedicated equipment which would be costly to deploy. And often, you want to be able to identify a document or a bank note immediately, on the spot”.

What’s more, the nanostructures are compatible with up-scalable fabrication processes such as roll-to-roll nanoimprint lithography (NIL) replication (from a master obtained through fast e-beam lithography technique).

An interesting application of the narrow light steering capability of these security markers is that multiple logos with different light out-coupling angles could be patterned within a given substrate area, so that as the smartphone scans the area, the logos could show up, one at a time. This enables animated logos, something brands may be keen to adopt to make their logo more interactive.

EPFL Lausanne –
Surys –

Related articles:
Counterfeit parts represent $169B annual risk

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