Researchers Deploy Metasurface Tech to Design of Encryption Devices

Researchers from the University of St. Andrews in Scotland have developed a metasurface-based device that produces multiple distinct holographic images depending on the surrounding medium and the wavelength of light used. The ability to store information that is only retrievable with the right set of keys — such as a certain light wavelength combined with wet conditions — could be further developed to design simple yet effective encryption devices.

The device demonstrated by the researchers revealed a holographic butterfly with 705-nm light in dry conditions, and a caterpillar when 750-nm light was combined with wet conditions. When 660-nm light was used in dry conditions, both images were visible. The selective response of the metasurfaces can be tailored to work with various pre-programmed conditions, the researchers said.

The work is part of an effort by researcher Andrea Di Falco and her team to develop photonic metasurfaces that operate in the visible range and that can be used for biomedical applications. These biomedical applications often involve wet environments.

“Holographic metasurfaces are particularly interesting because they can be used to deliver complex information by producing images that can be immediately interpreted, without the need for additional layers,” Di Falco said. “We are specifically interested in integrating different photonic functionalities into one device.

“We designed a relatively simple resonant meta-atom geometry that enhances the response of the holographic metasurface to the environmental conditions. This is an effective way to use the refractive index of the surrounding medium as an additional degree of freedom that enables multiplexing of information encoded in the metasurface.”

Researchers at the University of St. Andrews developed a metasurface-based device that produces holographic images that depend on the surrounding medium and the wavelength of light used. The device demonstrated by the researchers revealed a holographic butterfly with 705-nm light in dry conditions, and a caterpillar when 750-nm light was combined with wet conditions. Courtesy of Andrea Di Falco/University of St. Andrews.

Holographic metasurfaces can be used to create detailed images that depend on the properties of the light that they reflect or transmit. To create holographic metasurfaces that encode more than one image, the researchers used two types of meta-atoms, which act like pixels. One type of meta-atom only reflected light of a specific wavelength in dry conditions. The other meta-atom displayed high reflectivity only when a different wavelength was used in wet conditions.

To assess the response of the holographic metasurfaces, the researchers performed quantitative measurements of their efficiency for different wavelengths in the air and in water. These measurements showed that the metasurfaces produced very good contrast between the images, with little crosstalk. Only one image was visible for each set of parameters, which demonstrated two-tier information encoding.

According to Di Falco, the metasurface technology could be used as a humidity sensor or as part of more complex light-based biomedical systems, in addition to encryption applications. The holographic metasurface like that produced by the research team paves the way for multi-degree-of-freedom manipulations of holographic information.

Next, the researchers plan to increase the selectivity of the meta-atoms to create metasurfaces that distinguish between different media that have very similar optical properties. This could be used to create holographic sensors that produce different images when exposed to different concentrations of a particular substance.

Funding for the research was provided by the European Research Council under the Horizon 2020 Framework Programme Research and Innovation Program.

The research was published in Optics Express (www.doi.org/10.1364/OE.456843).