Researchers have developed a new meta­surface-based device that can produce 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. “Our new meta­surfaces give an immediate and simple readout of the environment sur­rounding the surface of the device,” said research team leader Andrea Di Falco from the University of St. Andrews. “In addition to encryption applications, this could be used as a humidity sensor or as part of more complex light-based biomedical systems.”

The researchers demonstrate a device that reveals a holo­graphic butterfly with 705-nanometer light in dry conditions and a caterpillar when 750-nanometer light is combined with wet condi­tions. When 660-nanometer light is used in dry conditions, both images are visible. The selective response of the metasurfaces can be tailored to work with various pre-programmed condi­tions. “Our holographic devices contri­bute to the worldwide effort to develop the physics and applications of photonic meta­surfaces, which are one of the most versatile photonic tools available to researchers to encode and manipulate infor­mation using light,” said Di Falco.

The new work is part of the Di Falco team’s ongoing effort to develop photonic meta­surfaces that operate in the visible range and can be used for biomedical applications, which 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,” said Di Falco. “We are specifically interested in inte­grating different photonic func­tionalities into one device.”

Holographic meta­surfaces can create detailed images that depend on the properties of the light 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 reflects light of a specific wavelength in dry conditions, while the other has high reflec­tivity only when a different wavelength is used in wet conditions.

“We designed a relatively simple resonant meta-atom geometry that enhances the response of the holo­graphic metasurface to the environ­mental conditions,” said Di Falco. “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.” The researchers assessed the response of the holo­graphic meta­surfaces by quantita­tively measuring their efficiency for different wavelengths in air and in water. The metasurfaces produced very good contrast between the images, with little crosstalk. In other words, only one image was visible for each set of parameters, which demons­trated two-tier infor­mation encoding.

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. (Source: Optica)

Reference: L. Yan et al.: Two-tier manipulation of holographic information, Opt. Exp. 30, 19145 (2022); DOI: 10.1364/OE.456843

Link: SUPA – School of Physics and Astronomy, University of St Andrews, St Andrews, Scotland, UK