Image multiplexing

Image multiplexing (the combination of information from several images into one) is a technique with potential application in the encryption of information or the security of optical documents, since multiplexed images are difficult to copy, easy to authenticate and impossible to alter them without destroying the multiplexing effect. A novel strategy to implement image multiplexing is based on the use of optical metasurfaces (thin nanostructured photonic layers that generate a strong interaction with light) in which metallic nanostructures with high precision ordered configurations, dimensions and positions allow the design of the optical properties of the metasurfaces. However, its use in industrial and large-scale processes has not been feasible because its manufacture requires the use of focused electron or ion beam lithography, very expensive techniques. An international team with the participation of the CSIC's Optical Institute has manufactured more economical and versatile metasurfaces capable of generating multiplexing using laser processing. The study is published at Advanced Functional Materials .




As Jan Siegel, CSIC researcher in the Group of Laser Processing (GPL) of the Institute of Optics (IO-CSIC) points out, “Femtosecond laser processing provides an alternative manufacturing method, especially for larger areas, combining high versatility and low cost, which allows the personalized printing of color images on a material ”.

“Unlike previous manufacturing techniques, which create nanostructures one by one, laser processing generates mechanisms that modify several parameters of the nanostructures simultaneously. They form what is called a random metasurface, which is a thin sheet of mesoporous titanium oxide with silver nanoparticles in the pores. It has an optical behavior like that of a classical metasurface but consists of randomly placed nanoparticles with certain statistical distributions of size, density and orientation, instead of perfectly ordered elements all with the same size, density and orientation ”, explains the scientist.

“The different colors are the result of a combination of absorption, interference and diffraction phenomena generated by laser-induced modification and self-organization of nanoparticles. By fine-tuning the energy of the laser pulse, the color of each pixel in the image can be independently controlled, ”he adds.

Three images in one

“This technique - points out Siegel - has great potential. Because laser processing is fast, cost-effective, flexible, and suitable for large-scale printing, this technology offers an innovative route to industrializing a new generation of security features for the fight against counterfeiting, as well as for the optical storage of data, pattern recognition or optical encryption ”.

Laser processing also makes it possible to increase the number of images that can be combined in image multiplexing, making copying more difficult and providing documents with greater security. “By independently controlling the effects of absorption and interference, the colors in reflection and transmission can be varied independently, resulting in a multiplexing of two images under white light. By generating an asymmetry in the shape of the nanoparticles, a third image can be multiplexed and revealed through polarization changes ”, explains the CSIC scientist.

Link to article


Link to the press release

The work has been led by Nathalie Destouches, from the Laboratoire Hubert Curien, from the Jean Monnet-Saint-Étienne University and the CNRS (France); And it has had the participation of the company HID Global CID (France) and the National Institute of Applied Sciences of Lyon (INSA Lyon), belonging to the University of Lyon (France).