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Research Activities

Holography and security holograms

Focusing on the degree of freedom represented by the decoding beam, new types of security holograms capable of decoding visual information only by means of light with definite spatial distribution of intensity, phase and polarization state have been engineered. The security holograms developed belong to two typologies: in the first one, constituted by a multilevel pixelated structure, the light that illuminates the hologram is modeled by particular phase masks (Spiral Phase Plates) in order to generate a unique decoding key allowing the correct reconstruction of the image. Traditional illumination will not succeed in decoding the image. A second type of security-hologram relies on the properties of Metasurfaces. Many works are devoted to the control of light with 2D metamaterials. They take advantage of resonant scattering by nanostructures instead of phase-accumulation through propagation. By tailoring shape and size of the nanostructures, hence modifying the properties of the material, we are no more limited to the normal response of naturally existing materials to electromagnetic waves: metasurfaces offer new abilities to control optical beams. Devices of general interest such as beam deflectors, holograms, high-performance metalenses at visible wavelengths and polarizing interfaces can be realized with those syntethic materials. An example of metasurface used as security-hologram is made up of subwavelength gratings with space-variant orientation capable of selective image reconstruction. By illuminating the hologram with a specific polarization, a first image is detected, while switching the polarization to its orthogonal state allows the reconstruction of a second image. These novel holographic devices are currently optimized and tested for industrial scaling.

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