# Application of Photo-Induced Chirality in Cover Authentication
###### Abstract  
A new technology to write and read covert information in authentication labels is described. This technology uses the phenomenon of photo-induced chirality in Ge\({}_{2}\)Sb\({}_{2}\)Te\({}_{5}\) thin films to encode the left- or right-circular or linear polarization of the laser beam used to write the label. The written polarization can be revealed by a simple reading device, which is demonstrated to provide the same qualitative information as reading based on cyclotron circular dichroism spectroscopy and imaging. The suggested method, while based on existing manufacturing approaches, offers a balance between technological complexity for writing and simplicity for reading, and may be advantageous as a new authentication technology. authentication labels; chirality; polarization; circular dichroism +
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directly on PET substrates that can be used as security labels. The stored images can then be visualized by illumination with a coherent light source [2]. New technologies for use in authentication labels are required and are constantly being proposed, each with corresponding advantages and disadvantages. For example, it has been suggested that unclonable security labels can be printed using ink-jet technology with different quantum dots that emit red, green and blue light to achieve full-color fluorescence in flower-like patterns. These labels can then be read with the help of artificial intelligence [3]. One of the approaches to increasing label security is to store covert information with higher levels of sophistication. As an example, it has been shown that current two-dimensional surface-enhanced Raman scattering (SERS)-based security labels can be extended to three dimensions. The reported 3D pillar microstructures allowed encoding for at least three layers of information within the same area along an axis perpendicular to the plane. These layers can represent predesigned covert images which can only be fully recovered by SERS imaging at predetermined depths [4]. This approach, however, requires complicated equipment to read the labels. Using polarization of light to encode covert information in security labels is a promising option. Recently, a liquid crystal (LC)-based optical device has been demonstrated, which can selectively reveal images by using polarized illumination. This device consisted of a surface fabricated from patterns of helical photonic crystals of different chirality that encode two different images. The LC layer on top acted as a tunable phase retarder, and by applying a suitable voltage across the LC layer, either a right- or left-circular polarization image could be viewed [5]. Another approach to utilizing circularly polarized light in security labels has been reported using a double-layer arc as 3D chiral color pixels. By varying the arc lengths, a range of colors can be achieved, which differ for left- or right-handed circularly polarized light (CPL). This can be used to produce images with varying intensity and different colors when the label is illuminated by differently handed circularly polarized light [6]. A drawback to this approach is that it requires expensive lithography technology to prepare chiral arc arrays that constitute the chiral pixels. Assembling luminescent quantum dots in a helical array within a matrix of suitable LC with a chiral dopant presents the possibility of preparing circularly polarized luminescence materials. These materials can be part of a multimodal-responsive security materials that can show responses to light activation, light polarization, temperature, voltage, pressure and viewing angle. The preparation of such materials requires high-quality quantum dots and relies on a multistage synthetic route [7].