Optimization of DIfractive Structures for optical SEcurity Applications – ODISSEA

The security holograms used for the authentication of documents like, for example, passports or driver licences, are composed of diffraction gratings. Furthermore, their chromatic response arises as a direct consequence of the interaction of the gratings with light. This mechanism for the generation of color is known as « structural coloration » and presents several advantages over pigment-based coloration, the most significant is that it only depends on the knowledge of the opto-geometrical parameters that characterize the diffraction grating. This fact suggests the possibility to generate specific colors through the optimization of the grating’s geometry and this objective can be achieved employing suitable modern optimization and machine learning techniques, like genetic algorithms and neural networks, that have proven successful for the resolution of this kind of optimization problems and can take into account feasibility restrictions, generally imposed by the fabrication processes.

The results obtained in this project can be classified in two main sets. The first concerns the application of metaheuristic optimization combined with neural networks to tailor, prior to fabrication, the chromatic and visual response of the holograms used for optical document security applications. The computational tool developed opens the possibility to take into account the feasibility constraints imposed by the mass fabrication process of our industrial partner. The second set of results can be considered as more fundamental, as it answers several open questions concerning the chromatic response of existing diffractive structures fabricated by our industrial partner. That is, it provided an explanation of the origin of certain effects that had been observed. Furthermore, it allowed to enhance the capabilities of the Fourier Modal Method and the Differential Method for the characterization of complex diffraction gratings and their application for color generation.

From an application point of view, the use of modern optimization techniques, combined with Artificial Intelligence, opens the way to our industrial partner for the design of optimal diffracting structures entirely satisfying the constraints imposed by the fabrication process. This approach allows to define and control the visual response of a diffracting structure prior to fabrication, which avoids the waste of time and materials consequence of the fabrication of multiple prototypes. The computing tool developed is completely generic and its modular structure allows the extension of its usage to other applications about documents security.

As for the more exploratory aspect, there are no restrictions on extending the application of the numerical computing tools developed in areas other than documents security. In particular, the new implementation of the MD-FFF method is an efficient solution to simulate diverse structures. These results encourage us to develop a 3D solution to compete with more traditional methods.

Other exploratory perspectives of the ODISSEA project are the development of a so-called phantom security imagery or holography, where images or holograms can be created thanks to quantum light, in particular to quantum entanglement. Also, the multi-objective optimization module developed can be used for the study of multi-physics problems. In particular, the characterization and optimization of the mechanical and optical properties of bio-inspired structures such as artificial nacre, whose hardness and transparency are examples of properties often exploited for industrial applications.

The results of this project have been reported in 6 peer-reviewed international publications, 7 international conferences, 5 national conferences and 1 published patent. It is noteworthy to mention that 4 of the peer-reviewed publications concern the optimization of diffractive gratings for color reproduction; whereas 2 are directly related with the improvements on the electromagnetic method used to compute the diffracted spectra generated by the gratings.

In a recent joint report, the European Police Office (EUROPOL) and the Office for Harmonization in the Internal Market (OHIM) pointed out the disastrous economical (200 billion USD per year) and health-related consequences of goods counterfeit. The dreadful events that have recently happened in Europe made evident that travel and identity documents such as passports or ID cards are among the most counterfeited products. Counterfeiters have also benefit of the recent development in fabrication and characterization technologies that paradoxically, had led, to important advances in the Optical Document Security (ODS) domain. There is therefore a need for even more innovative optical security devices involving complex designs and new materials difficult, if not impossible, to fabricate without specialized laboratory equipment. ODISSEA lies within this context. The project aims at developing the first stage towards innovative ODS. That is, a computational tool that should serve to settle the basis for a more efficient and intelligent way to characterize the Diffracted Optically Variable Image Device (DOVID) required for the security applications just mentioned.

The ultimate goal of the project is two-folded. From an applicative point of view, we aim at going much further into the analysis of the devices and working principles. We expect to have a modular toolbox that should serve as a starting point for the modeling, characterization and optimization of the relevant optical and material parameters involved in the visual response of DOVIDs. Here we will couple rigorous numerical methods with optimization techniques. A selection of the most suitable numerical tools, commercial or in-house, will be done through a comparison of their performance when applied to reference diffractive/periodic structures provided by SURYS. By combining the most efficient modeling and optimization codes, we expect to design original and efficient DOVIDs, compatible with a mass production. To test the optimized design, a few of the structures will be fabricated. The consortium is going to lean on the fabrication process developed by SURYS (Recombining, roll to roll, layer deposition or varnish deposition) as well as the fast prototyping techniques developed at UTT. To ensure a real feedback between the modeling and the experiment, the fabricated structure must be perfectly known or at least should have an opto-geometrical shape close to that of the structure used in the numerical stage. Other research ways are going to be explored thanks to the partner expertises on the study of the spectral response of nano-particules or the effect of surface roughness on the spectral response of layered structures well suited for mass production.

From a more fundamental point of view, we also aim to explore further the capabilities of plasmonic structures combined or not with dielectric resonant waveguides for the design of more efficient and secure DOVIDs. This new and high potential design will rely on the academic partners’ expertise in integrated near field optics. We will explore the possibilities and limitations of combining current holographic technologies, which usually make use of flexible polymeric substrates, with glass integrated optics for security applications such as labeling, optical sensing or filtering, where the use of holographic nano-structured membranes deposed on a glass substrate could be a way to add new functionalities to the low cost glass waveguide technology and further enlarge the application scope for our industrial partner.