Following the demand for functionalization of optical materials, laser-induced photoinscription opens up new perspectives for emerging photonic applications. The challenge is to design dielectric properties and add specific optical functions to compact, integrated optical devices. Nevertheless, the result of the laser action depends on the nature of the material, and the balance of electronic and structural transformations associated with refractive index change under light exposure defines the resultant optical conversion. Therefore, mastering the induced modifications requires extensive knowledge on the nature of light-generated matter transformation. We propose an advanced material structuring approach based on adaptive pulse spatio-temporal manipulation for optimizing ultrafast laser-induced processes inside transparent materials. The dynamic control over the energy delivery will lead to a significant upgrade in the quality of the laser-induced structures and will enable precise refractive index engineering. The technique allows to detect and influence dynamical processes on molecular and mesoscopic scale in a controllable and optimal manner towards user-defined outputs. This approach is aimed to extend the knowledge basis and to correlate irradiation conditions and material properties in a synergetic manner for optimal laser processing applications.

The present project supports the creation, in 2010 at LabHC, of a team-project entitled « nanoparticles and sol-gel for photonics » that involves the participants in this “young researcher” project and that is led by the project carrier. The work we propose to achieve aims at developing: • a know-how for the fast inscription of permanent or updatable multicolour micro-patterns by an “all-optics” way and by an “electrical” way. The “all-optics” approach is based on the use of lasers emitting at different wavelengths (UV and visible) and enables the treatment of large surfaces with a resolution of the order of few micrometers. The electrical writing is based on the use of an Atomic Force Microscope (AFM) and allows the achievement of coloured patterns with a sub-micrometric resolution. This technique can be, eventually, coupled with the optical approach. This know-how is based on the use of titanium dioxide matrices, in which we will reduce and oxidize metallic nanoparticles, successively. • a knowledge, on the light-matter interactions that lead to the formation, deformation and destruction of the metallic nanoparticles, on the colour of sets of nanoparticles (heterogeneous and diluted NP; homogeneous NP with particular shapes; NP in strong interaction), and on the plasmonic coupling between close metallic nanoparticules. The developed know-how will provide a technique for direct inscription of updatable colour patterns requiring neither chemical nor thermal post-treatment. It is based on the control of the growth conditions of metal nanoparticles in solid matrices of porous titanium dioxide and benefits from the photochromic behaviour of these compounds. This know-how, based on the synthesis of sol-gel thin films, the control of the photo and electro-inscription processes and an appropriate modelling, presents the following innovative features: - inscription of multicolour patterns on the same sample - Sub-micrometer spatial resolution - reversible or permanent patterns (optional). - Fast inscription (of the order of few seconds). The writable or re-writable, high-resolution and coloured materials that will be developed, well complement the available solutions for photo-inscription. They should find applications in areas such as image reproduction, data storage or product-marking for anticounterfeiting and traceability. One strength of this project lies on the multidisciplinary approach. The project involves : -chemists for the sol-gel elaboration of mesoporous films and their structural charcaterization at atomic and nanometric scales; -physicists and opticists for the development of inscription processes and for the optical and structural characterization of nanoparticles grown in the matrices; -theoreticians in electromagnetic optics for the modelling of the optical properties of the nanomaterials.

In order to manage some of the huge data sets that are now available, and more particularly to classify, recognize or search through these sets, one needs a representation system which is rich enough to describe the data while allowing an efficient and mathematically well understood exploitation. This sort of representation is both well defined and nicely computed when data are numerical values, or more generally vectors of numerical values. - However, many objects are poorly modelled with such vectors of numerical values that cannot express notions such as sequentiality or relationships between attributes. In particular, this project aims at representing and exploiting thumbnail images such as those returned by search engines like Google. If much work has been done on images having high definition levels, none concerns the question of filtering these small images, the definition of which is too low to allow a segmentation into regions and/or the exploitation of wide support local measures. - An appealing alternative lays in modelling images by extracting and symbolically structuring salient points: salient points, corresponding to the image high contrast points, may be easily detected in thumbnail images; we propose to structure them by means of strings, trees, or more generally graphs, in order to integrate information on saliency degree or spatial relationships. - We propose in this project to study the capabilities of such salient point structuring to model and exploit thumbnail images. This goal implies the definition of a new paradigm for analysing and statistically characterizing symbolic structured data, at odds with classical approaches used for numerical data. - To achieve this goal, a first step is to choose a symbolic structuring and define a relevant distance measure, such that the distance between the structures that model images actually reflects image similarity. A main difficulty relies in the very special semantics of our structures. In particular, when chaining salient points by decreasing saliency, the obtained strings have the unusual property that symbols at the beginning of the string are much more important than symbols at the end. - A second sub-goal is to define a statistical characterization of a set of symbolic structures (strings, trees, or graphs) modelling images: to perform clustering or filtering, one needs statistical characterizations of sets of structures such as mean or median structures, but also measures that evaluate the dispersion of the structures within the set, such as variance, standard deviation, or probability density. - A third sub-goal is to propose algorithms for computing the statistical measures introduced previously. The challenge relies here in the fact that the sets to characterize may contain a huge number of structures, that may contain thousands of symbols, and that most of the problems to solve are NP-hard. - Finally, we shall validate our work on tasks dealing with thumbnail images, typically classification and filtering. The low definition of these images forbids us to use classical pattern recognition approaches; the huge number of thumbnail images available on the web imposes to design efficient algorithm with nice scale-up properties. - ...

The project is focused at the improvement of the laser assisted APT and in the widening of its capacities in terms of the resolution, size of the analyzed region, materials to be analyzed, and precision. The main scientific objective is to establish the relationship between (i) illumination characteristics, (ii) material properties, (iii) electron or electron-hole pair excitation mechanisms and carrier dynamics (iv) subsequent material ejection. To achieve these goals, a better understanding of both ultra-fast femtosecond interactions and field extraction processes is required. In particular, this understanding should lelp to provide a possibility to explain the time of flight spectra based on the confrontation of the experimental findings and numerical models; a possibility to study electron excitation, ionization and recombination in dielectric and semiconductor materials looking at the ions evaporation time, a possibility to reduce the absorption and thermal damage optimizing the laser parameters according to the numerical calculations of absorption distribution and tip temperature evolution and a possibility to increase the evaporation rate changing laser parameters (wavelength, polarization, pulse width) according to the field extraction theory. The main challenge of this project is in the development of a protocol to analyze all materials and new nanoelectronics devices with atomic resolution in space. All diagnostics need theoretical support to extract the mechanisms of the atomic ejection and to enhance the flux. The objectives of the project require addressing the following scientific questions: (i) Excitation and ionization mechanisms occurring during the laser pulse (~100 fs) in the presence of a strong external field; (ii) Electron energy relaxation, electron-ion/matrix interactions and heating (femto- to picosecond time scale, ~1 ps); (iii) Subsequent material ejection and field extraction (up to nanoseconds). These processes occur at all time scales, from several femtoseconds.to nanoseconds Therefore, corresponding numerical models will be developed to provide a multi-scale simulation of the involved physical processes and, as a result to elucidate the basic mechanisms of material extraction. In particular, such techniques as detailed ab initio calculations of the density of states and the properties of the electronic sub-system, kinetic modeling of laser excitation and electronic transport, two-temperature thermo-dynamical calculations and molecular dynamics simulations of material ejection will be performed. As a result of the modeling, a better interpretation of the experimental results will be provided.