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Understanding the spontaneous emergence of patterns on laser-irradiated surface has been a topic of interest for many years. Brought far from equilibrium under ultrafast photoexcitation, the material exhibits topographic surfaces that reveal signatures from multiphysical coupling, interrogating on the principal mechanism that drives the organisation nature. Some laser-induced periodic surface structures reveal periods typic from electromagnetics, nonlinear optics and plasmonic origin whilst others are characteristic of fluid dynamics or even thermochemical reactions. However, upon multi-shot irradiation, liquid flows are driven by laser-induced energy gradients and patterns are unstable leading to a variety of possible states, including hybrid states of bistable patterns and chaos. Stable spatially-periodic patterns typically arise through small, localized perturbations of the underlying optical coupling on random nanoreliefs. These perturbations grow in amplitude until nonlinear saturation takes over while, at the same time, the nascent topography disturbs the optical response. Whereas the spatially uniform growth into a pattern that can be described within the classic Maxwell and Navier-Stokes equations, the propagation, spatial competition of modes and bifurcation regime require nonlinear dynamics that mimic the behavior of the excited systems near the onset of convective instabilities. One of the challenges is to develop a general and efficient model that inherit relevant symmetry and scale invariance properties and that contain the stochasticity (emergence) and nonlinear properties (dynamics) able to reproduce dissipative structures in spatially extended systems. A stochastic Swift-Hohenberg modelling is then proposed to reproduce hydrodynamic fluctuations at the convective instability threshold that has been recently demonstrated as the very nature of the laser-induced self-organized nanopatterns.

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International audience; Periodic self-organization of matter beyond the diffraction limit is a puzzling phenomenon, typicalboth for surface and bulk ultrashort laser processing. Here we compare the mechanisms of periodicnanostructure formation on the surface and in the bulk of fused silica. We show that volumenanogratings and surface nanoripples having subwavelength periodicity and oriented perpendicularto the laser polarization share the same electromagnetic origin. The nanostructure orientation isdefined by the near-field local enhancement in the vicinity of the inhomogeneous scattering centers.The periodicity is attributed to the coherent superposition of the waves scattered at inhomogeneities.Numerical calculations also support the multipulse accumulation nature of nanogratings formationon the surface and inside fused silica. Laser surface processing by multiple laser pulses promotes thetransition from the high spatial frequency perpendicularly oriented nanoripples to the low spatialfrequency ripples, parallel or perpendicular to the laser polarization. The latter structures also share theelectromagnetic origin, but are related to the incident field interference with the scattered far-field ofrough non-metallic or transiently metallic surfaces. The characteristic ripple appearances are predictedby combined electromagnetic and thermo-mechanical approaches and supported by SEM images of thefinal surface morphology and by time-resolved pump-probe diffraction measurements.

International audience; Generation of periodic arrangements of matter on materials irradiated by laser fields of uniform and isotropic energy distribution is a key issue in controlling laser structuring processes below the diffractive limit. Using 3D-FDTD methods, we evaluate energy deposition patterns below a material's rough surface and in bulk dielectric materials containing randomly distributed nano-inhomogeneities. We show that both surface and volume patterns can be attributed to spatially ordered electromagnetic solutions of linear and nonlinear Maxwell equations. Transient electronic response is also analyzed and show that for nonplasmonic metals, ultrafast carrier excitation can drastically affect electronic structures, driving a transient surface plasmonic state.

This thesis is focused on the numerical modeling of ultrashort laser interaction with transparent dielectrics. More particularly, the phenomenon of self-organized volume nanogratings in fused silica bulk is discussed and a multiphysical model is proposed to explain the mechanism of their formation. Volume nanogratings are sub-wavelength periodic nanostructures, consisting of less dense material, which are commonly induced by multipulse femtosecond laser irradiation in some glasses, crystals and indirect semiconductors. Their physical origin as well as the laser irradiation conditions for theirformation and erasure are investigated in this theoretical work. To model the nonlinear propagation inside glass, Maxwell's equations are coupled with rate equation. It is shown that three-dimensional periodic nanoplasmas are formed during ultrashort laser interaction with fused silica inhomogeneities. Laser-induced nanopores are proposed to play the role of inhomogeneous scattering centers. Subwavelength periodicity and polarization dependent orientation of the nanoplasmas, revealed in this thesis, make them a strong candidate for explaining volume nanogratings formation. Additionally, it is demonstrated that the nanoripples on fused silica surface and volume nanogratings have similar formation mechanisms. To justify the presence of nanopores in laser-irradiated fused silica bulk, glass decomposition processes are investigated. Firstly, the temperature profiles are found by incorporating the electron-ion temperature model. Then, based on the calculated temperatures, criteria for cavitation and nucleation in glass and also hydrodynamic Rayleigh-Plesset equation, the conditions for nanopores formation and for volume nanogratings survival are elucidated. To define the laser parameter dependencies on the volume nanogratings formation/erasure, a selfconsistent multiphysical approach is developed including ultrafast laser propagation in glass, multiple rate equation to take into account excitation/relaxation processes and two-temperature model. The numerical results provide a laser parameter window as a function of laser pulse energy, laser pulse duration and repetition rate for volume nanogratings consistent with numerous independent experiments. The performed work not only provides new insights into the formation mechanisms of periodic nanostructures but also improves our knowledge of the optimal laser parameter control over ultrafast material response, opening new opportunities in ultrashort laser processing of dielectrics, Cette thèse se concentre sur l'étude numérique de l'interaction laser ultrabref avec les diélectriques transparents. En particulier, le phénomène d'auto-organisation des nanoréseaux dans la silice est discuté et un modèle multiphysique est proposé pour expliquer le mécanisme de leur formation. Les nanoréseaux en volume sont des nanostructures périodiques de périodicité sub-longueur d'onde, qui consistent en un matériau moins dense et sont générés par une irradiation laser multi-impulsionnelle femtoseconde dans certains verres, cristaux et semiconducteurs. Leur origine physique ainsi que les conditions d'irradiation laser pour leur formation et leur effacement sont investiguées dans ce travail théorique. Pour simuler la propagation nonlinéaire dans les verres, les équations de Maxwell sont couplées avec l'équation d'évolution de la densité électronique. Il est démontré que les nanoplasmas périodiques 3D sont formés pendant l'interaction laser ultrabref avec les inhomogénéités de la silice fondue. Les nanopores induits par laser sont supposés jouer le rôle de centres inhomogènes de diffusion. La périodicité sub-longueur d'onde et l'orientation des nanoplasmas dépendante de la polarisation, révélées dans cette thèse, font d'eux un excellent candidat pour expliquer la formation des nanoréseaux en volume. En plus, il est demontré que les nano-ripples sur la surface de silice fondue et les nanoréseaux en volume ont des mécanismes de formation similaires. Pour justifier la présence de nanopores dans la silice fondue irradiée par laser, les processus de décomposition du verre sont étudiés. Premièrement, les profils de température sont calculés sur la base d'un modèle électron-ion. Ensuite, à partir des températures calculées, des critères de cavitation et de nucléation dans le verre ainsi que des équations hydrodynamiques de Rayleigh-Plesset, les conditions pour la formation des nanopores et la survie des nanoréseaux en volume sont élucidées. Pour établir les dépendances des paramètres du laser de formation et d'effacement des nanoréseaux en volume, l'approche multiphysique est développée comprenant la propagation du laser ultrabref dans le verre, les processus d'excitation/relaxation électroniques et le modèle à deux températures. Les résultats numériques fournissent les paramètres du laser en fonction de l'énergie de l'impulsion, sa durée et le taux de répétition pour induire des nanoréseaux en volume, en bon accord avec les expériences nombreuses et indépendantes de la littérature. Le travail réalisé a non seulement permis de déterminer les mécanismes de formation des nanostructures périodiques mais améliore également notre connaissance du contrôle optimal des paramètres du laser sur la réponse ultrarapide d matériau, en ouvrant des nouvelles opportunités de traitement des diélectriques par laser ultrabref

International audience; Using an electromagnetic approach based on three-dimensional Maxwell's equations coupled with electron density equation, we investigate femtosecond laser interaction with fused silica containing randomly distributed inhomogeneities. By irradiating with linearly, radially, azimuthally, circularly and mixed polarized beams, we visualize and analyze the resulting electron density distributions in glass. The numerical calculations demonstrate the local material excitation to the polarization state of light and underline the nanostructure orientation perpendicular to the local laser polarization. The advantages of using azimuthal- and radial-variant polarizations to produce the nanogratings with desirable characteristics are discussed. Circularly polarized beams are shown to be, on the contrary, beneficial to prevent the nanostructure self-organization.

International audience; Short and ultrashort laser pulses are used for a wide range of micro- and nano-machining applications. Among the most promising ones, the formation of periodic and non-periodic 3D nanostructures inside glasses attract particular attention. These structures can be used for optical memories, as optical components, metamaterials, photonics crystals, for laser-based image inscriptions in security, in counterfeiting fight, as well as in nanofluidics, medicine, etc. Interestingly, ultra-short-laser induced periodic volume nanogratings (VNGs), for instance, are still considered to be the smallest embedded structures ever created by light [1-4]. Previous experimental investigations of the phenomenon revealed numerous interesting features of ultra-short laser processing of dielectric materials [1-3]. In particular, it was found that laser parameters should correspond to a particular window in order to obtain the desired results. In fact, depending on the laser irradiation conditions, different types of permanent material modification could be induced in glasses by ultra-short laser pulses. Single pulse irradiation leads either to smooth medication characterized by positive index change or to void-like rarefaction regions. After multi-pulse laser irradiation, self-organized nanogratings were found at certain laser conditions in fused silica glass and few other glasses. Many questions concerning the formation mechanisms that explain the required parameter rangers are still open. To better control over the laser-induced processing, resulting modifications are experimentally investigated as a function of laser pulse energy, pulse duration, numerical aperture, laser wavelength, repetition rate, and the number of applied pulses. In addition, numerical model based on Maxwell equations coupled with non-linear ionization equations is developed [4].Here, in addition, we consider the main photo-thermal processes taking place during ultra-short laser irradiation and thermo-mechanical wave propagation. As a result, spatiotemporal evolutions of temperature, pressure and density are calculated, which accompany the refractive index change over a broad range of timescales from femtoseconds to few microseconds as a function of laser parameters such as pulse energy, pulse duration, laser wavelength, numerical aperture, number of pulses and repetition rate. By applying this model, we study the role of the laser experimental conditions and the one of material properties. The physical processes such as pressure wave generation, thermal diffusion and heat accumulation are investigated and the parameter window required for 3D laser treatment is explained. References:[1] M. Beresna, M. Gecevi_cius, and P. G. Kazansky, Opt. Mater. Express 1, 783 (2011),[2] Y. Shimotsuma, M. Sakakura, P. G. Kazansky, M. Beresna, J. Qiu, K. Miura, and K. Hirao, AdvancedMaterials 22, 4039 (2010) [3]B. Poumellec, M. Lancry, R. Desmarchelier, E. Herve, F. Brisset, and J. Poulin, Opt. Mater. Express 3,1586 (2013)[4] A. Rudenko, J-Ph. Colombier, T. E. Itina, From random inhomogeneities to periodic nanostructures induced in bulk silica by ultrashort laser, Phys. Rev. B93 (7), 075427.