Search

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.

Surface structuring of amorphous and crystals materials may find interest in many leading technological applications. The one we target is related to laser-driven surface topography modification, however, so far, no exhaustive study was made to really give a clear picture of the different mechanisms involved in this complex process. The main originality of this work is to investigate the ultrafast phenomena and the subsequent structural modifications triggered byfemtosecond laser pulses. In this context, classical molecular dynamics approach is proposed to capture solid-solid phase transformations in alloys. To manage our investigations, we have used LAMMPS to lead atomistic simulation, which is adapted to catch phenomena occurring at a picosecond scale. To model this laser-matter interaction a suitable hybrid simulation combining Molecular Dynamics and Two Temperature Model is used to predict the involved dynamics of the electrons and ions. This combination allows us the control of laser most important processing parameters, namely the fluence and pulse duration. We have to highlight that this work deals with metallic and amorphous based alloys structures. Dealing with such structures is motivatedby two reasons, the experimental evidence that this alloys compositions are stable and the availability of a robust Embedded Atomic Method potential which faithfully reproduces the properties required to determine the thermodynamic path for laser-induced polymorphic phasetransition.

International audience

International audience

Surface structuring of both amorphous and crystals materials may find interest in many leading technological applications. The one we target is related to laser-driven surface topography modification, however, so far, no exhaustive study was made to really give a clear picture of the different mechanisms involved in this complex process. The main originality of this work is to investigate the ultrafast phenomena and the subsequent structural modifications triggered by femtosecond laser pulses (< 100 fs). In this context, classical molecular dynamics approach is proposed. The challenge is to capture solide-solide phase transformations in both amorphous and crystal matrices.

International audience; A promising way to tailor the chemical and electronic properties of graphene layers is the doping of the carbon honeycomb using heteroatoms such as nitrogen or boron. In the domain of electrochemical sensor, this may allow to control both conductivity and chemical activity. Nitrogen doped graphene was produced by the thermal annealing of nitrogen doped amorphous carbon (a-C:N) deposited either on a nickel thin film on a silicon substrate or between the silicon substrate and the nickel catalyst layer. The a-C:N thin films were produced by femtosecond pulsed laser ablation of graphite in nitrogen, while the nickel thin film was thermally evaporated. The nitrogen content and bonding configuration in the produced films were evaluated by XPS spectroscopy, with a particular attention to the ratio of pyridinic and pyrrolic bondings, the former being more interesting to control the conductivity and chemical activity of the film. The structure of the graphene was studied through microRaman spectroscopy using Raman mapping. Those results were compared to the electrochemical properties of the produced N-doped graphene films, studied through cyclic voltammetry using ferrocene as a redox probe. Electron transfer properties are compared to non-doped graphene films, which have already shown promising electrografting perspectives for advanced compound detection (1).(1) P. Fortgang et al., ACS Applied Materials and Interfaces 8 (2016) 1424-1433