Understanding spontaneous pattern emergence on laser-irradiated materials is a long-standing interest. Periodic surface structures arise from multiphysical coupling: electromagnetics, nonlinear optics, plasmonics, fluid dynamics, or thermochemical reactions. Multi-shot irradiation with ultrafast laser pulses generates stable periodic patterns arising from localized perturbations influenced by disturbances and nonlinear saturation. Describing pattern growth requires nonlinear dynamics beyond classic equations. The challenge is developing an efficient model with symmetry breaking, scale invariance, stochasticity, and nonlinear properties to reproduce dissipative structures. Stochastic Swift-Hohenberg modeling replicates hydrodynamic fluctuations near the convective instability threshold, inherent in laser-induced self-organized nanopatterns. We will demonstrate that a deep convolutional networks can learn pattern complexity, connecting model coefficients to experimental parameters for designing specific patterns. The model predicts patterns accurately, even with limited non-time series data. It identifies laser parameter regions and could predict novel patterns independently.
Ultrafast-laser irradiated surface is a typical paragon of a self-organizing system, which emerge and organize complex micropatterns and even nanopatterns. An astounding exhibition of dissipative structures consists of various types of randomly and periodically generated nanostructures that originate from a homogeneous metal surface. The formation of nanopeaks, nanobumps, nanohumps and nanocavities patterns with 20–80 nm transverse size unit and up to 100 nm height are reported under femtosecond laser irradiation with a regulated energy dose. We shed the light on the originality of the nanopeaks, having an exceptional aspect ratio on the nanoscale. They are primarily generated on the crests grown between the convective cells formed on the very first pulses. The production of these distinct nanostructures can enable unique surface functionalizations toward the control of mechanical, biomedical, optical, or chemical surface properties on a nanometric scale.
Medical implants are frequently used to replace damaged organs, structures and tissues in human body. It is essential to ensure a perfect implantation hence paramount to optimize surface topography of the implants for desired integration. For dental implants, this implies reducing bacteria settlement near the prosthesis and increasing roughness to improve implant-bone interaction and thus osseointegration. For other types of implants like bone fracture fixtures and cardiac pacemakers, reduced infection and adhesion (scar tissue) are highly desirable. Ultrafast laser is a powerful tool for modifying medical implant surfaces, at the micro- and/or nano-scale, towards improving or limiting their cell adhesions.
Ultrashort laser irradiation of metal targets results in a variety of coupled processes, such as energy deposition
on surface, electron-ion heating and diffusion, as well as thermal ablation and plasma expansion, mechanical
rupture below the surface, and melt flow, modifying the initial surface morphology on micro/nanometric scales.
Multidimensional simulations capable to predict the consequences of inhomogeneous absorption on hydrodynamic
processes are performed in order to elucidate the mechanisms of surface micro/nanostructure formation and
material removal during multipulse laser ablation in regimes below, near and above laser ablation threshold. On
one hand, the numerical results suggest new ways of control over the properties of periodic and aperiodic surface
structures. On the other hand, the strategies to reduce the surface roughness and to improve the quality and the effciency of ultrashort laser ablation are discussed.
The availability of compact table top amplified femtosecond lasers and the technical simplicity of experimental
design have opened the way to many recent and fast developments towards thin film elaboration by Pulsed Laser
Deposition (PLD) with ultra short laser pulses, with the aim of producing materials of high quality previously
unattainable or attainable only through more complex means.
The first developments of PLD using femtosecond lasers were made on Diamond-Like Carbon thin films elaboration,
with the attempt to reach high sp3 content. PLD with ultra short pulses was used recently to deposit several systems
such as quasicrystals or oxides with a transfer of the target composition to the deposited films, even for compounds with
complex stoechiometry. Femtosecond laser ablation from solid targets has shown its capability in producing
nanoparticles of different materials, even in high vacuum conditions. Nanostructured films of doped Diamond-Like
Carbon were obtained recently, opening the way to large applications towards functional materials.
The characteristics of the plasma are a well-suited signature of the physics of laser-matter interaction and plasma plume
creation and expansion. Recent studies on the control of the film growth and femtosecond PLD processes will be
reported.
Emphasis on actual capability of the existing sources to elaborate high quality materials will be questioned in terms of
energy per pulse, time width, repetition rates but also in the need for further source development and beam shaping
improvement.
Pulsed laser ablation is a well-known technique used for thin film deposition, extending from oxydes to hard and wear resistant Diamond-Line Carbon (DLC) films. Most of the previous studies devoted to DLC thin films elaboration have used pulsed duration in the nanosecond range. The present study concerns femtosecond (10-15 s range) laser ablation of a graphite target for the elaboration of Diamond-Like Carbon. Compared to conventional nanosecond laser ablation, femtosecond laser pulses allow the production of high energy (up to a few keV) ions in the plasma, which may strongly affect the structure and properties of the deposited films. DLC films have been deposited under vacuum onto (100) p-type silicon substrates at room temperature, by ablating graphite targets with femtosecond laser pulses. The nature and properties of the film have been characterized by various techniques, including Raman, XPS and AFM. Discussion will be focused on the comparison between present results obtained using femtosecond laser pulses, with previously published results related to DLC films deposited using nanosecond laser pulses. Especially, Raman spectra of DLC films obtained by nanosecond laser ablation always show the two well-known D and G bands (located respectively at around 1350 cm-1 and 1550 cm-1), whereas some DLC films obtained when using femtosecond laser pulses exhibit an intense peak at 1140 cm-1, which may be attributed to nanocrystalline diamond.
The advent of table top high repetition rate regeneratively amplified femtosecond lasers has opened the way to many recent and fast developments towards applications of economical interest. The most well known are microprocessing, thin film elaboration, waveguide photoinscription, surface treatment, dentistry, ophthalmology. Recent studies on microprocessing and laser-matter interaction using femtosecond lasers are reported. This is done using largely presently performed work in Saint-Etienne including investigations on Heat Affected Zone (HAZ), plume expansion characterization, and thin film elaboration. Indeed specific characters appear as compared to what is obtained using multipicosecond/nanosecond laser pulses : Typical submicronic HAZ lengths have been evidenced and particle energies of plasma plume ranging up to a few KeV (carbon target) using typical pulse energies of 1 mJ (150 fs, 800 nm, 1 KHz), creating specific conditions for deposition. The concept of the often vocabled 'athermal' interaction is discussed. Emphasis on actual microprocessing capability of the existing sources to approach industrial applications are questioned in terms of energy per pulse, timewidth, repetition rates and the need for further source development and control beam improvement stressed. A brief review of the progresses under way in these fields and their capability to answer to actual large scale commercial applications are given.
The laser induced plasma plume expansion in vacuum is studied by a Monte Carlo simulation. The global shape of the plume created above a copper target is followed in time by using a 3D algorithm. An original method has allowed the simulation with no restrictions on laser spot width or ablated depth. Particles evaporation from the sample surface during the laser pulse is done by introducing in the model a radial distribution of laser energy and a high surface temperature induced by the vapor pressure. The effects of the later energy absorption by the evaporated particles appear to be dominating parameters on the expansion process. An approximation of these effects has been done by considering a fraction of energetic species, corresponding to the recombination of ionic species by kinetic energy transfer in the cloud. Results of this simulation are compared with experimental results obtained by time of flight measurements and fast photography of the luminous component of the plume.
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