Hydrodynamic simulations of metal ablation by femtosecond laser irradiation

Ablation of Cu and Al targets has been performed with 170 fs laser pulses in the intensity range of 10^12-10^14 W/cm^2. We compare the measured removal depth with 1D hydrodynamic simulations. The electron-ion temperature decoupling is taken into account using the standard "two-temperature model". The influence of the early heat transfer by electronic thermal conduction on hydrodynamic material expansion and mechanical behavior is investigated. A good agreement between experimental and numerical matter ablation rates shows the importance of including solid-to-vapor evolution of the metal in the current modeling of the laser matter interaction

Optimization of the Energy Deposition in Glasses with Temporally-Shaped Femtosecond Laser Pulses

International audienceBulk machining of glasses with femtosecond laser pulses enables the fabrication of embedded optical functions. Due to the nonlinear character of the laser-matter interaction, structural modifications can occur within the focal region. To reach a full control of the process, ways of controlling the deposition of the laser energy inside the material have to be unveiled. From static and time-resolved pictures of bulk-excitation of a-SiO2 and borosilicate glass, we show that particular laser temporal shapes such as picosecond sequences can better confine the energy deposition than the femtosecond sequence by reducing the propagation artifacts

Comparison of Heat Affected Zone due to nanosecond and femtosecond laser pulses using Transmission Electronic Microscopy

International audienceThis letter presents a method aimed at quantifying the dimensions of the heat-affected zone ~HAZ!, produced during nanosecond and femtosecond laser–matter interactions. According to this method, 0.1 mm thick Al samples were microdrilled and observed by a transmission electronic microscopy technique. The holes were produced at laser fluences above the ablation threshold in both nanosecond and femtosecond regimes ~i.e., 5 and 2 J/cm2, respectively!. The grain size in the samples was observed near the microholes. The main conclusion is that a 40 mm wide HAZ is induced by the nanosecond pulses, whereas the femtosecond regime does not produce any observable HAZ. It turns out that the width of the femtosecond HAZ is less than 2 mm, which is our observation limit

Time-Resolved Observation of Energy Deposition in Fused Silica by Ultrashort Laser Pulses in Single and Cumulative Regime

International audienceWhen femtosecond laser pulses are focused in the bulk of transparent materials (glasses), deposition of energy on a restricted volume can occur owing to the non linear character of the laser matter interaction. As a consequence, the possibility to generate micrometer-sized structural modifications arises. Those local changes are often associated with a minute variation in the refractive index which, when positive, enables the fabrication of light guiding components in three dimensions through simple laser translation. Although the first corresponding experimental demonstration approaches fifteen years of age, the complete picture of the dynamics and the proc- esses leading to the local refractive index changes has still to be drawn to reach an optimal control of the laser-induced modification process. In this report, the laser-dielectric interaction is followed on an ultrashort time scale with the help of a unique time-resolved side-imaging technique allow- ing for absorption and phase contrast detection. Experimental observation of an absorptive elec- tronic cloud in the first moments of the interaction along with the launch of a pressure wave after a few ns is reported. These physical objects are shown to be reliable indicators of the success of the energy transfer to the lattice which largely depends on the pulse temporal envelope

Efficacy and Tolerance of Post-operative Hypo-Fractionated Stereotactic Radiotherapy in a Large Series of Patients With Brain Metastases

Purpose: The aim of this study was to assess, in a large series, the efficacy and tolerance of post-operative adjuvant hypofractionated stereotactic radiation therapy (HFSRT) for brain metastases (BMs).Materials and Methods: Between July 2012 and January 2017, 160 patients from 2 centers were operated for BM and treated by HFSRT. Patients had between 1 and 3 BMs, no brainstem lesions or carcinomatous meningitis. The primary endpoint was local control. Secondary endpoints were distant brain control, overall survival (OS) and tolerance to HFSRT.Results: 73 patients (46%) presented with non-small cell lung cancer (NSCLC), 23 (14%) had melanoma and 21 (13%) breast cancer. Median age was 58 years (range, 22–83 years). BMs were synchronous in 50% of the cases. The most frequent prescription regimens were 24 Gy in 3 fractions (n = 52, 33%) and 30 Gy in 5 fractions (n = 37, 23%). Local control rates at 1 and 2 years were 88% [95%CI, 81–93%] and 81% [95%CI, 70–88%], respectively. Distant control rate at 1 year was 48% [95%CI, 81–93%]. In multivariate analysis, primary NSCLC was associated with a significant reduction in the risk of death compared to other primary sites (HR = 0.57, p = 0.007), the number of extra-cerebral metastatic sites (HR = 1.26, p = 0.003) and planning target volumes (HR = 1.15, p = 0.012) were associated with a lower OS. There was no prognostic factor of time to local progression. Median OS was 15.2 months [95%CI, 12.0–17.9 months] and the OS rate at 1 year was 58% [95% CI, 50–65%]. Salvage radiotherapy was administered to 72 patients (45%), of which 49 received new HFSRT. Ten (7%) patients presented late grade 2 and 4 (3%) patients late grade 3 toxicities. Thirteen (8.9%) patients developed radiation necrosis.Conclusions: This large multicenter retrospective study shows that HFSRT allows for good local control of metastasectomy tumor beds and that this technique is well-tolerated by patients

Etats excites des atomes et excimeres de krypton: cinetique, spectroscopie, et calcul ab initio des potentiels moleculaires

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 77707 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

High efficiency GHz laser processing with long bursts

Bursts of GHz repetition rate pulses involve more ablation mechanisms than single femtosecond pulses. Efficient ablation by GHz laser pulses is a multi-step process, consisting of a first thermal incubation phase, followed by a highly efficient ablation phase. GHz ablation therefore combines thermal and non-thermal ablation mechanisms. With an optimal choice of the burst duration, the ablation efficiency can be highly enhanced. Long bursts, comprising tens of pulses to hundreds of pulses, are needed to take full advantage of the increase in ablation efficiency

Théorie et simulation de l'interaction des impulsions laser ultracourtes à flux modéré avec un solide métallique

Les systèmes laser ultracourts concentrent une énergie de quelques microjoules dans une impulsion d'une centaine de femtosecondes, de telle sorte que les intensités atteignent 1012 à 1015 W/cm2. Lors de l'irradiation d'un métal, la matière est éjectée du milieu d'origine avec une très grande précision, ce qui confère au système des qualités indéniables pour des applications industrielles. Dans ce travail, nous avons adopté une démarche théorique en proposant une modélisation et une simulation des effets engendrés par ce type d'impulsion. La mise en mouvement ultrarapide des électrons libres insuffle une dynamique puissante de destruction du métal. Des modèles optiques, thermiques et hydrodynamiques adaptés, réalisant la transition entre l'état dégénéré de la matière condensée vers un régime plasma chaud non-dégénéré, sont ici développés. Nous les avons insérés dans un code Lagrangien de simulation hydrodynamique. Nous montrons que des états thermodynamiques extrêmes, hors d'équilibre, peuvent être engendrés et nous avons comparé les taux d'ablation obtenus aux résultats d'expérience. Une conductivité électrique hors d'équilibre a également été développée afin de rendre compte des effets produits par la dynamique électronique sur les propriétés d'absorption optique. Plusieurs types d'expériences numériques, impliquant notamment des dispositifs pompe-sonde, sont ensuite exposés afin d'améliorer notre compréhension des processus de transport (électron-électron et électron-phonon) dans ce régime. Nous avons enfin appliqué cette modélisation aux effets produits par une impulsion mise en forme temporellement afin d'optimiser les expériences d'ablation.Ultrashort laser systems focuses an energy of few microjoules within a pulse of hundred of femtoseconds, so that intensities reach 1012 to 1015 W/cm2. Such ultrashort laser pulses allows accurate metal micro-machining and provides first rate beam parameters for industrial purpose. In this thesis, we propose a theoretical approach to model and simulate the effects induced by such an impulsion. The ultrafast electronic response drives a strong damage dynamic of the metal lattice. We have developped several models to describe the optical, thermal and hydrodynamical phenomena involved in a material irradiated by a femtosecond laser. Special cares have been devoted to the physical descriptions of the phase transition between the degenerated condensed cold phase and the hot non degenerated plasma. These models have been implemented into a 1-D Lagrangian hydrodynamic code. Comparison between computed ablation rates and experimental results shown good agreements. We also discuss the existence of extreme out of equilibrium states. An original electrical conductivity model is also set up to take into account the effects of the electron dynamics on the optical properties. Several numerical experiments involving, among others, pump-probe diagnostics, are performed to improve our transport processes (electron-electron and electron-phonon) knowledge in this field. Finally, in order to enhance ablation rate, we used our model to examine temporal pulse shaping effects.ST ETIENNE-BU Sciences (422182103) / SudocSudocFranceF

Etude des procédés laser femtoseconde pour le micro-usinage à effets thermiques limités

L'ensemble des études présentées dans ce mémoire souligne le réel potentiel des lasers femtoseconde pour des applications de micro-usinage et insiste sur la maîtrise de l'utilisation des chaînes laser amplifiées. Les dispositifs expérimentaux mis en place pour contrôler l'usinage nécessitent la maîtrise de nombreux paramètres, qu'il soient mécaniques, informatiques ou optiques. Différentes techniques ont été élaborées pour accroître la qualité d'usinage et de perçage et des études systématiques ont été entreprises dans le cas des métaux sur l'effet de la fluence et de la cadence. Une comparaison des effets thermiques sur les bords d'usinage en mode femtoseconde et nanoseconde a été entreprise d'un point de vue expérimental (MET) et par l'intermédiaire d'un modèle à 2 température. Enfin des résultats plus exploratoires, en particulier sur le silicium ainsi que des essais de photoinscription de guide d'onde illustrent le potentiel applicatif considérable des impulsions ultra-courtesThe studies presented in this thesis underline the real potential of femtosecond lasers for micromachining applications and insist on the control and use of the amplified laser chains. The experimental devices for the control of micromachining require the control of many parameters, as mechanical, data-processing or optical. Various techniques of machining were worked out to increase the quality of machining and drilling and systematic studies were undertaken in the case of metals including fluence and repetition rate dependence. A comparison of the heating effects on the edges of holes drilled in femtosecond and nanosecond regime was performed with an experimental approach (TTM measurements) and with a modelization. This was based on the use of a 2 temperatures model. Finally complementary results, in particular on silicon micromachining and tests of wave guide photowritting illustrate the wide applicability field of the ultra-short laser pulsesST ETIENNE-BU Sciences (422182103) / SudocSudocFranceF