Ultrashort pulse laser cutting of glass by controlled fracture propagation

International audienceLaser induced controlled fracture propagation has great potential in cutting brittle materials such as glass or sapphire. In this paper we demonstrate that the use of ultrashort pulse laser sources may be advantageous since it allows to overcome several restrictions of the convenient method

Solid-liquid phase transition induced by fast heating of a thin metal film

International audienceFor fast heating, solid-liquid phase transition is generally assumed to be an isochoric process related to the ion temperature. However, the experimental studies of fast melting process do not completely agree with the theory. We discuss the validity of the isochoric assumption for fast heating by considering the fast melting of aluminum and gold thin films. The results show that solid-liquid phase transition can occur due to the foil expansion in a picosecond time scale, without significant ion heating, for deposited energies of the order of the melting enthalpy. The estimated melting time for gold thin film irradiated by a short laser pulse is in agreement with experimental measurements. Copyright (C) EPLA, 201

Ultra-short laser induced electron excitation/relaxation kinetics

Laser-induced electronic excitation, absorption and relaxation are the key issues in ultra-short laser interactions with dielectric materials. To numerically analyze these processes, several approaches are typically used. First, several detailed non-equilibrium models are based on a system of the detailed kinetic Boltzmann equations. Then, Fokker-Planck equations are also used. Finally, much more simplified rate equations are typically used in engineering modelling [1,2]. These models require additional sub-models to account for photo-ionization, electron-impact ionization, defect formation, recombination and other relaxation processes. In these sub-models, many parameters are rather unknown and are calculated based on additional considerations. One of such parameters is electron collision frequency [1-3], which was found to be crucial in determination of laser absorption and hence of laser damage.Boltzmann-based calculations are performed including all possible collisional processes. As a result, electron energy distributions are obtained allowing a better analysis of ultra-short laser interactions. The results reveal an effect of the laser-field on collision frequencies resulting in smaller free-carriers absorption than the one predicted by commonly used rate-equation models. Both electron-electron and electron-phonon relaxation are then examined, and the mean energy density of the electron sub-system is investigated as a function of laser fluence and pulse duration. Because efficient bond breaking requires energy, these calculations provide the required thresholds [4]. The dependency of the calculated damage threshold on laser pulse duration is compared with the available experimental data. The developed model is useful for many laser applications including high precision in laser treatment, laser-assisted atomic probe analysis, and for the development of new powerful laser systems.References[1] B. Chimier, O. Utéza, N. Sanner, M. Sentis, T. Itina, P. Lassonde, F. Légaré, F. Vidal, and J. C. Kieffer. "Damage and ablation thresholds of fused-silica in femtosecond regime." Physical Review B 84 (9), 094104, (2011).[2]T.E. Itina, N.S. Shcheblanov, N. Electronic excitation in femtosecond laser interactions with wide-band-gap materials. Applied Physics A, 98(4), 769-775 (2010).[3] C. Xie, V. Jukna, C. Milián, R. Giust, I. Ouadghiri-Idrissi, T. Itina, J. M. Dudley, A. Couairon, F. Courvoisier. "Tubular filamentation for laser material processing. Scientific reports, 5 (2015).[4] N. S. Shcheblanov, T. E. Itina, Appl. Phys. Femtosecond laser interactions with dielectric materials: insights of a detailed modeling of electronic excitation and relaxation processes, Appl. Phys. A, 110(3), 579-583 (2013).Surface & Interface Science & Engineerin

Modeling the electron collision frequency during solid-to-plasma transition of polystyrene ablator for direct-drive inertial confinement fusion applications

Within the inertial confinement fusion (ICF) framework, the solid-to-plasma transition of the ablator arouses increasingly interest, in particular due to the laser-imprint issue. Phase evolution of the ablator is linked to the evolution of the electron collision frequency which is of crucial importance since it drives electron heating by laser energy absorption, and lattice-ion heating due to collisions between electrons and the lattice-ion system. Thus, an accurate description of electron collisions over the whole temperature range occurring in ICF, starting from a few tens of kelvins (solid state) up to tens of millions of kelvins (plasma state), is necessary. In this work, polystyrene ablator is considered and a model of chemical fragmentation is presented to describe the heated polystyrene evolution. Electron collisions are described by electron-phonon collisions in the solid state, and by electron-ion and electron-neutral collisions in plasma state. An effective electron collision frequency valid over the whole range of temperatures reached in ICF experiments is established and discussed. Thermal conductivity is also deduced from collisions in the plasma state and shows a good agreement with the one evaluated by ab initio calculations

Improved modeling of the solid-to-plasma transition of polystyrene ablator for laser direct-drive inertial confinement fusion hydrocodes

International audienc

Absorption of femtosecond laser pulse in fused silica: experiments and modelling

We present experimental and theoretical investigations of interaction of a femtosecond laser (450 fs pulse at 1025nm) with dielectric materials (fused silica) for the single-shot laser regime. The aim is to analyze and understand the complex physical mechanisms of laser energy absorption yielding to damage and/or ablation. We outline the distinction between the ablation and the damage thresholds for dielectric materials. The evolution of the reflection, transmission and absorption signals is studied as a function of fluence. The experimental curves are accompanied by a modelling, which takes into account the photoionization and avalanche ionization depicting absorption of the laser energy by the material. The incident pulse propagation into the material, the temporal evolution of the electron density, reflection and transmission illustrate the beginning and the duration of the laser pulse absorption. The magnitude of the absorption process is energy density sensitive and, with the increase of the deposited fluence, the onset of absorption is moved temporally to the beginning of the pulse. We show the influence of the effective electron collision frequency on the calculated values of reflection, transmission and absorption. The results are particularly relevant to high micromachining industrial processes

Femtosecond laser pulse absorption by dielectrics: Surface experiments and modelling

Experimental and theoretical investigations are performed to measure and understand the physical mechanisms of laser energy absorption at the surface of a dielectric material excited by a femtosecond laser

Modeling the time-dependent electron dynamics in dielectric materials induced by two-color femtosecond laser pulses: Applications to material modifications

Controlling the electron dynamics during laser-matter interactions is a key factor to control the energy deposition and subsequent material modifications induced by femtosecond laser pulses. One way to achieve this goal is to use two-color femtosecond laser pulses. In this paper, the electron dynamics in dielectric materials induced by two-color femtosecond laser pulses is studied by solving dedicated optical Bloch equations. This model includes photo- and impact ionization, the laser heating of conduction electrons, their recombination to the valence band, and their collisions with phonons. The influence of photon energies, laser intensities, and pulse-to-pulse delay is analyzed. Depending on the interaction process, colors cooperate to excite electrons or drive them independently. For the given laser parameters, an optimal pulse-to-pulse delay is found which enhances significantly the energy deposition into the material, in agreement with experimental observations