Femtosecond laser interactions with dielectric materials: insights of a detailed modeling of electronic excitation and relaxation processes (original) (raw)
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Femtosecond Laser Interactions with Semiconductor and Dielectric Materials
International Symposium on High Power Laser Ablation 2012, 2012
Electronic excitation-relaxation processes induced by ultra-short laser pulses are studied numerically for semiconductors and dielectric materials (Si, quartz). A detailed kinetic approach is used in the calculations accounting for electron-photon-phonon, electron-phonon and electron-electron scatterings. In addition, both laser field ionization ranging from multiphoton to tunneling one, and electron impact (avalanche) ionization processes are included in the model. Based on the performed calculations we study the relaxation time as a function of laser parameters. It is shown that this time depends on the density of the created free carriers, which in turn is a nonlinear function of laser intensity. In addition, a simple damage criterion is proposed based on the mean electron energy density rather than on critical free electron density. This criterion gives a reasonable agreement with the available experimental data practically without adjustable parameters. Furthermore, the performed modeling provides energy absorbed in the target, conditions for damage of dielectric materials, as well as conditions for surface plasmon excitation and for periodic surface structure formation on the surface of semiconductor materials.
Numerical modeling of electronic excitation processes induced by ultra-short laser pulses in dielectric materials is performed. The developed model is based on a detailed kinetic description and accounts for the absence of equilibrium in the electronic subsystem. The photoionization process is first analyzed for different laser intensities. It is shown that the probability of this process depends strongly on the Keldysh parameter and effective ionization potential, which are calculated as a function of the laser field. Electron energy distributions are calculated and the average energy is analyzed as a function of laser parameters. For laser intensities corresponding to the maximum of this dependency, only around 1% of electrons are shown to achieve the energy required for the impact ionization.
Interaction of femtosecond laser pulses with dielectric materials: insights from numerical modelling
2009
To shed light on ultra-short laser interactions, we study the laser ionization processes leading to the energy absorption and reflection. In particular, we investigate the ratio of the energy deposited to the material to the total incident energy. The absorbed energy density is studied as a function of pulse width and laser intensity. It is shown that the maximum absorption takes place at a given incident laser intensity that is considered as ablation threshold. For pulses shorter than 100 fs, only a small fraction of laser energy is deposited to the matrix, causing heating and leading to the thermal and/or mechanical modifications of the target material. We connect these results with the electronic excitation and the ionization processes leading to the changes in reflectivity and consuming electron energy. The obtained numerical results explain several recent experiments.
Numerical Analysis of Ultra-Short Laser Interactions with Dielectric Materials
Laser-induced electronic excitation, absorption and relaxation are the key issues in ultra-short laser interactions with dielectric materials. To numerically analyze these processes, a detailed non-equilibrium model is developed based on the kinetic Boltzmann equations without any appeal to the classical Drude model. The calculations yield not only electron density in the conduction band, but also their energy distribution allowing a better analysis of the role of avalanche ionization. The calculations performed reveal a remarkable effect of the laser-field on collision frequencies resulting in smaller free-carriers absorption than the one predicted by commonly used rate-equation models. Furthermore, both electron-electron and electron-phonon relaxation are examined, and the energy 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.
Dynamics of femtosecond laser interactions with dielectrics
Applied Physics A, 2004
Femtosecond laser pulses appear as an emerging and promising tool for processing wide bandgap dielectric materials for a variety of applications. This article aims to provide an overview of recent progress in understanding the fundamental physics of femtosecond laser interactions with dielectrics that may have the potential for innovative materials applications. The focus of the overview is the dynamics of femtosecond laser-excited carriers and the propagation of femtosecond laser pulses inside dielectric materials.
Electron kinetics in semiconductors and metals irradiated with VUV-XUV femtosecond laser pulses
Damage to VUV, EUV, and X-ray Optics III, 2011
In solids under irradiation with femtosecond laser pulses, photoabsorption produces a strongly nonequilibrium highly energetic electrons gas. We study theoretically the ionization of the electronic subsystem of either a semiconductor (silicon) or a metal (aluminum) target, exposed to an ultra-short laser pulse (pulse duration ~10 fs) of VUV-XUV photons. We developed a numerical simulation technique, based on the classical Monte-Carlo method, to obtain transient distributions of electrons within conduction band. We extend the Monte-Carlo method in order to take into account quantum effects such as the electronic band structure, Pauli's exclusion principle for electrons in the conduction band and for holes within the valence band (for semiconductors), and free-free electron scattering (for metals).
Femtosecond Laser-Matter Interactions
2021
2.4.4 Electron-to-Ion Momentum and Energy Transfer 2.4.5 Electron-to-Ion Energy Exchange Time 2.4.5.1 Non-ideality effects 2.4.5.1 Effects of the oscillations in high-frequency electromagnetic field on the electrons' collision rate 2.5 Modification of the Electron Distribution Function: From the Fermi-Dirac to the Maxwell-Boltzmann 2.6 Electronic Heat Conduction 2.7 Summary 3. Interaction with Dielectrics 47 3.1 Ionization in the Strong High-Frequency Electric Field: Electrons Transfer from the Valence to Conduction Band and to Continuum 3.1.1 Tunnelling Ionization Rate in the Limit g << 1 Ablation ix Contents 3.1.1.1 Linear polarization 3.1.1.2 Tunnel ionization in the elliptically polarized electric field 3.
Dynamical rate equation model for femtosecond laser-induced breakdown in dielectrics
Physical Review B
Experimental and theoretical studies of laser-induced breakdown in dielectrics provide conflicting conclusions about the possibility to trigger ionization avalanche on the sub-picosecond time scale and the relative importance of carrier-impact ionization over field ionization. On the one hand, current models based on single ionization-rate equations do not account for the gradual heating of the charge carriers which, for short laser pulses, might not be sufficient to start an avalanche. On the other hand, models based on multiple rate equations that track the carriers kinetics rely on several free parameters, which limits the physical insight that we can gain from them. In this paper, we develop a model that overcomes these issues by tracking both the plasma density and carriers' mean kinetic energy as a function of time, forming a set of delayed rate equations that we use to match the laser-induced damage threshold of several dielectric materials. In particular, we show that this simplified model reproduces the predictions from the multiple rate equations, with a limited number of free parameters determined unambiguously by fitting experimental data. A side benefit of the delayed rate equations model is its computational efficiency, opening the possibility for large-scale, three-dimensional modelling of laser-induced breakdown of transparent media.