16 August 2000 Microscopical dynamics in solids absorbing a subpicosecond laser pulse
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Proceedings Volume 4065, High-Power Laser Ablation III; (2000); doi: 10.1117/12.407362
Event: High-Power Laser Ablation, 2000, Santa Fe, NM, United States
Abstract
Microscopic collision processes in solids occur on femtosecond time scales. A description of materials response to laser irradiation on this time scales should take these processes explicitly into account. Averaging descriptions are not applicable from the first. We calculate the distribution function of free electron gas in metals and insulators for the case of irradiation with a laser pulse of moderate intensity. A microscopical description on the basis of time-dependent Boltzmann equations is used. For the metal, photon absorption by free electrons, electron-electron collisions and electron-phonon collisions are considered each by a corresponding collision integral. In dielectrics, additional terms for two ionization processes (strong-electric-field ionization and impact ionization) are included. With this model, describing explicitly materials transient behavior, we are able to check the applicability of common averaging equations. For metals irradiated with sufficient high intensities about and above damage threshold the energy exchange between electrons and phonons can be described with the two temperature model, whereas for low excitation the non- equilibrium in the electron gas affects the electron-phonon coupling. For dielectrics we show that the commonly used rate equation for collisional ionization is not applicable for pulse durations below hundred femtoseconds. We propose an extended system of two rate equations taking the effect of energy dependence of impact ionization into account. This averaging approach can reproduce the evolution of free electron density in SiO2 with reasonable accuracy.
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Baerbel Rethfeld, Andreas Kaiser, Martin Vicanek, Gerhard Simon, "Microscopical dynamics in solids absorbing a subpicosecond laser pulse", Proc. SPIE 4065, High-Power Laser Ablation III, (16 August 2000); doi: 10.1117/12.407362; https://doi.org/10.1117/12.407362
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KEYWORDS
Ionization

Phonons

Absorption

Gas lasers

Metals

Dielectrics

Pulsed laser operation

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