Dynamics of femtosecond laser absorption of fused silica in the ablation regime

International audienceWe investigate the ultrafast absorption dynamics of fused silica irradiated by a single 500 fs laser pulse in the context of micromachining applications. A 60-fs-resolution pump-probe experiment that measures the reflectivity and transmissivity of the target under excitation is developed to reveal the evolution of plasma absorption. Above the ablation threshold, an overcritical plasma with highly non-equilibrium conditions is evidenced in a thin layer at the surface. The maximum electron density is reached at a delay of 0:5 ps after the peak of the pump pulse, which is a strong indication of the occurrence of electronic avalanche. The results are further analyzed to determine the actual feedback of the evolution of the optical properties of the material on the pump pulse. We introduce an important new quantity, namely, the duration of absorption of the laser by the created plasma, corresponding to the actual timespan of laser absorption by inverse Bremsstrahlung. Our results indicate an increasing contribution of plasma absorption to the total material absorption upon raising the excitation fluence above the ablation threshold. The role of transient optical properties during the energy deposition stage is characterized and our results emphasize the necessity to take it into account for better understanding and control of femtosecond laser-dielectrics interaction

Gallium transformation under femtosecond laser excitation: Phase coexistence and incomplete melting

The reversible phase transition induced by femtosecond laser excitation of Gallium has been studied by measuring the dielectric function at 775 nm with ~ 200 fs temporal resolution. The real and imaginary parts of the transient dielectric function were calculated from absolute reflectivity of Gallium layer measured at two different angles of incidence, using Fresnel formulas. The time-dependent electron-phonon effective collision frequency, the heat conduction coefficient and the volume fraction of a new phase were restored directly from the experimental data, and the time and space dependent electron and lattice temperatures in the layer undergoing phase transition were reconstructed without ad hoc assumptions. We converted the temporal dependence of the electron-phonon collision rate into the temperature dependence, and demonstrated, for the first time, that the electron-phonon collision rate has a non-linear character. This temperature dependence converges into the known equilibrium function during the cooling stage. The maximum fraction of a new phase in the laser-excited Gallium layer reached only 60% even when the deposited energy was two times the equilibrium enthalpy of melting. We have also demonstrated that the phase transition pace and a fraction of the transformed material depended strongly on the thickness of the laser-excited Gallium layer, which was of the order of several tens of nanometers for the whole range of the pump laser fluencies up to the damage threshold. The kinetics of the phase transformation after the laser excitation can be understood on the basis of the classical theory of the first-order phase transition while the duration of non-thermal stage appears to be comparable to the sub-picosecond pulse length.Comment: 28 pages, including 9 figs. Submitted to Phys. Rev. B 14 March 200

Laser beam quality of high pulse repetition frequency excimer lasers

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Surface oxide removal by XeCl laser for decontamination

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Absorption Dynamics of a Femtosecond Laser Pulse at the Surface of Dielectrics

International Symposium on High Power Laser Ablation (HPLA), Santa Fe, NM, APR 30-MAY 03, 2012International audienceWe study numerically the propagation of an ultrashort laser pulse (sub-100 fs) ionizing a dielectric material (e. g. fused silica). Helmholtz equation for the electric field is integrated in time, coupled to a plasma model taking into account basic processes in the conduction band like nonlinear photoionization, impact ionization, self-trapped exciton relaxation, electron collisions in the conduction band and inverse bremsstrahlung. A detailed model of the critical surface (n(e) = n(cr)) reflecting a high intensity laser pulse is crucial for understanding the excitation of the material, and thus the absorption of the radiation, potentially leading to optical breakdown. Finally, we link our results to recent experimental findings about ultrafast ablation of fused silica in order to optimize the energy deposition for micromachining processes

High temporal resolution and calibration in pump-probe experiments characterizing femtosecond laser-dielectrics interaction

International audienceIn the aim of performing pump-probe experiments with a high temporal accuracy, the determination of the perfect temporal overlap of the two incoming pulses is crucial. When addressing laser-dielectrics interaction, this `time zero' is often estimated from the dynamics of interest itself (e.g. when optical changes start). Here, we present an experimental approach to overcome this limitation, enabling in situ determination of the time zero independently from the observed dynamical process. We rely on an adapted frequency-resolved optical gating technique, which provides in addition the complete characterization of the probe pulse, measured directly in the focal plane of interaction. The designed set-up permits the observation of transient dynamics with a temporal sampling resolution smaller by a factor of eight compared to the pump pulse duration. For demonstration, we follow in time the change of transient optical properties at the surface of a fused silica sample irradiated by a single femtosecond pulse, during the pulse itself

Guidelines for efficient direct ablation of dielectrics with single femtosecond pulses down to 7 fs

Conference on Lasers and Electro-Optics (CLEO), San Jose, CA, JUN 08-13, 2014International audienceWe demonstrated that the measurement of the laser-induced ablation threshold and the fluence for maximum ablation efficiency, are only required to qualify the outcomes of laser ablation in an extended range of applied fluence

Dynamic photolytical actinometry of the vacuum-ultraviolet radiation produced by multichannel surface discharges of submicrosecond duration

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