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

Observation of magnetic circular dichroism in Fe L_{2,3} x-ray-fluorescence spectra

We report experiments demonstrating circular dichroism in the x-ray-fluorescence spectra of magnetic systems, as predicted by a recent theory. The data, on the L_{2,3} edges of ferromagnetic iron, are compared with fully relativistic local spin density functional calculations, and the relationship between the dichroic spectra and the spin-resolved local density of occupied states is discussed

Model-free classification of X-ray scattering signals applied to image segmentation

In most cases, the analysis of small-angle and wide-angle X-ray scattering (SAXS and WAXS, respectively) requires a theoretical model to describe the sample’s scattering, complicating the interpretation of the scattering resulting from complex heterogeneous samples. This is the reason why, in general, the analysis of a large number of scattering patterns, such as are generated by time- resolved and scanning methods, remains challenging. Here, a model-free classification method to separate SAXS/WAXS signals on the basis of their inflection points is introduced and demonstrated. This article focuses on the segmentation of scanning SAXS/WAXS maps for which each pixel corresponds to an azimuthally integrated scattering curve. In such a way, the sample composition distribution can be segmented through signal classification without applying a model or previous sample knowledge. Dimensionality reduction and clustering algorithms are employed to classify SAXS/WAXS signals according to their similarity. The number of clusters, i.e. the main sample regions detected by SAXS/WAXS signal similarity, is automatically estimated. From each cluster, a main representative SAXS/WAXS signal is extracted to uncover the spatial distribution of the mixtures of phases that form the sample. As examples of applications, a mudrock sample and two breast tissue lesions are segmented

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

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

Time-resolved femtosecond laser pulse absorption at the surface of transparent dielectrics

Conference on Lasers and Electro-Optics (CLEO), San Jose, CA, JUN 08-13, 2014International audienceTime-resolved absorption of a 500-fs laser pulse by fused silica is measured. The results evidence a strong contribution of linear absorption for high pump fluence, during the entire pulse length

Guidelines for efficient direct ablation of dielectrics with single femtosecond pulses

International audienceWe provide guidelines to femtosecond laser users to select ad hoc laser parameters, namely the fluence and pulse duration, in the context of the development of ablation processes at the surface of dielectrics using single femtosecond pulses. Our results and discussion are based on a comprehensive experimental and theoretical analysis of the energy deposition process at the surface of fused silica samples and of their postmortem ablation characteristics, in the range of intensities from 10(13) to 10(15) W/cm(2). We show experimentally and numerically that self-induced plasma transient properties at the pulse timescale dramatically determine the efficiency of energy deposition and affect the resulting ablation morphology. In practice, we determine that the precise measurement of two characteristic fluence values, namely the laser-induced ablation threshold F (th,LIAT) and the fluence F (opt) for maximum ablation efficiency, are only required to qualify the outcomes of laser ablation at the surface of a dielectric in an extended range of applied fluence

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

Absorption of a single 500 fs laser pulse at the surface of fused silica: Energy balance and ablation efficiency

International audienceAblation of fused silica by a single femtosecond laser pulse of 500 fs pulse duration is investigated from the perspective of efficiency of incident photons to remove matter. We measure the reflected and transmitted fractions of the incident pulse energy as a function of fluence, allowing us to recover the evolution of absorption at the material surface. At the ablation threshold fluence, 25% of incident energy is absorbed. At high fluences, this ratio saturates around 70% due to the appearance of a self-triggered plasma mirror (or shielding) effect. By using the energy balance retrieved experimentally and measurements of the ablated volume, we show that the amount of absorbed energy is far above the bonding energy of fused silica at rest and also above the energy barrier to ablate the material under non-equilibrium thermodynamic conditions. Our results emphasize the crucial role of transient plasma properties during the laser pulse and suggest that the major part of the absorbed energy has been used to heat the plasma formed at the surface of the material. A fluence range yielding an efficient and high quality ablation is also defined, which makes the results relevant for femtosecond micromachining processes. (c) 2013 AIP Publishing LLC