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

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 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