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

Reducing Amyloid Plaque Burden via Ex Vivo Gene Delivery of an Aβ-Degrading Protease: A Novel Therapeutic Approach to Alzheimer Disease

Matthew Hemming and colleagues describe the ex vivo gene delivery of an Aβ-degrading protease that reduces amyloid plaque burden in transgenic mice expressing human amyloid precursor protein

Realismo e transcendência: O mapa das minas do Grande sertão

Guimarães Rosa insistiu sempre sobre o valor metafísico e religioso de seus escritos. Com base nas chaves que Rosa daria no seu discurso de posse na Academia - o Hermetismo da tradição ocidental e o Taoísmo da tradição oriental, pretende-se analisar o percurso do protagonista de Grande sertão: veredas em função dos três grandes rios de Minas: o São Francisco, o Rio das Velhas e Urucuia. Para além da geografia "realista", decripta-se o substrato transcendental

Romanceiro da Inconfidência: Au-delà du christianisme

Publication disponible en ligne à l'adresse : http://www.univ-montp3.fr/llacs/publication-etude-sur-le-romanceiro-da-inconfidencia-francis-uteza/Documents sans référence de publicatio

O Sertão oriental-ocidental de João Guimarães Rosa

International audienc

Le "Romanceiro da Inconfidência": rénovation et symbolique

Séminaire réalisé en partenariat avec la Chaire Sá de Miranda (Ministère portugais des affaires étrangères) et organisé sous la forme de quatre journées d’étude (17 novembre 2008 ; 26 janvier 2009 ; 16 novembre 2009 ; 11 janvier 2010). La communication "Le "Romanceiro da Inconfidência" : rénovation et symbolique" s'est déroulée en janvier 2009.National audienc

Literatura e metafisica: oriente e Ocidente em "Grande Sertao: Veredas"

International audienc