Three-dimensional writing inside silicon using a 2-µm picosecond fiber laser

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Investigation of ultrashort laser excitation of aluminum and tungsten by reflectivity measurements

International audienceWe determine the laser-induced ablation threshold fluence in air of aluminum and tungsten excited by single near-infrared laser pulses with duration ranging from 15 fs to 100 fs. The ablation threshold fluence is shown constant for both metals, extending the corresponding scaling metrics to few-optical-cycle laser pulses. Meanwhile, the reflectivity is measured providing access to the deposited energy in the studied materials on a wide range of pulse durations and incident fluences below and above the ablation threshold. A simulation approach, based on the two-temperature model and the Drude-Lorentz model, is developed to describe the evolution of the transient thermodynamic and optical characteristics of the solids (lattice and electronic temperatures, reflectivity) following laser excitation. The confrontation between experimental results and simulations highlights the importance of considering a detailed description and evolution of the density of states in transition metals like tungsten

Electron Collision Rate in Ultrashort Laser – Metal Interaction Inferred from Reflectivity Measurements

International audienceWe study optical laser coupling of four metals (aluminum, copper, nickel and tungsten) upon single ultrashort 30-fs laser irradiation. Using dedicated experimental methodology, we carefully monitor the evolution of the material reflectivity integrated over the femtosecond pulse duration. We further apply the Drude-Lorentz formalism to infer the evolution of the effective electron collision rate eff as a function of applied fluence. The knowledge of this parameter is of paramount importance in any lasermatter interaction situation because of its immediate impact on laser energy coupling in material and on its subsequent macroscopic transformation

Internal laser writing inside semiconductors by ultrafast pulses

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Nonlinear Resolution: A Misconception in Femtosecond Laser Ablation

International audienceWe demonstrate a systematic one-to-one mapping between femtosecond laser ablation features and beam contours at a strict threshold-intensity. This is independent of the nonlinearity of interaction varied using various wavelengths and dielectric materials

Unravelling ultrashort laser excitation of nickel at 800 nm wavelength

International audienceThe optical response of nickel is studied in a wide range of laser fluence, below and above the ablation threshold, by selfreflectivity measurements of ultrashort 800-nm single laser pulses. At the ablation threshold, the reflectivity remains unchanged with respect to its unperturbed value irrespective of the pulse duration, from 15 to 100 fs, consistently with the steadiness of the laser-induced ablation threshold fluence Fth for all pulse durations tested. Until the ablation threshold (F Fth) and whatever the pulse duration, the disturbances caused to the initial structure of the electron gas distribution by the laser energy deposition are limited, having no significant impact on the transient optical response of nickel and on its ablation threshold. At higher laser fluences (F > Fth), the reflectivity becomes rapidly dominated by the contribution to the optical response of the fast-thermalized free electrons (4s-band) with energy largely above the Fermi energy level. In these conditions, the reflectivity decreases for all pulse durations enhancing laser energy coupling and larger optical absorption at the surface of nickel. The optical response of nickel under ultrashort (15-100 fs) irradiation is thus fully elucidated on a wide range of fluence (0.3 Fth-30 Fth) and for pulse duration down to few-optical-cycle pulse duration. As a key parameter for benchmarking laser-matter interaction in poorly known conditions yet, the evolution of the effective electron collision rate is determined as a function of fluence and pulse duration in very good consistency with experiments

Experimental investigation of size broadening of a Kα x-ray source produced by high intensity laser pulses

International audienceThe size of a hard K α x-ray source ( $${\mathrm{E}}_{{\rm{K}}_{\rm{\alpha }}}$$ E K α = 17.48 keV) produced by a high intensity femtosecond laser interacting with a solid molybdenum target is experimentally investigated for a wide range of laser intensity (I ~ 10 17 –2.8 × 10 19 W/cm 2 ) and for four values of the temporal contrast ratio (6.7 × 10 7 < CR < 3.3 × 10 10 ). Results point out the size enlargement of the x-ray source with the increase of laser intensity and with the deterioration of temporal contrast. It amounts up to sixteen times the laser spot size at the highest laser intensity and for the lowest temporal contrast ratio. Using hydrodynamic simulations, we evaluate the density scale length of the pre-plasma L/λ just before the main pulse peak. This allows us to show that a direct correlation with the laser absorption mechanisms is not relevant to explain the large size broadening. By varying the thickness of the molybdenum target down to 4 µm, the impact of hot electron scattering inside the solid is also proved irrelevant to explain the evolution of both the x-ray source size and the K α photon number. We deduce that the most probable mechanism yielding to the broadening of the source size is linked to the creation of surface electromagnetic fields which confine the hot electrons at the solid surface. This assumption is supported by dedicated experiments where the evolution of the size enlargement of the x-ray source is carefully studied as a function of the laser focal spot size for the highest contrast ratio

Modélisation de l'amplification d'une impulsion laser UV nanoseconde dans un milieu à excimères

Cet article présente un code numérique temporel décrivant le transport d'un champ de rayonnernent UV dans un milieu à excirnères arnplificateur XeCl et dans l'espace libre. Le modèle est ensuite appliqué à l'analyse de l'amplification d'un pulse court XeCl (∼ 3 ns) dans un milieu actif de volume 0.1 litre