We present an experimental and numerical study of the damage and ablation thresholds at the surface of a dielectric material, e.g., fused silica, using short pulses ranging from 7 to 300 fs. The relevant numerical criteria of damage and ablation thresholds are proposed consistently with experimental observations of the laser irradiated zone. These criteria are based on lattice thermal melting and electronic cohesion temperature, respectively. The importance of the three major absorption channels (multi-photon absorption, tunnel effect, and impact ionization) is investigated as a function of pulse duration (7-300 fs). Although the relative importance of the impact ionization process increases with the pulse duration, our results show that it plays a role even at short pulse duration (<50 fs). For few optical cycle pulses (7 fs), it is also shown that both damage and ablation fluence thresholds tend to coincide due to the sharp increase of the free electron density. This electron-driven ablation regime is of primary interest for thermal-free laser-matter interaction and therefore for the development of high quality micromachining processes.

International audienceThe paper is focused on the importance of accurate determination of surface damage/ablation threshold of a dielectric material irradiated with femtosecond laser pulses. We show that different damage characterization techniques and data treatment procedures from a single experiment provide complementary physical results characterizing laser–matter interaction. We thus compare and discuss two regression techniques, well adapted to the measurement of laser ablation threshold, and a statistical approach giving the laser damage threshold and further information concerning the deterministic character of femtosecond damage. These two measurements are crucial for laser micromachining processes and high peak-power laser technology in general

International audienceSurface ablation of a dielectric material (fused silica) by single femtosecond pulses is studied as a function of pulse duration (7-450 fs) and applied fluence (F (th)< F < 10F (th)). We show that varying the pulse duration gives access to high selectivity (with resolution similar to 10 nm) for axial removal of matter but does not influence the transverse ablation selectivity, which only depends on the normalized applied fluence F/F (th). The ablation efficiency is shown to be inversely dependent on the pulse duration and saturates with respect to the applied fluence earlier at ultra-short pulse durations (a parts per thousand currency sign30 fs). The deduced optimal fluence F (opt) corresponding to the highest ablation efficiency for each pulse width defines two regimes of laser application. Below F (opt), the removed material depth can be accurately adjusted in a large range (similar to 40-200 nm) as a function of the applied fluence and the morphology of the ablated pattern almost reproduces the Gaussian beam distribution. Above F (opt), the material removal depth tends to saturate and the morphology of the ablated pattern evolves to a top-hat distribution. The coupled evolution of depth and morphology is related to the dynamics of formation of dense plasma at the surface of the material, acting as an ultra-fast optical shutter

We describe the programmable spatial beam shaping of 100-kHz, 4-microJ amplified femtosecond pulses in a focal plane by wave-front modulation. Phase distributions are determined by a numerical iterative procedure. A nonpixelated optically addressed liquid-crystal light valve is used as a programmable wave-front tailoring device. Top-hat, doughnut, square, and triangle shapes of 20-microm size are obtained in a focal plane. Their suitability for femtosecond laser machining is demonstrated.

We introduce a quantitative measurement of the determinism of laser-induced damaging at the surface of a dielectric material, e.g., fused silica. Using laser pulses ranging from 7 to 300 fs, we demonstrate that laser damage occurrence tends to be dramatically deterministic at 7 fs, which is attributed to the increasing importance of tunneling ionization as the major channel for the generation of free-carriers in the conduction band.

Direct three-dimensional (3D) laser writing of waveguides is highly advanced in a wide range of bandgap materials, but has no equivalent in silicon so far. We show that nanosecond laser single-pass irradiation is capable of producing channel micro-modifications deep into crystalline silicon. With an appropriate shot overlap, a relative change of the refractive index exceeding 10^-3 is obtained without apparent nonuniformity at the micrometer scale. Despite the remaining challenge of propagation losses, we show that the created structures form, to the best of our knowledge, the first laser-written waveguides in the bulk of monolithic silicon samples. This paves the way toward the capability of producing 3D architectures for the rapidly growing field of silicon photonics.

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

To overcome the resolution limits in laser processing technologies, it is highly attractive to translate concepts used in advanced optical microscopy. In this prospect, the nonlinear nature of absorption in dielectrics with femtosecond lasers is recurrently taken as a direct advantage in an analogous way to excitation in multiphoton microscopy. However, we establish that no direct benefit in resolution can be expected when laser ablation is observed. We explore widely different nonlinear regimes using ultrashort pulses at different wavelengths (1550 and 515 nm) and target materials of various bandgaps (3.8-8.8 eV). We find in the experiments that the shapes of all ablation features correspond to a one-to-one mapping of the beam contours at a strict threshold intensity. The nonlinearity-independent response shows that the incorporation of extreme UV should provide a direct route to the nanoscale resolutions routinely achieved in lithography.