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

The metrology of laser-induced damage usually finds a single transition from 0% to 100% damage probability when progressively increasing the laser energy in experiments. We observe that picosecond pulses at 2-µm wavelength focused inside silicon provide a response that strongly deviates from this. Supported by nonlinear propagation simulations and energy flow analyses, we reveal an increased light delocalization for near critical power conditions. This leads to a nonmonotonic evolution of the peak delivered fluence as a function of the incoming pulse of the energy, a situation more complex than the clamping of the intensity until now observed in ultrafast regimes. Compared to femtosecond lasers, our measurements show that picosecond sources lead to reduced thresholds for three-dimensional (3D) writing inside silicon that is highly desirable. However, strong interplays between nonlinear effects persist and should not be ignored for the performance of future technological developments. We illustrate this aspect by carefully retrieving from the study the conditions for a demonstration of 3D data inscription inside a silicon wafer.

International audienceTwo-photon ionization by focused femtosecond laser pulses initiates the development of micrometer-scale plasmas in the bulk of silicon. Using pump-and-probe transmission microscopy with infrared light, we investigate the space-time characteristics of these plasmas for laser intensities up to 10(12) W/cm(2). The measurements reveal a self-limitation of the excitation at a maximum free-carrier density of congruent to 10(19) cm(-3), which is more than one order of magnitude below the threshold for permanent modification. The plasmas remain unchanged in the similar to 100 ps timescale revealing slow carrier kinetics. The results underline the limits in local control of silicon dielectric permittivity, which are inherent to the use of single near-infrared ultrashort Gaussian pulses. (C) 2014 AIP Publishing LLC

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.

International audienceUsing various band-gap materials and tightly focused femtosecond laser pulses with wavelengths in the range 1200-2200 nm, we show that nonlinear absorption is independent of the wavelength except for narrow gap semiconductor materials. This observation corresponds to a transition between multiphoton ionization and tunnel ionization for an adiabaticity parameter of about 3, which compares favorably with Keldysh predictions. Our results indicate that long wavelengths must open up an alternative to pulse shortening for ultraprecision optical breakdown in dielectrics

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.