Control of material removal of fused silica with single pulses of few optical cycles to sub-picosecond duration

Surface 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.