Evidencing the nonlinearity independence of resolution in femtosecond laser ablation.

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.