Femtosecond laser nano-ablation of glass surfaces and their self-scattering effects

Abstract

In this thesis, the interaction between a femtosecond laser and glasses in the ablation regime is studied. A pulsed femtosecond laser setup allows for surface ablation of glasses under strong focussing conditions. This setup is used to measure the reflectivity of four different glasses as a function of the laser fluence, in order to study the self action of the laser pulse. The craters produced by the laser pulse are examined using atomic force microscopy. These two measurements together are able to provide insights in the energy coupling of the laser pulse into the material. It is found that for increasing fluences, the selfreflectivity and crater depth both rise with increasing slopes. With a similar experimental setup, which uses a pump and probe technique, the dynamics of femtosecond laser matter interaction in fused silica are studied under weak focussing conditions. By illuminating the plasma generated by the pump beam at different times using a delayed probe beam, its evolution can be observed by its reflectivity. It is seen that in 1 ps, the plasma reaches its maximum reflectivity. The plasma's reflectivity remains constant for several picoseconds, before recombining with the glass. Well after the laser has interacted with the glass, signs of material ablation can be observed. A one-dimensional finite-difference time-domain simulation is performed to replicate the experiment's results. The simulation applies the Drude model and the often used single rate equation developed by Stuart et al. to model ionization. The simulated results do not agree with the experimental results, due to an overestimation of the ionization rate by the single rate equation. Instead, implementation of a multiple rate equation is suggested to model ionization.