Ultra-fast laser ablation dynamics using beam shaping

Abstract

The invention of high power lasers with extremely short pulses ranging from femto to nanosecond duration opened a plethora of new studies and industrial applications ranging from laser based surgery, laser processing of materials in the semiconductor industry and photonics. However, the spatial distribution of the laser light field is mostly limited to Gaussian shapes, which can prove restrictive for many applications. In this thesis, we study a combined approach of ultrafast laser ablation with spatial beam shaping, which opens up new roads to understand the fundamental physical process of highly non-linear interaction of light and matter. We elucidate key aspects of the physics of material removal by using time-resolved transient reflectivity microscopy combined with a spatial light modulator (SLM). The materials we study in this thesis are water and gel as they are simple substitutes to study the ablation of tissue. The visualisation of laser induced shockwaves in gas at a gas/liquid interface provides information about the flow and the energy released during the expansion of the vaporised material. Yet, it is extremely difficult to properly image weak shocks or subsonic fronts of compressed gas due to the low refractive index change caused by the expanding front. In this thesis, we present a novel method to visualize such gas flows, by inducing multiple excitations in close proximity and imaging the resulting stationary shockwave. We generate the required excitation patterns using a spatial light modulator (SLM). The use of SLM opens up the possibility of creating more exotic illumination patterns that would lead to more complex effects. An important quantity in ablation studies is the amount of removed material. If the amount of removed material is known, one can gain insight into the dominant processes during material removal, such as evaporation, liquid motion and resolidification. However, in the case of liquid materials, this presents a challenge as the surface recovers after each excitation. To solve this problem, we study the ablation of gel, as with a gel, a crater will remain visible in the aftermath. We find that water and gel have an extremely similar expansion dynamics. This similarity allows us to estimate the removed material during water ablation based on the gel ablation aftermath. From those measurements and finite-difference-time-domain calculations, we find that the removed mass and the absorbed energy follow a similar trend. From this, we argue that the (highly supersonic) expansion velocity of the ablation plume is almost independent of the laser pulse energy. In the final part of the thesis, we present a beam shaping method for laser processing of materials. As liquid motion inside the molten layer of the material contributes to the material removal process, it would be advantageous to control this liquid flow. One could control and study the flow using spatially shaped laser beams. We demonstrate high-fidelity TopHat patterns as a first step towards reaching this goal.