Basic concepts of laser drilling

The state of the technology of s pulse laser applications is dominated by single pulse drilling, percussion drilling and even trepanning used for high speed drilling with melt expulsion. However, short ps pulses have to be addressed anyway, since there are technical aspects in addition to achieve high speeds in drilling, namely, structuring and tapering while maintaining the mechanical integrity of operation. As an example, to avoid delamination of thermal barrier coatings while structuring the inlet of cooling holes in turbine manufacturing as well as to avoid cracking at the drilled wall forces the scientist needs to take into consideration the mechanisms of short pulse ablation at least in the case of ps pulses. The variety of intriguing physical phenomena span from recast formation well known from the action of s-pulses, via formation of cracks typical for ns- to ps-pulse duration, towards homogeneous expansion, phase explosion and spallation characteristic for fs-pulses. The numerous phenomena are related to physical models describing propagation and absorption of radiation, ionization, evaporation and non-linear transport of mass, momentum and energy. Technical achievements like lasers emitting 100 ps or shorter pulses and related experimental observations introduce the future need for simulations to cope also with kinetic properties of beam-matter interaction. Temperatures approaching the critical state during ablation with pulse durations in the range from some ps to a few hundred ns raise the question whether Equation of State phenomena are contributing to the overall appearance in drilling. In particular, beam aberrations instead of a free running or multiply reflected beam pattern are encountered in modelling independent of pulse duration. Beam aberrations are not only introduced by the action of beam guiding and forming optics, but also by spatially distributed feedback from the dynamical shape of the ablated material surface. Effects changing the phase distribution of the incident laser radiation are incorporated in the models for the first time: for example, some temporal and spatial changes of the density in the gaseous phase. In drilling, the dynamical phenomena governing the shape of the drilled hole are identified experimentally and can be related to the processing parameters theoretically