The Role of Defects in Pulsed Laser Matter Interaction

Laser nano- and micromachining exhibits multiple technological applications particularly in cyber physical production systems, where the integrity of production processes has to be maintained (e.g. in the implementation of Industry 4.0). Key characteristics of laser processing, such as the ablation threshold was commonly related to the fundamental parameters (fluence, pulse number, and irradiated area) by empirical adaptation and optimization. The description of the pulse number dependence (incubation, laser-induced defects) and the irradiation area dependence (intrinsic defects) of the threshold fluence still resorts to phenomenological models. This deficit can be resolved by a combined description of pulse number and beam radius based on a model involving high density and low density defects in the solid material. This extended defect model can describe single and multiple nanosecond and femtosecond pulse ablation experiments on various technological materials such as polystyrene, monocrystalline silicon, and stainless steel. While this model allows a quantification of the laser-induced threshold fluence in dependence of pulse number (incubation) and irradiated area (beam radius), the physical mechanisms involved in the interaction between light and defects, be it intrinsic or laser-generated, are still marginally understood. Further experimental and theoretical effort in this direction is aiming at the provision of the deterministic understanding required in any field applying intense laser light with multiple pulses and various spot sizes, which is essential in e.g. laser cutting, drilling, marking, engraving, hardening, and welding.