Investigation of air plasma in ultra-short laser processing of crystalline silicon

Ultra-short pulsed laser radiation has been shown to be effective for precision material processing and surface micro-modification. One of the advantages is the substantial reduction of the heat penetration depth, which can lead to minimal lateral damage. On the other hand, the peak laser power applied in this experiment is sufficiently high to induce air breakdown forming air plasma. The femtosecond-resolved imaging pump-and-probe experiments provide a distinct evidence for the occurrence of air plasma. This highly nonlinear phenomenon is triggered before the commencement of the ablation process, which is traced at elapsed time of the order of 10 ps with respect to the ablating pulse.