Interaction with Atmosphere

Abstract

Contrary to longer-pulsed laser irradiation, ultrashort laser pulses in the femto- and low picosecond-pulse duration domains are expected to be too short to interact directly with the material vapor they produce during ablation. Nonetheless, the very high intensities reached by focused ultrashort pulses cause a number of interaction phenomena of the pulses with the ambient atmosphere. The various effects are discussed with respect to their relevance to femtosecond laser machining. Optical breakdown, the threshold of which is easily reached with ultrashort pulses, can cause absorption losses despite the short irradiation times. When nonlinear interaction with the atmospheric gas takes place additionally at even higher intensities, the phenomenon of conical emission leads to beam-profile disruption and increased beam divergence. During ablation shock waves are created by the rapid vapor flow. Their visualization provides insight into individual ablation processes and allows the effects of laser parameter variations to be judged. In the case of multi-pulse machining, residual ablated matter left behind by previous pulses is accumulated in the atmosphere and can interact with subsequent pulses.