Effects of a gas medium on ultrafast laser beam delivery and materials processing

The propagation of a focused femtosecond laser pulse under the coupled effects of seff-focusing and plasma defocusing in a gas beam-delivery medium is investigated. The results show that a focused beam profile can be dramatically distorted in air but that this distortion can be minimized by use of an inert gas, particularly helium, to deliver the beam. Model predictions are in good agreement with previous experimental results for femtosecond laser micromachining of a copper sample in four gas environments: air, nitrogen, neon, and helium at ambient pressure. The best machining quality was obtained in helium; the worst, in air.