Two-wavelength interferometry on excimer laser induced vapour/plasma plumes during the laser pulse

Abstract

Ablation-using short-pulse lasers, e.g., excimer lasers and solid state lasers, is becoming an important technology for micro-machining, thin film formation and fine particle generation. Hence, there is a great interest to understand the interaction mechanisms between the radiation field and the evaporated material. Especially the laser-induced material vapour influences the efficiency and the quality of the ablation, as shown in earlier contributions [G. Callies, P. Berger, J. Kästle, H. Hügel, Proc. SPIE, Vol. 2502, p. 706; G. Callies, H. Schittenhelm, P. Berger, F. Dausinger, H. Hügel, Proc. SPIE, Vol. 2246, p. 126]. Two-wavelength interferometry, shadowgraphy and resonance absorption photography allowed us to investigate the whole laser induced region with each probe-laser pulse. The experiments were performed in ambient air, helium and argon at a pressure of 10 5 Pa. Earlier, shadowgraphy experiments indicated several discontinuities within the plume arising during the laser pulse. To get more information about the nature of these discontinuities and their expansion behaviour and to obtain the free electron density distributions within the shock wave, interferometry with two wavelengths was applied. The results show spatially-separated regions of high free electron densities and therefore, high temperatures within the plasma plume. The observed regions correspond to those found by shadowgraphy and resonance absorption photography: the region of material vapour directly behind the contact front, the plasma core near the target surface with high electron densities, and two more regions separated by discontinuities. A variation of the ambient gas causes a drastic change in the electron density. In an argon atmosphere, a formation of a laser supported detonation wave, instead of a shock wave, arises for energy densities higher than 20–25 J/cm 2. The interferometry yields, for this case, a very high electron density within the material vapour near the contact front. A comparison of the electron density distribution with ablation rates in helium and nitrogen indicate the independence of the interaction of the excimer laser radiation with the electrons.