Time-resolved interferometric investigations of the KrF-laser-induced interaction zone

In earlier ablation experiments the excimer laser-induced interaction zone was observed using Schlieren-photography. A spherical blast wave and, additionally, several regions separated by discontinuities behind the shock front were discovered during the pulse. This article will present experimental investigations using a Michelson interferometer in order to measure the phase front distortion of a probe laser radiation field crossing the mentioned interaction zone. The experiments were realized during the pulse (FWHM=30 ns, basis width=60 ns) under different ambient gas pressures (104–105 Pa), different energy densities (5–45 J/cm2) and with a time resolution of 500 ps. The index of refraction n was calculated using the optical path lengths obtained from the interference pattern, considering a radial symmetry. The results show a high refractive index at the shock front and a steep decrease towards the target closely behind the shock front. In this region the index of refraction tends to a lower value than the index of refraction of vacuum due to a high concentration of free electrons. A comparison with the results of Schlieren-photography gives reasonable explanations for the different regions within the interaction zone.