Expansion of laser-induced plume after the passage of a counter shock wave through a background gas

Double-pulsed laser ablation with two targets and lasers in a background gas is a method to form nanoparticle complex. Effects of pulse delay between two lasers on plume expansion dynamics are discussed. The germanium and silicon targets were set parallel to each other and irradiated by two YAG lasers. The germanium target was irradiated followed by irradiation of the silicon target with delay time, t(d). We found that the expansion distance of delayed silicon plume is enhanced for 2 mu s <= t(d) <= 50 mu s, compared to that when only the silicon target is irradiated. For t(d) = 200 mu s, the expansion distance of delayed silicon plume is similar to that when only the silicon target is irradiated. We discuss the expansion dynamics of the delayed silicon plume based on the effect of the density distribution induced by the primary germanium plume. Our results indicate that the effect of primary germanium plume remains up to about t(d) = 50 mu s, and it disappears by t(d) = 200 mu s.