Transition from expansion to shock compression in laser Irradiated Si by multiple shots

Picosecond time-resolved X-ray diffraction measurements have been performed on laser irradiated Si single crystal. Two types of experiments were performed in air. In one set of experiments, single shot irradiation on the silicon (111) crystal was performed at an irradiance range of 1 - 10 GW/cm2. The results of X-ray diffraction measurement showed both thermal expansion and transient elastic shock compression. It is deduced that the ablation threshold falls in the range in irradiance of 1 - 10 GW/cm2. The second set of experiments was performed with multiple shot irradiation in the range in irradiance of GW/cm2. Lattice compression due to the multiple laser irradiation was observed. This result indicates that multiple irradiation causes reduction of the ablation threshold