Figure 5.

(a) Temporal evolution of silicon needle formation visualized by employing an ultrahigh speed camera during the first vortex pulse irradiation. The temporal evolution is classified under three stages. (First stage) The silicon is melted by the optical vortex pulse deposition. (Second stage) After the vortex pulse is gone, the melted silicon is collected to the dark core of the optical vortex. (Third stage) The silicon is accumulated at the core. Also, the silicon droplets then fly away. The resulting silicon needle is established. The inset shows a magnified silicon droplets. They fly to form a straight line, and their diameter (radius) is typically estimated to be ~4 (~2) μm. (b) Temporal evolution of the silicon needle formation visualized by employing the ultrahigh speed camera during a twelfth vortex pulse irradiation onto the silicon target. The temporal evolution is classified under three stages. (First stage) The silicon is melted by the optical vortex pulse deposition. (Second stage) After the vortex pulse is gone, the melted silicon is accumulated onto the silicon needle. (Third stage) The undesired silicon droplets then fly away. The height of the silicon needle is reinforced. (c) Temporal evolution of the ablated silicon surface visualized by employing the ultrahigh speed camera upon the irradiation of a picosecond pulse with a Gaussian spatial form. The thermal explosion and melting of the silicon occurred within 200 ns, thus resulting in shallow crater formation without any needles.