Formation of Conical Microstructures of Silicon with Picosecond Laser Pulses in Air

5000) at laser fluence of 1.3 - 1.5 J/cm 2 . The microstructures are cones which grew toward the laser beam and surrounded by deep holes. The ps-formed microstructures are roughly 10 µm tall and separated by 5-10 µm in the center of the irradiation spot. Nanocolumns (or Nanodroplets) are formed on top of every cone of the laserinduced microstructures surface, reaching a height of ~1 µm and diameter of 100-250 nm. At higher applied laser fluence levels, no conical spikes formed and smooth surface is obtained due to high rate of thermal ablation. We will discuss the possible mechanism underlying these experimental observations.