The turbulence in the core of the plasma results due to the interactions between the highly energized plasma species due to incoming laser pulse absorption and nitrogen gas molecules. Due to the more turbulent interactions and excessive plasma material during 13-MHz repetition rate machining, the plasma species expand wider, and thus, the redeposition back to the target surface occurs over a larger surface area resulting in the formation of a much larger number of randomly oriented leaf-like nanotips,
as seen in Figure 6c. When the ablation is performed at the 8-MHz repetition rate, the plasma must have ideal condition in terms of the amount of the RG-7388 mouse turbulence and available ablated material resulting in the growth of highly populated and oriented narrower nanotips compared to 13 MHz, as seen in Figure 6b. For a low number of pulses, the plasma expansion and interaction with surrounding nitrogen gas is less turbulent. The plasma has more time to relax before the next pulse arrives. Thus, the plasma does Selleck MK5108 not expand outward as much resulting in the
plasma species being closer. This Givinostat order resulted in the formation of larger droplets of vapor content which get deposited over the target surface area. As a consequence, only a few nanotips are found to be growing randomly from large droplets for the 4-MHz repetition rate, as seen from Figure 6a. Figure 6 Effect of laser pulse repetition rate on plasma expansion and nanotip growth. Nanotips generated for the average laser power of 16 W for pulse repetition rates of (a) 4, (b) 8, and (c) PAK6 13 MHz; the dwell time was 0.5 ms. Effect of dwell time The dwell time study was performed for 214-fs pulse width and various repetition rates. Figure 7 shows the SEM images of the glass target machined at dwell times of 0.1, 0.25, and 0.5 ms for the 8-MHz repetition rate. The growth steps of the nanotips are clearly evident from these three images. As a result, it is obvious from Figure 7 that the growth of these nanostructures is dependent on the dwell time as much as on other laser parameters. For
example, at 0.1 ms, the plasma has very little vapor content resulting in the redeposition of the droplets on the target surface and the growth of stem for the nanotips, as seen in Figure 7a. Once the stem growth has started, the continuous redeposition of vapor condensates from plasma back to the surface provides the building material for tips to grow. At 0.25-ms dwell time, the plasma has just enough building material for the tips to start growing in a nanoscale to a micrometer length; the number of tips present on surface also increased. When the dwell time is further increased to 0.50 ms, the nanoscale tips grew to the length of 1 to 2 μm as well as their population increased on the target surface. Figure 7 Nanotip growth under different femtosecond laser irradiation dwell times.