# Experimental Investigations on Laser Ablation of Aluminum in Sub-Picosecond Regimes
## 4 Results and Discussion  
As it was previously stated, ablation mechanisms strongly depend on the laser beam parameters [2]. The methodology of the conducted experiments is presented in Table 3.  
In the materials undergoing laser micromachining, there are various markers that can be connected to the dynamical mechanisms and the ablation threshold. Among them, there are crater depth and width, local changes in material structure, cracks, surface debris, and colour change caused by melting, to mention a few. To analyse some of those features, we used a Nikon Eclipse metallographic microscope (E100, Nikon Corporation, Tokyo, Japan, 2006) and a 3D Hirox KH8700 microscope (Hirox Co. Ltd., Tokyo, Japan, 2011). Post-machining crater dimensions were measured and the heat-affected changes on the surface of the samples were observed. The 3D analysis was applied to examine a cross-section of profiles and material structure changes. The representative results are presented below (Figures 2-5). \begin{table}
\begin{tabular}{c c c} \hline
**Parameter** & **Sub-Picosecond** & **Femtosecond** \\ \hline Laser type & Yb:KYW & Ti:S \\ \hline Pulse duration (fs) & 650 & 35 \\ \hline Available wavelength (nm) & 345–1030 & 240–2600 \\ \hline Frequency (kHz) & 100–400 & 1–10 \\ \hline Pulse energy (\(\mu\)J) & 140 & 6000 \\ \hline \end{tabular}
\end{table} 
Table 2: The specifications of the two ultrafast lasers used in the experimental setup. Figure 1: The experimental setup for ultrafast laser micromachining. Figure 3: Aluminum sample irradiated with sub-picosecond Yb:KYW laser (57.5 \(\upmu\)J, 650 fs, 1030 nm, 200 kHz). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Type of Machining** & **Parameter Change** & **Results** \\ \hline \hline Scribing (1 cm line) & Pulse energy 7.5–57.5 \(\upmu\)J & Figure 2 \\ \hline \hline Prolonged irradiation & Time 1–15 s & Figure 3 \\ \hline \hline Scribing (1 cm line) & Frequency 100–400 kHz & Figures 4 and 5 \\ \hline \hline Scribing with increasing pulse energy & Wavelength 266–800 nm & Figure 6 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The summary of the experiment cases. Figure 2: Aluminum sample scribed with sub-picosecond Yb:KYW laser (650 fs, 1030 nm, 200 kHz). Figure 2 presents a 0.406 mm thick aluminum sample machined with the sub-picosecond laser. Each 1 cm line was scribed with different laser pulse energy. It was observed that crater width decreases with decreasing energy. There were no visible effects of the heat affected zone and no debris on the surface, even for much higher energies than the ablation threshold. Line edges were straight and clean, except for energies exceeding 37.5 \(\upmu\)J. The best results were obtained for 7.5-17.5 \(\upmu\)J. Crater depths of 1000 \(\upmu\)J.  
Figure 4: Aluminum sample scribed with sub-picosecond Yb:KYW laser (650 fs, 1030 nm). Figure 5: Crater profile of the aluminum sample scribed with sub-picosecond Yb:KYW laser (650 fs, 1030 nm). obtained from 3D analysis varied from 2 to 5 \(\upmu\)m and were decreasing with pulse energy as well. All samples were machined with 25 mm/s beam transition speed. Figure 3 presents aluminum samples treated with extended laser irradiation times of up to 15 s. The purpose of the experiment was to overheat the sample and enable the heat transfer to the area surrounding the target. Despite the prolonged time of laser-matter interaction and high pulse energy, no melting or significant color change (i.e., yellow to blue rings assigned to high temperature metal tempering) were noticed. The crater diameter was insignificantly increasing with the pulse energy. Figure 6: Ablation dependence on laser fluence for different wavelengths. Figure 7: Ablation dependence on laser fluence for sub-picosecond and femtosecond lasers. Figure 4 shows the crater width decreasing with the increasing laser frequency. Although higher frequency results in tight overlapping of the laser pulses (the beam transition speed was constant, at 25 mm/s), the pulse energy decreases, resulting in a thinner line.