In recent years, ultrashort pulsed laser material micromachining based on the burst mode has received extensive attention, and the burst mode is an effective method to improve the efficiency and quality of material removal. For higher processing efficiency, the subpulse repetition frequency is increased to GHz, which enables the precise regulation of both electron dynamics and thermal effects and thus the control and optimization of the ablation mechanism and effect. However, pulse train ablation involves complex physical processes and experimental phenomena. It is important to study the physical mechanisms of pulse train ablation. Research on laser micromachining of GHz pulse trains mainly focuses on femtosecond lasers; however, the ablation effect of pico-pulse trains is still less studied, although picosecond lasers are currently the most widely used in industry. Pico- and femto-pulses have significant ablation effect differences. Among them, dual-pulse (pump-probe) is the simplest form of the burst mode, which is the main method used to study the physical process of pulse train ablation. Therefore, the study of picosecond dual-pulse ablation is important to reveal the physical mechanism of picosecond burst ablation. In this study, we combined a dual-pulse train and temporal shaping to investigate the ablation process and mechanism of the K9 glass surface.

We proposed the temporal shaping of a dual-pulse train at the sub-nanosecond scale to study laser ablation on a glass surface. First, the time interval of the dual-pulse was fixed at 667 ps. The effects of single- and dual-pulses with various shapes on the laser ablation characteristics were studied by adjusting the energy ratio of the dual-pulse. The ablation morphology under different fluences was classified based on similarities with various dual-pulse shapes. Then, the distribution curve of the characteristic morphology with fluence was used to analyze the law of subpulse ablation. Second, we further reduced the time interval of the flat-shaped dual-pulse to 333 ps and analyzed the variation law and factors of the ablation characteristics under two delays. Finally, the physical process of surface ablation modulation based on the temporal shaping of the dual-pulse train is discussed based on the experimental results.

The experimental results show that the temporally shaped dual-picosecond pulse train at the sub-nanosecond scale has a significant effect on the ablation morphology, size, and threshold of K9 glass surface. With a subpulse interval of 667 ps, the ablation morphology at different fluences was grouped into five characteristic types based on the dual-pulse ablation morphology with various shape factors (Fig. 4). The ablation morphology depends mainly on the pump pulse fluence. The pump fluence below the ablation threshold has no effect or only weakly modified areas on the sample surface, and the ablation characteristics of dual-pulse are similar to those of a single pulse. With a pump pulse above the threshold, the dual-pulse ablation morphology has concentric rings (Fig. 5). The distribution curve of the characteristic morphology with fluence (Fig. 6) demonstrates that the characteristics of the core ring depend only on the pump pulse, whose size and threshold are similar to those of the single pulse, whereas the outer size is the result of the combined effect of the two sub-pulse ablations. Dual-pulses of various shapes have different ablation thresholds (Fig. 8), and the ramp-up-shaped train has a higher threshold. In addition, we compared the ablation morphology of flat-shaped dual-pulses with subpulse intervals of 333 and 667 ps (Fig. 9). A smaller interval has a lower threshold and higher ablation efficiency at a low fluence, whereas the ablation outcome of the pump pulse above the threshold prevents the energy deposition of the probe pulse (Fig. 10). A qualitative analysis of the schematic of the ablation process is shown in Fig. 11. By adjusting the fluence of the pump and probe pulses, the temporally shaped dual-pulse can flexibly control the initial ablation size and plumes caused by the pump pulse, laser propagation, and energy deposition of the probe pulse, resulting in various types of ablation morphologies.

Based on the dual-pulse with sub-nanosecond intervals, we investigate the regulation of dual-pulse temporal shaping on the picosecond ablation characteristics of the K9 glass surface, including ablation morphology, size, and threshold. First, with a subpulse interval of 667 ps, the dependence of the ablation morphology on the laser fluence is significantly different for various shapes of dual-pulses, and the pump pulse plays a critical role. When the pump fluence is below the threshold, the dual-pulse ablation characteristics are similar to those of a single pulse. When the pump fluence is near the threshold, the ablation of pump pulse on the surface at the submicron scale significantly enhances the ablation effect of the probe pulse. When the pump fluence is more than 1.3 times of the threshold, the pump pulse generates a shock wave near the surface, and the probe pulse is reflected and interfered by the high-density shock front, which produces concentric rings around the central ablation region. The core size of dual-pulse ablation is related to the pump fluence, whereas the outer diameter size is related to both the dual-pulse shape and fluence. Second, we compare the ablation morphology of flat-shaped dual-pulses with subpulse intervals of 333 and 667 ps. A smaller interval enhances the ablation effect at a low fluence, whereas the ablation outcome of a pump pulse above the threshold prevents the energy deposition of the probe pulse. The difference in concentric ring morphology between the two intervals reflects the transmission of the shock front caused by the pump pulse. Finally, the physical mechanism of surface ablation regulation by dual-pulse temporal shaping is discussed based on the experimental results, which contributes to further understanding and optimization of the laser ablation of transparent materials in GHz.