To understand the physics of double-pulse femtosecond laser ablation of metals, the authors have proposed and implemented a new ablation method for measuring the temporal change in the laser penetration length (LPL). The measurements were performed for titanium and platinum with femtosecond laser pulses with a central wavelength of 810 nm, a repetition rate of 10 Hz, and a pulse duration of 45 fs. The delay between the seed pulse and the ablation pulse was between 0.3 ps and 1 ns for titanium and between 0.3 ps and 30 ns for platinum. A low-fluence (below the ablation threshold) seed pulse and a high-fluence (above the ablation threshold) ablation pulse were used for the measurement. The seed pulse modifies the optical properties of the target surface, and the ablation pulse creates a crater on the modified surface. The LPL after seed-pulse irradiation was estimated by analyzing how the ablation rate depends on the laser fluences. With a delay of 300 ps, at which the ablation rate is the lowest for both titanium and platinum, the LPL for a titanium target was 40% of that for the target without seed-pulse irradiation, while the LPL for a platinum target was almost the same as that for the target without seed-pulse irradiation. The reduced LPL corresponded to suppressed ablation for titanium and platinum with a delay of 300 ps. The suppression ratio ${\eta }_{\sup }$ (ablation rate with seed pulse divided by ablation rate without seed pulse) for titanium (${\eta }_{\sup }=0.5$) was less than that for platinum (${\eta }_{\sup }=0.8$). The LPL measured using the proposed ablation method is a key parameter changed by seed-pulse irradiation. For platinum with a delay exceeding 1 ns, a characteristic increase of LPL was also measured.

中文翻译：

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新型烧蚀法测量双脉冲烧蚀钛和铂的激光穿透长度的时间变化

为了理解金属双脉冲飞秒激光烧蚀的物理原理，作者提出并实现了一种新的烧蚀方法，用于测量激光穿透长度（LPL）的时间变化。使用飞秒激光脉冲对钛和铂进行测量，飞秒激光脉冲的中心波长为810 nm，重复频率为10 Hz，脉冲持续时间为45 fs。对于钛，种子脉冲与消融脉冲之间的延迟在0.3 ps与1 ns之间，对于铂，在0.3 ps与30 ns之间。低通量（低于消融阈值）种子脉冲和高通量（高于消融阈值）消融脉冲用于测量。种子脉冲改变了目标表面的光学特性，而消融脉冲在改变后的表面上产生了凹坑。通过分析烧蚀速率如何取决于激光通量来估算种子脉冲照射后的LPL。在300 ps的延迟下，钛和铂的烧蚀率最低，钛靶材的LPL是未经种子脉冲辐照的靶材的LPL的40％，而铂金靶材的LPL几乎是与没有种子脉冲照射的靶相同。降低的LPL对应于钛和铂的抑制烧蚀，延迟为300 ps。抑制比 铂靶的LPL与没有种子脉冲辐射的靶的LPL几乎相同。降低的LPL对应于钛和铂的抑制烧蚀，延迟为300 ps。抑制比 铂靶的LPL与没有种子脉冲辐射的靶的LPL几乎相同。降低的LPL对应于钛和铂的抑制烧蚀，延迟为300 ps。抑制比${\eta }_{\mathrm{SUP}}$ 钛（带种子脉冲的烧蚀率除以不带种子脉冲的烧蚀率）${\eta }_{\mathrm{SUP}}=0.5$）小于铂金（）${\eta }_{\mathrm{SUP}}=0.8$）。使用提出的消融方法测量的LPL是种子脉冲辐照改变的关键参数。对于延迟超过1 ns的铂，还测量了LPL的特性增加。