Observation of damage generation induced by electron excitation and stress wave propagation during ultrashort pulse laser drilling of sapphire
Observation of damage generation induced by electron excitation and stress wave propagation during ultrashort pulse laser drilling of sapphire
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DOI：
10.1007/s00339-022-05686-8
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发表时间：
2022-06
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影响因子：
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通讯作者：
Huijie Sun;Yusuke Ito;Guoqi Ren;J. Hattori;K. Nagato;N. Sugita
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作者：
Huijie Sun;Yusuke Ito;Guoqi Ren;J. Hattori;K. Nagato;N. Sugita
超短脉冲激光加工作为一种加工蓝宝石的方法正在引起人们的广泛关注。然而，由于加工形状周围会产生严重损坏，其精确度是一个问题。在本研究中，为了阐明损伤产生的机制及其对脉冲持续时间的依赖性，采用了泵浦探针成像和高速相机相结合的成像技术。泵浦探针成像将皮秒和纳秒量级发生的超快现象可视化，例如电子激发和应力波传播，而高速相机则捕捉随着脉冲数量以毫秒量级增加而发生的现象变化。对多达 20 个脉冲的电子激发的观察表明，当脉冲持续时间超过 3 ps 时，蓝宝石内部的电子诱导损伤非常严重。高达 1000 个脉冲的高速观测表明，应力波从孔尖传播并导致应力引起的损坏。应力引起的损坏首先在尖端产生，然后随着脉冲数量的增加而保留在孔周围作为侧壁损坏。侧壁损伤逐渐扩大，最终传播到表面，造成表面损伤。基于不同脉冲持续时间的研究表明，当脉冲持续时间为 180 fs 时，由于应力波更强，应力引起的损伤更为突出。此外，我们发现最初产生的电子引起的损伤随着脉冲数量的增加而消融。因此，当脉冲持续时间较长时，可以实现更精确的处理。损伤产生的机理不仅有助于精密激光加工技术的发展，也有助于基础科学的进一步认识。
Ultrashort pulse laser processing is garnering significant attention as a method for processing sapphire. However, its preciseness is an issue owing to severe damage generated around a processed shape. In this study, to clarify the mechanism of damage generation and its dependence on pulse durations, an imaging technique combining pump-probe imaging and a high-speed camera is utilized. The pump-probe imaging visualizes ultrafast phenomena that occur in the order of picoseconds and nanoseconds, such as electron excitation and stress wave propagation, while the high-speed camera captures changes in the phenomena as the number of pulses increases in the order of milliseconds. Observations of electron excitations with up to 20 pulses show that when the pulse duration exceeds 3 ps, electron-induced damage inside sapphire is significant. High-speed observations up to 1000 pulses show that stress waves propagate from the tip of the hole and cause stress-induced damage. The stress-induced damage is first generated on the tip and then remains around the hole as sidewall damage as the number of pulses increases. The sidewall damage expands gradually and finally propagates to the surface, resulting in surface damage. Investigations based on varying pulse durations reveal that the stress-induced damage is more prominent when the pulse duration is 180 fs because of stronger stress waves. Furthermore, we discovered that the initially generated electron-induced damage ablates as the number of pulses increases; as such, more precise processing is achieved when the pulse duration is longer. The mechanisms of damage generation will contribute to not only the development of precision laser processing technology, but also to the further understanding of basic science.