Observation of damage generation induced by electron excitation and stress wave propagation during ultrashort pulse laser drilling of sapphire

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.