Underwater ablation using short and ultrashort laser pulses

Abstract

Laser ablation is a method used to create surface structures on materials by focusing laser energy onto a small area. Doing so requires a trade-off between high energy for fast production and low energy to maintain structure quality. Laser ablation through a water layer has shown promise in overcoming this challenge but to do so, optimal water layer and laser parameters are still largely unknown and require further examination. To aid in this examination, this thesis explores the production of craters on stainless steel and silicon surfaces using nano- and picosecond pulsed laser sources set to different pulse frequencies and pulse energies. Water layer thickness and flow over the ablation region are also varied. A series of morphological analysis show that different structures are formed during ablation under a water layer compared to ambient air. Additionally, a volume analysis identifies three pulse energy-dependent crater formation regimes. Results of varying the water layer thickness for the picosecond pulsed laser source are ambiguous whereas a 1-mm water layer yields most ablated volume for the nanosecond laser source. Finally, flow has a positive influence on ablated volume of craters, but only up to a pulse frequency of 1 kHz. At 1 kHz, crater volume decreases significantly. This decrease is attributed to bubbles formed during underwater laser ablation which negatively affect subsequent laser pulses and material removal rate in general, but especially at 1 kHz.