Understanding multi-regime dynamics observed with femtosecond laser ablation (Conference Presentation)

Abstract

Ultrafast laser micromachining has been extensively researched for its “clean, cold” cutting potential in fields from microelectronics to dentistry. It is clear that the mechanism of laser ablation with pulses shorter than about 500 fs differs significantly different from the light-to-heat dominated processes with longer pulsed (ns, ps) and CW laser machining. However, the details of the femtosecond laser ablation mechanism remain incompletely understood. The ablation threshold (J/cm^2) is widely used for characterizing laser machining efficiency. Unfortunately, it is not entirely clear what the ablation threshold means in the ultrashort pulse regime. For example, our diameter regression measurements of the ablation thresholds of several materials using 800 nm, 120 fs laser pulses reveal multiple distinct ablation regimes, each characterized by a different effective beam waist. Evidence of similar behavior can be found in the literature, however it is often unremarked upon. In this paper, we present thorough characterization of the ultrafast laser ablation for a diverse collection of materials (undoped silicon, sapphire, stainless steel and cortical bone). For example, for undoped silicon we find three ablation regimes each characterized by a different ablation threshold and apparent beam waist: (1) 1.56 J/cm^2, 11.8 µm; (2) 1.21 J/cm^2, 51.9 µm; and (3) 0.85 J/cm^2, 159.9 µm. We show the presence of up to three different ablation regimes that vary depending on the type of material. Using computational modeling, we address the mechanistic underpinnings of these observations, particularly the dependence upon pulse energy and spatial beam shape.