Ultrafast laser micromachining that utilises pulses on a femtosecond timescale is a rapidly growing area of research with applications in a wide variety of fields, from microelectronics to microsurgery. Femtosecond pulses are often praised for their ability to perform precise cutting of materials through a ‘cold-cutting’ mechanism which avoids mechanical and thermal collateral damage to the surrounding area. However, the high precision and clean ablation features associated with ultrafast laser micromachining can be counteracted through the intense plasma in air that is generated at high pulse energies. The highly reflective plasma generated above the sample surface can result in a distorted beam profile at the target machining plane, producing machined features with reduced edge quality and accuracy. In addition, the highly reflective plasma results in underutilised portions of the incident pulse energy, therefore decreasing machining efficiency.
We present the ablation threshold data and trends for a variety of materials including undoped silicon, stainless steel and sapphire laser machined under vacuum and other ambient conditions. Ablation thresholds were determined using the diameter regression technique with 130 fs, 800 nm laser pulses at a repetition rate of 500 Hz. Ablation features are analysed extensively to observe the impact of the ambient conditions on the resulting feature quality.
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.
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