  In laser micromachining, the ablation threshold (minimum fluence required to
cause ablation) is a key performance parameter and overall indicator of the
efficiency of material removal. For pulsed laser micromachining, this important
observable depends upon material properties, pulse properties and the number of
pulses applied in a complex manner that is not yet well understood. The
incubation effect is one example. It manifests as a change in the ablation
threshold as a function of number of laser pulses applied and is driven by
photoinduced defect accumulation in the material. Here, we study femtosecond
(800 nm, 110 fs, 0.1-1 mJ/pulse) micromachining of a material with well-defined
initial defect concentrations: doped Si across a range of dopant types and
concentrations. The single-pulse ablation threshold (Fth,1) was observed to
decrease with increasing dopant concentration, from a maximum of 0.70 J/cm2
(+/-0.02) for undoped Si to 0.51 J/cm2 (+/-0.01) for highly N-type doped Si.
The effect was greater for N-type doped Si than for P-type, consistent with the
higher carrier mobility of electrons compared to holes. In contrast, the
infinite-pulse ablation threshold (Fth,inf) was the same for all doping levels
and types. We attribute this asymptotic behaviour to a maximum defect
concentration that is independent of the initial defect concentration and type.
These results lend insight into the mechanism of multipulse, femtosecond laser
ablation.
