Single and multiple shot near-infrared femtosecond laser pulse ablation thresholds of copper

Direct observation of the temperature field during ablation of materials by multiple femtosecond laser pulses

“…Although the single-pulse peak fluence ranged from below to above the single-pulse melting threshold of steel (which was measured to be about 226 mJ cm À2 ), the accumulation of multiple fs pulses for all the chosen fluence levels (i.e., from 74 to 594 mJ cm À2 ) could ablate the steel specimen with crater observed after a period of time. This accumulation effect by sub-threshold pulses is similar to a recent study on copper specimens ablated by multiple fs laser pulses [20]. Depending on the laser fluence, a saturated condition (with negligible temperature fluctuation with time) was achieved 40-80 s after the laser was turned on.…”

Generation of subsurface voids and a nanocrystalline surface layer in femtosecond laser irradiation of a single-crystal Ag target

“…A notable observation from this simulation, however, is that the presence of sub-surface voids generated by the first laser pulse brings the absorbed fluence required for the material ejection below the single-pulse spallation threshold. This observation may be related to the incubation effect reported for multi-pulse irradiation of metal targets, when the laser fluence threshold for ablation/damage decreases with increasing number of laser pulses applied to the same area, e.g., [61,[65][66][67][68][69]. While the generation and accumulation of defects has been discussed in general terms as one of the mechanisms [12,[65][66][67]70] (along with the absorption enhancement due to roughening of the surface [61,68] and heat accumulation in high repetition rate irradiation [70]) responsible for the incubation effect, the current simulation results provide first direct insights into the laser-induced structural-modification processes that will also contribute to the reduction of the spallation threshold in the multi-pulse laser irradiation regime.…”

Femtosecond laser ablation of silver foil with single and double pulses

“…(1 À R) was five times higher than the classical value for pure silver. This adjustment of the Fresnel value has previously found to be necessary in calculations [33] and in matching multiple pulse with single pulse measured ablation thresholds [35]. The increase in absorptivity is probably due to roughened and damaged surfaces from previous pulses, but could also be influenced by the strong changes in electron temperature during the pulse, not included in the present analysis.…”