Influence of pulse separation on the ablation volume during femtosecond laser ablationWednesday (24.06.2020) 14:10 - 14:30 Room 1
In recent years, several works have been investigating the temporal pulse spacing with the aim of maximizing removal rate by applying double pulses or pulse bursts. When double pulse time spacing is exceeding the electron-phonon relaxation time, the second pulse suppresses the rarefaction wave induced by the first pulse resulting in a decreased ablation depth. The above findings can certainly explain the drop in double pulse ablation depth in the first few ps, whereby do not explain the double pulse ablation depth dropping below that of a single pulse, when temporal pulse spacing exceeds well above the electron-phonon relaxation time. For a separation time in a nanosecond domain it was argued that only the first pulse contributes to the formation of the ablation crater, while the second pulse is absorbed by the ejected ablation plume resulting into a re-deposition of ablation plume. Here we study the removal rate of double pulses in dependency of increasing pulse separation on aluminum bulk material. We focus especially on a pulse spacing range from tens of picoseconds up to one nanosecond to close an existing gap in the state of the art. To avoid incubation effects the double pulse irradiation is repeated only three times at one position. Ultrafast pump-probe ellipsometry and microscopy are applied to study the surface material motion after single laser pulse impact. Additionally, a hydrodynamic simulation of double pulse ablation supports the experiments. The comparison of time-resolved measurements and double pulse ablation volumes confirm the suggestion that a double pulse acts as a single pulse for double pulse spacing below 5 ps. Between 5 ps and 20 ps the interaction of the second pulse with the rarefaction wave decreases removal rate. The propagation of ablation layer is clearly visible in between 100 ps and 1 ns. Therefore, we can conclude that for longer double pulse spacing up to 1 ns the decrease in removal rate can be attributed to the re-deposition of ablated material back to the surface. The re-deposition is confirmed with a hydrodynamic simulation with a pulse spacing of 100 ps.
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