Emission signal enhancement of laser ablation of metals (aluminum and titanium) by time delayed femtosecond double pulses from femtoseconds to nanoseconds

Experimental study on double-pulse laser ablation of steel upon multiple parallel-polarized ultrashort-pulse irradiations

“…In regime 1, as observed for interpulse delays much shorter than 1 ps, the effect of double pulses was found to be very similar to single-pulse ablation of same energy [21]. In this timescale, the inter-pulse delay is shorter than the time for electron-phonon coupling; thus, electrons and ions do not interact and the double pulse behaves as a single pulse having twice the fluence [21,25,28]. For the longer inter-pulse delay closely to electron-ion relaxation, in the range between one and tens of picoseconds for metal, the removal rate decreases monotonically with increasing inter-pulse delay in ablation regime 2.…”

“…However, for pulse-train processing, the relevant literature identified up to four different characteristic ablation regimes that are distinguished based on their inter-pulse delay. In regime 1, as observed for interpulse delays much shorter than 1 ps, the effect of double pulses was found to be very similar to single-pulse ablation of same energy [21]. In this timescale, the inter-pulse delay is shorter than the time for electron-phonon coupling; thus, electrons and ions do not interact and the double pulse behaves as a single pulse having twice the fluence [21,25,28].…”

“…This was determined, for example, in silicon [22], silver foils [25], aluminum and copper [27] and copper films [28][29][30], steel [27,31], or nickel in vacuum [16]. On the other hand, by contrast, a few studies identified insignificant or no increase of metal ablation for double-pulse processing, as stated for aluminum [32], copper [19], and titanium [21]. Later studies have shown experimentally and by modeling that splitting the pulse energy was a way to be closer to the optimal regime for laser ablation with increased efficiency, without clear evidence in benefits of burst regime in laser ablation [33,34].…”

“…This effect can also be attributed to interaction of the different stress waves [28,35], as already mentioned above. For nanosecond time delay in ablation regime 4, the ablation rate increases again, reaching the level of single-pulse ablation for each individual pulse [21,35]. Therefore, it is argued that plasma expands with larger times between the pulses; thus, the plasma plume becomes transparent for the delayed pulse and a higher amount of energy is irradiating the sample [21,35,36].…”

“…For bursts with shorter inter-pulse delay, it was reported that plasma shielding limits the removal efficiency [14] while longer delays cause highenergy plasma generation that is explosively remelting the surface, in turn increasing surface roughness significantly [11]. In case of pulse-train processing with very short picosecond inter-pulse delay, by contrast, electron/ion yield [15][16][17], plasma intensity [18][19][20][21], and phonon temperature [17,[22][23][24][25][26] increase as a result of enhanced photon efficiency. For the latter, temperature simulations indicate that inter-pulse delay of 10 ps or less is optimal for maximum heating.…”