CC BY
15LMI-I-7
Influence of pulse duration and pulse separation on dynamics and efficiency of ultrafast laser ablation of metals
Jan Winter1, Maximilian Spellauge1, David Redka1, Heinz P. Huber1*
1-Munich University of Applied Sciences, Lasercenter, Lothstr. 34, 80335 Munich, Germany
*heinz.huber@hm.edu
A surface irradiated with an ultrashort pulse passes through a sequence of physical processes, occurring on a temporal range spanning from femtoseconds to microseconds. Open questions remain as to why the pulse duration and temporal pulse separation influence the energy specific ablation volume.
Pump-probe ellipsometry (PPE) [1] reveals changes to the complex refractive index for the first tens of picoseconds, while pump-probe microscopy (PPM) [2] gives access to changes of the relative reflectivity from the initial pulse impact to the final state after about 1 ^s. Fig. 1 displays PPM experiments on Cu, Al and AISI304 in air, showing the transient evolution of the relative reflectivity change AR/R for probe pulse delay times ranging between -12.5 ps and 10 ^s.
T-
0,000 0,025 '' 0,1 I 10 100 1000 10000
Time [ns]
Fig. 1 Transient reflectivity change AR/R of Cu (black open squares), Al (red open circles) and AISI304 (blue open triangles)
The experiments indicate an ultrafast density decrease at the surface of 30% within the first 5 ps, which creates a surface expansion [3] as well as separation and propagation of a spallation layer from about 100 ps to 1 ns. Finally, the spallation layer disintegration and particle generation occurs from 1 ns to 100 ns [4].
Our measurements and simulations support the conclusion that the ablation process is efficient and precise when stress-confinement is fulfilled, which means, when initiated with pulse durations shorter than the mechanical relaxation time of about 5 ps and left un-interrupted until the final state is approached after about 10 to 100 ns [5]. MHz-burst processing leads to particle shielding, GHz-burst processing to re-deposition.
[1] S. Rapp, M. Kaiser, M. Schmidt, H.P. Huber, Ultrafast pump-probe ellipsometry setup for the measurement of transient optical properties during laser ablation, Opt. Expr. 24 (2016) 17572
[2] M. Domke, S. Rapp, M. Schmidt, and H. Huber, Ultrafast pump-probe microscopy with high temporal dynamic range, Optics Express 20 (2012)10330
[3] J. Winter, S. Rapp, M. Schmidt and H.P. Huber, Ultrafast laser processing of copper: A comparative study of experimental and simulated transient optical properties, Applied Surface Science 417 (2017) 2
[4] J. Winter, S. Rapp, M. Spellauge, C. Eulenkamp, M. Schmidt, and H.P. Huber, Ultrafast pump-probe ellipsometry and microscopy reveal the surface dynamics of femtosecond laser ablation of aluminium and stainless steel, Applied Surface Science 511 (2020) 145514
[5] M. Spellauge, J. Winter, S. Rapp, C. McDonnell, F. Sotier, M. Schmidt, and H.P. Huber, Influence of stress confinement, particle shielding and re-deposition on the ultrashort pulse laser ablation of metals revealed by ultrafast time-resolved experiments, Applied Surface Science 545 (2021) 148930.
