Publications in Scientific Journals:

S. Bashir, M. Rafique, W. Husinsky:
"Identification of non-thermal and thermal processes in femtosecond laser-ablated aluminum";
Radiation Effects and Defects in Solids, 168 (2013), 1 - 10.

English abstract:
Non-thermal and thermal processes due to femtosecond laser ablation of aluminum (Al) at low, moderate,
and high-fluence regimes are identified by Atomic Force Microscope (AFM) surface topography investigations.
For this purpose, surface modifications of Al by employing 25 fs Ti: sapphire laser pulses at the
central wavelength of 800 nm have been performed to explore different nano- and microscale features such
as hillocks, bumps, pores, and craters. The mechanism for the formation of these diverse kinds of structures
is discussed in the scenario of three ablation regimes. Ultrafast electronic and non-thermal processes are
dominant in the lower fluence regime, whereas slow thermal processes are dominant at the higher fluence
regime. Therefore, by starting from the ablation threshold three different fluence regimes have been chosen:
a lower fluence regime (0.06-0.5 J cm−2 single-shot irradiation under ultrahigh vacuum condition and
0.25-2.5 J cm−2 single-shot irradiation in ambient condition), a moderate-fluence regime (0.25-1.5 J cm−2
multiple-shot irradiation), and a high-fluence regime 2.5-3.5 J cm−2 multiple-shot irradiation. For the lower
fluence (gentle ablation) regime, around the ablation threshold, the unique appearance of individual, localized
Nano hillocks typically a few nanometers in height and less than 100 nm in diameter are identified.
These Nano hillock-like features can be regarded as a nonthermal, electronically induced phase transition
process due to localized energy deposition as a result of Coulomb explosion or field ion emission by surface
optical rectification. At a moderate-fluence regime, slightly higher than ablation threshold multiple-pulse
irradiation produces bump-formation and is attributed to ultrafast melting (plasma formation). The highfluence
regime results in greater rates of material removal with highly disturbed and chaotic surface of
Al with an appearance of larger protrusions at laser fluence well above the ablation threshold. These nonsymmetrical
shapes due to inhomogeneous nucleation, cluster formation, and resolidification of a metallic
surface after melting are attributable to slow thermal processes (ps time scale).

nanohillocks; bumps; nonthemal and thermal ablation; femtosecond laser; Coulomb explosion; surface optical rectification

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