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S. Bashir, M. Rafique, C. Nathala, A. Ajami, W. Husinsky:
"SEM and Raman spectroscopy analyses of laser-induced periodic surface structures grown by ethanol-assisted femtosecond laser ablation of chromium";
Radiation Effects and Defects in Solids, 170 (2015), 5; 414 - 428.

The effect of fluence and pulse duration on the growth of nanostructures on chromium (Cr) surfaces has
been investigated upon irradiation of femtosecond (fs) laser pulses in a liquid confined environment of
ethanol. In order to explore the effect of fluence, targets were exposed to 1000 pulses at various peak
fluences ranging from 4.7 to 11.8 J cm-2 for pulse duration of ∼25 fs. In order to explore the effect of
pulse duration, targets were exposed to fs laser pulses of various pulse durations ranging from 25 to 100
fs, for a constant fluence of 11.8 J cm-2. Surface morphology and structural transformations have been
analyzed by scanning electron microscopy and Raman spectroscopy, respectively. After laser irradiation,
disordered sputtered surface with intense melting and cracking is obtained at the central ablated areas,
which are augmented with increasing laser fluence due to enhanced thermal effects. At the peripheral
ablated areas, where local fluence is approximately in the range of 1.4-4 mJ cm-2, very well-defined
laser-induced periodic surface structures (LIPSS) with periodicity ranging from 270 to 370 nm along with
dot-like structures are formed. As far as the pulse duration is concerned, a significant effect on the surface
modification of Cr has been revealed. In the central ablated areas, for the shortest pulse duration (25
fs), only melting has been observed. However, LIPSS with dot-like structures and droplets have been
grown for longer pulse durations. The periodicity of LIPSS increases and density of dot-like structures
decreases with increasing pulse duration. The chemical and structural modifications of irradiated Cr have
been revealed by Raman spectroscopy. It confirms the formation of new bands of chromium oxides and
enol complexes or Cr-carbonyl compounds. The peak intensities of identified bands are dependent upon
laser fluence and pulse duration.

chromium; femtosecond laser; pulse duration; fluence; nanostructures; laser-induced period surface structures; SEM; Raman spectroscopy