SEM and Raman spectroscopy analyses of laser-induced periodic surface structures grown by ethanol-assisted femtosecond laser ablation of chromium

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 11.8Jcm(-2) for pulse duration of approximate to 25fs. In order to explore the effect of pulse duration, targets were exposed to fs laser pulses of various pulse durations ranging from 25 to 100fs, for a constant fluence of 11.8Jcm(-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 1.4-4mJcm(-2), very well-defined laser-induced periodic surface structures (LIPSS) with periodicity ranging from 270 to 370nm 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 (25fs), 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.