Liquid assisted ablation of zirconium for the growth of LIPSS at varying pulse durations and pulse energies by femtosecond laser irradiation

Abstract

Investigations have been performed to explore the optimized conditions for the growth of Laser Induced Periodic Surface Structures (LIPSS) by varying pulse durations and pulse energies during ultrashort pulsed laser ablation of zirconium (Zr). The Ti: Sapphire laser with central wavelength of 800nm, maximum pulse energy of 1mJ is used to ablate Zr targets in the wet environment of ethanol. Scanning Electron Microscope (SEM) analysis was performed for central as well as the peripheral ablated area to characterize nano and microstructures formed on the Zr surface. Raman spectroscopy was carried out to explore the chemical and compositional changes produced in laser ablated Zr. In order to explore the effect of varying pulse durations ranging from 25 to 100fs, targets were exposed to 1000 succeeding pulses keeping the pulse energy constant at 600μJ. The micrographs of peripheral ablated areas reveal the formation of nano scale ripples or Laser Induced Periodic Surface Structures (LIPSS) for all pulse durations. LIPSS are more distinct and well organized for the shortest pulse duration of 25fs. Whereas, LIPSS become diffused and indistinct with the increase in the pulse duration. This is the clear indication that shortest pulse duration (in our case 25fs) is most suitable for the growth of nanoscale ripples. In order to explore the effect of varying pulse energies on the growth of LIPSS, targets were exposed to 1000 succeeding pulses with energies ranging from 200μJ to 600μJ for a pulse duration of 25fs. In the peripheral ablated areas LIPSS are grown for all pulse energies. For the lowest pulse energy of 200μJ, LIPSS are distinct and well defined. For intermediate energies of 300 and 400μJ they become diffused and indistinct. For higher pulse energies of 500 and 600μJ, their appearance again becomes well defined and distinct. For central ablated areas LIPSS are grown but their appearance diffuses with increasing pulse energies. For the highest pulse energy they are completely vanished. Raman spectroscopy reveals that in the presence of liquids the chemical reactivity of the target with liquid is significantly enhanced which is responsible for the growth of new phases and modification in the chemical composition of the irradiated Zr. The ethanol forms the carbonyl compounds with the Zr and induces C–C stretching. The reactivity of Zr with oxygen is responsible for the formation of tetragonal phase and monoclinic phase of zirconia.