fs Laser surface nano-structuring of high refractory ceramics to enhance solar radiation absorbance

The surface and structural modification of titanium (Ti) has been explored after the interaction of ultrashort laser pulses with the surface target. The targets were exposed by femtosecond Ti: Sapphire laser pulses in liquid (ethanol) and dry (air) environment. In order to explore the effect of pulse energy, the targets were exposed to 1,000 succeeding pulses for various pulse energies ranging from 200 to 500 μJ for pulse duration of 25 fs. SEM analyses were performed for central as well as the peripheral ablated areas of the target. It was found that in the case of ethanol (both for central and peripheral ablated areas) there is a grain growth along with nanoscale pores and dots when the target was irradiated for 200 μJ. For intermediate energies (300–400 μJ), grains of 1–2 μm with distinct boundaries are formed in the central ablated area. Whereas in the peripheral ablated area, laser-induced periodic surface structures (LIPSS) and globules are grown. For the highest pulse energy (500 μJ), distinct grains are observed for both regions. However, in the peripheral area the grains are of bigger size with cracks along the boundaries. In case of ablation in air, in the center of ablated areas, island-like structures with multiple ablative layer or LIPSS and nanoscale spheres are observed both for lower and intermediate pulse energies. For the highest pulse energy only nanoscale LIPSS could be observed. For ablation in air at the peripheral areas, well-defined, laser-induced periodic surface structures are observed for all pulse energies. Raman spectroscopy reveals that the liquid (ethanol) environment forms the carbonyl compounds with the metal and induces C–C stretching vibration, whereas in case of air, hydroxo complexes are formed. It has been found that surface treatment of Ti with ultrashort (25 fs) laser radiation in ethanol environment allows the growth of particular surface structures in the form of grains and simultaneously induces changes in its chemical composition.