The presence of surface oxides on the formation of laser-induced periodic surface structures (LIPSS) is regularly advocated to favor or even trigger the formation of high-spatial-frequency LIPSS (HSFL) during ultrafast laser-induced nano-structuring. This paper reports the effect of the laser texturing environment on the resulting surface oxides and its consequence for HSFLs formation. Nanoripples are produced on tungsten samples using a Ti:sapphire femtosecond laser under atmospheres with varying oxygen contents. Specifically, ambient, 10 mbar pressure of air, nitrogen and argon, and 10−7 mbar vacuum pressure are used. In addition, removal of any native oxide layer is achieved using plasma sputtering prior to laser irradiation. The resulting HSFLs have a sub-100 nm periodicity and sub 20 nm amplitude. The experiments reveal the negligible role of oxygen during the HSFL formation and clarifies the significant role of ambient pressure in the resulting HSFLs period.

In this work, we report the fabrication and characterization of large area micro-/nano-textured silicon surfaces using laser pulses of nanoseconds duration. An area of 6×6mm2 has been textured by the parallel line scanning method to create hierarchical structures, consisting of microscale channels and self-organized surface nano-capillaries decorated with randomly distributed silicon nanoparticles. The combination of micro-channels and nano-capillaries results in a superhydrophilic silicon surface, with the contact angle reduced substantially from about 80° to nearly 5°. In contrast to most of the reports given in the literature, the superhydrophilicity of the surface remains stable without a shift to hydrophobicity, even after exposure to the atmosphere for about three months. Thus, long-lasting and durable superhydrophilic silicon has been obtained by using maskless, compact, and cost-effective nanosecond laser writing, without the need to employ any chemical post-processing. Potential applications of these surfaces include heat exchangers, biosensors, cell adhesives, and self-cleaning solar cells.

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