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The irradiation of a steel surface in atmospheric pressure ambient was performed to study the surface nanostructuring resulting from the formation of a backward-deposited layer. The dynamics of the plume expansion and of the nanoparticle deposition process were investigated by in situ time-resolved optical analysis. Scanning electron microscopy and atomic force microscopy were employed to investigate the morphological characteristics of the backward deposited layer. The observations revealed a particular shape of plasma expansion. The latter is characterized by the formation of two vortices at the plasma plume periphery, where a high density of condensed nanoparticles are generated. It is shown that the surface nanostructuring is mainly due to a backward nanoparticles flux which leads to a deposition process during several tens of microseconds. The effects of laser wavelength on nanoparticle formation and surface nanostructuring are presented and discussed.

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