Periodic surface structures on titanium self-organized upon double femtosecond pulse exposures

Laser induced periodic surface structures (LIPSS) self-organized on Ti surface after irradiations by femtosecond laser beam composed by double pulses with a fixed time delay of 160 fs. The fluence of the first pulse (F-PP), responsible for surface plasma formation, was varied in the range 10-50 mJ cm(-2) and always kept below the LIPSS formation threshold fluence (F-LIPSS) on Ti for 50-single-shots exposure. The fluence of the delayed pulse (F-LP), responsible for LIPSS self-organization, was varied in the range 60-150 mJ cm(-2) and always kept above F-LIPSS. Regardless the specific fluence F-LP of the delayed pulse, the interspace of the grating structures increases with the increase of F-PP, that is an increase of the surface plasma density. This tendency suggests that a variation of the surface plasma density, due to a variation of F-PP, actually leads to a modification of the grating features. Moreover, we observed that the LIPSS periodicities after double pulse exposures are in quite good agreement with data on LIPSS periodicities after single 160 fs pulse irradiations on Ti surface and with the curve predicted by the parametric decay model. This experimental result suggests that the preformed plasma might be produced in the rising edge of the temporal profile of the laser pulse. (C) 2015 Elsevier B.V. All rights reserved.