Multi-phase titanium oxide LIPSS formation under fs laser irradiation on titanium thin films in ambient air

The formation of laser-induced periodic surface structures (LIPSS) on Ti thin films, their phase and stoichiometric evolution, as a function of well below ablation threshold laser fluence, and the number of pulses, is investigated. The experiments were carried out in ambient air by using a femtosecond laser at a wavelength of 1030 nm with a pulse duration of 270 fs, operating at a repetition rate of 18.6 kHz. On the one hand, the formation of LSFL composed by an either single phase (C-Ti2O3 or r-TiO2-x) or a two-phase mixture of titanium oxide (C-Ti2O3 +r-TiO2-x) is reported. The titanium oxide phases were obtained by micro-Raman spectroscopy. The orientation of the LSFL, either parallel or perpendicular with respect to the polarization of the incident beam, correlate well with the electronic nature of the titanium oxide phases. On the other hand, the results also show the formation of HSFL, this under low cumulative fluence, with periods of 398 and 460 nm with a two-phase mixture of titanium oxide in the form ofa-Ti2O3+ C-Ti2O3and C-Ti2O3+ r-TiO2-x, respectively. To the best of our knowledge this is the first time that single phase C-Ti2O3 or r-TiO2-x LIPSS are reported.