Nanometer-resolved chemical analyses of femtosecond laser-induced periodic surface structures on titanium.

The chemical characteristics of two different types of laser-induced periodic surface structures (LIPSS), so-called high and low spatial frequency LIPSS (HSFL and LSFL), formed upon irradiation of titanium surfaces by multiple femtosecond laser pulses in air (30 fs, 790 nm, 1 kHz), are analyzed by various optical and electron beam based surface analytical techniques, including micro- Raman spectroscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, and Auger electron spectroscopy. The latter method was employed in a high-resolution mode being capable of spatially resolving even the smallest HSFL structures featuring spatial periods below 100 nm. In combination with an ion sputtering technique, depths-resolved chemical information of superficial oxidation processes was obtained, revealing characteristic differences between the two different types of LIPSS. Our results indicate that a few tens of nanometer shallow HSFL are formed on top of a ~150 nm thick graded superficial oxide layer without sharp interfaces, consisting of amorphous TiO₂ and partially crystallized Ti₂O₃. The larger LSFL structures with periods close to the irradiation wavelength originate from the laser-interaction with metallic titanium. They are covered by a ~200 nm thick amorphous oxide layer, which consists mainly of TiO₂ (at the surface) and other titanium oxide species of lower oxidation states underneath. [ABSTRACT FROM AUTHOR]