Formation of nano-textured conical microstructures in titanium metal surface by femtosecond laser irradiation

We report for the first time that a regular array of sharp nano-textured conical microstructures are formed on the titanium metal surface by irradiation with ultrafast laser pulses of 130 fs duration, 800 nm wavelength in vacuum (∼1 mbar) or in 100 mbar He. The microstructures are up to 25 μm tall, and taper to about 500 nm diameters at the tip. Irradiation in the presence of SF₆, air or HCl creates a textured surface but does not create sharp conical microstructures. The surfaces of these microstructures exhibit periodic nano-texture of feature size comparable to the wavelength of light consistent with ripple formation. Contrary to pillar formation by femtosecond laser irradiation of silicon where the initial ripples evolve into the pillars and the ripples disappear, the ripples on titanium pillars have a much smaller periodicity than the pillars and remain on the surface of the pillars. The textured surface is pitch black compared to its original silver-grayish color, i.e, it exhibits greatly reduced reflectivity throughout the measured visible spectrum. PACS 52.38.Mf; 79.20.Ds; 81.07.-b; 81.16.-c; 82.53.-k