Femtosecond laser nanostructuring of metals: sub100-nm one-dimensional surface gratings

One-dimensional quasi-periodic structures whose period is much smaller than the wavelength of exciting optical radiation have been obtained on a titanium surface under the multi-shot action of linearly polarized femtosecond laser radiation at various surface energy densities. As the radiation energy density increases, the one-dimensional surface nanogratings oriented perpendicularly to the radiation polarization evolve from quasi-periodic ablative nano-grooves to regular lattices with sub-wavelength periods (90-400 nm). In contrast to the preceding works for various metals, the period of lattices for titanium decreases with increasing energy density. The formation of the indicated surface nanostructures is explained by the interference of the electric fields of incident laser radiation and a surface electromagnetic wave excited by this radiation, as shown by our transient reflectivity measurements and modeling, because the length of the surface electromagnetic wave for titanium with significant interband absorption decreases versus increasing electron excitation of the material.