Femtosecond-laser-induced quasiperiodic nanostructures on TiO2 surfaces

High-spatial frequency, quasiperiodic structures (HSFL, Nanoripples) of 170 nm feature size were induced in rutile-type titanium dioxide surfaces by focused 150 fs Ti:sapphire laser pulses at wavelengths around 800 nm. The ripple formation is distinctly visible for numbers of pulses of N=100-1000. At lower number of pulses (N=10), a significant surface roughening appears instead of ripples which is characterized by randomly meandering nanostructures. These observations confirm an essential contribution of early stage irregular material modifications to the dynamics of quasiperiodic ripple formation. The threshold fluence for ripple generation is estimated on the basis of the conventional theory of laser-induced surface structuring. The decrease in the threshold fluence from 0.34 to 0.24 J/cm(2), as it was found for an increase in the number of pulses from N=100 to N=1000, is attributed to a damage accumulation effect. Nanostructuring of spatially extended regions was enabled by utilizing a controlled sample translation at optimized energy and repetition rate of the laser pulses. A significant enhancement of the periodicity of the nanostructures was confirmed by two-dimensional Fourier transform of scanning electron microscopy data. At second harmonic wavelengths around 400 nm, the generation of nanoripples even on the sub-100 nm scale was demonstrated. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3117509]