GHz burst mode femtosecond (fs) laser processing that contains a series of fs laser pulse (intra-pulse) trains with a pulse interval of several hundred picoseconds, has been attracting considerable attention, since it can offer some distinct characteristics in material processing compared with the conventional fs laser irradiation scheme (single-pulse mode). Specifically, laser ablation with the GHz burst mode can be achieved higher-quality with enhanced ablation efficiency than that by the single-pulse mode. Most of the research using GHz burst mode to date has been focused on ablation of materials. The objective of this study is to examine the possibility of GHz burst mode fs laser processing for the fabrication of novel nanostructures, that is different from laser-induced periodic surface structures (LIPSS) by the conventional single-pulse mode.
In the experiment, linearly-polarized fs laser pulses (central wavelength 1030 nm, pulse width 220 fs) were focused onto crystalline silicon and titanium surfaces. It’s well known that the single-pulse mode creates nanoripple structures on both substrates, whose direction is perpendicular to the laser polarization direction.  Interestingly, we found that GHz burst mode fs laser pulses can form unique two-dimensional LIPSS on both substrates with periodic structures perpendicular and parallel to the laser polarization direction. The formation mechanism of 2D LIPSS is discussed and their surface properties are characterized.