On the fundamental structure of femtosecond laser-induced nanogratings

The nanoscale structure of femtosecond laser-induced modifications known as "nanogratings" has been the subject of speculation and intensive debate throughout the decade since their discovery. The aim of this work is to gain dependable information on the three-dimensional (3D) substructure of nanogratings down to the nanometre scale. To this end, non-destructive small angle X-ray scattering (SAXS) was employed to determine the characteristic sizes associated with the smallest features over a wide range of inscription parameters. The characteristic size of these cavities is 30 × 200 × 300 nm3 and largely independent of the exposure parameters, whereas prolonged exposure to laser pulses leads to an increase in their total number. Subsequently, focused ion beam (FIB) milling was used to dissect an extended volume and for the first time directly observe the 3D structure of nanogratings with nanometre resolution. The experiments clearly show that hollow cavities are the primary constituents of nanogratings and that their sheet-like arrangement gives rise to the well-known periodicity.