The underlying structure of ultrashort pulse laser-induced nanogratings

Sub-wavelength structures are a crucial ingredient for modern optics. A class of ultrashort laser pulse induced, selforganized modifications in bulk transparent materials have attracted particular interest in recent years. Despite the multitude of potential applications of these so-called "nanogratings", their underlying structure on the nanometer scale has been the subject of intensive debate throughout the decade since their discovery: Are they merely continuous modulation patterns of the material density, or do they consist of a substructure of hollow cavities? As nanogratings are embedded within the bulk material the conventional visualization technique relies on polishing and subsequent etching to excavate the modifications. However, such invasive sample preparation effectively erases sub-100 nm features. Moreover, they only provide access to two-dimensional cross sections. To overcome these limitations, we employed small angle X-ray scattering (SAXS), focused ion beam (FIB) milling and scanning electron microscopy (SEM) to reveal the underlying three-dimensional structure of nanogratings. Our results show that small cavities are the primary constituents of the nanogratings. These cavities grow predominantly during the first 100 laser pulses and reach a final size of about 30x200x300 nm3. Prolonged exposure to laser pulses increases the absolute number of cavities. Their threedimensional arrangement forms characteristic periodic planes of nanogratings.