The onset of ultrashort pulse-induced nanogratings

The fundamental formation process of nanogratings induced by focused ultrashort laser pulses inside the bulk of transparent materials is still object of intense debate. In particular the initial evolutionary steps, including the formation of anisotropic pores, alignment and arrangement in highly periodic grating planes could not been clarified up to now. To this end we have performed a comprehensive investigation of this process by optical retardance measurements, small-angle X-ray scattering, and focused ion beam milling in combination with scanning electron microscopy. The results show that the formation of nanogratings starts with isotropic, deterministic voids after the first incident laser pulse which start to elongate perpendicular to the laser scan direction with further illumination and the appearance of cracks along the scan direction. For increasing pulse overlap these cracks as well as randomly aligned voids build the template for the subsequent pore growth perpendicular to the laser polarization. The process of grating formation and subsequent period reduction occurs via increasing number of sheets and mutual realignment forming a more porous glass matrix.