Ultrashort laser pulse induced nanogratings in borosilicate glass

We report on nanogratings inscribed by repetitive femtosecond laser pulses into the bulk of borosilicate glass. The irradiation produces small nanopores (10–20 nm thick) which start to self-organize in gratings as well as elongated sheets of up to 400 nm length. A quantitative description of the grating structure and its development are obtained by a combination of focused ion beam milling, scanning electron microscopy, and small angle X-ray scattering (SAXS). The SAXS partial invariant of the thin sheets is found to correlate well with the measured optical retardance. Compared to fused silica nanogratings borosilicate glass shows a much smaller retardance due to re-annealing of pores. In addition, the nanograting period strongly deviates from the well-known λ/2n prediction. We could observe periods down to 60 nm (at an inscribing wavelength of 800 nm). This has not been observed yet in other glasses.