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 lambda/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.