Nanoscale femtosecond laser milling and control of nanoporosity in the normal and anomalous regimes of GeO_2-SiO_2 glasses

Glass modifications on the nanoscale occurring after femtosecond laser irradiation give rise to strong form birefringence. This birefringence is related to the so-called nanogratings. By observing induced tracks in various germanosilicate glasses using scanning electron microscopy (SEM), we demonstrate that porous nanoplanes can be formed not only in silicate glasses with anomalous density behaviour with fictive temperature, but also within glassy systems with normal density behaviour. The nanoporous oxide is likely due to fast decomposition and volume expansion along with glassy condensation of the oxide creating extreme conditions far from equilibrium. The porosity filling factor and the average pore size significantly decreases when increasing the GeO2 content. Precise laser translation and control of these nanoporous structures allows arbitrary milling, tuning and positioning within the glass, an important top-down approach to control micro and nanostructure and consequently optical properties for molecular sieves, catalysts, composites and optoelectronics applications. At a fundamental level, femtosecond laser milling of glass allows access to glassy regimes that may have no obvious natural counterpart.