Crystalline / glass nanoscale chemical separation induced by femtosecond laser pulses in aluminosilicate glass

An aluminosilicate glass with a molar composition of 60%Al$_2$O$_3$–40%SiO$_2$ fabricated by the aerodynamic levitation technique was irradiated in volume using an infrared femtosecond laser. Optical retardance was observed and measured upon a wide range of varying laser parameters including pulse energy, repetition rate, pulse duration, writing speed, and laser polarization. A similar analysis was performed in silica glass (Suprasil CG) taken as a reference. We measured retardance values as high as 100 nm in the aluminosilicate glass, which is approximately 2x higher than previously investigated aluminosilicate glasses. Raman spectroscopy and electron microscopy techniques revealed the presence of an orientable lamellar structure, commonly referred to as nanogratings, composed of alternate SiO$_2$-enriched amorphous layers with Al$_2$O$_3$-enriched ones. The latter are essentially crystallized in a 3Al$_2$O$_3$:2SiO$_2$ Mullite phase. This internal structuring, the first one in a congruent glass, brings additional insights into the mechanisms leading to nanogratings formation, and may be proved useful for future glass-based functional optical devices with very large thermal stability.